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Characterization of the key aroma compounds in three types of bagels by means of the sensomics approach

Characterization of the key aroma compounds in three types of bagels by means of the sensomics... Background: To evaluate the impact of cold fermentation time on bagel rolls, the key aroma-active compounds in the volatile fractions obtained from three different bagel rolls through solvent assisted flavor evaporation (SAFE) were sequentially characterized by an aroma extract dilution analysis (AEDA), quantified by stable isotope dilution and analyzed by odor activity values (OAVs) respectively. Results: Findings revealed 40 aroma-active compounds with flavor dilution (FD) factor ranges of 2–1024. Of these, 22 compounds (FD ≥ 16) were quantified by stable isotope dilution assays (SIDA). Subsequent analysis of the 22 compounds by odor activity values (OAVs) revealed 14 compounds with OAVs ≥ 1 and the highest concentrations were obtained for 2,3-butanedione, 2-phenylethanol, 3-methylbutanal and acetoin respectively. Two recombina- tion models of the bagels (i.e. 24 h and 48 h bagels) showed similarity to the corresponding bagels. Omission tests confirmed that 2,3-butanedione (buttery), acetoin (buttery), 2-acetyl-1-pyrroline (roasty), 5-methyl-2-furanmethanol (bread-like), (Z)-4-heptenal (biscuit-like) and 4-hydroxy-2,5-dimethyl-3(2H)-furanone, were the key aroma compounds. Additionally, acetic acid, butanoic acid, 2-phenylethanol (honey-like), 3-methylbutanoic acid, 2/3-methylbutanal, vanil- lin, 3-methylbutanol, methional were also important odorants of the bagel. Conclusion: Whilst the long, cold fermented bagels exhibited roasty, malty, buttery, baked potato-like, smoky and biscuit-like notes, the control bagels produced similar but less intense odor notes. Keywords: Bagel, Aroma-active compounds, Cold fermentation, Sensomics approach Introduction of the dough used in bagel production. Traditional bagels are often produced with high protein (13–16%) spring Bagels are one of the most widely consumed bread rolls wheat flour [2]. In addition, the long, cold fermentation in the United States. Recent statistics have shown that step called retardation gives the traditional bagels a dis 204 million Americans consumed bagels in 2019 [1]. This - figure is projected to increase to 210 million in 2023 [1]. tinctive crust and flavor not found in the regular bread Bagels have a very simple formulation similar to simple rolls. The quality of bread is normally defined by its vol - bread or roll formulas (i.e. flour, salt, yeast, and water). ume, texture, color and flavor [3]. However, the aroma of However, what differentiates bagel from the rest of the bread is undoubtedly one of the most important qualities rolls are the flour quality and the long, cold fermentation that influence its acceptance by consumers [4]. Bread flavor appreciation is one of the first evalua - tion signals encountered by consumers during bread *Correspondence: olaniny56@gmail.com consumption [5]. The flavor of bread is engendered by Department of Food Technology, University Putra Malaysia, UPM, the interaction of a large number of compounds, which 43400 Serdang, Malaysia exhibit different olfactive characteristics, tactile oral and Full list of author information is available at the end of the article © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://crea- tivecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdo- main/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Lasekan et al. BMC Chemistry (2021) 15:16 Page 2 of 13 trigeminal sensations. Some of these compounds include; regular bread. Therefore, elucidating the flavor chemistry alcohols, aldehydes, esters, ketones, acids, hydrocarbons, of bagels could improve their quality control and process- pyrazines, pyrrolines, furans etc. [3, 4]. Over 300 vola- ing of bagels. The objective of this study was to charac - tile compounds have been reported in white bread [6]. terize the key aroma compounds in long, cold fermented In addition, the odour quality of bread depends on many bagels using the sensomics approach. factors like; type of flour, type of fermentation [7] and dough improvers [8] used during bread production. The Materials and methods production process and storage are also known to influ - Bagel production ence the flavor of bread [9]. Bagel doughs were made by employing three processes Analysis of volatile compounds in a food matrix is differing in their cold fermentation conditions, and quite complex and several extraction methods have been the time required for boiling the bagel dough in water reported, ranging from solvent extraction [10], headspace (Fig. 1). The dough recipes contained high protein wheat solid phase micro-extraction (HS-SPME) [11], dynamic flour 13% (enriched bakers patent flour from Pastry Prod - headspace extraction (DHE) [4], multiple headspace uct Sdn., Malaysia) (2000  g); cold water, 1100  g; instant solid phase micro-extraction (MH-SPME) [12], solvent dry yeast, 30 g; salt, 30 g; granulated sugar, 60 g, shorten- assisted flavor evaporation (SAFE) [13] and vacuum ing; 60 g and malt flour (high diastatic malted barley flour sublimation [14]. In the same vein, many identification 185 Lintner minimum) 60 g. The ingredients were made techniques have been employed to provide aroma pro- into dough by mixing it for 3  min in a mixer (Model files for different types of breads. Some of the techniques VCM-44A-1, Stephan, Hameln, Germany). The dough involved the use of gas chromatography–mass spec- was subsequently divided into 3 equal parts (dough A, trometry alone [15] or coupled with a comprehensive B & C). Dough A (control) was kneaded for 10  min and bi-dimensional gas chromatography–time of flight mass allowed to develop for about 1  h. After 1  h, the dough spectrometry (GC  ×  GC–TOFMS) [16], electronic nose was further kneaded a dozen times and divided into eight [17], artificial mouth [18]. And proton-transfer-reaction pieces. Each dough piece was rolled into a rope and the mass spectrometry (PTR-MS) [19], which only provides two ends were joined together to form a circle with a the chemical identities of breads. diameter of approximately 1–2 inches. The bagels were Recently, the sensomics approach, which includes, gas dropped into a large boiling water pot and allowed to boil chromatography–olfactometry (GC–O), sensory analy- for 2  min with constant turning. The boiled bagels were sis, aroma extraction dilution analysis (AEDA), iden- baked in a pre-heated oven at 218 °C for 20 min. Dough tification experiments, quantitation by stable isotope B was kneaded (10  min) as in dough A and allowed to dilution assays (SIDAs), calculation of odour activity val- develop for 1 h. After the kneading operation, the dough ues (OAVs) and aroma recombination and omission tests mass was returned into a large bowl, covered tightly and to validate analytical data, has proven a useful method kept in a chiller (5 °C) for 48 h. After, 48 h of cold fermen- for characterizing the potent aroma constituents of food tation, the dough was brought out and kneaded for about [20]. Sensomics is a multi-step analytical procedure used 3 min and it was divided into eight pieces and made into for identifying and quantifying key odorants in a food eight bagels as described above. The bagels were boiled matrix as well as defining their sensory impact on the in water for 2  min and later baked in a pre-heated oven overall food aroma [21, 22]. Sensomics approaches help at 218 °C for 20 min. Dough C was kneaded as in dough to identify potent aroma compounds as well as taste A and allowed to develop for 1  h. After the kneading, components in food [23]. Furthermore, the sensomic the dough was divided into eight bagels. The pre-formed approach combines separation-based chromatographic bagels were kept in the chiller (5 °C) for 24 h. After 24 h, methods with reconstitution and omission experiments the bagels were subjected to the same boiling and baking to evaluate the role of specific compounds in the per - protocols as described above. ceived aroma of a mixture addition. The implication of this that the sensomics approach is able to produce a Chemicals flavor-cum taste signatures of food [24]. In addition, the Pure chemical standards with purity ranging between 97 sensomics approach has been applied in the characteriza- and 99% were used. The chemical standards included; tion of aroma compounds in yeast dough dumpling [22] acetic acid, butanoic acid, benzyl alcohol, 2,3-butan- and the crust of soft pretzels [25]. edione (diacetyl), heptanoic acid, 3-hydroxy-2-butanone Although there are many reported studies on the (acetoin), and octanoic acid which were purchased characteristic aroma profiles of different wheat breads, from Merck (Darmstadt, Germany). Ethyl nonanoate, however, there has been no reported study on bagels. In ethyl octanoate, 4-hydroxy-2,5-dimethyl-3(2H)-fura- addition, bagel processing is slightly different from that of none (HDMF), (E)-2-nonenal, 2-phenyl ethanol, phenyl Lasek an et al. BMC Chemistry (2021) 15:16 Page 3 of 13 Fig. 1 Flow diagram for the production of bagels acetaldehyde, 3-methylbutanoic acid, 3-methylbutanol, Lancaster (Eastgate, Morecombe, UK). Ethanol (40% v/v) vanillin, 1-butanol, propionic acid, hexanoic acid, benza- was of food grade. The following labelled compounds ldehyde, (E,E)-2,4-decadienal, furfural, 2-methylbutanal, (Table  1) were synthesized according to the literature 2 2 2,3-hexanedione, 2-heptanone, 5-methyl-2-furanmeth-cited; [ H ]-butanoic acid [26]; 2-[ H ]-phenylethanol 2 2 2 2 anol, decanol, 4,5-epoxy-(E)-2-decanal, 1-octen-3-one, [27]; 3-[ H ]-methylbutanol [27]; [ H ]-ethyl octanoate 2 2 13 2 methional, 2-acetyl-1-pyrolline, sodium citrate dehy- [27]; [ C ]-acetic acid [28]; 3-[ H ]-methylbutanal [29]; 2 2 2 13 drate, and citric acid were from Aldrich (Steinheim, Ger- [ H ]-2-acetyl-1-pyrroline [30]; [ C ]-2,3-butanedione 2 4 2 13 many) and 4-vinyl-2-methoxyphenol was purchased from [31]; [ H ]-3-methylbutanoic acid [32]; [ C ]-2-methoxy 2 6 Table 1 Selected ions, and calibration factors used for the quantification of aroma compounds in three bagels by stable isotope dilution assays No. Compounds Selected ions (m/z) Internal standards Selected ions (m/z) Calibration factor 1 Phenylethanol 105 2-[ H] phenylethanol 107 1.02 2 Butanoic acid 89 [ H] butanoic acid 91 0.89 3 3-Methylbutanol 71 3[ H] methylbutanol 73 0.87 4 Acetic acid 61 [ C ] acetic acid 63 1.00 5 Ethyl octanoate 173 [ H] ethyl octanoate 176 1.00 6 4-Hydroxy-2,5-dimethyl-3(2H)- 129 4-Hydroxy-2[ C] methyl-5-methyl-3(2H)- 131 1.00 furanone furanone 7 2,3-Butanedione 87 [ C ]-2,3-butanedione 91 0.90 8 3-Methylbutanal 87 [ H]-3-methylbutanal 89 1.00 9 3-Methylbutanoic acid 60 [ H]-3-methylbutanoic acid 62 1.00 10 Methional 105 [ H]-methional 108 1.00 11 2-Acetyl-1-pyrroline 112 [ H]-2-acetyl-1-pyrroline 114 1.00 12 Phenylacetaldehyde 121 [ H]-2-phenylacetaldehde 123 0.85 13 2-Methoxy-4-vinylphenol 150 [ C ]-2-methoxy-4-vinylphenol 156 0.85 14 Vanillin 137 [ H]-vanillin 139 0.98 15 (Z)-4-Heptenal 95 [ H]-(Z)-4-heptenal 97 0.98 b a Calibration factors and compounds were determined as earlier reported by Guth and Grosch [32] and Lasekan et al. [41] respectively Lasekan et al. BMC Chemistry (2021) 15:16 Page 4 of 13 2 2 vinyl phenol [33]; [ H]-vanillin [33]; [ H ]-phenyl acetal- Mulheim, Germany) connected to a Trace Ultra 1300 2 13 dehyde [34]; [ H ]-heptenal [35] and 4-hydroxy-2[ H ]- gas chromatograph (Thermos Scientific, Waltham, MA, 2 2 methyl-5-methyl-3(2H)-furanone [36]. Citrate buffer USA) was used. The GC–O system was fitted with a (0.1  M, pH 6.0) was prepared as follow: sodium citrate DB-FFAP column (30  m × 0.32  mm i.d; film thickness, dehydrate (12.044 g, 0.04 M) was added to 800 mL of dis- 0.25  µm, Scientific Instrument Services, Inc., Ringoes, tilled water in a liter round bottom flask with constant NJ). The GC–O conditions are the same as reported in stirring. Subsequently, citric acid (11.341  g, 0.06  M) was “Analysis of volatile constituents” section. The effluent added to the solution and the solution was adjusted to a was split 1:1. Sniffing was conducted as described previ - pH 6.0 with 0.1 N HCl. ously [39]. Three experienced panelists (two females and a male) with strong gustative and olfactory responses in Isolation of volatile constituents earlier sessions were used for the sniffing test. The sniff - Freshly baked bagels were sliced into pieces, frozen in liq- ing analysis was divided into three sessions of 20 min and uid nitrogen and pulverized in a Waring blender. The pul - each assessor participated in the exercise. All analyses verized bagel (300 g) was extracted with dichloromethane were repeated in triplicate by each assessor. (700  mL) at room temperature (29  °C) for 2  h and the obtained extract was subjected to solvent-assisted flavor Aroma extracts dilution analysis (AEDA) extraction (SAFE) distillation [13] at 40  °C. To separate The flavor dilution (FD) factors of the aroma compounds the acidic volatiles from the neutral-basic fraction, the were determined by GC–O as reported by Lasekan and −1 extract was treated four times with 50 mL of 0.5 mol  L Yap [39]. The original extracts (200  µL) containing the aqueous sodium bicarbonate. The combined aqueous neutral/basic as well as the acidic volatile compounds −1 solutions were adjusted to pH 2 with HCl (2 mol  L ) and obtained from the crumbs (300  g) were diluted in a extracted with 50 mL of dichloromethane (4×) to obtain stepwise fashion by the addition of dichloromethane as the acidic fractions. Subsequently, the solutions (i.e. the described earlier [39]. Three panelists evaluated all dilu - acidic or the neutral-basic) were concentrated to 2 mL at tions in triplicate. Only the aroma compounds detected 40  °C using a small size Vigreux column [37]. The con - by more than two panelists were recorded. The flavor centrated extract was further concentrated to 200  μL dilution factors obtained by AEDA [40] were plotted [38]. All analyses were repeated in triplicate. against the retention index values of the corresponding aroma compound (FD chromatogram). Analysis of volatile constituents The GC–MS was performed by means of a gas chroma - Aroma compound quantification by stable isotope dilution tograph type QP-5050A (Shimadzu, Kyoto, Japan) using assays (ACQSIDA) the following capillary columns: DB-5 (30  m × 0.25  mm Labelled standards (20–50  µg) previously dissolved in I.D; 0.25  µm film thickness; Scientific Instrument Ser - dichloromethane (5  mL) were added to each crumb vices, Inc., Ringoes, NJ); DB-FFAP (30  m, 0.32  mm I.D; (100  g). The obtained extract was subjected to SAFE 0.25  µm film thickness, Scientific Instrument Services, distillation as described earlier in “Isolation of vola- Inc., Ringoes, NJ). The extracts (2  µL) were applied tile constituents”. Aliquots (0.5  µL) of the concentrates by the on-column injection technique at 230  °C. The were analyzed by means of two dimensional GC–MS −1 temperature of the oven was raised at 40  °C  min to as described previously [41]. Calibration factor for each 50  °C, held for 2  min isothermally and then raised at compound was determined by analyzing mixtures of −1 4 °C  min to 250 °C. The flow rate of the carrier helium defined quantity of the labelled compounds in five dif - −1 was 2.0 mL  min . The retention indices (RI) of the com - ferent mass ratios (1:5, 1:3, 1:1, 3:1, and 5:1) using the pounds were calculated as described previously [37]. GC–MS. The obtained response factors from the peak Mass spectra were recorded in the electron impact pos- area and the amounts of labelled compound are shown in itive mode (EI) over a scan ranges of m/z 40–270 (scan Table  1. The concentration of compounds quantified by frequency 5.8  Hz) applying electron energy of 70  eV. the selected stable isotopologues is reported in Table 3. Total run time was 45 min. Source and transfer line tem- peratures were 200 and 240° C respectively. Mass spectra Orthonasal aroma analysis of bagel were evaluated by using the Xcalibur software (Thermos One hour after baking, the bagels (approximately 8 g with Scientific, Dreieich, Germany). similar crust covering) were placed inside glass beakers (height 7 cm, volume 45 mL) with three random digitals GC–olfactometry and were orthonasally evaluated by panel members at To further identify the aroma constituents in the bagel room temperature (29 ± 2 °C). In addition, samples were extracts, an olfactory detection port ODP-3 (Gerstel, rotated among panelists to prevent carry-over effects. Lasek an et al. BMC Chemistry (2021) 15:16 Page 5 of 13 The panel consisted of 10 members, aged between 24 temperature 29  °C. The aroma model was evaluated and 35  years and were made up of seven women and orthonasally in comparison with the 24 and 48  h bagels three men. These panelists have participated in a weekly as described above (“Orthonasal aroma analysis of bagel” sensory training session for at least a year to be able to section). recognize and describe different aroma qualities. The sensory analyses were conducted in a sensory room fol- Omission experiments lowing the International Standard (ISO 8589, 2007) [42] A triangle test was performed to determine the signifi - protocols with individual booths equipped with uni- cance of one odorant on the aroma recombination mod- form and glare free white light (D65). Descriptors used els (24  h and 48  h) reported in Table  5. For each of the were determined in preliminary sensory experiments as models a glass of the mixture (20  mL) was prepared by described by Steinhaus et  al. [43]. The panelists started omitting one or a group of selected odorants from the with seven descriptors and when all panelists were able complete recombination model (Table  6). This mixture to achieve complete agreement on a descriptor such a and two other glasses containing the complete recom- descriptor was chosen. Each descriptor used was defined bination models were presented to the sensory panel on the basis of the odour of the selected aqueous solu- in a triangle test [45]. The results of the Triangle tests tion of reference compounds. The reference compounds were analyzed by comparing the total number of cor- −1 used as stimuli were; 10  μg  L of 2-acetyl-1-pyrroline rect responses with the minimum number of responses −1 −1 (roasty); 100 μg  L of 3-methylbutanal (malty); 70 μg  L required for statistical significance (ISO, 4120, 2004) [46]. −1 of 2,3-butanedione (buttery); 50  μg  L of (Z)-4-hepte- Panel performance was obtained by applying analysis of −1 nal (biscuit-like); 10  μg  L of 4-vinyl-2-methoxyphenol variance (ANOVA) to the sensory profile data. The data −1 (smoky); 100 μg  L of methional (baked potato-like). were analyzed using SAS Statistical software (SAS Insti- During evaluation, the panelists had 5  min to rest after tute, Inc. 1996). The significance α was calculated accord - each set of samples was tested. All samples were repeated ing to the method of Callejo et al. [45] in triplicate. The intensities of the attributes were rated on a 7-point linear scale (i.e. 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) Results and discussion from 0 (not perceivable) to 3 (strongly perceivable) in Identification of aroma‑active compounds in control steps of 0.5 by the panelists. The sensory data were ana - bagels lyzed by one-way analysis of variance (ANOVA) using A combined total of 40 aroma compounds were identified SPSS 20.0 (SPSS Inc., Chicago, IL., USA). ANOVA with in the three differently processed bagels (i.e. control; 24 h Duncan’s multiple comparison tests were performed to cold fermented bagels: and bagels produced from 48  h determine whether there were differences among indi - cold fermented dough mass). Among these compounds, vidual samples. The differences were considered to be 10 aldehydes, 9 alcohols, 7 acids, 6 ketones, 5 heterocy- significant at p < 0.05 (Table  4). In addition, the ethi- clic compounds and 3 esters were positively identified cal standards as instituted by the institutional and/or (Table  2). To reveal the differences between the flavors national research committee according to the 1964 Hel- of the bagels, the volatile fractions of their crumbs were sinki declaration and its later amendments or comparable subjected to AEDA. In the control bagels, 40 aroma com- ethical Standards on studies involving human subjects pounds were detected in the FD factor range of 2 to 256 were adhered to. The study protocol and consent pro - respectively (Table  2). Furthermore, the results revealed cedure received ethical approval from the Institutional 2-acetyl-1-pyrroline (roasty), methional (baked potato- Review Board (IRB) of the University Putra Malaysia. like), vanillin (vanilla-like), 2,3-butanedione (buttery) Informed consent was obtained from all individual par- and 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) ticipants included in the study. as compounds with the highest FD values in the control bagel. These aroma-active compounds exhibited high FD Aroma model recombinant of the 24 and 48 h bagels factors (128–256) (Fig.  1). Other important aroma com- Reference standards of key aroma compounds (Table  5) pounds in the control bagels were butanoic acid (sweaty), were prepared in ethanolic solution [44]. The combined acetoin (buttery), benzaldehyde (almond-like), furfural ethanolic stock solutions of the 17 aroma compounds (bread-like), 2/3-methyl butanoic acid (sweaty), acetyl made up of 15 compounds with OAVs > 1 and two com- pyrazine (toasty), phenyl acetaldehyde (honey-like), pounds (i.e. acetic acid and acetoin) with significantly 2-phenylethanol (honey-like), octanoic acid (fatty, soapy), high concentrations (Table  5) (500 μL) was added to cit- 4-vinyl-2-methoxyphenol (smoky), acetic acid (sour), −1 rate buffer (30  mL; pH 5.6; 0.1  mol  L ) and free corn 3-methyl butanol (malty), and 2-methylpyrazine (nutty) starch (30  g) respectively in a closed Teflon cup. The all of which exhibited FD factors ranging from 16 to 32 Teflon cup was stirred continuously for 15  min at room (Fig. 2). Lasekan et al. BMC Chemistry (2021) 15:16 Page 6 of 13 Table 2 Aroma compounds identified in cold fermented and control bagels a b No. Compound Retention Retention Odour description Fractions FD FD FD Identification method index on index on CB BF BF 24 48 DB-5 FFAP 1 Acetic acid 605 1443 Sweaty A 32 64 64 MS/RI/O/ST 2 2,3-Butanedione (diacetyl) 606 993 Buttery NB 128 512 1024 MS/RI/O/ST 3 1-Butanol 636 1179 Sweaty/buttery NB 4 4 8 MS/RI/O/ST 4 2/3-Methylbutanal 647 936 Malty NB 8 16 16 MS/RI/O/ST 5 Propionic acid 668 1540 Sweaty/pungent A 8 8 8 MS/RI/O/ST 6 Butanoic acid 718 1619 Sweaty A 16 64 64 MS/RI/O/ST 7 Acetoin 720 1275 Buttery NB 16 128 128 MS/RI/O/ST 8 3-Methyl butanol 769 1067 Malty NB 32 32 32 MS/RI/O/ST 9 2,3-Hexanedione 792 ND ND NB ND ND ND MS/RI/ST 10 Furfural 826 1457 Bread-like NB 16 16 16 MS/RI/O/ST 11 2-Methyl pyrazine 827 1298 Nutty, roasty NB 32 32 32 MS/RI/O/ST 12 2/3-Methylbutanoic acid 831 1661 Sweaty A 16 64 128 MS/RI/O/ST 13 Isoamyl acetate 878 1124 Fruity NB 8 16 16 MS/RI/O/ST 14 2-Heptanone 889 1182 ND NB ND ND ND MS/RI/ST 15 Heptanol 896 1174 Citrusy NB 2 4 8 MS/RI/O/ST 16 Methional 919 1449 Baked potato NB 256 256 256 MS/RI/O/ST 17 2-Acetyl-1-pyrroline 922 1331 Roasty NB 256 256 256 MS/RI/O/ST 18 Benzaldehyde 936 1196 Almond-like NB 16 32 32 MS/RI/O/ST 19 5-Methyl-2-furanmethanol 953 1723 Bread like NB 8 8 8 MS/RI/O/ST 20 (Z)-4-Heptenal 960 1287 Biscuit-like NB 8 16 32 MS/RI/O/ST 21 Hexanoic acid 961 1842 Sweaty A 4 8 8 MS/RI/O/ST 22 1-Heptanol 970 ND ND NB ND ND ND MS/RI/ST 23 1-Octen-3-one 971 1297 Mushroom-like NB 2 4 4 MS/RI/O/ST 24 2,3,5-Trimethylpyrazine 985 1395 Broth-like NB 2 4 4 MS/RI/O/ST 25 2-Pentyl furan 992 ND Fruity, sweet NB 4 4 8 MS/RI/O/ST 26 Acetyl pyrazine 1020 1662 Toast NB 16 16 32 MS/RI/O/ST 27 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 1022 2038 Sweet/caramel A 128 512 1024 MS/RI/O/ST 28 Benzyl alcohol 1039 1866 Sweet/flowery NB 8 16 16 MS/RI/O/ST 29 Phenyl acetaldehyde 1042 1653 Honey, rose NB 16 16 16 MS/RI/O/ST 30 Heptanoic acid 1077 1949 Rancid A 2 8 8 MS/RI/O/ST 31 2-Phenyl ethanol 1136 1911 Honey-like NB 16 16 16 MS/RI/O/ST 32 (E)-2-Nonenal 1164 1568 Fatty, green NB 8 8 8 MS/RI/O/ST 33 Octanoic acid 1182 2047 Fatty, soapy A 16 32 32 MS/RI/O/ST 34 Ethyl octanoate 1194 1428 Fruity, fatty NB 8 8 8 MS/RI/O/ST 35 Decanol 1269 ND Fatty NB 4 8 8 MS/RI/O/ST 36 Ethyl nonanoate 1296 ND Fruity, tropical NB 4 4 4 MS/RI/O/ST 37 (E,E)-2,4-Decadienal 1313 1684 Fatty NB 2 4 8 MS/RI/O/ST 38 4-Vinyl-2-methoxyphenol 1317 2174 Smoky A 16 32 32 MS/RI/O/ST 39 4,5-Epoxy-(E)-2-decenal 1360 1970 Metallic NB 4 8 8 MS/RI/O/ST 40 Vanillin 1410 2601 Vanilla-like A 256 256 256 MS/RI/O/ST ND not detected, CB control bagel, BF 24 h fermented bagel, BF 48 h fermented bagel, NB neutral basic fraction, A acidic fraction 24 48 Compound identified by comparison of its odour quality and intensity, retention indices on capillaries DB-5 and FFAP as well as mass spectra in EI with data of reference compounds. MS, RI, O, ST represents mass spectra, retention indices, olfactometry and standard odorants respectively Odour quality as perceived at the sniffing port Aroma‑active compounds in long, cold fermented bagels FD factors (4–1024) than the control bagel (Table 2). For Application of long, cold fermentation (5  °C, 24  h and instance, the FD factor of 2,3-butanedione in the 24  h 48  h) produced bagels that exhibited a wider range of and 48 h fermented bagels increased by almost (4 times) Lasek an et al. BMC Chemistry (2021) 15:16 Page 7 of 13 Fig. 2 Flavour dilution chromatogram obtained by the application of AEDA on a distillate of unfermented bagel (control). Compounds with an FD factor ≥ 32 are displayed. Numbering is identical with that in Table 2 and 8 times the value obtained in the control bagel. Other bread exhibited similar amounts of Strecker aldehydes compounds exhibiting higher FD factors in the long, cold (i.e. 2-methylpropanal, 2-methylbutanal and 3-meth- fermented bagels were; acetic acid (sweaty), 2/3-meth- ylbutanal) as obtained with the artisanal process. This ylbutanal (malty), 2,3-butanedione (buttery), propionic observation is probably due to a longer proteolysis which acid (sweaty/pungent), butanoic acid (sweaty), acetoin leads to the formation of amino acids that participates (buttery), 3-methylbutanol (malty), furfural (bread-like), in the Strecker reactions as well as the Ehrlich pathway 2-methyl pyrazine (nutty), 2/3-methyl butanoic acid to produce the aldehydes. It is worthy of note that both (sweaty), methional (baked potato-like), 2-acetyl-1-pyr- 2,3-butanedione and HDMF which exhibited the highest roline (roasty), benzaldehyde (almond-like), (Z)-4-hep- FD factors in the cold fermented bagels as well as many tenal (biscuit-like), acetyl-pyrazine (toasty), 4-HDMF other key aroma compounds such as: 2/3-methylbutanal, (sweet/caramel), benzyl alcohol (sweet/flowery), phenyl acetoin, 3-methylbutanol, furfural, 2-methyl pyrazine, acetaldehyde (rose-like), 2-phenyl ethanol (honey-like), isoamyl acetate, methional, 2-acetyl-1-pyrroline, benza- octanoic acid (fatty), 4-vinyl-2-methoxyphenol (smoky) ldehyde, (Z)-4-heptenal, acetyl pyrazine, phenyl acetal- and vanillin (vanilla-like) all of which exhibited FD fac- dehyde and vanillin have been identified in the crumb of tors from16 to 1024. While long, cold fermented bagels wheat bread [3, 11, 47]. Also, various acids such as acetic generally exhibited higher FD values than the control, the acid, butanoic acid, 2/3-methyl butanoic acid and octa- 48  h bagel also showed higher FD values in some com- noic acid which exhibited high FD factors ≥ 16 in the cold pounds (i.e. diacetyl, 1-butanol, 2/3-methylbutanoic acid, fermented bagels have been reported in bread [50, 51]. heptanol, (Z)-4-heptenal and 2-pentyl furan) compared to the 24 h bagels. Quantitation and odour‑activity values (OAVs) The influence of fermentation temperatures on the for - of aroma‑active compounds in bagels mation of volatile compounds in bread crust and crumb To have an insight into the contribution of each com- has been well documented [47–49]. While high fer- pounds to the overall aroma of bagels, 22 aroma-active mentation temperatures (≥ 27  °C) are more suitable for compounds with FD factors ≥ 16 were selected for fur- generating more complete volatile profiles, most bread ther investigation. For each of the selected compound, industries are more favorable to employing longer fer- a stable isotopologue (Table  1) was employed as an mentation time or using sourdough that needs time to internal standard to quantify it. As expected the long ferment. For instance, Zehentbauer and Grosch [48] cold fermented bagels produced compounds with sig- observed that when bread is prepared from dough sub- nificantly (p < 0.05) high concentrations (Table  3). The −1 jected to an initial 2  h of fermentation at 22  °C and an highest concentrations (1126–12,950  μg  kg ) were additional 18  h of fermentation at 4  °C, the resulting determined for 2,3-butanedione, 2-phenylethanol, Lasekan et al. BMC Chemistry (2021) 15:16 Page 8 of 13 Table 3 Concentrations, odour thresholds and odour activity values (OAVs) of key aroma compounds (FD factor ≥ 16) in cold fermented bagels −1 No. Compounds Concentration (μg kg ) Threshold in Odour activity values −1 c starch (μg kg ) (OAVs) CB BF BF CB BF BF 24 48 24 48 c b a 1 Acetic acid 300 ± 2.0 480 ± 2.0 510 ± 2.0 31,140 < 1 < 1 < 1 c b a 2 2,3-Butanedione (diacetyl) 710 ± 5.1 11,800 ± 12.0 12,950 ± 15.5 6.5 109 1815 1992 c b a a 3 2/3-Methyl butanal 164 ± 2.1 321 ± 2.1 434 ± 2.0 32 5 10 14 c b a a 4 Butanoic acid 113 ± 1.0 201 ± 1.0 317 ± 1.0 100 1 2 3 c b a 5 Acetoin 1140 ± 4.5 1245 ± 5.0 1276 ± 4.0 Nf nd nd nd c b a 6 3-Methyl butanol 647 ± 3.1 1126 ± 7.8 1364 ± 10.0 102 6 11 13 b a a 7 Furfural 101 ± 1.0 126 ± 1.0 124 ± 1.0 Nf nd nd nd c b a 8 2-Methylpyrazine 30 ± 0.2 54 ± 0.2 76 ± 0.2 Nf nd nd nd c b a 9 3-Methylbutanoic acid 64 ± 1.0 276 ± 2.1 314 ± 2.0 24 3 12 13 c b a 10 Methional 16 ± 0.1 24 ± 0.1 43 ± 0.1 0.27 59 89 160 ab a a a 11 2-Acetyl-1-pyrroline 17 ± 0.1 19 ± 0.1 18 ± 0.1 0.0073 2329 2603 2466 c b a 12 Benzaldehyde 174 ± 3.0 920 ± 4.5 1121 ± 6.0 350 < 1 3 3 c b a b 13 5-Methyl-2-furanmethanol 46 ± 0.1 52 ± 0.1 58 ± 0.1 11.9 4 4 5 c b a 14 (Z)-4-Heptenal 51 ± 0.1 135 ± 0.2 234 ± 0.2 3 17 45 78 c b a 15 Acetyl pyrazine 171 ± 1.0 186 ± 1.0 193 ± 1.0 nf nd nd nd c b a b 16 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 234 ± 2.0 347 ± 2.1 453 ± 3.0 13 18 27 35 c b a 17 Benzyl alcohol 115 ± 1.0 176 ± 2.0 182 ± 2.1 nf nd nd nd b a a b 18 Phenyl acetaldehyde 15 ± 0.0 18 ± 0.1 17 ± 0.1 28 < 1 < 1 < 1 c b a b 19 2-Phenyl ethanol 1101 ± 5.0 1134 ± 5.0 1512 ± 5.0 125 9 9 12 c b a 20 Octanoic acid 87 ± 2.1 102 ± 2.0 116 ± 2.0 Nf nd nd nd c b a b 21 4-Vinyl-2-methoxyphenol 146 ± 2.0 305 ± 2.1 512 ± 4.0 18 8 17 28 c b a b 22 Vanillin 56 ± 0.1 73 ± 0.1 95 ± 0.1 4.6 12 16 21 nf not found, nd not determined, CB control bagel, BF 24 h fermented bagels, BF 48 h fermented bagels; Mean ± SD; superscripts with different letters in a row are 24 48 significantly (p < 0.05) different Reference; Zehentbauer and Grosch [48] Reference; Rychlik and Grosch [10] OAV on the basis of odour thresholds in starch 3-methylbutanal and acetoin respectively (Table 3). The noticed with the methional, acetyl pyrazine, HDMF, −1 lowest concentrations (17–43  μg  kg ) were obtained 4-vinyl-2-methoxyphenol, vanillin, 2/3-methylbutanal, for phenyl acetaldehyde, methional and 2-acetyl-1-pyr- 2-phenyl ethanol, butanoic acid, 3-methylbutanol and roline respectively. A comparative analysis of the aroma benzaldehyde. However, acetic acid, phenyl acetalde- potencies between the three differently produced hyde had OAVs below 1. bagels revealed some differences. Cold fermented While some of the bagel aroma compounds were bagels showed more potencies for the buttery smell- already present in the wheat flour and were thus trans - ing 2,3-butanedione, baked potato-like methional and ferred into the bagel. Others such as 3-methylbutanol, the toasty-like 2-acetyl-1-pyrroline as revealed by their 2-phenyl ethanol and 2,3-butanedione were probably respective high odour-activity values (Table  3). For formed during biochemical reactions in the yeast metab- example, 2-acetyl-1-pyrroline exceeded its threshold olism during the dough fermentation [27]. On the other by factors of 2603 and 2466 in the 24  h and 48  h cold hand the nitrogen-containing compounds such as the fermented bagels respectively. 2-Acetyl-1-pyrroline roasty 2-acetyl-1-pyrroline and acetyl pyrazine were only exceeded its threshold by a factor of 2329 in the formed via the reaction of free amino acids l-ornithine or control bagels. Similarly, 2,3-butanedione exceeded its l-proline with dihydroxyacetone phosphate [52]. In addi - threshold by factors of 1815 and 1992 in the 24  h and tion to the nitrogen-containing compounds, aldehydes, 48  h cold fermented bagels respectively. On the other such as 2/3-methylbutanal (malty), phenyl acetaldehyde hand 2,3-butanedione only exceeded its threshold by (rose/floral) and methional (baked potato-like) were a factor of 109 in the control bagel. Similar trend was formed by the Strecker degradation of valine, isoleucine, Lasek an et al. BMC Chemistry (2021) 15:16 Page 9 of 13 leucine, phenylalanine and methionine respectively [53]. descriptors, all the panelists had to achieve complete Moreover the caramel-like 4-Hydroxy-2,5-dimethyl- agreement on any given descriptor for such descriptor 3(2H)-furanone (HDMF) can be formed by the Maillard to be chosen. The aroma profiles of the cold fermented reaction [54]. 4-Hydroxy-2,5-dimethyl-3(2H)-furanone bagels were characterized as roasty, biscuit-like, malty, is mainly formed via Maillard reaction of pentoses with smoky and buttery. The control bagel exhibited similar the amino acids glycine and alanine, respectively. Alter- but less intense aroma notes as compared to the cold natively, 4-hydroxy-2,5-dimethyl-3(2H)-furanone can fermented bagels. However, the 24  h and 48  h bagels also be produced without the direct interaction of flavor profiles were similar with the exception of the glycine [36]. Furthermore, certain aldehydes such as biscuit-like aroma note (Table 4). The statistical analysis (E,E)-2,4-decadienal, (E)-2-nonenal, and (E)-4,5-epoxy- results (Table  4) showed that the six attributes (roasty, (E)-2-decenal were formed by autoxidation and thermal malty, buttery, biscuit-like, smoky and baked potato degradation of fatty acids respectively [53]. like) with different superscripts provided a clearer explanation of the aroma characteristics of the differ - Sensory analysis and aroma reconstitution evaluation ent bagels. To confirm this observation, recombina - The results of sensory evaluation of the different bagels tion experiments were carried out by mixing solutions (i.e. control, 24  h fermented and 48  h fermented) are of the pure reference compounds in the same amounts shown in (Fig.  3a, Table  4). In order to select the final as indicated for both 24  h and 48  h bagels respectively Roasty a b BF24 BF 24 Model Roasty 2.5 2.5 Baked potato Malty 1.5 Baked potato Malty 1.5 0.5 0.5 Smoky Buttery Smoky Buttery Biscuit-like Biscuit-like Control BF24 BF48 BF48 BF48 Model Roasty 2.5 Baked potato Malty 1.5 0.5 Smoky Buttery Biscuit-like Fig. 3 a Aroma profiles of bagels; control bagels (blue line), 24 h fermented bagels (red line) and 48 h fermented bagels (green line). b A comparative aroma profiles of 24 h bagels (red colour) and its aroma model (green colour). c Aroma profiles of 48 h bagel (red colour) and its aroma model (green colour) Lasekan et al. BMC Chemistry (2021) 15:16 Page 10 of 13 Table 4 The mean scores of the six attributes for the three bagels and the aroma models generated. (Supplementary) Sensory attribute Bagels Mean scores of bagel’s and their aroma models Control BF BF BF BF Model BF BF Model 24 48 24 24 48 48 A A A a a a a Roasty 3.0 ± 0.21 3.0 ± 0.70 3.0 ± 0.91 3.0 ± 0.42 3.0 ± 0.50 3.0 ± 0.72 3.0 ± 0.23 B A A a a a a Malty 1.5 ± 0.05 2.0 ± 0.23 2.0 ± 0.60 2.0 ± 0.23 1.9 ± 0.14 2.0 ± 0.33 1.9 ± 0.24 B A A a a a a Buttery 1.0 ± 0.04 1.5 ± 0.02 1.5 ± 0.13 1.5 ± 0.05 1.5 ± 0.25 1.5 ± 0.15 1.5 ± 0.21 B B A a a a a Biscuit-like 2.5 ± 0.81 2.5 ± 0.33 3.0 ± 0.56 2.5 ± 0.50 2.5 ± 0.71 3.0 ± 0.30 3.0 ± 0.80 A A A a a a a Smoky 0.5 ± 0.02 0.5 ± 0.01 0.5 ± 0.03 0.5 ± 0.04 0.5 ± 0.12 0.5 ± 0.03 0.5 ± 0.05 A A A a a a a Baked potato 0.5 ± 0.01 0.5 ± 0.03 0.5 ± 0.01 0.5 ± 0.02 0.5 ± 0.04 0.5 ± 0.01 0.5 ± 0.03 A, B, C : a, b, c Different letters within the same row represents significant differences (p < 0.05) using Duncan’s multiple comparison test (n = 30, 10 panellists with 3 replications) BF 24 h fermented bagel, BF 48 h fermented bagel 24 48 Omission tests (Table  5). A parallel evaluation of the recombination The contributions of some key aroma compounds models of the freshly baked 24  h and 48  h bagels was to the flavor of the bagels, was evaluated by omission conducted. Results showed that the recombinant model tests. Omission tests are used to assess the contribution imitated well the flavor of the freshly baked bagels of individual compound to the overall aroma of a given (Fig.  3b, c, Table  4). The aroma of the recombination food [54]. Eleven aroma omission models (M1–M11), models had good similarities for all the odor notes such containing either single or a group of compounds, were as roasty, baked potato-like, smoky and biscuit-like. prepared. Each of the omission models was analyzed The roasty and biscuit-like aroma notes were perceived in triangular experiments with two complete recom- as equally intense in the aroma models as well as in the bination models (Table  6). Results showed that, the bagels. omission of the entire group of acids (M1) from the complete recombination model could be distinguished by 9 out of the 10 assessors. This shows that these acids Table 5 Aroma models composition for bagels produced from (i.e. acetic acid, butanoic acid and 3-methyl butanoic 24 and 48 h cold fermentation acid) play an important role in the overall aroma of b the long, cold fermented bagels. In the second group, No. Compounds Concentration (μg −1 kg ) the ketones (2,3-butanedione and acetoin) with char- acteristic buttery nuance were omitted. Acetoin was BF BF 24 48 included in this group because of its high concentra- 1 Acetic acid 480 510 tion. Result of the omission of the entire ketones from 2 2,3-Butanedione (diacetyl) 11,800 12,950 the complete recombination model showed that all 10 3 2/3-Methyl butanal 321 434 assessors could detect between the omission model 4 Butanoic acid 201 317 and the complete recombination models. This shows 5 Acetoin 1245 1276 that 2,3-butanedione and acetoin greatly influence the 6 3-Methyl butanol 1126 1364 overall aroma of the bagel. When the aldehydes (M3) 7 3-Methylbutanoic acid 276 314 (2,3-methyl butanal, methional, benzaldehyde, (Z)- 8 Methional 24 43 4-heptenal, phenyl acetaldehyde and vanillin) were 9 2-Acetyl-1-pyrroline 19 18 omitted, only 8 assessors were able to detect the dif- 10 Benzaldehyde 920 1121 ference (p < 0.01). Similar trend was observed when the 11 5-Methyl-2-furanmethanol 52 58 entire group of alcohols (M4) was omitted. In model 12 (Z)-4-Heptenal 135 234 5, 4-vinyl-2-methoxyphenol was omitted because of 13 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 347 453 its high concentration and the result showed that only 14 Phenyl acetaldehyde 18 17 7 assessors were able to detect the difference between 15 2-Phenyl ethanol 1134 1512 the omission model and the complete recombination 16 4-Vinyl-2-methoxyphenol 305 512 models. In model 6, 4-hydroxy-2,5-dimethyl-3(2H)- 17 Vanillin 73 95 furanone was omitted and this resulted in significant a (p ≤ 0.001) reduction in the characteristic aroma of Acetoin was included in the model even though its threshold in starch was not found the bagels. In addition, 9 of the assessors were able to Ethanolic solutions of aroma compounds dissolved in free corn starch Lasek an et al. BMC Chemistry (2021) 15:16 Page 11 of 13 Table 6 Omission analysis on the bagel aroma models (BF and BF ) 24 48 Odorant groups Aroma note Compounds omitted No of correct No of correct Significance a a judgments judgments BF BF 24 48 Acids (M1) Sweaty Acetic acid, butanoic acid, 3-methylbutanoic acid 9/10 9/10 *** Ketones (M2) Buttery 2,3-Butanedione, acetoin 10/10 10/10 *** Acetaldehydes (M3) Malty, baked potato, 2,3-Methylbutanal, methional, benzaldehyde, (Z)-4-hep- 8/10 8/10 ** almond-like, biscuit-like, tenal, phenyl acetaldehyde, vanillin vanilla Alcohols (M4) Malty, bread-like, honey 3-Methylbutanol, 5-methyl-2-furanmethanol, 2-phenyl 8/10 8/10 ** ethanol Phenol (M5) Smoky 4-Vinyl-2-methoxyphenol 7/10 7/10 * (M6) Sweat, caramel 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 9/10 9/10 *** (M7) Floral, honey 2-Phenyl ethanol 8/10 8/10 ** (M8) Cooked potato-like Methional 8/10 8/10 ** (M9) Biscuit-like (Z)-4-Heptenal 9/10 9/10 *** (M10) Bread-like 5-Methyl-2-furanmethanol 10/10 10/10 *** (M11) Popcorn-like 2-Acetyl-1-pyrroline 10/10 10/10 *** M1–M11 Models Number of correct judgments from 10 assessors Significance: * significant (α ≤ 0.05); **, highly significant (α ≤ 0.01); ***, very highly significant (α ≤ 0.001) distinguish its omission from the complete recombina- of cold fermentation on bakery products found in many tion models. Similar observation was obtained when world cuisines. other single compounds such as 2-phenyl ethanol, Acknowledgements methional, (Z)-4-heptenal, 5-methyl-2-furanmethanol The authors wish to thank the Faculty of Food Science & Technology, Univer- and 2-acetyl-1-pyrroline were omitted from the com- sity Putra Malaysia for supplying the facilities for this study. plete recombination models respectively. However, the Authors’ contributions omission of 5-methyl-2-furanmethanol and 2-acetyl- OL: Conceptualize, funding acquisition, supervised and reviewed the initial 1-pyrroline was detected by all 10 assessors. and final manuscript. FD, MM, HJ: formal analysis, data collection, writing of draft, AL: reviewed initial draft and provided necessary information on bagel. All authors reviewed the manuscript. All authors read and approved the final manuscript. Conclusion Funding This study has revealed the key aroma-active compounds Financial support for this research was provided by the University Putra Malay- responsible for the characteristic aroma of the long, sia research scheme (Grant No. 9478500). cold fermented bagels. The results of the OAVs and sen - Availability of data and materials sory studies showed distinct differences in the aroma All data generated or analyzed during this study are included in this published notes of the cold fermented and control bagels. Whilst article. the cold fermented bagels exhibited roasty, malty, but- Declarations tery, baked potato-like, smoky and biscuit-like notes, the odour notes in the control bagels were similar to the Ethics approval and consent to participate other bagels but less intense. Aroma compounds such as The study protocol and consent procedure received ethical approval from the Institutional review board of the University Putra Malaysia. Informed consent 2,3-butanedione (buttery), acetoin (buttery), 2-acetyl- was obtained from all individual participants included in the study. 1-pyrroline (roasty), 5-methyl-2-furanmethanol (bread- like), (Z)-4-heptenal (biscuit-like) and HDMF, were the Consent for publication Not applicable. key aroma compounds. In addition, vanillin (vanilla), 2/3-methylbutanal (malty), 3-methyl butanoic acid Competing interests (sweaty), 3-methylbutanol (malty), methional (baked The authors declare no competing interests. potato-like), 2-phenyl ethanol (honey-like), benzaldehyde Author details (almond-like), and butanoic acid (sweaty) were identified 1 Department of Food Technology, University Putra Malaysia, UPM, 43400 Ser- as important aroma compounds of bagels. These find - dang, Malaysia. Department of Food Service and Management, University Putra Malaysia, UPM, 43400 Serdang, Malaysia. 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Frasse P, Lambert S, Richard-Molard D, Chiron H (1993) The influence of fermentation on volatile compounds in French bread dough. LWT-Food Sci Technol 26:126–132 Re Read ady y to to submit y submit your our re researc search h ? Choose BMC and benefit fr ? Choose BMC and benefit from om: : fast, convenient online submission thorough peer review by experienced researchers in your field rapid publication on acceptance support for research data, including large and complex data types • gold Open Access which fosters wider collaboration and increased citations maximum visibility for your research: over 100M website views per year At BMC, research is always in progress. Learn more biomedcentral.com/submissions http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Chemistry Central Journal Springer Journals

Characterization of the key aroma compounds in three types of bagels by means of the sensomics approach

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Springer Journals
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Copyright © The Author(s) 2021
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2661-801X
DOI
10.1186/s13065-021-00743-4
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Abstract

Background: To evaluate the impact of cold fermentation time on bagel rolls, the key aroma-active compounds in the volatile fractions obtained from three different bagel rolls through solvent assisted flavor evaporation (SAFE) were sequentially characterized by an aroma extract dilution analysis (AEDA), quantified by stable isotope dilution and analyzed by odor activity values (OAVs) respectively. Results: Findings revealed 40 aroma-active compounds with flavor dilution (FD) factor ranges of 2–1024. Of these, 22 compounds (FD ≥ 16) were quantified by stable isotope dilution assays (SIDA). Subsequent analysis of the 22 compounds by odor activity values (OAVs) revealed 14 compounds with OAVs ≥ 1 and the highest concentrations were obtained for 2,3-butanedione, 2-phenylethanol, 3-methylbutanal and acetoin respectively. Two recombina- tion models of the bagels (i.e. 24 h and 48 h bagels) showed similarity to the corresponding bagels. Omission tests confirmed that 2,3-butanedione (buttery), acetoin (buttery), 2-acetyl-1-pyrroline (roasty), 5-methyl-2-furanmethanol (bread-like), (Z)-4-heptenal (biscuit-like) and 4-hydroxy-2,5-dimethyl-3(2H)-furanone, were the key aroma compounds. Additionally, acetic acid, butanoic acid, 2-phenylethanol (honey-like), 3-methylbutanoic acid, 2/3-methylbutanal, vanil- lin, 3-methylbutanol, methional were also important odorants of the bagel. Conclusion: Whilst the long, cold fermented bagels exhibited roasty, malty, buttery, baked potato-like, smoky and biscuit-like notes, the control bagels produced similar but less intense odor notes. Keywords: Bagel, Aroma-active compounds, Cold fermentation, Sensomics approach Introduction of the dough used in bagel production. Traditional bagels are often produced with high protein (13–16%) spring Bagels are one of the most widely consumed bread rolls wheat flour [2]. In addition, the long, cold fermentation in the United States. Recent statistics have shown that step called retardation gives the traditional bagels a dis 204 million Americans consumed bagels in 2019 [1]. This - figure is projected to increase to 210 million in 2023 [1]. tinctive crust and flavor not found in the regular bread Bagels have a very simple formulation similar to simple rolls. The quality of bread is normally defined by its vol - bread or roll formulas (i.e. flour, salt, yeast, and water). ume, texture, color and flavor [3]. However, the aroma of However, what differentiates bagel from the rest of the bread is undoubtedly one of the most important qualities rolls are the flour quality and the long, cold fermentation that influence its acceptance by consumers [4]. Bread flavor appreciation is one of the first evalua - tion signals encountered by consumers during bread *Correspondence: olaniny56@gmail.com consumption [5]. The flavor of bread is engendered by Department of Food Technology, University Putra Malaysia, UPM, the interaction of a large number of compounds, which 43400 Serdang, Malaysia exhibit different olfactive characteristics, tactile oral and Full list of author information is available at the end of the article © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://crea- tivecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdo- main/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Lasekan et al. BMC Chemistry (2021) 15:16 Page 2 of 13 trigeminal sensations. Some of these compounds include; regular bread. Therefore, elucidating the flavor chemistry alcohols, aldehydes, esters, ketones, acids, hydrocarbons, of bagels could improve their quality control and process- pyrazines, pyrrolines, furans etc. [3, 4]. Over 300 vola- ing of bagels. The objective of this study was to charac - tile compounds have been reported in white bread [6]. terize the key aroma compounds in long, cold fermented In addition, the odour quality of bread depends on many bagels using the sensomics approach. factors like; type of flour, type of fermentation [7] and dough improvers [8] used during bread production. The Materials and methods production process and storage are also known to influ - Bagel production ence the flavor of bread [9]. Bagel doughs were made by employing three processes Analysis of volatile compounds in a food matrix is differing in their cold fermentation conditions, and quite complex and several extraction methods have been the time required for boiling the bagel dough in water reported, ranging from solvent extraction [10], headspace (Fig. 1). The dough recipes contained high protein wheat solid phase micro-extraction (HS-SPME) [11], dynamic flour 13% (enriched bakers patent flour from Pastry Prod - headspace extraction (DHE) [4], multiple headspace uct Sdn., Malaysia) (2000  g); cold water, 1100  g; instant solid phase micro-extraction (MH-SPME) [12], solvent dry yeast, 30 g; salt, 30 g; granulated sugar, 60 g, shorten- assisted flavor evaporation (SAFE) [13] and vacuum ing; 60 g and malt flour (high diastatic malted barley flour sublimation [14]. In the same vein, many identification 185 Lintner minimum) 60 g. The ingredients were made techniques have been employed to provide aroma pro- into dough by mixing it for 3  min in a mixer (Model files for different types of breads. Some of the techniques VCM-44A-1, Stephan, Hameln, Germany). The dough involved the use of gas chromatography–mass spec- was subsequently divided into 3 equal parts (dough A, trometry alone [15] or coupled with a comprehensive B & C). Dough A (control) was kneaded for 10  min and bi-dimensional gas chromatography–time of flight mass allowed to develop for about 1  h. After 1  h, the dough spectrometry (GC  ×  GC–TOFMS) [16], electronic nose was further kneaded a dozen times and divided into eight [17], artificial mouth [18]. And proton-transfer-reaction pieces. Each dough piece was rolled into a rope and the mass spectrometry (PTR-MS) [19], which only provides two ends were joined together to form a circle with a the chemical identities of breads. diameter of approximately 1–2 inches. The bagels were Recently, the sensomics approach, which includes, gas dropped into a large boiling water pot and allowed to boil chromatography–olfactometry (GC–O), sensory analy- for 2  min with constant turning. The boiled bagels were sis, aroma extraction dilution analysis (AEDA), iden- baked in a pre-heated oven at 218 °C for 20 min. Dough tification experiments, quantitation by stable isotope B was kneaded (10  min) as in dough A and allowed to dilution assays (SIDAs), calculation of odour activity val- develop for 1 h. After the kneading operation, the dough ues (OAVs) and aroma recombination and omission tests mass was returned into a large bowl, covered tightly and to validate analytical data, has proven a useful method kept in a chiller (5 °C) for 48 h. After, 48 h of cold fermen- for characterizing the potent aroma constituents of food tation, the dough was brought out and kneaded for about [20]. Sensomics is a multi-step analytical procedure used 3 min and it was divided into eight pieces and made into for identifying and quantifying key odorants in a food eight bagels as described above. The bagels were boiled matrix as well as defining their sensory impact on the in water for 2  min and later baked in a pre-heated oven overall food aroma [21, 22]. Sensomics approaches help at 218 °C for 20 min. Dough C was kneaded as in dough to identify potent aroma compounds as well as taste A and allowed to develop for 1  h. After the kneading, components in food [23]. Furthermore, the sensomic the dough was divided into eight bagels. The pre-formed approach combines separation-based chromatographic bagels were kept in the chiller (5 °C) for 24 h. After 24 h, methods with reconstitution and omission experiments the bagels were subjected to the same boiling and baking to evaluate the role of specific compounds in the per - protocols as described above. ceived aroma of a mixture addition. The implication of this that the sensomics approach is able to produce a Chemicals flavor-cum taste signatures of food [24]. In addition, the Pure chemical standards with purity ranging between 97 sensomics approach has been applied in the characteriza- and 99% were used. The chemical standards included; tion of aroma compounds in yeast dough dumpling [22] acetic acid, butanoic acid, benzyl alcohol, 2,3-butan- and the crust of soft pretzels [25]. edione (diacetyl), heptanoic acid, 3-hydroxy-2-butanone Although there are many reported studies on the (acetoin), and octanoic acid which were purchased characteristic aroma profiles of different wheat breads, from Merck (Darmstadt, Germany). Ethyl nonanoate, however, there has been no reported study on bagels. In ethyl octanoate, 4-hydroxy-2,5-dimethyl-3(2H)-fura- addition, bagel processing is slightly different from that of none (HDMF), (E)-2-nonenal, 2-phenyl ethanol, phenyl Lasek an et al. BMC Chemistry (2021) 15:16 Page 3 of 13 Fig. 1 Flow diagram for the production of bagels acetaldehyde, 3-methylbutanoic acid, 3-methylbutanol, Lancaster (Eastgate, Morecombe, UK). Ethanol (40% v/v) vanillin, 1-butanol, propionic acid, hexanoic acid, benza- was of food grade. The following labelled compounds ldehyde, (E,E)-2,4-decadienal, furfural, 2-methylbutanal, (Table  1) were synthesized according to the literature 2 2 2,3-hexanedione, 2-heptanone, 5-methyl-2-furanmeth-cited; [ H ]-butanoic acid [26]; 2-[ H ]-phenylethanol 2 2 2 2 anol, decanol, 4,5-epoxy-(E)-2-decanal, 1-octen-3-one, [27]; 3-[ H ]-methylbutanol [27]; [ H ]-ethyl octanoate 2 2 13 2 methional, 2-acetyl-1-pyrolline, sodium citrate dehy- [27]; [ C ]-acetic acid [28]; 3-[ H ]-methylbutanal [29]; 2 2 2 13 drate, and citric acid were from Aldrich (Steinheim, Ger- [ H ]-2-acetyl-1-pyrroline [30]; [ C ]-2,3-butanedione 2 4 2 13 many) and 4-vinyl-2-methoxyphenol was purchased from [31]; [ H ]-3-methylbutanoic acid [32]; [ C ]-2-methoxy 2 6 Table 1 Selected ions, and calibration factors used for the quantification of aroma compounds in three bagels by stable isotope dilution assays No. Compounds Selected ions (m/z) Internal standards Selected ions (m/z) Calibration factor 1 Phenylethanol 105 2-[ H] phenylethanol 107 1.02 2 Butanoic acid 89 [ H] butanoic acid 91 0.89 3 3-Methylbutanol 71 3[ H] methylbutanol 73 0.87 4 Acetic acid 61 [ C ] acetic acid 63 1.00 5 Ethyl octanoate 173 [ H] ethyl octanoate 176 1.00 6 4-Hydroxy-2,5-dimethyl-3(2H)- 129 4-Hydroxy-2[ C] methyl-5-methyl-3(2H)- 131 1.00 furanone furanone 7 2,3-Butanedione 87 [ C ]-2,3-butanedione 91 0.90 8 3-Methylbutanal 87 [ H]-3-methylbutanal 89 1.00 9 3-Methylbutanoic acid 60 [ H]-3-methylbutanoic acid 62 1.00 10 Methional 105 [ H]-methional 108 1.00 11 2-Acetyl-1-pyrroline 112 [ H]-2-acetyl-1-pyrroline 114 1.00 12 Phenylacetaldehyde 121 [ H]-2-phenylacetaldehde 123 0.85 13 2-Methoxy-4-vinylphenol 150 [ C ]-2-methoxy-4-vinylphenol 156 0.85 14 Vanillin 137 [ H]-vanillin 139 0.98 15 (Z)-4-Heptenal 95 [ H]-(Z)-4-heptenal 97 0.98 b a Calibration factors and compounds were determined as earlier reported by Guth and Grosch [32] and Lasekan et al. [41] respectively Lasekan et al. BMC Chemistry (2021) 15:16 Page 4 of 13 2 2 vinyl phenol [33]; [ H]-vanillin [33]; [ H ]-phenyl acetal- Mulheim, Germany) connected to a Trace Ultra 1300 2 13 dehyde [34]; [ H ]-heptenal [35] and 4-hydroxy-2[ H ]- gas chromatograph (Thermos Scientific, Waltham, MA, 2 2 methyl-5-methyl-3(2H)-furanone [36]. Citrate buffer USA) was used. The GC–O system was fitted with a (0.1  M, pH 6.0) was prepared as follow: sodium citrate DB-FFAP column (30  m × 0.32  mm i.d; film thickness, dehydrate (12.044 g, 0.04 M) was added to 800 mL of dis- 0.25  µm, Scientific Instrument Services, Inc., Ringoes, tilled water in a liter round bottom flask with constant NJ). The GC–O conditions are the same as reported in stirring. Subsequently, citric acid (11.341  g, 0.06  M) was “Analysis of volatile constituents” section. The effluent added to the solution and the solution was adjusted to a was split 1:1. Sniffing was conducted as described previ - pH 6.0 with 0.1 N HCl. ously [39]. Three experienced panelists (two females and a male) with strong gustative and olfactory responses in Isolation of volatile constituents earlier sessions were used for the sniffing test. The sniff - Freshly baked bagels were sliced into pieces, frozen in liq- ing analysis was divided into three sessions of 20 min and uid nitrogen and pulverized in a Waring blender. The pul - each assessor participated in the exercise. All analyses verized bagel (300 g) was extracted with dichloromethane were repeated in triplicate by each assessor. (700  mL) at room temperature (29  °C) for 2  h and the obtained extract was subjected to solvent-assisted flavor Aroma extracts dilution analysis (AEDA) extraction (SAFE) distillation [13] at 40  °C. To separate The flavor dilution (FD) factors of the aroma compounds the acidic volatiles from the neutral-basic fraction, the were determined by GC–O as reported by Lasekan and −1 extract was treated four times with 50 mL of 0.5 mol  L Yap [39]. The original extracts (200  µL) containing the aqueous sodium bicarbonate. The combined aqueous neutral/basic as well as the acidic volatile compounds −1 solutions were adjusted to pH 2 with HCl (2 mol  L ) and obtained from the crumbs (300  g) were diluted in a extracted with 50 mL of dichloromethane (4×) to obtain stepwise fashion by the addition of dichloromethane as the acidic fractions. Subsequently, the solutions (i.e. the described earlier [39]. Three panelists evaluated all dilu - acidic or the neutral-basic) were concentrated to 2 mL at tions in triplicate. Only the aroma compounds detected 40  °C using a small size Vigreux column [37]. The con - by more than two panelists were recorded. The flavor centrated extract was further concentrated to 200  μL dilution factors obtained by AEDA [40] were plotted [38]. All analyses were repeated in triplicate. against the retention index values of the corresponding aroma compound (FD chromatogram). Analysis of volatile constituents The GC–MS was performed by means of a gas chroma - Aroma compound quantification by stable isotope dilution tograph type QP-5050A (Shimadzu, Kyoto, Japan) using assays (ACQSIDA) the following capillary columns: DB-5 (30  m × 0.25  mm Labelled standards (20–50  µg) previously dissolved in I.D; 0.25  µm film thickness; Scientific Instrument Ser - dichloromethane (5  mL) were added to each crumb vices, Inc., Ringoes, NJ); DB-FFAP (30  m, 0.32  mm I.D; (100  g). The obtained extract was subjected to SAFE 0.25  µm film thickness, Scientific Instrument Services, distillation as described earlier in “Isolation of vola- Inc., Ringoes, NJ). The extracts (2  µL) were applied tile constituents”. Aliquots (0.5  µL) of the concentrates by the on-column injection technique at 230  °C. The were analyzed by means of two dimensional GC–MS −1 temperature of the oven was raised at 40  °C  min to as described previously [41]. Calibration factor for each 50  °C, held for 2  min isothermally and then raised at compound was determined by analyzing mixtures of −1 4 °C  min to 250 °C. The flow rate of the carrier helium defined quantity of the labelled compounds in five dif - −1 was 2.0 mL  min . The retention indices (RI) of the com - ferent mass ratios (1:5, 1:3, 1:1, 3:1, and 5:1) using the pounds were calculated as described previously [37]. GC–MS. The obtained response factors from the peak Mass spectra were recorded in the electron impact pos- area and the amounts of labelled compound are shown in itive mode (EI) over a scan ranges of m/z 40–270 (scan Table  1. The concentration of compounds quantified by frequency 5.8  Hz) applying electron energy of 70  eV. the selected stable isotopologues is reported in Table 3. Total run time was 45 min. Source and transfer line tem- peratures were 200 and 240° C respectively. Mass spectra Orthonasal aroma analysis of bagel were evaluated by using the Xcalibur software (Thermos One hour after baking, the bagels (approximately 8 g with Scientific, Dreieich, Germany). similar crust covering) were placed inside glass beakers (height 7 cm, volume 45 mL) with three random digitals GC–olfactometry and were orthonasally evaluated by panel members at To further identify the aroma constituents in the bagel room temperature (29 ± 2 °C). In addition, samples were extracts, an olfactory detection port ODP-3 (Gerstel, rotated among panelists to prevent carry-over effects. Lasek an et al. BMC Chemistry (2021) 15:16 Page 5 of 13 The panel consisted of 10 members, aged between 24 temperature 29  °C. The aroma model was evaluated and 35  years and were made up of seven women and orthonasally in comparison with the 24 and 48  h bagels three men. These panelists have participated in a weekly as described above (“Orthonasal aroma analysis of bagel” sensory training session for at least a year to be able to section). recognize and describe different aroma qualities. The sensory analyses were conducted in a sensory room fol- Omission experiments lowing the International Standard (ISO 8589, 2007) [42] A triangle test was performed to determine the signifi - protocols with individual booths equipped with uni- cance of one odorant on the aroma recombination mod- form and glare free white light (D65). Descriptors used els (24  h and 48  h) reported in Table  5. For each of the were determined in preliminary sensory experiments as models a glass of the mixture (20  mL) was prepared by described by Steinhaus et  al. [43]. The panelists started omitting one or a group of selected odorants from the with seven descriptors and when all panelists were able complete recombination model (Table  6). This mixture to achieve complete agreement on a descriptor such a and two other glasses containing the complete recom- descriptor was chosen. Each descriptor used was defined bination models were presented to the sensory panel on the basis of the odour of the selected aqueous solu- in a triangle test [45]. The results of the Triangle tests tion of reference compounds. The reference compounds were analyzed by comparing the total number of cor- −1 used as stimuli were; 10  μg  L of 2-acetyl-1-pyrroline rect responses with the minimum number of responses −1 −1 (roasty); 100 μg  L of 3-methylbutanal (malty); 70 μg  L required for statistical significance (ISO, 4120, 2004) [46]. −1 of 2,3-butanedione (buttery); 50  μg  L of (Z)-4-hepte- Panel performance was obtained by applying analysis of −1 nal (biscuit-like); 10  μg  L of 4-vinyl-2-methoxyphenol variance (ANOVA) to the sensory profile data. The data −1 (smoky); 100 μg  L of methional (baked potato-like). were analyzed using SAS Statistical software (SAS Insti- During evaluation, the panelists had 5  min to rest after tute, Inc. 1996). The significance α was calculated accord - each set of samples was tested. All samples were repeated ing to the method of Callejo et al. [45] in triplicate. The intensities of the attributes were rated on a 7-point linear scale (i.e. 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) Results and discussion from 0 (not perceivable) to 3 (strongly perceivable) in Identification of aroma‑active compounds in control steps of 0.5 by the panelists. The sensory data were ana - bagels lyzed by one-way analysis of variance (ANOVA) using A combined total of 40 aroma compounds were identified SPSS 20.0 (SPSS Inc., Chicago, IL., USA). ANOVA with in the three differently processed bagels (i.e. control; 24 h Duncan’s multiple comparison tests were performed to cold fermented bagels: and bagels produced from 48  h determine whether there were differences among indi - cold fermented dough mass). Among these compounds, vidual samples. The differences were considered to be 10 aldehydes, 9 alcohols, 7 acids, 6 ketones, 5 heterocy- significant at p < 0.05 (Table  4). In addition, the ethi- clic compounds and 3 esters were positively identified cal standards as instituted by the institutional and/or (Table  2). To reveal the differences between the flavors national research committee according to the 1964 Hel- of the bagels, the volatile fractions of their crumbs were sinki declaration and its later amendments or comparable subjected to AEDA. In the control bagels, 40 aroma com- ethical Standards on studies involving human subjects pounds were detected in the FD factor range of 2 to 256 were adhered to. The study protocol and consent pro - respectively (Table  2). Furthermore, the results revealed cedure received ethical approval from the Institutional 2-acetyl-1-pyrroline (roasty), methional (baked potato- Review Board (IRB) of the University Putra Malaysia. like), vanillin (vanilla-like), 2,3-butanedione (buttery) Informed consent was obtained from all individual par- and 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) ticipants included in the study. as compounds with the highest FD values in the control bagel. These aroma-active compounds exhibited high FD Aroma model recombinant of the 24 and 48 h bagels factors (128–256) (Fig.  1). Other important aroma com- Reference standards of key aroma compounds (Table  5) pounds in the control bagels were butanoic acid (sweaty), were prepared in ethanolic solution [44]. The combined acetoin (buttery), benzaldehyde (almond-like), furfural ethanolic stock solutions of the 17 aroma compounds (bread-like), 2/3-methyl butanoic acid (sweaty), acetyl made up of 15 compounds with OAVs > 1 and two com- pyrazine (toasty), phenyl acetaldehyde (honey-like), pounds (i.e. acetic acid and acetoin) with significantly 2-phenylethanol (honey-like), octanoic acid (fatty, soapy), high concentrations (Table  5) (500 μL) was added to cit- 4-vinyl-2-methoxyphenol (smoky), acetic acid (sour), −1 rate buffer (30  mL; pH 5.6; 0.1  mol  L ) and free corn 3-methyl butanol (malty), and 2-methylpyrazine (nutty) starch (30  g) respectively in a closed Teflon cup. The all of which exhibited FD factors ranging from 16 to 32 Teflon cup was stirred continuously for 15  min at room (Fig. 2). Lasekan et al. BMC Chemistry (2021) 15:16 Page 6 of 13 Table 2 Aroma compounds identified in cold fermented and control bagels a b No. Compound Retention Retention Odour description Fractions FD FD FD Identification method index on index on CB BF BF 24 48 DB-5 FFAP 1 Acetic acid 605 1443 Sweaty A 32 64 64 MS/RI/O/ST 2 2,3-Butanedione (diacetyl) 606 993 Buttery NB 128 512 1024 MS/RI/O/ST 3 1-Butanol 636 1179 Sweaty/buttery NB 4 4 8 MS/RI/O/ST 4 2/3-Methylbutanal 647 936 Malty NB 8 16 16 MS/RI/O/ST 5 Propionic acid 668 1540 Sweaty/pungent A 8 8 8 MS/RI/O/ST 6 Butanoic acid 718 1619 Sweaty A 16 64 64 MS/RI/O/ST 7 Acetoin 720 1275 Buttery NB 16 128 128 MS/RI/O/ST 8 3-Methyl butanol 769 1067 Malty NB 32 32 32 MS/RI/O/ST 9 2,3-Hexanedione 792 ND ND NB ND ND ND MS/RI/ST 10 Furfural 826 1457 Bread-like NB 16 16 16 MS/RI/O/ST 11 2-Methyl pyrazine 827 1298 Nutty, roasty NB 32 32 32 MS/RI/O/ST 12 2/3-Methylbutanoic acid 831 1661 Sweaty A 16 64 128 MS/RI/O/ST 13 Isoamyl acetate 878 1124 Fruity NB 8 16 16 MS/RI/O/ST 14 2-Heptanone 889 1182 ND NB ND ND ND MS/RI/ST 15 Heptanol 896 1174 Citrusy NB 2 4 8 MS/RI/O/ST 16 Methional 919 1449 Baked potato NB 256 256 256 MS/RI/O/ST 17 2-Acetyl-1-pyrroline 922 1331 Roasty NB 256 256 256 MS/RI/O/ST 18 Benzaldehyde 936 1196 Almond-like NB 16 32 32 MS/RI/O/ST 19 5-Methyl-2-furanmethanol 953 1723 Bread like NB 8 8 8 MS/RI/O/ST 20 (Z)-4-Heptenal 960 1287 Biscuit-like NB 8 16 32 MS/RI/O/ST 21 Hexanoic acid 961 1842 Sweaty A 4 8 8 MS/RI/O/ST 22 1-Heptanol 970 ND ND NB ND ND ND MS/RI/ST 23 1-Octen-3-one 971 1297 Mushroom-like NB 2 4 4 MS/RI/O/ST 24 2,3,5-Trimethylpyrazine 985 1395 Broth-like NB 2 4 4 MS/RI/O/ST 25 2-Pentyl furan 992 ND Fruity, sweet NB 4 4 8 MS/RI/O/ST 26 Acetyl pyrazine 1020 1662 Toast NB 16 16 32 MS/RI/O/ST 27 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 1022 2038 Sweet/caramel A 128 512 1024 MS/RI/O/ST 28 Benzyl alcohol 1039 1866 Sweet/flowery NB 8 16 16 MS/RI/O/ST 29 Phenyl acetaldehyde 1042 1653 Honey, rose NB 16 16 16 MS/RI/O/ST 30 Heptanoic acid 1077 1949 Rancid A 2 8 8 MS/RI/O/ST 31 2-Phenyl ethanol 1136 1911 Honey-like NB 16 16 16 MS/RI/O/ST 32 (E)-2-Nonenal 1164 1568 Fatty, green NB 8 8 8 MS/RI/O/ST 33 Octanoic acid 1182 2047 Fatty, soapy A 16 32 32 MS/RI/O/ST 34 Ethyl octanoate 1194 1428 Fruity, fatty NB 8 8 8 MS/RI/O/ST 35 Decanol 1269 ND Fatty NB 4 8 8 MS/RI/O/ST 36 Ethyl nonanoate 1296 ND Fruity, tropical NB 4 4 4 MS/RI/O/ST 37 (E,E)-2,4-Decadienal 1313 1684 Fatty NB 2 4 8 MS/RI/O/ST 38 4-Vinyl-2-methoxyphenol 1317 2174 Smoky A 16 32 32 MS/RI/O/ST 39 4,5-Epoxy-(E)-2-decenal 1360 1970 Metallic NB 4 8 8 MS/RI/O/ST 40 Vanillin 1410 2601 Vanilla-like A 256 256 256 MS/RI/O/ST ND not detected, CB control bagel, BF 24 h fermented bagel, BF 48 h fermented bagel, NB neutral basic fraction, A acidic fraction 24 48 Compound identified by comparison of its odour quality and intensity, retention indices on capillaries DB-5 and FFAP as well as mass spectra in EI with data of reference compounds. MS, RI, O, ST represents mass spectra, retention indices, olfactometry and standard odorants respectively Odour quality as perceived at the sniffing port Aroma‑active compounds in long, cold fermented bagels FD factors (4–1024) than the control bagel (Table 2). For Application of long, cold fermentation (5  °C, 24  h and instance, the FD factor of 2,3-butanedione in the 24  h 48  h) produced bagels that exhibited a wider range of and 48 h fermented bagels increased by almost (4 times) Lasek an et al. BMC Chemistry (2021) 15:16 Page 7 of 13 Fig. 2 Flavour dilution chromatogram obtained by the application of AEDA on a distillate of unfermented bagel (control). Compounds with an FD factor ≥ 32 are displayed. Numbering is identical with that in Table 2 and 8 times the value obtained in the control bagel. Other bread exhibited similar amounts of Strecker aldehydes compounds exhibiting higher FD factors in the long, cold (i.e. 2-methylpropanal, 2-methylbutanal and 3-meth- fermented bagels were; acetic acid (sweaty), 2/3-meth- ylbutanal) as obtained with the artisanal process. This ylbutanal (malty), 2,3-butanedione (buttery), propionic observation is probably due to a longer proteolysis which acid (sweaty/pungent), butanoic acid (sweaty), acetoin leads to the formation of amino acids that participates (buttery), 3-methylbutanol (malty), furfural (bread-like), in the Strecker reactions as well as the Ehrlich pathway 2-methyl pyrazine (nutty), 2/3-methyl butanoic acid to produce the aldehydes. It is worthy of note that both (sweaty), methional (baked potato-like), 2-acetyl-1-pyr- 2,3-butanedione and HDMF which exhibited the highest roline (roasty), benzaldehyde (almond-like), (Z)-4-hep- FD factors in the cold fermented bagels as well as many tenal (biscuit-like), acetyl-pyrazine (toasty), 4-HDMF other key aroma compounds such as: 2/3-methylbutanal, (sweet/caramel), benzyl alcohol (sweet/flowery), phenyl acetoin, 3-methylbutanol, furfural, 2-methyl pyrazine, acetaldehyde (rose-like), 2-phenyl ethanol (honey-like), isoamyl acetate, methional, 2-acetyl-1-pyrroline, benza- octanoic acid (fatty), 4-vinyl-2-methoxyphenol (smoky) ldehyde, (Z)-4-heptenal, acetyl pyrazine, phenyl acetal- and vanillin (vanilla-like) all of which exhibited FD fac- dehyde and vanillin have been identified in the crumb of tors from16 to 1024. While long, cold fermented bagels wheat bread [3, 11, 47]. Also, various acids such as acetic generally exhibited higher FD values than the control, the acid, butanoic acid, 2/3-methyl butanoic acid and octa- 48  h bagel also showed higher FD values in some com- noic acid which exhibited high FD factors ≥ 16 in the cold pounds (i.e. diacetyl, 1-butanol, 2/3-methylbutanoic acid, fermented bagels have been reported in bread [50, 51]. heptanol, (Z)-4-heptenal and 2-pentyl furan) compared to the 24 h bagels. Quantitation and odour‑activity values (OAVs) The influence of fermentation temperatures on the for - of aroma‑active compounds in bagels mation of volatile compounds in bread crust and crumb To have an insight into the contribution of each com- has been well documented [47–49]. While high fer- pounds to the overall aroma of bagels, 22 aroma-active mentation temperatures (≥ 27  °C) are more suitable for compounds with FD factors ≥ 16 were selected for fur- generating more complete volatile profiles, most bread ther investigation. For each of the selected compound, industries are more favorable to employing longer fer- a stable isotopologue (Table  1) was employed as an mentation time or using sourdough that needs time to internal standard to quantify it. As expected the long ferment. For instance, Zehentbauer and Grosch [48] cold fermented bagels produced compounds with sig- observed that when bread is prepared from dough sub- nificantly (p < 0.05) high concentrations (Table  3). The −1 jected to an initial 2  h of fermentation at 22  °C and an highest concentrations (1126–12,950  μg  kg ) were additional 18  h of fermentation at 4  °C, the resulting determined for 2,3-butanedione, 2-phenylethanol, Lasekan et al. BMC Chemistry (2021) 15:16 Page 8 of 13 Table 3 Concentrations, odour thresholds and odour activity values (OAVs) of key aroma compounds (FD factor ≥ 16) in cold fermented bagels −1 No. Compounds Concentration (μg kg ) Threshold in Odour activity values −1 c starch (μg kg ) (OAVs) CB BF BF CB BF BF 24 48 24 48 c b a 1 Acetic acid 300 ± 2.0 480 ± 2.0 510 ± 2.0 31,140 < 1 < 1 < 1 c b a 2 2,3-Butanedione (diacetyl) 710 ± 5.1 11,800 ± 12.0 12,950 ± 15.5 6.5 109 1815 1992 c b a a 3 2/3-Methyl butanal 164 ± 2.1 321 ± 2.1 434 ± 2.0 32 5 10 14 c b a a 4 Butanoic acid 113 ± 1.0 201 ± 1.0 317 ± 1.0 100 1 2 3 c b a 5 Acetoin 1140 ± 4.5 1245 ± 5.0 1276 ± 4.0 Nf nd nd nd c b a 6 3-Methyl butanol 647 ± 3.1 1126 ± 7.8 1364 ± 10.0 102 6 11 13 b a a 7 Furfural 101 ± 1.0 126 ± 1.0 124 ± 1.0 Nf nd nd nd c b a 8 2-Methylpyrazine 30 ± 0.2 54 ± 0.2 76 ± 0.2 Nf nd nd nd c b a 9 3-Methylbutanoic acid 64 ± 1.0 276 ± 2.1 314 ± 2.0 24 3 12 13 c b a 10 Methional 16 ± 0.1 24 ± 0.1 43 ± 0.1 0.27 59 89 160 ab a a a 11 2-Acetyl-1-pyrroline 17 ± 0.1 19 ± 0.1 18 ± 0.1 0.0073 2329 2603 2466 c b a 12 Benzaldehyde 174 ± 3.0 920 ± 4.5 1121 ± 6.0 350 < 1 3 3 c b a b 13 5-Methyl-2-furanmethanol 46 ± 0.1 52 ± 0.1 58 ± 0.1 11.9 4 4 5 c b a 14 (Z)-4-Heptenal 51 ± 0.1 135 ± 0.2 234 ± 0.2 3 17 45 78 c b a 15 Acetyl pyrazine 171 ± 1.0 186 ± 1.0 193 ± 1.0 nf nd nd nd c b a b 16 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 234 ± 2.0 347 ± 2.1 453 ± 3.0 13 18 27 35 c b a 17 Benzyl alcohol 115 ± 1.0 176 ± 2.0 182 ± 2.1 nf nd nd nd b a a b 18 Phenyl acetaldehyde 15 ± 0.0 18 ± 0.1 17 ± 0.1 28 < 1 < 1 < 1 c b a b 19 2-Phenyl ethanol 1101 ± 5.0 1134 ± 5.0 1512 ± 5.0 125 9 9 12 c b a 20 Octanoic acid 87 ± 2.1 102 ± 2.0 116 ± 2.0 Nf nd nd nd c b a b 21 4-Vinyl-2-methoxyphenol 146 ± 2.0 305 ± 2.1 512 ± 4.0 18 8 17 28 c b a b 22 Vanillin 56 ± 0.1 73 ± 0.1 95 ± 0.1 4.6 12 16 21 nf not found, nd not determined, CB control bagel, BF 24 h fermented bagels, BF 48 h fermented bagels; Mean ± SD; superscripts with different letters in a row are 24 48 significantly (p < 0.05) different Reference; Zehentbauer and Grosch [48] Reference; Rychlik and Grosch [10] OAV on the basis of odour thresholds in starch 3-methylbutanal and acetoin respectively (Table 3). The noticed with the methional, acetyl pyrazine, HDMF, −1 lowest concentrations (17–43  μg  kg ) were obtained 4-vinyl-2-methoxyphenol, vanillin, 2/3-methylbutanal, for phenyl acetaldehyde, methional and 2-acetyl-1-pyr- 2-phenyl ethanol, butanoic acid, 3-methylbutanol and roline respectively. A comparative analysis of the aroma benzaldehyde. However, acetic acid, phenyl acetalde- potencies between the three differently produced hyde had OAVs below 1. bagels revealed some differences. Cold fermented While some of the bagel aroma compounds were bagels showed more potencies for the buttery smell- already present in the wheat flour and were thus trans - ing 2,3-butanedione, baked potato-like methional and ferred into the bagel. Others such as 3-methylbutanol, the toasty-like 2-acetyl-1-pyrroline as revealed by their 2-phenyl ethanol and 2,3-butanedione were probably respective high odour-activity values (Table  3). For formed during biochemical reactions in the yeast metab- example, 2-acetyl-1-pyrroline exceeded its threshold olism during the dough fermentation [27]. On the other by factors of 2603 and 2466 in the 24  h and 48  h cold hand the nitrogen-containing compounds such as the fermented bagels respectively. 2-Acetyl-1-pyrroline roasty 2-acetyl-1-pyrroline and acetyl pyrazine were only exceeded its threshold by a factor of 2329 in the formed via the reaction of free amino acids l-ornithine or control bagels. Similarly, 2,3-butanedione exceeded its l-proline with dihydroxyacetone phosphate [52]. In addi - threshold by factors of 1815 and 1992 in the 24  h and tion to the nitrogen-containing compounds, aldehydes, 48  h cold fermented bagels respectively. On the other such as 2/3-methylbutanal (malty), phenyl acetaldehyde hand 2,3-butanedione only exceeded its threshold by (rose/floral) and methional (baked potato-like) were a factor of 109 in the control bagel. Similar trend was formed by the Strecker degradation of valine, isoleucine, Lasek an et al. BMC Chemistry (2021) 15:16 Page 9 of 13 leucine, phenylalanine and methionine respectively [53]. descriptors, all the panelists had to achieve complete Moreover the caramel-like 4-Hydroxy-2,5-dimethyl- agreement on any given descriptor for such descriptor 3(2H)-furanone (HDMF) can be formed by the Maillard to be chosen. The aroma profiles of the cold fermented reaction [54]. 4-Hydroxy-2,5-dimethyl-3(2H)-furanone bagels were characterized as roasty, biscuit-like, malty, is mainly formed via Maillard reaction of pentoses with smoky and buttery. The control bagel exhibited similar the amino acids glycine and alanine, respectively. Alter- but less intense aroma notes as compared to the cold natively, 4-hydroxy-2,5-dimethyl-3(2H)-furanone can fermented bagels. However, the 24  h and 48  h bagels also be produced without the direct interaction of flavor profiles were similar with the exception of the glycine [36]. Furthermore, certain aldehydes such as biscuit-like aroma note (Table 4). The statistical analysis (E,E)-2,4-decadienal, (E)-2-nonenal, and (E)-4,5-epoxy- results (Table  4) showed that the six attributes (roasty, (E)-2-decenal were formed by autoxidation and thermal malty, buttery, biscuit-like, smoky and baked potato degradation of fatty acids respectively [53]. like) with different superscripts provided a clearer explanation of the aroma characteristics of the differ - Sensory analysis and aroma reconstitution evaluation ent bagels. To confirm this observation, recombina - The results of sensory evaluation of the different bagels tion experiments were carried out by mixing solutions (i.e. control, 24  h fermented and 48  h fermented) are of the pure reference compounds in the same amounts shown in (Fig.  3a, Table  4). In order to select the final as indicated for both 24  h and 48  h bagels respectively Roasty a b BF24 BF 24 Model Roasty 2.5 2.5 Baked potato Malty 1.5 Baked potato Malty 1.5 0.5 0.5 Smoky Buttery Smoky Buttery Biscuit-like Biscuit-like Control BF24 BF48 BF48 BF48 Model Roasty 2.5 Baked potato Malty 1.5 0.5 Smoky Buttery Biscuit-like Fig. 3 a Aroma profiles of bagels; control bagels (blue line), 24 h fermented bagels (red line) and 48 h fermented bagels (green line). b A comparative aroma profiles of 24 h bagels (red colour) and its aroma model (green colour). c Aroma profiles of 48 h bagel (red colour) and its aroma model (green colour) Lasekan et al. BMC Chemistry (2021) 15:16 Page 10 of 13 Table 4 The mean scores of the six attributes for the three bagels and the aroma models generated. (Supplementary) Sensory attribute Bagels Mean scores of bagel’s and their aroma models Control BF BF BF BF Model BF BF Model 24 48 24 24 48 48 A A A a a a a Roasty 3.0 ± 0.21 3.0 ± 0.70 3.0 ± 0.91 3.0 ± 0.42 3.0 ± 0.50 3.0 ± 0.72 3.0 ± 0.23 B A A a a a a Malty 1.5 ± 0.05 2.0 ± 0.23 2.0 ± 0.60 2.0 ± 0.23 1.9 ± 0.14 2.0 ± 0.33 1.9 ± 0.24 B A A a a a a Buttery 1.0 ± 0.04 1.5 ± 0.02 1.5 ± 0.13 1.5 ± 0.05 1.5 ± 0.25 1.5 ± 0.15 1.5 ± 0.21 B B A a a a a Biscuit-like 2.5 ± 0.81 2.5 ± 0.33 3.0 ± 0.56 2.5 ± 0.50 2.5 ± 0.71 3.0 ± 0.30 3.0 ± 0.80 A A A a a a a Smoky 0.5 ± 0.02 0.5 ± 0.01 0.5 ± 0.03 0.5 ± 0.04 0.5 ± 0.12 0.5 ± 0.03 0.5 ± 0.05 A A A a a a a Baked potato 0.5 ± 0.01 0.5 ± 0.03 0.5 ± 0.01 0.5 ± 0.02 0.5 ± 0.04 0.5 ± 0.01 0.5 ± 0.03 A, B, C : a, b, c Different letters within the same row represents significant differences (p < 0.05) using Duncan’s multiple comparison test (n = 30, 10 panellists with 3 replications) BF 24 h fermented bagel, BF 48 h fermented bagel 24 48 Omission tests (Table  5). A parallel evaluation of the recombination The contributions of some key aroma compounds models of the freshly baked 24  h and 48  h bagels was to the flavor of the bagels, was evaluated by omission conducted. Results showed that the recombinant model tests. Omission tests are used to assess the contribution imitated well the flavor of the freshly baked bagels of individual compound to the overall aroma of a given (Fig.  3b, c, Table  4). The aroma of the recombination food [54]. Eleven aroma omission models (M1–M11), models had good similarities for all the odor notes such containing either single or a group of compounds, were as roasty, baked potato-like, smoky and biscuit-like. prepared. Each of the omission models was analyzed The roasty and biscuit-like aroma notes were perceived in triangular experiments with two complete recom- as equally intense in the aroma models as well as in the bination models (Table  6). Results showed that, the bagels. omission of the entire group of acids (M1) from the complete recombination model could be distinguished by 9 out of the 10 assessors. This shows that these acids Table 5 Aroma models composition for bagels produced from (i.e. acetic acid, butanoic acid and 3-methyl butanoic 24 and 48 h cold fermentation acid) play an important role in the overall aroma of b the long, cold fermented bagels. In the second group, No. Compounds Concentration (μg −1 kg ) the ketones (2,3-butanedione and acetoin) with char- acteristic buttery nuance were omitted. Acetoin was BF BF 24 48 included in this group because of its high concentra- 1 Acetic acid 480 510 tion. Result of the omission of the entire ketones from 2 2,3-Butanedione (diacetyl) 11,800 12,950 the complete recombination model showed that all 10 3 2/3-Methyl butanal 321 434 assessors could detect between the omission model 4 Butanoic acid 201 317 and the complete recombination models. This shows 5 Acetoin 1245 1276 that 2,3-butanedione and acetoin greatly influence the 6 3-Methyl butanol 1126 1364 overall aroma of the bagel. When the aldehydes (M3) 7 3-Methylbutanoic acid 276 314 (2,3-methyl butanal, methional, benzaldehyde, (Z)- 8 Methional 24 43 4-heptenal, phenyl acetaldehyde and vanillin) were 9 2-Acetyl-1-pyrroline 19 18 omitted, only 8 assessors were able to detect the dif- 10 Benzaldehyde 920 1121 ference (p < 0.01). Similar trend was observed when the 11 5-Methyl-2-furanmethanol 52 58 entire group of alcohols (M4) was omitted. In model 12 (Z)-4-Heptenal 135 234 5, 4-vinyl-2-methoxyphenol was omitted because of 13 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 347 453 its high concentration and the result showed that only 14 Phenyl acetaldehyde 18 17 7 assessors were able to detect the difference between 15 2-Phenyl ethanol 1134 1512 the omission model and the complete recombination 16 4-Vinyl-2-methoxyphenol 305 512 models. In model 6, 4-hydroxy-2,5-dimethyl-3(2H)- 17 Vanillin 73 95 furanone was omitted and this resulted in significant a (p ≤ 0.001) reduction in the characteristic aroma of Acetoin was included in the model even though its threshold in starch was not found the bagels. In addition, 9 of the assessors were able to Ethanolic solutions of aroma compounds dissolved in free corn starch Lasek an et al. BMC Chemistry (2021) 15:16 Page 11 of 13 Table 6 Omission analysis on the bagel aroma models (BF and BF ) 24 48 Odorant groups Aroma note Compounds omitted No of correct No of correct Significance a a judgments judgments BF BF 24 48 Acids (M1) Sweaty Acetic acid, butanoic acid, 3-methylbutanoic acid 9/10 9/10 *** Ketones (M2) Buttery 2,3-Butanedione, acetoin 10/10 10/10 *** Acetaldehydes (M3) Malty, baked potato, 2,3-Methylbutanal, methional, benzaldehyde, (Z)-4-hep- 8/10 8/10 ** almond-like, biscuit-like, tenal, phenyl acetaldehyde, vanillin vanilla Alcohols (M4) Malty, bread-like, honey 3-Methylbutanol, 5-methyl-2-furanmethanol, 2-phenyl 8/10 8/10 ** ethanol Phenol (M5) Smoky 4-Vinyl-2-methoxyphenol 7/10 7/10 * (M6) Sweat, caramel 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 9/10 9/10 *** (M7) Floral, honey 2-Phenyl ethanol 8/10 8/10 ** (M8) Cooked potato-like Methional 8/10 8/10 ** (M9) Biscuit-like (Z)-4-Heptenal 9/10 9/10 *** (M10) Bread-like 5-Methyl-2-furanmethanol 10/10 10/10 *** (M11) Popcorn-like 2-Acetyl-1-pyrroline 10/10 10/10 *** M1–M11 Models Number of correct judgments from 10 assessors Significance: * significant (α ≤ 0.05); **, highly significant (α ≤ 0.01); ***, very highly significant (α ≤ 0.001) distinguish its omission from the complete recombina- of cold fermentation on bakery products found in many tion models. Similar observation was obtained when world cuisines. other single compounds such as 2-phenyl ethanol, Acknowledgements methional, (Z)-4-heptenal, 5-methyl-2-furanmethanol The authors wish to thank the Faculty of Food Science & Technology, Univer- and 2-acetyl-1-pyrroline were omitted from the com- sity Putra Malaysia for supplying the facilities for this study. plete recombination models respectively. However, the Authors’ contributions omission of 5-methyl-2-furanmethanol and 2-acetyl- OL: Conceptualize, funding acquisition, supervised and reviewed the initial 1-pyrroline was detected by all 10 assessors. and final manuscript. FD, MM, HJ: formal analysis, data collection, writing of draft, AL: reviewed initial draft and provided necessary information on bagel. All authors reviewed the manuscript. All authors read and approved the final manuscript. Conclusion Funding This study has revealed the key aroma-active compounds Financial support for this research was provided by the University Putra Malay- responsible for the characteristic aroma of the long, sia research scheme (Grant No. 9478500). cold fermented bagels. The results of the OAVs and sen - Availability of data and materials sory studies showed distinct differences in the aroma All data generated or analyzed during this study are included in this published notes of the cold fermented and control bagels. Whilst article. the cold fermented bagels exhibited roasty, malty, but- Declarations tery, baked potato-like, smoky and biscuit-like notes, the odour notes in the control bagels were similar to the Ethics approval and consent to participate other bagels but less intense. Aroma compounds such as The study protocol and consent procedure received ethical approval from the Institutional review board of the University Putra Malaysia. Informed consent 2,3-butanedione (buttery), acetoin (buttery), 2-acetyl- was obtained from all individual participants included in the study. 1-pyrroline (roasty), 5-methyl-2-furanmethanol (bread- like), (Z)-4-heptenal (biscuit-like) and HDMF, were the Consent for publication Not applicable. key aroma compounds. In addition, vanillin (vanilla), 2/3-methylbutanal (malty), 3-methyl butanoic acid Competing interests (sweaty), 3-methylbutanol (malty), methional (baked The authors declare no competing interests. potato-like), 2-phenyl ethanol (honey-like), benzaldehyde Author details (almond-like), and butanoic acid (sweaty) were identified 1 Department of Food Technology, University Putra Malaysia, UPM, 43400 Ser- as important aroma compounds of bagels. These find - dang, Malaysia. Department of Food Service and Management, University Putra Malaysia, UPM, 43400 Serdang, Malaysia. 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Frasse P, Lambert S, Richard-Molard D, Chiron H (1993) The influence of fermentation on volatile compounds in French bread dough. LWT-Food Sci Technol 26:126–132 Re Read ady y to to submit y submit your our re researc search h ? Choose BMC and benefit fr ? Choose BMC and benefit from om: : fast, convenient online submission thorough peer review by experienced researchers in your field rapid publication on acceptance support for research data, including large and complex data types • gold Open Access which fosters wider collaboration and increased citations maximum visibility for your research: over 100M website views per year At BMC, research is always in progress. Learn more biomedcentral.com/submissions

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