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www.nature.com/npjscifood ARTICLE OPEN Food nanoparticles from rice vinegar: isolation, characterization, and antioxidant activities 1 1 1 1 1 1 1✉ 2 1 1 Zhaoshuo Yu , Ying Tan , Sihao Luo , Jingru Zhou , Tianhao Xu , Jianqiao Zou , Lijing Ke ,JiYu , Suyun Zhang , Jianwu Zhou , 1 3 Pingfan Rao and Jiaxing Li Abundant nanostructures have been constantly found in various foods, like vinegar, tea, coffee, and milk. However, these structures largely remain unexplored and even been eliminated for stability reasons in food industry. Here we report the isolation, characterization, and antioxidant activities of food nanoparticles (NPs) carrying polyphenols from Chinese rice vinegar. Using a gel- chromatography-based isolation protocol, the vinegar was separated into three major fractions. They were identified as spherical NPs (P1), lollipop-like NPs (P2) and spherical microparticles (P3) with average hydrodynamic diameter of 210, 245,1643 nm, separately. The former two fractions accounted for the major parts of dry matter in the vinegar. The P1-NPs fraction was composed of proteins, carbohydrates, and a high number of polyphenols (15 wt%), demonstrated potent antioxidant activity as determined by ABTS and ORAC assays. Moreover, they effectively quenched peroxyl free radicals in peritoneal macrophages and promoted cellular growth. The P2 fraction contained majority of organic acids, esters and mineral elements of the vinegar. It demonstrated the NPs are bioactive units of the rice vinegar, inspiring the development of novel functional nanomaterials with nutraceutical and pharmaceutical applications. npj Science of Food (2022) 6:1 ; https://doi.org/10.1038/s41538-021-00118-y INTRODUCTION bioactivities and functions. Yet, in what way that polyphenols in vinegar interact with human body remains unknown. The omnipresence of self-assembled micro/nano-particles (MNPs) In order to improve the stability of liquid food, the industry has within foods or traditional Chinese medicines has enjoyed a surge 1–3 by all means eliminated the precipitation, in which, however, of attention lately , from chicken soup, coffee, tea, fruit to herbal 4 might involve the micro/nanostructures, leading to the loss of decoction . These MNPs, composed of polar and nonpolar benefits of vinegar and other foods alike. Therefore, there is an compositions are incidentally formed during food processing as urge need to isolate and characterize the micro/nanostructures in the results of chemical reactions and physical interactions. They vinegar and unveil their biological activities, offering a more often serve as functional units of foods, exhibiting various comprehensive insights into food nanoparticles. In this study, a biological activities. For example, MNPs from porcine bone soup separation protocol using gel-chromatography coupled with can directly interact with oral and peritoneal macrophages and dynamic light scattering (DLS) was established for isolation of 5,6 inhibit polarization of cells . Many bioactive compounds have vinegar NPs. The NPs were subsequently determined for their been fabricated into MNPs by various approaches, in the hope to morphological profiles, colloidal properties, compositions, antiox- facilitate the functions of controlled-release, targeted distribution idant activities, and cytotoxicity. 7,8 and elevated bioavailability . However, all these efforts are still far from application in food products, owing to the uncertainty in food safety and cost efficiency. RESULT AND DISCUSSIONS Besides these NPs identified from the neutral or weak acidic Gel-chromatography separation and characterization of food matrix, an abundant amount of NPs have been found in the vinegar MNPs 9,10 acidic food, vinegar . The major steps of producing Chinese rice Three fractions of Chinese aged rice vinegar were successfully vinegar includes steaming of raw materials, alcohol fermentation, separated by gel-chromatography coupled with UV absorbance acetic acid fermentation, and ageing. The whole process usually (Fig. 1a) and online-DLS detector (Fig. 1b). The combined protocol lasts for more than one year, allowing the occurrence of extensive 14,15 has been successfully applied to the particle characterization chemical reactions between carbohydrates and proteins or amino and here was used to monitor the isolation of food nanoparticles acids to form a great amount of Maillard reaction products (MRPs) from complex food matrix. In order to minimize the damage to the and micro-/nano-scale aggregates of multiple components. fractionate structures, mobile phase has to be preliminarily tested Beyond a daily used condiment, the Chinese aged rice vinegar for its potential impacts on the colloidal properties of vinegar. The is embraced as a traditional functional food for its health- test results of several buffers and deionized water were promoting benefits, such as hypolipidemic, anti-obesity effects , determined (Supplementary Data 1). Among them, deionized 12,13 and microbiome regulation . Such benefits are believed to be water possessed the least effects on the vinegar particles while attributed to a great number of antioxidant components, the stability might be vulnerable to the ions in buffers. Once being including polyphenols, together with MRPs. The interaction of diluted by deionized water, the particles’ Debye screening length bioactive components and food or drug matrix could affect their became larger and therefore the Coulomb repulsion between the 1 2 SIBS-Zhejiang Gongshang University Joint Centre for Food and Nutrition Sciences, Zhejiang Gongshang University, Hangzhou 310012, China. Institute of Food Science, Jishou University, Jishou 416000 Hunan, China. Hunan Salt Industry Co., Ltd., Changsha 410004, China. email: lijingke@zjgsu.edu.cn Published in partnership with Beijing Technology and Business University 1234567890():,; Z. Yu et al. Fig. 1 Isolation of the micro/nano-particles in vinegar with combination of size-exclusive chromatography and dynamic light scattering. a The isolation chromatogram. The Sepharose CL-4B column (1.0 cm × 13 cm) was used, equilibrated, and eluted with deionized water at a flow rate of 0.5 mL/min with UV absorbance at 280 nm (black line) and 420 nm (brown line). b Light scattering intensity of isolated micro/ nanoparticles determined by the Zetasizer Nano-ZS. c Chromatogram obtained mixture of molecule weight standards (Dextran, Bromophenol blue). d–f Size distribution of P1-NPs, P2-NPs, and P3-MNPs. Table 1. Colloidal properties of vinegar and fractions. P1 P2 P3 Vinegar ** ** ** Z-average (nm) 210.4 ± 3.0 245.4 ± 6.1 1643.0 ± 73.4 486.7 ± 2.0 ** * PDI 0.30 ± 0.03 0.44 ± 0.02 0.44 ± 0.13 0.38 ± 0.02 ** ** ** ζ-potential (mV) −32.20 ± 0.65 −1.37 ± 0.78 −0.76 ± 0.02 −3.40 ± 0.54 9 ** ** ** Concentration (10 particles/mL fraction) 14.60 ± 0.42 2.97 ± 0.63 0.86 ± 0.10 142.80 ± 2.07 Data are presented as mean ± standard deviation (n = 3). *p < 0.05 vs. P1 and **p < 0.01 vs. P1. particles would prevent the particles from aggregating and keep zero, as a result, P1-NPs gained larger electrostatic repulsion with relatively stable. Therefore, deionized water was used as the column packings and then eluted before other fractions. mobile phase in the gel-chromatography separation. The first two Compared with size distribution of vinegar and other fractions, P1-NPs exhibited narrower distribution (Fig. 1d–f) and contained peaks were named as P1 (elution time 15–27 min) and P2 (elution time 34–50 min), respectively, and the shoulder near P2 was less microparticles. TEM observation (Fig. 2) confirmed the particle named P3 (elution time 50–54 min). UV absorbance at 420 nm was size measured by DLS and revealed some well-defined morpho- logical details of the nano-assemblies: spherical nanoparticles in used to monitor distribution of MRPs or polyphenol oxides, and UV absorbance at 280 nm were implying the presence of protein. P1, lollipop-like nanoparticles in P2 and microparticles in P3. In addition, a significant number of fluorescent components were The components in P1 scattered the laser lights most intensively among three fractions (Fig. 1b) and possessed the highest particle found to be distributed mainly in P2 (Fig. 3). The difference of fluorescent intensity lying in the vinegar fractions implied that at concentration in vinegar according to the particle number determination by Nanosight (Table 1). least two types of MRPs existing in vinegar. Furthermore, as Right after the collection, the colloidal properties of all fractions fluorescent carbon nanodots with size less than 20 nm have been and original vinegar were measured with DLS as shown in Table 1. identified in the vinegar , it is quite possible that P2 contained The average D of P1-NPs, P2-NPs, and P3-NPs were 210, 245, and such nanodots, which might be derived from MRPs and could 1643 nm, separately. Notably, the average D of P1-NPs is smaller further aggregate into nanoparticles . than P2-NPs and P3-NPs. However, P1-NPs, P2-NPs exhibited the The content of P1-NPs was about 4.5 mg per milliliter vinegar, same elution time with dextran (Mw = ~500 kDa) and bromophe- which was over 1000 times higher than the MNPs from vinegar reported previously . Low yield of these relatively small NPs nol blue (Mw = ~670 Da), respectively (Fig. 1c), which suggested that P1-NPs should have larger size than P2-NPs and P3-NPs. One (~20 nm) from the same type of vinegar may be attributed to the sample extraction using ethanol, which would cause agglomera- possible explanation was that the difference in the electrostatic interaction between fractions and the column packings, i.e., tion or decomposition of the native NPs. The chromatography-DLS separation employed here provides a less-invasive approach for Sepharose led to the isolation results. The ζ-potential of P1-NPs was −32.2 mV while those of P2-NPs and P3-NPs were both near isolating nanoparticles from complex food matrix. npj Science of Food (2022) 1 Published in partnership with Beijing Technology and Business University 1234567890():,; Z. Yu et al. Table 2. Major composition of the fractions. P1 P2 P3 Carbohydrate (%) 31.9 ± 1.1 37.0 ± 0.4** 36.6 ± 0.9** Protein (%) 53.1 ± 0.7 1.5 ± 0.1** 0** Total acid (%) 0 39.1 ± 1.4** 14.0 ± 0.8** Total ester 2.6 ± 0.6 12.3 ± 1.7** 20.3 ± 1.3** Mineral element (%) 2.6 ± 0.4 9.1 ± 0.1** 23.8 ± 1.1** Mass weight (mg) 4.5 ± 0.1 185.0 ± 4.3** 2.0 ± 0.2** Data are presented as mean ± standard deviation (n = 3). *p < 0.05 vs. P1 and **p < 0.01 vs. P1. magnesium, calcium, and iron. These differences may affect the particle formation and stability of the vinegar components. P1- NPs hardly contained minerals and then its ζ-potential enlarged due to the growing Debye screening length with dilution, while other fractions had high composition of minerals which might enable the particles tend to be aggregated. In this sense, P2-NPs Fig. 2 TEM micrographs of the fractions isolated from vinegar. and P3-NPs should be the precursors of precipitate in vinegar. It a P1-NPs, original magnification ×40,000, b P2-NPs, original provides an explanation as well to the dramatical changes in the magnification ×40,000, c P3-MNPs, original magnification ×10,000, and d Selectively amplifying scope of P2-NPs. properties of vinegar caused by addition of buffers (Supplemen- tary Data 1). The size of particles in P3 fraction is not stable during the separation. It was less than 20 nm determined by online-DLS, but over 1 μm determined by static DLS. As its content in vinegar is extremely low, it was not further studied in this work. Bioactive compounds Phenolic compounds are widely regarded as the main bioactives of vinegar exhibiting various health benefits, including antibacter- ial, antioxidant, and hypolipidemic activities. As shown in Table 4, the contents of polyphenols and flavonoids in P1 and P2 were 15.5% and 2.0%, respectively. It is rather unusual since the phenolic compounds were small molecules and not supposed to be eluted in the free form together with P1-NPs during size- Fig. 3 Fluorescence imaging of the isolated fractions. The images were photographed with a fluorescence stereomicroscope FluorVivo exclusive separation, unless they were bound to the NPs. This under daylight and the excitation of blue light 410–440 nm, binding was tough enough to resist the mechanical forces and respectively, and treated with pseudo-color. intermolecular interactions during chromatographic separation. In this sense, phenolic compounds possibly took part in the assembly of nanostructures. Compositions of vinegar NPs Among over ten kinds of phenolic compounds have been The major compositions of vinegar NPs were determined and identified in the vinegar, half of which are catechins .The content of phenolic compounds in P1 was 7.5 times of that in P2. presented in Table 2. Carbohydrate and protein accounted for over 85% of P1-NPs compositions, forging the protein- Vinegar NPs here serve as nanocarriers with desirably high carbohydrate hybrid nanostructures. In contrast, P2 was mainly loading capacity for bioactive compounds compared to other composed of carbohydrates which might be low-molecular- delivery platforms, being expected to elevate bioaccessibility and weight, e.g., mono/oligosaccharides, organic acids, esters and a bioavailability. As a matter of fact, the majority of existing micro/ nanocarriers have a rather low loading capacities (up to 4.0 wt % ) small proportion of proteins. Organic acids and esters, as the main flavor substances of vinegar, were mainly distributed in P2 and P3, while enduring complicated fabrication processes and safety risks. In addition, phenolic compounds can attach to MRPs via non- but rarely in P1. The Chinese aged rice vinegar is rich in organic acids, i.e. acetic acid, lactic acid, succinic acid, oxalic acid, and covalent bonds to reinforce their antioxidant activities . There- fore, the co-existence of polyphenols and MRPs in P1 and P2 NPs aromatic acids such as sinapic acid . At the same time, in the fermentation process of vinegar, the esters formed by amino acid may enhance antioxidant capacity of the vinegar. Meanwhile, the polyphenols may facilitate the self-assemble of components may be amphiphilic since they have both amino and 19,20 ester bonds, resulting in the generation of specific structures such micro/nanostructures via covalent bonds reacted with MRPs or non-covalent bonds, e.g. π-π stacking interaction with as micelles or microemulsions. 22 23 Like the organic acids and esters, the majority of minerals of hydrophobic moiety of proteins . Moreira, et al. also reported vinegar was found in P2 and P3, while less than 1% of which was the sugar chains, i.e., galactomannan, could polymerize with in P1 (Table 3). Among all the metal ions, sodium and potassium phenolic compounds in MRPs. One of the possible products of this are the most dominant ones in all the three fractions, followed by polymerization is lollipop-like aggregates, like the NPs in P2. Published in partnership with Beijing Technology and Business University npj Science of Food (2022) 1 Z. Yu et al. Table 3. Major mineral element composition of the fractions. Mineral element (mg/L) Detection limit P1 P2 P3 Na 0.0015 104.20 ± 0.92 8554.67 ± 36.95** 314.13 ± 0.92** K 0.0021 0.28 ± 0.07 5600.00 ± 84.66** 166.67 ± 1.22** Mg 0.0009 0.24 ± 0.00 2154.67 ± 18.48** 84.53 ± 0.46** Ca 0.0093 0 430.40 ± 2.08** 23.03 ± 0.20** Fe 0.0007 11.16 ± 0.70 92.80 ± 1.93** 3.57 ± 0.04** Zn 0.0002 0.51 ± 0.01 35.44 ± 1.20** 1.68 ± 0.01** Al 0.0015 1.49 ± 0.26 16.28 ± 0.69** 0.71 ± 0.01** Total / 117.88 ± 1.95 16884.25 ± 145.99** 594.31 ± 2.87** Data are presented as mean ± standard deviation (n = 3). *p < 0.05 vs. P1 and **p < 0.01 vs. P1. CAA Table 4. Contents of flavonoids and phenolic compounds and In CAA assay, DCFH-DA derived fluorescence is selectively antioxidant activities of P1 and P2. reporting the level of intracellular reactive oxygen species (ROS). As shown in Fig. 5a, b, the AAPH-induced intracellular ROS of P1 P2 macrophages were significantly decreased by P1 and P2, respectively, indicating potent cellular antioxidant activities in Total flavonoid (%) 6.82 ± 0.56 0.87 ± 0.02** both P1 and P2. As shown in Fig. 5c, both P1 and P2 showed dose- Total phenols (%) 8.67 ± 0.80 1.17 ± 0.08** dependent antioxidant activities on macrophages, e.g. CAA value ABTS (mmol/g) 0.83 ± 0.11 0.29 ± 0.07** over 60 unites at 200 μg/mL. The CAA of P1 was higher than that ORAC (mmol/g) 1.19 ± 0.07 0.16 ± 0.03** of P2, though marginally, implying the important contribution of NPs to bioactivities of the vinegar. Despite the much lower Data are presented as mean ± standard deviation (n = 3). content of polyphenol, P2 exhibited similar cellular antioxidant *p < 0.05 vs. P1 and **p < 0.01 vs. P1. activities with P1. For the CAA, the free radical scavenging capacity and initial concentration of bioactives did not waltz alone. The Fourier transform infrared (FT-IR) spectroscopy cellular uptake, distribution and metabolism of bioactive compo- FT-IR spectra of P1 and P2 was shown in Fig. 4. The absorbance nents, often altered by ζ-potential and micro-/nano-structure of −1 27,28 peak around 3414 cm is ascribed to O–H vibration and the NPs, all affect their final biological impacts . −1 Macrophages play essential roles in the innate immune peaks at 2930 cm are ascribed to CH and CH , respectively. 3 2 −1 response and tissue homeostasis . Food-derived NPs have Bands around at 1650, 1540 and 1400 cm could be assigned to demonstrated regulatory effects on immune cells including Amide I (C=O group), Amide II (N–H bending) and Amide III (C–N macrophages. Colloidal particles in bone soup were quickly stretching), separately. One major difference between P1 and P2 −1 −1 engulfed by oral and peritoneal macrophages and maintain fell in the range of 1620–1656 cm . The peak around 1620 cm normal mitochondrial metabolism . NPs from grape could protect of P2 was attributed to –COOH vibration , which is corresponded mice against dextran sulfate sodium induced colitis by modulating with its high content of organic acid. In contrast, P1 contains no stem cells and macrophages . The modulation of macrophage carboxylic acids as determined in Table 2. −1 proliferation and intracellular ROS by vinegar NPs is indicating Bands between 1047 and 1037 cm were caused by C–O and 24 their potential immune-regulatory effects. C–C stretching of carbohydrates, polysaccharides, or flavonoids . −1 At around at 671 cm ,C–H external bending vibration of Cytotoxicity aromatic hydrocarbons was observed, validating existence of phenolic compounds in vinegar NPs. As shown in Fig. 5d, the influences of P1 and P2 on viability of murine peritoneal macrophages were determined with MTT assay. P1-NPs showed no cytotoxicity at all the assayed concentrations, Antioxidant activities as high as 400 μg/mL while promoted cell proliferation by 35% at To verify the antioxidant activity of P1 and P2, the ABTS cation the lowest concentration of 25 μg/mL. While the active principles radical reducing activity and ORAC were determined. As shown in in P1 NPs responsible for their cell proliferation promoting activity Table 4, the activities of P1 and P2 on scavenging ABTS cation are awaiting to be identified, one may expect the rich content of radicals were 830 ± 110 mmol trolox equivalent (TE)/g and 290 ± polyphenols in these NPs could be one of the underlying reasons. 70 mmol TE/g, respectively. Their ORAC activities were 1190 ± In contrast, P2 was nontoxic to the cells at the concentration range 70 mmol TE/g and 160 ± 30 mmol TE/g, respectively. It elucidates of 25–200 μg/mL, while inhibited cell proliferation by over 30% at that P1 and P2 are contributing to the antioxidant capacity of 400 μg/mL. The high content of organic acids in P2 should be vinegar. The antioxidant activity of P1 was a few times higher than responsible for its cytotoxicity at the high concentration. The that of P2, echoing the 7.5 times higher content of polyphenols in tested concentration (25–400 μg/mL) of P1 and P2 was roughly P1. The difference in ABTS and ORAC might be attributed to the equivalent to 6–90 μL and 0.1–2.2 μL of vinegar, respectively. It different capacity of vinegar components on scavenging ABTS suggested that the particles in vinegar may directly affect the cation radicals and peroxyl radicals . Besides phenolic com- mucosal cells in the upper part of alimentary tract, e.g., pounds, oligosaccharides and organic acids may reduce ABTS macrophages in the mouth, even at a very low dose. radicals, too. Furthermore, the binding of phenolic compounds to The nanocarriers often provide protection, targeted delivery, the MNPs may improve the former’s physicochemical stability and elevated bioavailability, and bioactivities to their cargo composi- bioavailability. tions . Moreover, the bioatives in vinegar are highly concentrated npj Science of Food (2022) 1 Published in partnership with Beijing Technology and Business University Z. Yu et al. Fig. 4 IR spectra of P1 and P2 isolated from vinegar. Main absorbance peaks were described using annotations below the spectrum. Fig. 5 The effects of P1 and P2 on the cellular redox status and viability of peritoneal macrophages. a Kinetic curves of AAPH-induced DCF fluorescence and the inhibition of oxidation by P1 or b P2 on peritoneal macrophages from mice. c Cellular antioxidant activity of P1 or P2 with different concentration on peritoneal macrophages. d Viability of peritoneal macrophages incubated with P1 or P2 for 24 h and measured by MTT assay (n = 3). Data are presented as mean±standard deviation. Statistics: *p < 0.05 vs. P1 and **p < 0.01 vs. P1, t-test. on the nanoparticles rather than dispersed evenly in the aqueous original foods would inevitably be affected. Beyond single phase. Taking polyphenols in vinegar as example, they were molecules, the food nanoparticles acquire a revisit to the quality condensed in NPs with a concentration over 14,000 times higher and function evaluation of vinegar, expanding our understanding than that of overall vinegar dispersion, as the NPs only occupy 1/ of this traditional fermented food and other liquid foods such as 14000 of dispersion’s total volume. The effect of concentrated tea, coffee, and fruit juice. Moreover, utilizing the food nanopar- bioactives bound to NPs validates the new possibility that NPs at a ticles as functional carriers, a novel approach is emerging for low dose can exert considerable bio-functions and provides with a development of effective, safe, and sustainable nano-scale food new interaction mechanism between food NPs within human ingredients. bodies. As a price, the bioactive payloads may be unintentionally discarded during the production and consumption as the vinegar NPs gradually agglomerated and precipitated, possibly triggered METHODS by the low surface potential of particles in the acidic solution. Materials Although the vinegar NPs have shown significant impact on Vinegar was provided by Jiangsu Hengshun Vinegar-Industry Co., Ltd peritoneal macrophages in vitro, the real situation when they (Jiangshu, China), which is one of the representative rice vinegars in China. encounter the mucosal cells in digestive tract, together with other Deionized water was prepared by a Millipore Milli-Q water system components of food and mucosal fluids, is a fascinating and (Millipore, Bedford, MA, USA). Blue Dextran (MW: ~500 kDa), and meaningful subject for the future studies. Bromophenol blue (MW: ~670 Da), 2,2-azobis(2-amidinopropane) hydro- In conclusion, spherical nanoparticles, lollipop-like nanoparticles chloride (AAPH), 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium and microparticles were successively isolated from vinegar by DLS bromide (MTT) and DCFH-DA ( ≥ 97% purity) were purchased from Sigma- coupled gel-chromatography. Vinegar NPs were nano-vehicles Aldrich Co., Ltd. (Shanghai, China). ABTS kit was purchased from Beyotime with high loading capacity for hydrophobic and hydrophilic Biotechnology Co., Ltd (Shanghai, China). DMEM, FBS, streptomycin, PBS antioxidants, exerting potent oxygen-free radical scavenging and penicillin were purchased from Thermo Fisher Scientific (China) Co., capacity and cellular antioxidant activity on mucosal macrophages Ltd (Shanghai, China). All the chemicals and solvents, unless otherwise (Fig. 6). Once these NPs are removed, the function and stability of noted, used were of AR grade, and Sepharose CL-4B column (60–2×10 Published in partnership with Beijing Technology and Business University npj Science of Food (2022) 1 Z. Yu et al. Fig. 6 Graphical illustration of the isolation, characterization, and antioxidant activities detection for vinegar nanoparticles. Briefly, the vinegar was centrifuged at 5,000 g for 15 min to remove the precipitates. The supernatant was then injected into the gel-chromatography column (1.0 cm × 13.0 cm) coupled with DLS and UV detection. Subsequently, three fractions were observed in the chromatogram. The colloidal properties, chemical composition, and representative TEM image of fractions were determined. At last, the effects of the major fractions on the cellular redox status and viability of peritoneal macrophages from SD mice were explored. kDa) were purchased from Sinopharm Chemical Reagent Co., Ltd Carbohydrate concentration was determined by anthrone-sulfuric acid (Shanghai, China). assay . Briefly, 0.5 mL sample or glucose standard of different concentra- tions (0–80 μg/mL) were added to 5 mL of anthrone reagent (0.75% anthrone and 84% sulfuric acid). The samples were shaken with a Vortex Isolation of vinegar NPs by gel-chromatography coupled DLS mixer (Thermofisher, USA) to ensure complete uniformity of the dispersion The vinegar was centrifuged at 5000 × g for 15 min to remove the and heated for exactly 10 min in a boiling-water bath. They were cooled in precipitates. The supernatant was collected and stored at −4 °C until an ice-water bath and measured the absorbance at 590 nm. analyzed. To minimize the damage to the fractionate structures, mobile The total acid and ester content was conducted by titration method phase was preliminarily tested for its potential impacts on the colloidal (AOAC, 1984) . Briefly, 50.0 mL sample with 2 drops of phenolphthalein properties of vinegar. One milliliter of vinegar was diluted ten times with indicator solution was pipetted into a 250 mL Erlenmeyer flask. To different buffers, separately, and the impacts of buffers on the vinegar neutralize the acid the sample, 0.1 mol/L sodium hydroxide standard stability then were determined by DLS (Zatasizer Nano-ZS, Malven solution was slowly added until the color became reddish. The recorded instruments Ltd, UK). One milliliter vinegar was subjected to a pre- consumption of sodium hydroxide solution in milliliters then convert the equilibrated gel-chromatography column (Sepharose CL-4B, 1.0 cm × total acid content. The esters in the samples were saponified with 13.0 cm) equipped with a UPLC (Chromatography System, BIO-RAD, USA) abundant and 0.1 mol/L NaOH solution, and then the residual NaOH was system at flow rate 0.5 mL/min, using deionized water as mobile phase and titrated with 0.1 mol/L H SO solution. The content of esters could be UV monitor at 280 and 420 nm, respectively, and coupled with a DLS 2 4 calculated by the quantitative relationship with actual consumed H SO in instrument (Zetasizer Nano-ZS, Malven instruments Ltd, UK) equipped with 2 4 a flow-cell. Elutes were collected by an automatic fraction collector at the chemical reaction. The mineral composition was evaluated by inductively coupled plasma mass 2 mL/tube (NGC Fraction Collector, BIO-RAD, USA). As shown in Fig. 1, the spectrometry (Thermo CAPQ ICP-MS, Thermo Electron, Waltham, MA, USA). fractions with strong UV absorbance were collected and labeled as below: P1 (elution time 15–27 min), P2 (elution time 34–50 min), and P3 (elution Briefly, 1.0 mL of hydrogen peroxide and 3.0 mL of nitric acid were added to time 50 to 54 min). The fractions were lyophilized by a lyophilizer 0.5 mL of sample in closed microwave digestion tank. Then the digestion was (VCP63MV, Martin Christ GmbH, Osterode, Germany) and stored at −30 °C. according to the following procedure that microwave power was 1500 W, temperature programming from 0 to 120 °C within 5 min (held for 5 min), continued temperature programming to 200 °C within 5 min (held for 30 min). Colloid properties and morphology A multi-element stock standard solution containing all the analytical mineral The hydrodynamic diameter, polymer dispersity index (PDI), and ζ- elements was used to prepare a standard curve. The digested samples were potential of the three fractions and vinegar were characterized by DLS dissolved in deionized water and analyzed using ICP-MS. (Zatasizer Nano-ZS, Malven instruments Ltd, UK). The particle numbers and size distribution of these samples were determined by Nanosight LM10 (Malven instruments Ltd, UK). One milliliter of the supernatant, without Bioactive compounds analysis dilution, was gently injected to the cuvette for measurement at 25 °C. The total phenols contents in P1 and P2 were determined by Folin–Ciocalteu Viscosity of 0.8872 and refractive index of 1.330 were used in the method, and the total flavonoid contents (TFC) in P1 and P2 were measurement and analysis. determined by AlCl method . Briefly, samples were diluted ten times with TEM observation of fractions was performed with JEOL JEM-1230 (TEM, deionized water, and 0.2 mL of diluted sample was mixed with 0.8 mL Japan). The dispersion was transferred to the 230 mesh copper grid, stained Folin–Ciocalteu reagent. After 5 min, 1.5 mL 10% Na CO (w/v) solution was with phosphotungstic acid, and then observed under the TEM at 80 kV. 2 3 mixed and then deionized water was added to obtain the final volume of 10 mL. The mixture was measured at 765 nm after 120 min in the dark with Fluorescence imaging Gallic acid used as a reference. Besides, every sample was neutralized with Three fractions were photographed with a fluorescence stereomicroscope 2% NaOH solution and diluted 10 times with deionized water. Two milliliters FluorVivo (INDEC BioSystems, USA) under daylight and the excitation of of diluted sample, 8 mL of distilled water and 1 mL of 5% NaNO solution blue light 410–440 nm. The fluorescent images were treated with pseudo- were mixed. After 6 min, 1 mL of 5% Al (NO ) solution was added and stood 3 3 color according to the fluorescent intensity. for 6 min. Finally, 4 mL of 20% NaOH solution was added and made up to 25 mL with deionized water. After 15 min, the absorbance was measured at Major composition analysis 510 nm with Rutin used as a reference. All the data was recorded in a UV–visible Spectrophotometer (U-5100, Hitachi, Japan). Protein concentration was determined by Coomassie Brilliant Blue assay . Briefly, 0.1 mL sample or protein standard of different concentrations (0–0.8 mg/mL) and five milliliters of protein reagent (0.01% Coomassie Fourier transform infrared spectrometer Brilliant Blue G-250, 4.7% ethanol, and 8.5% phosphoric acid) were added A Fourier transform infrared spectrometer (FT-IR-8400, Shimadzu, Kyoto, to the test tube and the contents mixed by inversion. The absorbance at 595 nm was measured in the 3 mL cuvette with ultraviolet spectro- Japan) was used to record Fourier transform infrared spectra of P1 and P2 −1 photometer (UV-5100, HITACHI, Japan). with an accumulation of 32 scans and a resolution of 1 cm . npj Science of Food (2022) 1 Published in partnership with Beijing Technology and Business University Z. Yu et al. Extracellular antioxidant activity DATA AVAILABILITY ABTS assay. The antioxidant activities of P1 and P2 were measured by The authors declare that the data supporting the findings of this study are available ABTS assay kit (Beyotime) according to the manufacturer’s protocols . The within the article. absorbance at 734 nm was measured with a FlexStation 3 Plate Reader (Molecular Devices, USA). The data were expressed as TE/g of samples. 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Attribution 4.0 International License, which permits use, sharing, 37. Wu, S. L. et al. Genome-wide analysis of YB-1-RNA interactions reveals a novel adaptation, distribution and reproduction in any medium or format, as long as you give role of YB-1 in miRNA processing in glioblastoma multiforme. Nucleic Acids Res. appropriate credit to the original author(s) and the source, provide a link to the Creative 17, 8516–8528 (2015). Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly ACKNOWLEDGEMENTS from the copyright holder. To view a copy of this license, visit http://creativecommons. This study was supported by the National Key R&D Program of China org/licenses/by/4.0/. (2016YFD0400202). The authors thank Professor Chengjin Ma from Jishou University for valuable discussions, and Professor Yue Zhang from Zhejiang Gongshang University for editing the manuscript. © The Author(s) 2022 npj Science of Food (2022) 1 Published in partnership with Beijing Technology and Business University
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Published: Jan 11, 2022
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