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Impact of ventilation design on the precooling effectiveness of horticultural produce—a review

Impact of ventilation design on the precooling effectiveness of horticultural produce—a review Optimizing the ventilation design of packaging system is of crucial importance for improving the efficiency of the forced-air precooling process to maintain the quality of horticultural produce and extend the shelf life in food cold chain. Many efforts had been devoted to the study about the impact of ventilation design on airflow and temperature distribution inside ventilated packages. This paper reviews relevant research methods, commonly used quantities for the measurement of precooling effectiveness, attractive design parameters, and their impact on precooling effectiveness. These allow us to know exactly the characteristic and deficiency of each research method, identify dominant design parameters, and seek a promising way for the future improvement of the ventilated packaging system. Key words: horticultural produce; precooling effectiveness; ventilation design; forced-air precooling technique; packaging system Among the many postharvest technologies, precooling of horti- Introduction cultural produce is the most important technique for maintaining As perishable foods, horticultural products are regional and sea- desirable, fresh, and marketable products. It can remove field heat sonal. Consequently, postharvest handling is a very important from fresh agricultural produce, thereby slowing down metab- step in maintaining the quality of agricultural produce and ex- olism and reducing deterioration before storage and transportation tending the shelf life (Dehghannya et al., 2010; Yan et al., 2019). (Dehghannya et al., 2010). The average mass loss of the cold chain Respiration and transpiration of horticultural produce are im- without precooling is about 23% more than that of the cold chain portant causes for loss of organic material and moisture (Verboven based on precooling (Wu and Defraeye, 2018). et al., 2004b; Ambaw et al., 2013; Aghdam et al., 2018). There are Forced-air cooling is one of the most widely used precooling tech- many factors influencing the physiological and biological changes niques due to its advantages of rapid cooling rate, high efficiency, of horticultural produce (Brosnan and Sun, 2001; Aghdam et al., and low cost (Kader, 2002; Vigneault and Goyette, 2002; Delele 2019; Xu et  al., 2019), such as product thermal properties, tem- et  al., 2010). The schematic diagram of the forced-air precooling perature, concentration of ethylene, oxygen, carbon dioxide, system is shown in Figure 1. This technique uses a fan to generate the etc. It is noteworthy that temperature is the most important en- necessary driving force to create a pressure difference, which forces vironmental factor affecting the postharvest life of horticultural the cold air through the inside of the box. The cold air removes produce. For example, if fresh strawberries are left for 2 h in the heat from fruits and vegetables when passing through the ventilated 30°C heat, only 80% of the fruits are considered suitable for sale packaging system, and thereby achieves the goal of rapid precooling. (Mitchell et al., 1972). © The Author(s) 2020. Published by Oxford University Press on behalf of Zhejiang University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by- nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 30 Y. Cao et al., 2020, Vol. 4, No. 1 Experimental measurement dominated the early research on the forced-air precooling of horticultural produce. Many efforts had been devoted to the experimental study about the impact of the ventilation design of packaging system on the precooling effectiveness of fresh fruits and vegetables. The most attractive design parameters include airflow rate, vent area, vent position, internal tray, line film, etc. The frequently encountered parameters defined to measure precooling ef- fectiveness include cooling rate, cooling uniformity, air pressure drop, energy consumption, half-cooling time, seven-eighths cooling time, heat transfer coefficient, cooling efficiency, etc. Some representative experimental researches on the impact of ventilation design in the forced-air precooling process of horticultural produce are summar- Figure 1. Typical forced-air cooling system of horticultural produce (Delele ized in Table 1. In order to ensure the reliability of test data obtained et al., 2010). in repeated experiments and in the meanwhile reduce experimental cost, simulators (Castro et al., 2004a, 2004b, 2005; Vigneault et al., Cooling heterogeneity is a prevalent phenomenon during the 2005, 2006, 2007) were usually used in experimental studies rather forced-air precooling process (Alvarez and Flick, 1999a; Alvarez than horticultural produce, such as solid polymer balls, polychlor- et al., 2003), and consequently the ventilation design of packaging inated vinyl balls filled with water, etc. In the rest few experiments system has been a research hotspot in the past 20 years. The adop- (Leyte and Forney, 1999; Fikiin et al., 1999; Anderson et al., 2004; tion of packaging system keeps horticultural produce clean, protects Vigneault et  al., 2004a, 2004b; Ferrua and Singh, 2011; Ngcobo horticultural produce from mechanical damage, minimizes moisture et al., 2012; Mukama et al., 2017; Wu et al., 2018), the precooling loss, and delays microbial decay (Ngcobo et al., 2012). However, the process of horticultural produce, such as table grapes, strawberries, packaging material also hinders the direct contact between horticul- blueberry, etc., was studied. In available experimental studies, the tural produce and the cooling airflow and consequently increases typical package types include carton, clamshell, wooden container, the resistance of the cooling airflow and the energy consumption container formed by acrylic plates, etc. The main results obtained in of the precooling process (Delele et  al., 2013a, 2013b). Therefore, some representative researches are summarized in Table  1. Due to the ventilation design of the packaging system should balance the the complex internal structure of ventilated packages filled with trays relationship between the cooling performance and the mechanical and products, some physical phenomena are difficult to measure with strength of packages. At the same time, the complex internal struc- the high spatial and temporal resolution, such as airflow and heat ture of the package filled with trays and products complicates the transfer characteristics, convective heat transfer coefficient, and tem- airflow and heat transfer characteristics, resulting in serious cooling perature change of the entire product. However, there were still many heterogeneity, such as local under-cooling and over-cooling of prod- researchers who made unremitting efforts for accurate measurements. ucts. These heterogeneous phenomena threaten the quality of horti- Vigneault et al. (2007) developed the Square Cross-Section Velocity cultural produce and the shelf life in food cold chain. Therefore, the (SCSV) measurement method and proved that SCSV can provide optimization of the ventilation design of the packaging system is an a more accurate measurement of airflow rates than the previously active topic in the food industry. developed Circular Approach Velocity method. Ferrua and Singh This review focuses on available studies about the influence of (2011) used nonintrusive flow measurement techniques [Particle ventilation design (airflow rate, vent area, vent position, etc.) on Image Velocimetry (PIV)] to determine the internal airflow field in a the precooling effectiveness of horticultural produce. The rest of package with a container-to-product diameter ratio of less than 10. the review paper is organized as follows. In the Available Research The use of PIV provided a valuable understanding and quantitative Methods section, available research methods about the forced- description of the local behaviour of fluid flow inside the packaging air precooling process of horticultural produce in the ventilated system. Nevertheless, experimental studies for improving ventilation packaging system are summarized, along with some representative design had been limited due to the fluctuations in physical properties researches and results. Both the merits and flaws of each method of horticultural produce in repeated experiments, the difficulties in are described. In the Impact of Ventilation Design on Precooling handling biological materials, and the inherent drawbacks of experi- Effectiveness section, available discussions about the impact of ven- mental studies, such as expensive, time-consuming, etc. tilation design on precooling effectiveness are elaborated. Finally, the With the rapid development of computer technology, numer- research trends and future challenges about the ventilation design of ical simulation has grown to be an alternative research method packaging system are pointed out in the Conclusions section. for the ventilation design of packaging system used in the forced- air precooling of fresh foods. Different from experimental studies, the numerical simulation method is capable of providing detailed Available Research Methods airflow and temperature distribution inside the packaging system In order to ensure the quality and safety of horticultural produce and and avoiding the effect of the fluctuation in the physical proper - extend the storage and shelf life, the critical step in the postharvest ties of fresh foods in a series of tests. Generally, available numerical cold chain is to rapidly precool horticultural produce by removing methods about the forced-air precooling process can be divided into field heat (Han et  al., 2015). Poor postharvest handling techniques two categories: the porous-medium approach and the direct compu- (mainly poor temperature management) can affect the rate of res- tational fluid dynamics (CFD) simulation. piration and thereby lead to decay of horticultural produce and Due to the complex internal structure of ventilated packages postharvest losses (Saenmuang et al., 2012). The postharvest loss and and the limitation of computation resources, the porous-medium waste in fruit and vegetable production supply chains even can be as approach was the dominant numerical method in the early stage. high as 13%–38% before reaching consumers (Defraeye et al., 2015). It allows the simplification of mathematical models by treating the Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 31 Table 1. Representative experimental researches and results about the impact of ventilation design in the forced-air precooling of horti- cultural produce Reference Produce or simulator used Container or Design parameters Effectiveness parameters package type Leyte and Forney, Highbush blueberry Plastic clamshell Vent position Cooling time Remark: Results indicate that the most rapid cooling occurs in the clamshells with vents on the top. While adding vents at the bottom has no effect on the cooling rate. Fikiin et al., 1999 Five horticultural produce Trays of pine wooden Airflow rate Heat transfer coefficient material Remark: Results indicate that the surface heat transfer coefficient of the fruit increases with the airflow rate. Anderson et al., 2004 Strawberry Different clamshell Vent area; clamshell and Cooling time and tray combinations tray combinations Remark: Results indicate that the integral optimization of the combination of clamshell and tray is an effective way to maxi- mize the contact between the cold air and the product. Vigneault et al., 25 different horticultural produce Wooden container Product size; product Air pressure drop 2004a, 2004b shape; vent position Remark: Results indicate that both the size and shape of horticultural produce have a significant impact on the pressure drop. In order to achieve the goal of minimizing the pressure drop, the vents should be uniformly distributed on the sides (perpendicular to airflow) of the container. Castro et al., 2004a Solid polymer ball Container formed by Airflow rate; vent area Cooling efficiency; air pressure acrylic plates drop Remark: Results indicate that the increase of total vent area (0.67%, 2%, 6%) leads to an enhancement of cooling effi- ciency, while increasing the airflow rate (from 0.125 to 3.9 L/(s*kg)) results in a greater air pressure drop. In order to obtain a relatively satisfactory cooling efficiency, the opening area should be more than 6%. Castro et al., 2004b Polychlorinated vinyl ball filled with Wooden tunnel Airflow rate; vent area Cooling time; cooling rate; cooling water/agar-agar solution uniformity; air pressure drop; en- ergy consumption Remark: Results indicate that the airflow rate is an important factor influencing the half-cooling time. At the maximum airflow rate, neither vent area nor vent position has significant impacts on half-cooling time. Using the ventilated pack- age with the total vent area of 14% has the same cooling rate as the fully open structure. Although increasing airflow rate compensates for the negative effects of the low vent area, it also increases the pressure drop and energy consumption of the precooling process. Vigneault et al., 2005 Solid polymer ball Container formed by Vent area; vent position Cooling rate; cooling uniformity; acrylic plates energy consumption Remark: Results indicate that in the viewpoint of optimizing energy consumption, the optimal vent area should be between 8% and 16%, and the vents should be avoided as far as possible in the corner. Castro et al., 2005 Solid polymer ball Container formed by Vent area; vent position Cooling rate; cooling uniformity; acrylic plates air pressure drop Remark: Results indicate that cooling uniformity increases as the vents move from the corner of ventilated package to the centre of the package surface. Vigneault et al., 2006 Solid polymer ball Container formed by Slat width; airflow rate; Cooling time; cooling uniformity; acrylic plates vent area; vent position air pressure drop Remark: Results indicate that the effect of vent position on cooling uniformity is pronounced at low airflow rate, in com- parison with vent area. The vent area and slat width are suggested to be larger than 2.4% and 200 mm, respectively. Vigneault et al., 2007 Solid polymer ball Container formed by Airflow rate; vent area Cooling rate acrylic plates Remark: This research developed the Square Cross Section Velocity measurement method, which can measure the airflow rate more accurately than the previously developed Circular Approach Velocity method. Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 32 Y. Cao et al., 2020, Vol. 4, No. 1 Table 1. Continued Reference Produce or simulator used Container or Design parameters Effectiveness parameters package type Ferrua and Singh, Strawberry Clamshell Vent area; design of tray Cooling rate; cooling uniformity; 2011 energy consumption Remark: Results indicate that the optimal design of tray has a considerable contribution to the improvement of precooling effectiveness. Ngcobo et al., 2012 Table grape Carton Different components in Cooling rate; airflow resistance the package Remark: Results indicate that the line films have the greatest resistance to airflow compared to other components of the grape package. Mukama et al., 2017 Pomegranate Carton Plastic liner; stack orien- Cooling rate; energy consumption tation Remark: Results indicate that the direction of the stack (relative to the direction of the cooling airflow) affects energy con- sumption. The plastic liner has the greatest impact on the precooling process, and the energy consumption is up to three times higher than the unlined stack. Wu et al., 2018 Citrus fruit Carton Packaging type; wrap- Cooling rate; cooling uniformity ping; fruit size Remark: Results indicate that cooling heterogeneity mainly occurs in the flow direction. Fruit wrapping significantly reduces the cooling rate and increases cooling heterogeneity. The Nova mandarin fruit in the open top carton cooled 24% faster on the inflow side of the pallet and 42% faster on the outflow side than the Eureka lemon in the Supervent carton. complex structure inside ventilated packages, such as the area be- The direct CFD simulation has grown to be the most attractive tween the product air zone of the bulk package and that between method for the numerical study about the forced-air precooling trays of the layered package, as a porous media (Verboven et  al., process of horticultural produce in ventilated packaging system in 2004b). Consequently, the complicated treatment about geometry the past 10 years. Different from the porous-medium approach, the details is easily avoided, and the computation time and simulation direct CFD simulation allows an accurate description of explicit cost are dramatically reduced. In the past 20 years, many efforts had geometries of the complex internal structure of packaging system, been devoted to the study about the forced-air precooling process by including the shape and size of the product, the gap of neigh- using the porous-medium approach. Some representative researches bouring products, the location and material of trays, etc. Although and results are summarized in Table  2. In the porous-medium ap- the grid generation becomes complicated and the computation ef- proach, a pressure drop induced by the porous media is added to fort has a dramatic increase, the direct CFD simulation is capable the momentum conservation equation. The general relationship be- of providing accurate local information about the detailed airflow tween permeability and inertial loss coefficient is determined by the and temperature distribution inside packages (Zhao et  al., 2016). Ergun equation. For some products, the calculated pressure drop by Furthermore, it is not limited by the confinement ratio (Ferrua and the Ergun equation shows good agreement with the experimental Singh, 2007). Therefore, many efforts had been devoted to the de- results. For example, the results obtained by Van der Sman (2002) velopment of efficient CFD schemes for the forced-air precooling demonstrated the validity of the Ergun equation in the precooling process of horticultural produce. Based on the measurements from of potatoes and that of oranges, with the deviation of the numeric- some packaging or cold storage, a new zoning-model approach ally predicted value from experimental measurement being less than was developed to predict the heat and mass transfer processes in 6%. However, due to the difference in individual characteristics, refrigerated horticultural packaging (Tanner et  al., 2002a, 2002b, such as product shape, surface roughness, and porosity, it may also 2002c). By neglecting the buoyancy effect and assuming the air- give poor predictions. The inconsistency between the experimental flow to be incompressible, the mass conservation equation can be data and the prediction from the Ergun equation is attributed to decoupled from the energy conservation equation. On the basis of the uncertainty of the porosity values caused by the relatively small the decoupling method, Ferrua and Singh (2009a, 2009b, 2009c) confinement ratio of the packed bed. Verboven et al. (2004a) pointed performed a numerical analysis of the forced-air precooling process out that the impact of confinement should be considered when the for strawberry packaging and reported that the difference between confinement ratio is less than 10. By taking into account the confine- the average fruit temperature prediction and the experimental curve ment ratio, the deviation of the numerically predicted pressure drop for each clamshell was less than 0.7°C. Delele et al. (2013a) devel- from the experimental measurement can be reduced from 65% to oped a three-dimensional CFD model for airflow and heat transfer 20% (Verboven et al., 2004a). In addition, a spatial average method processes in packaged horticultural produce. The prediction results is required in the porous-medium approach, and the fluid velocity are in good agreement with the measured results, with the average is generally characterized by a superficial velocity (Verboven et al., relative errors of predicted pressure drop and product temperature 2006). Therefore, the porous-medium approach is unlikely to predict being 13.80% and 16.27%, respectively. The SST k−ω turbulence the detailed flow behaviour around an individual product and the model combined with the wall function had been demonstrated accurate temperature distribution inside ventilated packages. to perform best (Defraeye et al., 2013) and was widely used in the Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 33 Table 2. Representative researches based on the porous-medium approach Reference Material Time dependency Effectiveness parameters Xu and Burfoot, 1999 Potato Transient Cooling time; water vapour concentration Remark: This paper presented a transient three-dimensional numerical model for the heat and mass transfer of porous bulk pellet foods. The Ergun equation was used to describe the interaction between the airflow and the porous media. The conservation equations were solved to predict the airflow, temperature, and humidity. Results indicate that the predicted temperature profile is highly consistent with the measured one, with the maximum difference of 1.4°C. Alvarez et al., 2003 PVC sphere Steady Heat transfer coefficient Remark: This paper presented a single equation model for a two-dimensional problem. The Darcy–Forchheimer equation was adopted to describe the influence of the porous media on the airflow. Results indicate that the dis- crepancy between the predicted heat transfer coefficient and the experimental measurement is about 6%. Verboven et al., 2004a Apple and chicory root Steady Air pressure drop Remark: This paper presented a modified numerical scheme, in which the impacts of the product shape, surface roughness, and confinement were taken into account. The precooling process of apples and that of chicory roots were numerically studied, respectively. Results indicate that the deviation of the calculated pressure drop from the experimental measurement is reduced from 65% to 20%. Zou et al., 2006a, 2006b Apple Steady and transient Airflow pattern; temperature profile Remark: This paper presented the numerical prediction about both the airflow pattern and temperature profile in the case of bulk products and layered products, respectively. Results indicate that the consistency between the model prediction and the experimental measurement is satisfactory. Dehghannya et al., 2008 Solid polymer ball Transient Cooling uniformity Remark: This paper presented the study about the impact of vent area on cooling uniformity. Results indicate that a more uniform airflow distribution is obtained by increasing the vent area from 2.4% to 12.1%, and the most non-uniform cooling occurs in the vicinity of vents. Delele et al., 2013c Table grape Transient Cooling time; moisture loss; relative humidity Remark: This paper presented the discussion about the impacts of internal packaging components (bunch carry bag and plastic liners) and product stacking modes on airflow, heat, and mass transfer characteristics. Results indicate that the presence of the bunch carry bag increases the half-cooling time and seven-eighths cooling time by 61.09% and 97.34%, respectively, compared to the cooling of the bulk grape bunch. Adding a plastic liner to bunch carry increases the half-cooling time and seven-eighths cooling time by 168.90% and 185.22%, respectively. optimization design of ventilation packaging. Although respiration cooling, but others exposed to high airflow are over-cooled, causing is the most important life activity of fruits and vegetables, available chilling injury. The ventilation design of packaging system is the studies (Campanone et al., 1995; Gowda et al., 1997; Tanner et al., direct cause of heterogeneous airflow distribution (Vigneault and 2002c; Han et al., 2017a) indicated that respiratory heat is unlikely Goyette, 2003; Vigneault et  al., 2005). The reasonably distributed to significantly affect the precooling effectiveness. During typical venting holes allow the field heat of horticultural produce to escape, precooling, the rate of heat generated by respiration is often low so that there are good airflow patterns, proper temperature, and (about 0.5% of the total heat load). However, considering respir- relative humidity inside the package to ensure long shelf life and ation heat can improve the accuracy of numerical models (Vigneault good quality of fresh produce (Opara and Zou, 2006). However, the et al., 2007). Available results about the impact of ventilation design impact of the ventilation design of packaging system on precooling on precooling effectiveness, obtained by using the direct CFD simu- effectiveness is complicated. lation, are very rich. Some representative researches are summarized Due to the complex structure inside the ventilated package filled in Table 3. with horticultural produce, the internal airflow and temperature distribution exhibit serious non-uniformity during the forced-air precooling process. Consequently, cooling heterogeneity is the most Impact of Ventilation Design on Precooling important unresolved issue, which threatens both the quality and Effectiveness shelf life of fresh fruits and vegetables seriously. Alvarez and Flick Optimized design of forced-air precooling is critical to maintaining (1999a, 1999b) observed the non-uniform distribution of airflow quality and minimizing postharvest losses of horticultural produce in the ventilated packaging system through wind tunnel experi- (Castro et  al., 2006). However, due to poor temperature manage- ments, which resulted in very serious cooling heterogeneity, with ment, serious cooling heterogeneity will occur in the process of the difference in local heat transfer coefficient as high as 40%. The forced-air precooling (Alvarez and Flick, 1999a; Alvarez et  al., non-uniform distribution of airflow was manifested on the large 2003). The products located behind blind walls have insufficient speed appearing at the inlet and outlet, and the zero-speed on the Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 34 Y. Cao et al., 2020, Vol. 4, No. 1 Table 3. Representative studies based on direct computational fluid dynamics (CFD) simulation Reference Material Time dependency Effectiveness parameters Delele et al., 2008 Sphere Steady Pressure drop Remark: In this study, a random stack of spherical products in a package was created by using discrete-element method. The impact of ventilation design on pressure drop was discussed. The involved parameters include confinement ratio, stacking pattern, product size, vent area, and randomness of filling. The numerical prediction is in good agreement with experimental measurement. Results indicate that the air resistance of random filling is much smaller. Tutar et al., 2009 Sphere Steady and transient Cooling uniformity; cooling time Remark: In this paper, a numerical scheme was proposed to predict the airflow pattern and temperature distri- bution in the stack of circular products in the precooling process. The influencing factors discussed are flow di- mension, turbulence intensity, opening size, vent ratio, and airflow rate. Results indicate that the airflow rate has the greatest impact on precooling effectiveness. Properly increasing the opening area at the bottom or on side walls can considerably improve the vertical airflow. The intensity of imported turbulence is not an important factor, which only has a slight impact on the surface average heat flux of the product. Ferrua and Singh, 2009d Strawberry Steady and transient Cooling uniformity; cooling time Remark: This study provided quantitative results to emphasize the necessity of adding ventilation design. Results indicate that vent area has a significant impact on cooling time, but has no effect on cooling uniformity. Remov- ing the vents on the outside of the clamshells results in the increase of cooling time by 20%. Dehghannya et al., 2011 Polymer sphere Transient Cooling uniformity Remark: In this study, the impact of the number of vents (1, 3, and 5) on temperature distribution was investi- gated under the condition that the area of a single vent was 2.4%. Results indicate that increasing the number of vents leads to a significant improvement in cooling uniformity. Dehghannya et al., 2012 Polymer sphere Transient Cooling time; cooling rate; cooling uniformity Remark: In this paper, the influences of vent area and vent position on precooling effectiveness were studied nu- merically. Results indicate that increasing the vent area beyond a certain level does not have a positive impact on cooling uniformity or cooling time. If vent holes are not properly distributed on the package wall, they can even increase the cooling time. Delele et al., 2013a, 2013b PVC ball filled with water Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, a three-dimensional CFD model of the precooling process of packaged horticultural products was developed, in which the effects of product respiration heat and buoyancy were considered. The nu- merical prediction is in good agreement with the experimental measurement, with the average relative errors of pressure drop and product temperature being 13.80% and 16.27%, respectively. Results indicate that the vent area has the greatest impact on precooling effectiveness. Defraeye et al., 2013, 2014 Citrus Steady and transient Cooling time; cooling uniformity; energy consumption Remark: In this paper, the direct CFD simulation was used to evaluate the cooling performance and energy consumption of the existing container and two new designs. Results indicate that the existing container is prone to chilling injury, while the adoption of the two new designs not only increases the cooling rate and cooling uni- formity, but also reduces energy consumption. Berry et al., 2016 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, the cooling performance and energy consumption of four packaging designs were evalu- ated numerically on the premise of the total vent area of 4%. The impact of trays on airflow resistance was also studied. Results indicate that the adoption of the two designs with trays not only improves the cooling uniform- ity between the fruit layers, but also reduces the energy consumption by 27% and 26%, respectively. O’Sullivan et al., 2016 Kiwifruit Transient Cooling rate Remark: In this paper, a three-dimensional CFD model was proposed to study the precooling process in a pallet- ized polylined kiwifruit package. The model considers the effect of natural convection on both the flow and heat transfer characteristics in polyliners. Results indicate that the maximum volumetric flowrate through the pallet is much lower than the recommended flowrate of non-polylined produce. The continuous increase in flowrate results in an increasingly diminished reduction of cooling rate. Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 35 Table 3. Continued Reference Material Time dependency Effectiveness parameters Berry et al., 2017 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the impacts of vent configuration, vent area, and corrugated fibreboard grade on cooling effectiveness and energy consumption were evaluated. Results indicate that the carton strength is negatively cor- related with the vent area. As the total vent area increases, the cooling heterogeneity decreases. Han et al., 2017b Apple Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the cooling performances of 10 different apple box samples used in China were evaluated by using a three-dimensional numerical model. The maximum deviation between the predicted and experimen- tal values of the apple surface temperature was 18.69%. Results indicate that the cooling rate increases with the airflow rate, and the optimal air-inflow velocity is 0.4 m/s. The uniform and symmetric distribution of ventila- tion holes has a more significant contribution to the improvement of precooling effectiveness than the increase of vent area, especially when handling packages with more than three layers. In addition, polyvinyl chloride foam has a relatively smaller airflow resistance. Gruyters et al., 2018 Apple Transient Heat transfer coefficient; air pressure drop Remark: In this study, the realistic product shapes were used in the numerical simulation of the forced-air precooling process of apples. Results indicate that the main factor contributing to the overall pressure drop is the packaging design rather than the shape of the product. However, the shape of the product has a significant impact on the local air velocity and the convective heat transfer coefficient. The real apple shape has a lower surface heat transfer coefficient. Han et al., 2018 Apple Transient Cooling rate; cooling uniformity; energy consumption Remark: In this study, the forced-air precooling process of apple was simulated by a three-dimensional numer- ical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the optimal air-inflow rate is 2.31  L/(s*kg). Wang et al., 2019 Strawberry Transient Cooling rate; cooling time; cooling uniformity Remark: In this study, the forced-air precooling process of strawberry was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the combination of 9.4% area of box vent and 8.5% area of clamshell vent in the commercial packaging system has the best precooling effect on strawberry. surface of horticultural produce (Ferrua and Singh, 2008). Using insufficient, the precooling process is decelerated, and consequently a coupled airflow and heat transfer model for aerodynamic and the rate of product quality deterioration increases (Verboven et al., thermal analysis of the forced-air precooling process, Dehghannya 2004a). Increasing airflow rate can compensate for the negative ef- et al. (2008) pointed out that the most non-uniform cooling appears fect of low open area. As the airflow rate increases, the surface heat in the vicinity of the inlet and outlet of a ventilated package. The transfer coefficient increases (Fikiin et al., 1999). Under the condi- numerical results by Delele et  al. (2013a) indicated that the area tion that the vent area is given constant, increasing airflow rate is an near the package vent showed relatively high cooling air velocity effective way to reduce the cooling time (Vigneault et al., 2006; Han and turbulence intensity. The typical airflow characteristics and tem- et al., 2018). perature distribution are shown in Figures 2 and 3, respectively. The However, increasing airflow rate also has a negative effect on coldest place is behind the entrance vents. The pressure drops across precooling effectiveness. For example, a high airflow rate leads to the inlet and outlet are 51.1% and 45.2%, respectively. There is a excessive water loss of fresh foods (Verboven et al., 2004a). Pressure good correlation between the airflow rate and the product tempera- drop also increases with the airflow rate, and consequently the ture distribution. The coldest zone corresponds to the zone with the energy consumption increases greatly (Verboven et  al., 2004a). highest cooling air velocity. Therefore, optimization of the ventila- Furthermore, the impact of air-inflow rate on precooling effective- tion design of packaging system had been regarded as a promising ness is in tight association with the location of vents. Any increase way to improve precooling effectiveness and maintain the quality of in airflow rate would result in an obvious pressure drop and air - horticultural produce in the postharvest cold chain. flow non-uniformity if the open area was formed by non-uniformly distributed vents (Vigneault et al., 2005). As the air-inflow rate in- Impact of air-inflow rate creases to 2.31 L/(s*kg), the cooling rate and uniformity are reason- ably improved. Any further increase in air-inflow rate will result in It is commonly accepted that the inlet airflow velocity has the a relatively low increase in cooling rate and uniformity, but energy greatest impact on the cooling rate (Castro et al., 2004a; Tutar et al., consumption and cooling damage will increase significantly (Han 2009; Han et  al., 2018) since the heat transfer coefficient of the et al., 2018). fruit surface is related to the airflow rate. When the airflow rate is Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 36 Y. Cao et al., 2020, Vol. 4, No. 1 Figure 2. Airflow characteristics inside a ventilated package for a superficial velocity of 0.3 m/s: (a) velocity vector, (b) velocity contour, (c) pressure contour, and (d) turbulent kinetic energy contour (Delele et al., 2013a). vents for blueberry plastic clamshell can increase the cooling rates of Impact of vent area both 6-oz and 1-pt clamshell by 10%–40% and 15%–55%, respect- Vent area is an important factor in the optimization of ventilation ively (Leyte et al., 1999). When the area of a single vent is constant, design. To maximize cooling uniformity, vent area should be large increasing the number of ventilation holes from two to four halves enough not to restrict airflow. In practical application, however, vent the cooling time (Han et al., 2015). The ±20% change of vent area area should have an upper limit to balance the relationship between had a significant impact on both the central and near-exit area of a the precooling effectiveness of horticultural produce and the mech- ventilated package rather than the near-entrance area (Opara and anical strength of the packaging system. Zou, 2007). Vent area has an optimal value. The further increase Many efforts had been devoted to the discussion about the impact of vent area beyond its optimal value has no obvious contribution of vent area on cooling rate. Generally, increasing the area of ven- to the cooling rate (Tutar et al., 2009). The wind tunnel experiment tilation holes can accelerate the cooling rate of products (Vigneault about the precooling of solid polymer balls (Castro et  al., 2004b) et  al., 2006; Ferrua and Singh, 2009d). For example, adding top Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 37 properly distributed, increasing the vent area may no longer be suf- ficient to increase the cooling rate (Dehghannya et  al., 2012). The design of four vents distributed in corners resulted in poor cooling uniformity and at least 1.75 times more energy consumption in com- parison with uniformly distributed vents (Vigneault et  al., 2005). Therefore, optimizing vent position is of essential importance for cooling uniformity and energy saving. Impact of other factors The box material and internal packaging, such as tray, bunch carry bag, plastic liner, etc., also influence precooling effectiveness of horti- cultural produce. Ventilated package is the main cause for pressure drop compared to the product itself (Delele et  al., 2013a, 2013b) since it hinders the direct contact of horticultural produce with the cooling airflow and increases the flow resistance. The comparison study by Han et al. (2017b) indicates that polyvinyl chloride foam plastics have a low airflow resistance in comparison with corrugated boards. In addition, Opara and Zou (2007) studied the impact of the gap width between the tray and the package wall on precooling effectiveness. Results indicate that the product centre temperature is very sensitive to the change in the gap width, especially at the product layer away from the airflow inlet. Internal packaging, such as bunch carry bag, plastic liners, polyliners, etc., was often used to prevent the moisture loss in the precooling process. However, these internal packaging materials had a significant contribution to the increase of both pressure drop Figure 3. Temperature distribution within a ventilated package after 2  h of cooling from an initial temperature of 21°C using air with a superficial and cooling time (Delele et  al., 2013c). Quantitatively, the pres- velocity of 0.3 m/s and temperature of −5°C (Delele et al., 2013a). sure drop produced by the non-perforated plastic liner accounts for 83.34% ± 2.13% of the total pressure drop, while the pressure indicates that the vent area should exceed 6%. As the total opening drop of the grape itself only accounts for 1.40% ± 0.01% to 9.41% area exceeds 8%, increasing vent area has no significant impact on ± 1.23% (Ngcobo et al., 2012). The maximum volumetric flowrate the cooling rate (Castro et al., 2004a). For strawberry, the cooling in a palletized polylined kiwifruit package is 0.34  L/(s*kg), much uniformity is best as the vent area is about 9.4% (Wang et al., 2019). lower than the recommended flowrate of non-polylined produce Vent area also affects cooling uniformity and air pressure drop. (O’Sullivan et  al., 2016). That is, in order to achieve the same A more uniform airflow distribution can be achieved by increasing precooling effectiveness, a lot more energy is needed under the the vent area from 2.4% to 12.1%, with the heterogeneity index circumstances that complex internal packaging is adopted. In the decreasing gradually from 108% to 0% (Dehghannya et al., 2008). precooling process, the quality loss of fruit is mainly affected by Similarly, increasing the number of vent holes could also improve cooling time. The plastic liner increased the average 7/8 cooling cooling uniformity (Dehghannya et al., 2011). In addition, Van der time of the CT1 and CT2 stacks from 4.0 and 2.5  h to 9.5 and Sman (2002) confirmed the hypothesis of the power-law relationship 8.0  h, respectively (Ambaw et  al., 2017). In addition, energy −1.5 (Δp ∼ O ) between the pressure drop and the box vent ratio (O). tot consumption is up to three times higher than the unlined stack That is, as the vent area increases, the air pressure drop through the (Mukama et al., 2017). package decreases (Delele et al., 2008), and consequently the energy consumption is reduced (Ferrua and Singh, 2011). From the view- Conclusions point of energy saving, the optimal vent area was suggested to be between 8% and 16% (Vigneault et al., 2005). This paper reviews available studies about the ventilation design of the packaging system for the forced-air precooling of horticultural Impact of vent position produce. The function of packaging system is to protect horticultural Vent position is also an important factor influencing precooling ef- produce from mechanical damage during storage and transporta- fectiveness. Changing the vent position will change the airflow pat- tion, while vent holes are designed on the package wall to achieve tern and consequently influence the heat transfer characteristic and the purpose of rapid and uniform removal of field heat by ensuring temperature distribution inside the ventilated packaging system. adequate airflow through the surface of horticultural produce. Berry et  al. (2016) pointed out that packages with multiple vents Relevant research methods are summarized, along with representa- were more energy-efficient as the total vent area was given constant. tive researches and results. Both the merits and flaws of each method Han et al. (2017b) compared 10 typical apple carton samples used are described. Based on the limited and valuable experimental re- in China and found that uniformly and symmetrically distributed sults, developing reliable and efficient numerical simulation schemes vents were beneficial to the cooling uniformity of multilayer venti- seems to be the most hopeful direction of future research. lated packaging, especially when handling more than three layers of Ventilation packaging plays a key role in the postharvest packaging. handling of horticultural produce. The ventilation design is of Moreover, the position of ventilation holes is critical for cooling critical importance in the forced-air precooling process and con- uniformity when the air velocity is low. When vent holes are not sequently has been one of the research hotspots of the food cold Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 38 Y. Cao et al., 2020, Vol. 4, No. 1 Brosnan,  T., Sun,  D.  W. (2001). Precooling techniques and applications for chain. The impacts of the air-inflow rate, the vent area, the vent horticultural products—a review. International Journal of Refrigeration, position, and other factors, such as the box material and internal 24: 154–170. packaging, on precooling effectiveness are summarized from the Campanone, L. A., Giner, S. A., Mascheroni, R. H. (1995). The use of a simu- viewpoint of cooling rate, cooling time, cooling uniformity, air pres- lation software to optimize cooling times and to lower weight losses in sure drop, and energy consumption. It is found that the influence of fruit refrigeration. Proceedings of 19th International Congress of Refriger- ventilation design on precooling effectiveness is complicated. For ation, 1: 121–128. example, increasing air-inflow rate can accelerate the cooling rate, Castro, L. R. D., Cortez, L. A. B., Vigneault, C. (2006). Effect of sorting, re- but it will also increase energy consumption. Increasing vent area frigeration and packaging on tomato shelf life. International Journal of tends to increase the cooling rate and improve cooling uniformity, Food, Agriculture and Environment, 4: 70–74. but it also leads to a reduction in the mechanical strength of the Castro, L. R. D., Vigneault, C., Cortez, L. A. B. (2004a). Container opening design for horticultural produce cooling efficiency. Journal of Food Agri- ventilated packaging system. Other factors, such as the variety in culture and Environment, 2: 135–140. horticultural produce, the various options for box material and size, Castro, L. R. D., Vigneault, C., Cortez, L. A. B. (2004b). Effect of container the internal packaging, etc., further complicate the optimization of opening area on air distribution during precooling of horticultural ventilation design. Therefore, multi-parameter analysis seems to be produce. 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Impact of ventilation design on the precooling effectiveness of horticultural produce—a review

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Oxford University Press
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Abstract

Optimizing the ventilation design of packaging system is of crucial importance for improving the efficiency of the forced-air precooling process to maintain the quality of horticultural produce and extend the shelf life in food cold chain. Many efforts had been devoted to the study about the impact of ventilation design on airflow and temperature distribution inside ventilated packages. This paper reviews relevant research methods, commonly used quantities for the measurement of precooling effectiveness, attractive design parameters, and their impact on precooling effectiveness. These allow us to know exactly the characteristic and deficiency of each research method, identify dominant design parameters, and seek a promising way for the future improvement of the ventilated packaging system. Key words: horticultural produce; precooling effectiveness; ventilation design; forced-air precooling technique; packaging system Among the many postharvest technologies, precooling of horti- Introduction cultural produce is the most important technique for maintaining As perishable foods, horticultural products are regional and sea- desirable, fresh, and marketable products. It can remove field heat sonal. Consequently, postharvest handling is a very important from fresh agricultural produce, thereby slowing down metab- step in maintaining the quality of agricultural produce and ex- olism and reducing deterioration before storage and transportation tending the shelf life (Dehghannya et al., 2010; Yan et al., 2019). (Dehghannya et al., 2010). The average mass loss of the cold chain Respiration and transpiration of horticultural produce are im- without precooling is about 23% more than that of the cold chain portant causes for loss of organic material and moisture (Verboven based on precooling (Wu and Defraeye, 2018). et al., 2004b; Ambaw et al., 2013; Aghdam et al., 2018). There are Forced-air cooling is one of the most widely used precooling tech- many factors influencing the physiological and biological changes niques due to its advantages of rapid cooling rate, high efficiency, of horticultural produce (Brosnan and Sun, 2001; Aghdam et al., and low cost (Kader, 2002; Vigneault and Goyette, 2002; Delele 2019; Xu et  al., 2019), such as product thermal properties, tem- et  al., 2010). The schematic diagram of the forced-air precooling perature, concentration of ethylene, oxygen, carbon dioxide, system is shown in Figure 1. This technique uses a fan to generate the etc. It is noteworthy that temperature is the most important en- necessary driving force to create a pressure difference, which forces vironmental factor affecting the postharvest life of horticultural the cold air through the inside of the box. The cold air removes produce. For example, if fresh strawberries are left for 2 h in the heat from fruits and vegetables when passing through the ventilated 30°C heat, only 80% of the fruits are considered suitable for sale packaging system, and thereby achieves the goal of rapid precooling. (Mitchell et al., 1972). © The Author(s) 2020. Published by Oxford University Press on behalf of Zhejiang University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by- nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 30 Y. Cao et al., 2020, Vol. 4, No. 1 Experimental measurement dominated the early research on the forced-air precooling of horticultural produce. Many efforts had been devoted to the experimental study about the impact of the ventilation design of packaging system on the precooling effectiveness of fresh fruits and vegetables. The most attractive design parameters include airflow rate, vent area, vent position, internal tray, line film, etc. The frequently encountered parameters defined to measure precooling ef- fectiveness include cooling rate, cooling uniformity, air pressure drop, energy consumption, half-cooling time, seven-eighths cooling time, heat transfer coefficient, cooling efficiency, etc. Some representative experimental researches on the impact of ventilation design in the forced-air precooling process of horticultural produce are summar- Figure 1. Typical forced-air cooling system of horticultural produce (Delele ized in Table 1. In order to ensure the reliability of test data obtained et al., 2010). in repeated experiments and in the meanwhile reduce experimental cost, simulators (Castro et al., 2004a, 2004b, 2005; Vigneault et al., Cooling heterogeneity is a prevalent phenomenon during the 2005, 2006, 2007) were usually used in experimental studies rather forced-air precooling process (Alvarez and Flick, 1999a; Alvarez than horticultural produce, such as solid polymer balls, polychlor- et al., 2003), and consequently the ventilation design of packaging inated vinyl balls filled with water, etc. In the rest few experiments system has been a research hotspot in the past 20 years. The adop- (Leyte and Forney, 1999; Fikiin et al., 1999; Anderson et al., 2004; tion of packaging system keeps horticultural produce clean, protects Vigneault et  al., 2004a, 2004b; Ferrua and Singh, 2011; Ngcobo horticultural produce from mechanical damage, minimizes moisture et al., 2012; Mukama et al., 2017; Wu et al., 2018), the precooling loss, and delays microbial decay (Ngcobo et al., 2012). However, the process of horticultural produce, such as table grapes, strawberries, packaging material also hinders the direct contact between horticul- blueberry, etc., was studied. In available experimental studies, the tural produce and the cooling airflow and consequently increases typical package types include carton, clamshell, wooden container, the resistance of the cooling airflow and the energy consumption container formed by acrylic plates, etc. The main results obtained in of the precooling process (Delele et  al., 2013a, 2013b). Therefore, some representative researches are summarized in Table  1. Due to the ventilation design of the packaging system should balance the the complex internal structure of ventilated packages filled with trays relationship between the cooling performance and the mechanical and products, some physical phenomena are difficult to measure with strength of packages. At the same time, the complex internal struc- the high spatial and temporal resolution, such as airflow and heat ture of the package filled with trays and products complicates the transfer characteristics, convective heat transfer coefficient, and tem- airflow and heat transfer characteristics, resulting in serious cooling perature change of the entire product. However, there were still many heterogeneity, such as local under-cooling and over-cooling of prod- researchers who made unremitting efforts for accurate measurements. ucts. These heterogeneous phenomena threaten the quality of horti- Vigneault et al. (2007) developed the Square Cross-Section Velocity cultural produce and the shelf life in food cold chain. Therefore, the (SCSV) measurement method and proved that SCSV can provide optimization of the ventilation design of the packaging system is an a more accurate measurement of airflow rates than the previously active topic in the food industry. developed Circular Approach Velocity method. Ferrua and Singh This review focuses on available studies about the influence of (2011) used nonintrusive flow measurement techniques [Particle ventilation design (airflow rate, vent area, vent position, etc.) on Image Velocimetry (PIV)] to determine the internal airflow field in a the precooling effectiveness of horticultural produce. The rest of package with a container-to-product diameter ratio of less than 10. the review paper is organized as follows. In the Available Research The use of PIV provided a valuable understanding and quantitative Methods section, available research methods about the forced- description of the local behaviour of fluid flow inside the packaging air precooling process of horticultural produce in the ventilated system. Nevertheless, experimental studies for improving ventilation packaging system are summarized, along with some representative design had been limited due to the fluctuations in physical properties researches and results. Both the merits and flaws of each method of horticultural produce in repeated experiments, the difficulties in are described. In the Impact of Ventilation Design on Precooling handling biological materials, and the inherent drawbacks of experi- Effectiveness section, available discussions about the impact of ven- mental studies, such as expensive, time-consuming, etc. tilation design on precooling effectiveness are elaborated. Finally, the With the rapid development of computer technology, numer- research trends and future challenges about the ventilation design of ical simulation has grown to be an alternative research method packaging system are pointed out in the Conclusions section. for the ventilation design of packaging system used in the forced- air precooling of fresh foods. Different from experimental studies, the numerical simulation method is capable of providing detailed Available Research Methods airflow and temperature distribution inside the packaging system In order to ensure the quality and safety of horticultural produce and and avoiding the effect of the fluctuation in the physical proper - extend the storage and shelf life, the critical step in the postharvest ties of fresh foods in a series of tests. Generally, available numerical cold chain is to rapidly precool horticultural produce by removing methods about the forced-air precooling process can be divided into field heat (Han et  al., 2015). Poor postharvest handling techniques two categories: the porous-medium approach and the direct compu- (mainly poor temperature management) can affect the rate of res- tational fluid dynamics (CFD) simulation. piration and thereby lead to decay of horticultural produce and Due to the complex internal structure of ventilated packages postharvest losses (Saenmuang et al., 2012). The postharvest loss and and the limitation of computation resources, the porous-medium waste in fruit and vegetable production supply chains even can be as approach was the dominant numerical method in the early stage. high as 13%–38% before reaching consumers (Defraeye et al., 2015). It allows the simplification of mathematical models by treating the Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 31 Table 1. Representative experimental researches and results about the impact of ventilation design in the forced-air precooling of horti- cultural produce Reference Produce or simulator used Container or Design parameters Effectiveness parameters package type Leyte and Forney, Highbush blueberry Plastic clamshell Vent position Cooling time Remark: Results indicate that the most rapid cooling occurs in the clamshells with vents on the top. While adding vents at the bottom has no effect on the cooling rate. Fikiin et al., 1999 Five horticultural produce Trays of pine wooden Airflow rate Heat transfer coefficient material Remark: Results indicate that the surface heat transfer coefficient of the fruit increases with the airflow rate. Anderson et al., 2004 Strawberry Different clamshell Vent area; clamshell and Cooling time and tray combinations tray combinations Remark: Results indicate that the integral optimization of the combination of clamshell and tray is an effective way to maxi- mize the contact between the cold air and the product. Vigneault et al., 25 different horticultural produce Wooden container Product size; product Air pressure drop 2004a, 2004b shape; vent position Remark: Results indicate that both the size and shape of horticultural produce have a significant impact on the pressure drop. In order to achieve the goal of minimizing the pressure drop, the vents should be uniformly distributed on the sides (perpendicular to airflow) of the container. Castro et al., 2004a Solid polymer ball Container formed by Airflow rate; vent area Cooling efficiency; air pressure acrylic plates drop Remark: Results indicate that the increase of total vent area (0.67%, 2%, 6%) leads to an enhancement of cooling effi- ciency, while increasing the airflow rate (from 0.125 to 3.9 L/(s*kg)) results in a greater air pressure drop. In order to obtain a relatively satisfactory cooling efficiency, the opening area should be more than 6%. Castro et al., 2004b Polychlorinated vinyl ball filled with Wooden tunnel Airflow rate; vent area Cooling time; cooling rate; cooling water/agar-agar solution uniformity; air pressure drop; en- ergy consumption Remark: Results indicate that the airflow rate is an important factor influencing the half-cooling time. At the maximum airflow rate, neither vent area nor vent position has significant impacts on half-cooling time. Using the ventilated pack- age with the total vent area of 14% has the same cooling rate as the fully open structure. Although increasing airflow rate compensates for the negative effects of the low vent area, it also increases the pressure drop and energy consumption of the precooling process. Vigneault et al., 2005 Solid polymer ball Container formed by Vent area; vent position Cooling rate; cooling uniformity; acrylic plates energy consumption Remark: Results indicate that in the viewpoint of optimizing energy consumption, the optimal vent area should be between 8% and 16%, and the vents should be avoided as far as possible in the corner. Castro et al., 2005 Solid polymer ball Container formed by Vent area; vent position Cooling rate; cooling uniformity; acrylic plates air pressure drop Remark: Results indicate that cooling uniformity increases as the vents move from the corner of ventilated package to the centre of the package surface. Vigneault et al., 2006 Solid polymer ball Container formed by Slat width; airflow rate; Cooling time; cooling uniformity; acrylic plates vent area; vent position air pressure drop Remark: Results indicate that the effect of vent position on cooling uniformity is pronounced at low airflow rate, in com- parison with vent area. The vent area and slat width are suggested to be larger than 2.4% and 200 mm, respectively. Vigneault et al., 2007 Solid polymer ball Container formed by Airflow rate; vent area Cooling rate acrylic plates Remark: This research developed the Square Cross Section Velocity measurement method, which can measure the airflow rate more accurately than the previously developed Circular Approach Velocity method. Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 32 Y. Cao et al., 2020, Vol. 4, No. 1 Table 1. Continued Reference Produce or simulator used Container or Design parameters Effectiveness parameters package type Ferrua and Singh, Strawberry Clamshell Vent area; design of tray Cooling rate; cooling uniformity; 2011 energy consumption Remark: Results indicate that the optimal design of tray has a considerable contribution to the improvement of precooling effectiveness. Ngcobo et al., 2012 Table grape Carton Different components in Cooling rate; airflow resistance the package Remark: Results indicate that the line films have the greatest resistance to airflow compared to other components of the grape package. Mukama et al., 2017 Pomegranate Carton Plastic liner; stack orien- Cooling rate; energy consumption tation Remark: Results indicate that the direction of the stack (relative to the direction of the cooling airflow) affects energy con- sumption. The plastic liner has the greatest impact on the precooling process, and the energy consumption is up to three times higher than the unlined stack. Wu et al., 2018 Citrus fruit Carton Packaging type; wrap- Cooling rate; cooling uniformity ping; fruit size Remark: Results indicate that cooling heterogeneity mainly occurs in the flow direction. Fruit wrapping significantly reduces the cooling rate and increases cooling heterogeneity. The Nova mandarin fruit in the open top carton cooled 24% faster on the inflow side of the pallet and 42% faster on the outflow side than the Eureka lemon in the Supervent carton. complex structure inside ventilated packages, such as the area be- The direct CFD simulation has grown to be the most attractive tween the product air zone of the bulk package and that between method for the numerical study about the forced-air precooling trays of the layered package, as a porous media (Verboven et  al., process of horticultural produce in ventilated packaging system in 2004b). Consequently, the complicated treatment about geometry the past 10 years. Different from the porous-medium approach, the details is easily avoided, and the computation time and simulation direct CFD simulation allows an accurate description of explicit cost are dramatically reduced. In the past 20 years, many efforts had geometries of the complex internal structure of packaging system, been devoted to the study about the forced-air precooling process by including the shape and size of the product, the gap of neigh- using the porous-medium approach. Some representative researches bouring products, the location and material of trays, etc. Although and results are summarized in Table  2. In the porous-medium ap- the grid generation becomes complicated and the computation ef- proach, a pressure drop induced by the porous media is added to fort has a dramatic increase, the direct CFD simulation is capable the momentum conservation equation. The general relationship be- of providing accurate local information about the detailed airflow tween permeability and inertial loss coefficient is determined by the and temperature distribution inside packages (Zhao et  al., 2016). Ergun equation. For some products, the calculated pressure drop by Furthermore, it is not limited by the confinement ratio (Ferrua and the Ergun equation shows good agreement with the experimental Singh, 2007). Therefore, many efforts had been devoted to the de- results. For example, the results obtained by Van der Sman (2002) velopment of efficient CFD schemes for the forced-air precooling demonstrated the validity of the Ergun equation in the precooling process of horticultural produce. Based on the measurements from of potatoes and that of oranges, with the deviation of the numeric- some packaging or cold storage, a new zoning-model approach ally predicted value from experimental measurement being less than was developed to predict the heat and mass transfer processes in 6%. However, due to the difference in individual characteristics, refrigerated horticultural packaging (Tanner et  al., 2002a, 2002b, such as product shape, surface roughness, and porosity, it may also 2002c). By neglecting the buoyancy effect and assuming the air- give poor predictions. The inconsistency between the experimental flow to be incompressible, the mass conservation equation can be data and the prediction from the Ergun equation is attributed to decoupled from the energy conservation equation. On the basis of the uncertainty of the porosity values caused by the relatively small the decoupling method, Ferrua and Singh (2009a, 2009b, 2009c) confinement ratio of the packed bed. Verboven et al. (2004a) pointed performed a numerical analysis of the forced-air precooling process out that the impact of confinement should be considered when the for strawberry packaging and reported that the difference between confinement ratio is less than 10. By taking into account the confine- the average fruit temperature prediction and the experimental curve ment ratio, the deviation of the numerically predicted pressure drop for each clamshell was less than 0.7°C. Delele et al. (2013a) devel- from the experimental measurement can be reduced from 65% to oped a three-dimensional CFD model for airflow and heat transfer 20% (Verboven et al., 2004a). In addition, a spatial average method processes in packaged horticultural produce. The prediction results is required in the porous-medium approach, and the fluid velocity are in good agreement with the measured results, with the average is generally characterized by a superficial velocity (Verboven et al., relative errors of predicted pressure drop and product temperature 2006). Therefore, the porous-medium approach is unlikely to predict being 13.80% and 16.27%, respectively. The SST k−ω turbulence the detailed flow behaviour around an individual product and the model combined with the wall function had been demonstrated accurate temperature distribution inside ventilated packages. to perform best (Defraeye et al., 2013) and was widely used in the Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 33 Table 2. Representative researches based on the porous-medium approach Reference Material Time dependency Effectiveness parameters Xu and Burfoot, 1999 Potato Transient Cooling time; water vapour concentration Remark: This paper presented a transient three-dimensional numerical model for the heat and mass transfer of porous bulk pellet foods. The Ergun equation was used to describe the interaction between the airflow and the porous media. The conservation equations were solved to predict the airflow, temperature, and humidity. Results indicate that the predicted temperature profile is highly consistent with the measured one, with the maximum difference of 1.4°C. Alvarez et al., 2003 PVC sphere Steady Heat transfer coefficient Remark: This paper presented a single equation model for a two-dimensional problem. The Darcy–Forchheimer equation was adopted to describe the influence of the porous media on the airflow. Results indicate that the dis- crepancy between the predicted heat transfer coefficient and the experimental measurement is about 6%. Verboven et al., 2004a Apple and chicory root Steady Air pressure drop Remark: This paper presented a modified numerical scheme, in which the impacts of the product shape, surface roughness, and confinement were taken into account. The precooling process of apples and that of chicory roots were numerically studied, respectively. Results indicate that the deviation of the calculated pressure drop from the experimental measurement is reduced from 65% to 20%. Zou et al., 2006a, 2006b Apple Steady and transient Airflow pattern; temperature profile Remark: This paper presented the numerical prediction about both the airflow pattern and temperature profile in the case of bulk products and layered products, respectively. Results indicate that the consistency between the model prediction and the experimental measurement is satisfactory. Dehghannya et al., 2008 Solid polymer ball Transient Cooling uniformity Remark: This paper presented the study about the impact of vent area on cooling uniformity. Results indicate that a more uniform airflow distribution is obtained by increasing the vent area from 2.4% to 12.1%, and the most non-uniform cooling occurs in the vicinity of vents. Delele et al., 2013c Table grape Transient Cooling time; moisture loss; relative humidity Remark: This paper presented the discussion about the impacts of internal packaging components (bunch carry bag and plastic liners) and product stacking modes on airflow, heat, and mass transfer characteristics. Results indicate that the presence of the bunch carry bag increases the half-cooling time and seven-eighths cooling time by 61.09% and 97.34%, respectively, compared to the cooling of the bulk grape bunch. Adding a plastic liner to bunch carry increases the half-cooling time and seven-eighths cooling time by 168.90% and 185.22%, respectively. optimization design of ventilation packaging. Although respiration cooling, but others exposed to high airflow are over-cooled, causing is the most important life activity of fruits and vegetables, available chilling injury. The ventilation design of packaging system is the studies (Campanone et al., 1995; Gowda et al., 1997; Tanner et al., direct cause of heterogeneous airflow distribution (Vigneault and 2002c; Han et al., 2017a) indicated that respiratory heat is unlikely Goyette, 2003; Vigneault et  al., 2005). The reasonably distributed to significantly affect the precooling effectiveness. During typical venting holes allow the field heat of horticultural produce to escape, precooling, the rate of heat generated by respiration is often low so that there are good airflow patterns, proper temperature, and (about 0.5% of the total heat load). However, considering respir- relative humidity inside the package to ensure long shelf life and ation heat can improve the accuracy of numerical models (Vigneault good quality of fresh produce (Opara and Zou, 2006). However, the et al., 2007). Available results about the impact of ventilation design impact of the ventilation design of packaging system on precooling on precooling effectiveness, obtained by using the direct CFD simu- effectiveness is complicated. lation, are very rich. Some representative researches are summarized Due to the complex structure inside the ventilated package filled in Table 3. with horticultural produce, the internal airflow and temperature distribution exhibit serious non-uniformity during the forced-air precooling process. Consequently, cooling heterogeneity is the most Impact of Ventilation Design on Precooling important unresolved issue, which threatens both the quality and Effectiveness shelf life of fresh fruits and vegetables seriously. Alvarez and Flick Optimized design of forced-air precooling is critical to maintaining (1999a, 1999b) observed the non-uniform distribution of airflow quality and minimizing postharvest losses of horticultural produce in the ventilated packaging system through wind tunnel experi- (Castro et  al., 2006). However, due to poor temperature manage- ments, which resulted in very serious cooling heterogeneity, with ment, serious cooling heterogeneity will occur in the process of the difference in local heat transfer coefficient as high as 40%. The forced-air precooling (Alvarez and Flick, 1999a; Alvarez et  al., non-uniform distribution of airflow was manifested on the large 2003). The products located behind blind walls have insufficient speed appearing at the inlet and outlet, and the zero-speed on the Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 34 Y. Cao et al., 2020, Vol. 4, No. 1 Table 3. Representative studies based on direct computational fluid dynamics (CFD) simulation Reference Material Time dependency Effectiveness parameters Delele et al., 2008 Sphere Steady Pressure drop Remark: In this study, a random stack of spherical products in a package was created by using discrete-element method. The impact of ventilation design on pressure drop was discussed. The involved parameters include confinement ratio, stacking pattern, product size, vent area, and randomness of filling. The numerical prediction is in good agreement with experimental measurement. Results indicate that the air resistance of random filling is much smaller. Tutar et al., 2009 Sphere Steady and transient Cooling uniformity; cooling time Remark: In this paper, a numerical scheme was proposed to predict the airflow pattern and temperature distri- bution in the stack of circular products in the precooling process. The influencing factors discussed are flow di- mension, turbulence intensity, opening size, vent ratio, and airflow rate. Results indicate that the airflow rate has the greatest impact on precooling effectiveness. Properly increasing the opening area at the bottom or on side walls can considerably improve the vertical airflow. The intensity of imported turbulence is not an important factor, which only has a slight impact on the surface average heat flux of the product. Ferrua and Singh, 2009d Strawberry Steady and transient Cooling uniformity; cooling time Remark: This study provided quantitative results to emphasize the necessity of adding ventilation design. Results indicate that vent area has a significant impact on cooling time, but has no effect on cooling uniformity. Remov- ing the vents on the outside of the clamshells results in the increase of cooling time by 20%. Dehghannya et al., 2011 Polymer sphere Transient Cooling uniformity Remark: In this study, the impact of the number of vents (1, 3, and 5) on temperature distribution was investi- gated under the condition that the area of a single vent was 2.4%. Results indicate that increasing the number of vents leads to a significant improvement in cooling uniformity. Dehghannya et al., 2012 Polymer sphere Transient Cooling time; cooling rate; cooling uniformity Remark: In this paper, the influences of vent area and vent position on precooling effectiveness were studied nu- merically. Results indicate that increasing the vent area beyond a certain level does not have a positive impact on cooling uniformity or cooling time. If vent holes are not properly distributed on the package wall, they can even increase the cooling time. Delele et al., 2013a, 2013b PVC ball filled with water Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, a three-dimensional CFD model of the precooling process of packaged horticultural products was developed, in which the effects of product respiration heat and buoyancy were considered. The nu- merical prediction is in good agreement with the experimental measurement, with the average relative errors of pressure drop and product temperature being 13.80% and 16.27%, respectively. Results indicate that the vent area has the greatest impact on precooling effectiveness. Defraeye et al., 2013, 2014 Citrus Steady and transient Cooling time; cooling uniformity; energy consumption Remark: In this paper, the direct CFD simulation was used to evaluate the cooling performance and energy consumption of the existing container and two new designs. Results indicate that the existing container is prone to chilling injury, while the adoption of the two new designs not only increases the cooling rate and cooling uni- formity, but also reduces energy consumption. Berry et al., 2016 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, the cooling performance and energy consumption of four packaging designs were evalu- ated numerically on the premise of the total vent area of 4%. The impact of trays on airflow resistance was also studied. Results indicate that the adoption of the two designs with trays not only improves the cooling uniform- ity between the fruit layers, but also reduces the energy consumption by 27% and 26%, respectively. O’Sullivan et al., 2016 Kiwifruit Transient Cooling rate Remark: In this paper, a three-dimensional CFD model was proposed to study the precooling process in a pallet- ized polylined kiwifruit package. The model considers the effect of natural convection on both the flow and heat transfer characteristics in polyliners. Results indicate that the maximum volumetric flowrate through the pallet is much lower than the recommended flowrate of non-polylined produce. The continuous increase in flowrate results in an increasingly diminished reduction of cooling rate. Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 35 Table 3. Continued Reference Material Time dependency Effectiveness parameters Berry et al., 2017 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the impacts of vent configuration, vent area, and corrugated fibreboard grade on cooling effectiveness and energy consumption were evaluated. Results indicate that the carton strength is negatively cor- related with the vent area. As the total vent area increases, the cooling heterogeneity decreases. Han et al., 2017b Apple Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the cooling performances of 10 different apple box samples used in China were evaluated by using a three-dimensional numerical model. The maximum deviation between the predicted and experimen- tal values of the apple surface temperature was 18.69%. Results indicate that the cooling rate increases with the airflow rate, and the optimal air-inflow velocity is 0.4 m/s. The uniform and symmetric distribution of ventila- tion holes has a more significant contribution to the improvement of precooling effectiveness than the increase of vent area, especially when handling packages with more than three layers. In addition, polyvinyl chloride foam has a relatively smaller airflow resistance. Gruyters et al., 2018 Apple Transient Heat transfer coefficient; air pressure drop Remark: In this study, the realistic product shapes were used in the numerical simulation of the forced-air precooling process of apples. Results indicate that the main factor contributing to the overall pressure drop is the packaging design rather than the shape of the product. However, the shape of the product has a significant impact on the local air velocity and the convective heat transfer coefficient. The real apple shape has a lower surface heat transfer coefficient. Han et al., 2018 Apple Transient Cooling rate; cooling uniformity; energy consumption Remark: In this study, the forced-air precooling process of apple was simulated by a three-dimensional numer- ical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the optimal air-inflow rate is 2.31  L/(s*kg). Wang et al., 2019 Strawberry Transient Cooling rate; cooling time; cooling uniformity Remark: In this study, the forced-air precooling process of strawberry was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the combination of 9.4% area of box vent and 8.5% area of clamshell vent in the commercial packaging system has the best precooling effect on strawberry. surface of horticultural produce (Ferrua and Singh, 2008). Using insufficient, the precooling process is decelerated, and consequently a coupled airflow and heat transfer model for aerodynamic and the rate of product quality deterioration increases (Verboven et al., thermal analysis of the forced-air precooling process, Dehghannya 2004a). Increasing airflow rate can compensate for the negative ef- et al. (2008) pointed out that the most non-uniform cooling appears fect of low open area. As the airflow rate increases, the surface heat in the vicinity of the inlet and outlet of a ventilated package. The transfer coefficient increases (Fikiin et al., 1999). Under the condi- numerical results by Delele et  al. (2013a) indicated that the area tion that the vent area is given constant, increasing airflow rate is an near the package vent showed relatively high cooling air velocity effective way to reduce the cooling time (Vigneault et al., 2006; Han and turbulence intensity. The typical airflow characteristics and tem- et al., 2018). perature distribution are shown in Figures 2 and 3, respectively. The However, increasing airflow rate also has a negative effect on coldest place is behind the entrance vents. The pressure drops across precooling effectiveness. For example, a high airflow rate leads to the inlet and outlet are 51.1% and 45.2%, respectively. There is a excessive water loss of fresh foods (Verboven et al., 2004a). Pressure good correlation between the airflow rate and the product tempera- drop also increases with the airflow rate, and consequently the ture distribution. The coldest zone corresponds to the zone with the energy consumption increases greatly (Verboven et  al., 2004a). highest cooling air velocity. Therefore, optimization of the ventila- Furthermore, the impact of air-inflow rate on precooling effective- tion design of packaging system had been regarded as a promising ness is in tight association with the location of vents. Any increase way to improve precooling effectiveness and maintain the quality of in airflow rate would result in an obvious pressure drop and air - horticultural produce in the postharvest cold chain. flow non-uniformity if the open area was formed by non-uniformly distributed vents (Vigneault et al., 2005). As the air-inflow rate in- Impact of air-inflow rate creases to 2.31 L/(s*kg), the cooling rate and uniformity are reason- ably improved. Any further increase in air-inflow rate will result in It is commonly accepted that the inlet airflow velocity has the a relatively low increase in cooling rate and uniformity, but energy greatest impact on the cooling rate (Castro et al., 2004a; Tutar et al., consumption and cooling damage will increase significantly (Han 2009; Han et  al., 2018) since the heat transfer coefficient of the et al., 2018). fruit surface is related to the airflow rate. When the airflow rate is Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 36 Y. Cao et al., 2020, Vol. 4, No. 1 Figure 2. Airflow characteristics inside a ventilated package for a superficial velocity of 0.3 m/s: (a) velocity vector, (b) velocity contour, (c) pressure contour, and (d) turbulent kinetic energy contour (Delele et al., 2013a). vents for blueberry plastic clamshell can increase the cooling rates of Impact of vent area both 6-oz and 1-pt clamshell by 10%–40% and 15%–55%, respect- Vent area is an important factor in the optimization of ventilation ively (Leyte et al., 1999). When the area of a single vent is constant, design. To maximize cooling uniformity, vent area should be large increasing the number of ventilation holes from two to four halves enough not to restrict airflow. In practical application, however, vent the cooling time (Han et al., 2015). The ±20% change of vent area area should have an upper limit to balance the relationship between had a significant impact on both the central and near-exit area of a the precooling effectiveness of horticultural produce and the mech- ventilated package rather than the near-entrance area (Opara and anical strength of the packaging system. Zou, 2007). Vent area has an optimal value. The further increase Many efforts had been devoted to the discussion about the impact of vent area beyond its optimal value has no obvious contribution of vent area on cooling rate. Generally, increasing the area of ven- to the cooling rate (Tutar et al., 2009). The wind tunnel experiment tilation holes can accelerate the cooling rate of products (Vigneault about the precooling of solid polymer balls (Castro et  al., 2004b) et  al., 2006; Ferrua and Singh, 2009d). For example, adding top Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 Impact of ventilation design on the precooling effectiveness of horticultural produce, 2020, Vol. 4, No. 1 37 properly distributed, increasing the vent area may no longer be suf- ficient to increase the cooling rate (Dehghannya et  al., 2012). The design of four vents distributed in corners resulted in poor cooling uniformity and at least 1.75 times more energy consumption in com- parison with uniformly distributed vents (Vigneault et  al., 2005). Therefore, optimizing vent position is of essential importance for cooling uniformity and energy saving. Impact of other factors The box material and internal packaging, such as tray, bunch carry bag, plastic liner, etc., also influence precooling effectiveness of horti- cultural produce. Ventilated package is the main cause for pressure drop compared to the product itself (Delele et  al., 2013a, 2013b) since it hinders the direct contact of horticultural produce with the cooling airflow and increases the flow resistance. The comparison study by Han et al. (2017b) indicates that polyvinyl chloride foam plastics have a low airflow resistance in comparison with corrugated boards. In addition, Opara and Zou (2007) studied the impact of the gap width between the tray and the package wall on precooling effectiveness. Results indicate that the product centre temperature is very sensitive to the change in the gap width, especially at the product layer away from the airflow inlet. Internal packaging, such as bunch carry bag, plastic liners, polyliners, etc., was often used to prevent the moisture loss in the precooling process. However, these internal packaging materials had a significant contribution to the increase of both pressure drop Figure 3. Temperature distribution within a ventilated package after 2  h of cooling from an initial temperature of 21°C using air with a superficial and cooling time (Delele et  al., 2013c). Quantitatively, the pres- velocity of 0.3 m/s and temperature of −5°C (Delele et al., 2013a). sure drop produced by the non-perforated plastic liner accounts for 83.34% ± 2.13% of the total pressure drop, while the pressure indicates that the vent area should exceed 6%. As the total opening drop of the grape itself only accounts for 1.40% ± 0.01% to 9.41% area exceeds 8%, increasing vent area has no significant impact on ± 1.23% (Ngcobo et al., 2012). The maximum volumetric flowrate the cooling rate (Castro et al., 2004a). For strawberry, the cooling in a palletized polylined kiwifruit package is 0.34  L/(s*kg), much uniformity is best as the vent area is about 9.4% (Wang et al., 2019). lower than the recommended flowrate of non-polylined produce Vent area also affects cooling uniformity and air pressure drop. (O’Sullivan et  al., 2016). That is, in order to achieve the same A more uniform airflow distribution can be achieved by increasing precooling effectiveness, a lot more energy is needed under the the vent area from 2.4% to 12.1%, with the heterogeneity index circumstances that complex internal packaging is adopted. In the decreasing gradually from 108% to 0% (Dehghannya et al., 2008). precooling process, the quality loss of fruit is mainly affected by Similarly, increasing the number of vent holes could also improve cooling time. The plastic liner increased the average 7/8 cooling cooling uniformity (Dehghannya et al., 2011). In addition, Van der time of the CT1 and CT2 stacks from 4.0 and 2.5  h to 9.5 and Sman (2002) confirmed the hypothesis of the power-law relationship 8.0  h, respectively (Ambaw et  al., 2017). In addition, energy −1.5 (Δp ∼ O ) between the pressure drop and the box vent ratio (O). tot consumption is up to three times higher than the unlined stack That is, as the vent area increases, the air pressure drop through the (Mukama et al., 2017). package decreases (Delele et al., 2008), and consequently the energy consumption is reduced (Ferrua and Singh, 2011). From the view- Conclusions point of energy saving, the optimal vent area was suggested to be between 8% and 16% (Vigneault et al., 2005). This paper reviews available studies about the ventilation design of the packaging system for the forced-air precooling of horticultural Impact of vent position produce. The function of packaging system is to protect horticultural Vent position is also an important factor influencing precooling ef- produce from mechanical damage during storage and transporta- fectiveness. Changing the vent position will change the airflow pat- tion, while vent holes are designed on the package wall to achieve tern and consequently influence the heat transfer characteristic and the purpose of rapid and uniform removal of field heat by ensuring temperature distribution inside the ventilated packaging system. adequate airflow through the surface of horticultural produce. Berry et  al. (2016) pointed out that packages with multiple vents Relevant research methods are summarized, along with representa- were more energy-efficient as the total vent area was given constant. tive researches and results. Both the merits and flaws of each method Han et al. (2017b) compared 10 typical apple carton samples used are described. Based on the limited and valuable experimental re- in China and found that uniformly and symmetrically distributed sults, developing reliable and efficient numerical simulation schemes vents were beneficial to the cooling uniformity of multilayer venti- seems to be the most hopeful direction of future research. lated packaging, especially when handling more than three layers of Ventilation packaging plays a key role in the postharvest packaging. handling of horticultural produce. The ventilation design is of Moreover, the position of ventilation holes is critical for cooling critical importance in the forced-air precooling process and con- uniformity when the air velocity is low. When vent holes are not sequently has been one of the research hotspots of the food cold Downloaded from https://academic.oup.com/fqs/article-abstract/4/1/29/5810753 by DeepDyve user on 13 May 2020 38 Y. Cao et al., 2020, Vol. 4, No. 1 Brosnan,  T., Sun,  D.  W. (2001). Precooling techniques and applications for chain. The impacts of the air-inflow rate, the vent area, the vent horticultural products—a review. International Journal of Refrigeration, position, and other factors, such as the box material and internal 24: 154–170. packaging, on precooling effectiveness are summarized from the Campanone, L. A., Giner, S. A., Mascheroni, R. H. (1995). The use of a simu- viewpoint of cooling rate, cooling time, cooling uniformity, air pres- lation software to optimize cooling times and to lower weight losses in sure drop, and energy consumption. It is found that the influence of fruit refrigeration. Proceedings of 19th International Congress of Refriger- ventilation design on precooling effectiveness is complicated. For ation, 1: 121–128. example, increasing air-inflow rate can accelerate the cooling rate, Castro, L. R. D., Cortez, L. A. B., Vigneault, C. (2006). Effect of sorting, re- but it will also increase energy consumption. Increasing vent area frigeration and packaging on tomato shelf life. International Journal of tends to increase the cooling rate and improve cooling uniformity, Food, Agriculture and Environment, 4: 70–74. but it also leads to a reduction in the mechanical strength of the Castro, L. R. D., Vigneault, C., Cortez, L. A. B. (2004a). Container opening design for horticultural produce cooling efficiency. Journal of Food Agri- ventilated packaging system. Other factors, such as the variety in culture and Environment, 2: 135–140. horticultural produce, the various options for box material and size, Castro, L. R. D., Vigneault, C., Cortez, L. A. B. (2004b). Effect of container the internal packaging, etc., further complicate the optimization of opening area on air distribution during precooling of horticultural ventilation design. Therefore, multi-parameter analysis seems to be produce. 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