Chemical and Sensory Characteristics of Soy Sauce: A Review

Soy sauce is a fermented product, and its flavor is a complex mixture of individual senses which, in combination, create a strong palatable condiment for many Eastern and Western dishes. This Review focuses on our existing knowledge of the chemical compounds present in soy sauce and their potential relevance to the flavor profile. Taste is dominated by umami and salty sensations. Free amino acids, nucleotides, and small peptides are among the most important taste-active compounds. Aroma is characterized by caramel-like, floral, smoky, malty, and cooked potato-like odors. Aroma-active volatiles are chemically diverse including acids, alcohols, aldehydes, esters, furanones, pyrazines, and S-compounds. The origin of all compounds relates to both the raw ingredients and starter cultures used as well as the parameters applied during production. We are only just starting to help develop innovative studies where we can combine different analytical platforms and chemometric analysis to link flavor attributes to chemical composition.

Enhancing Activities of Salt-Tolerant Proteases Secreted by Aspergillus oryzae Using Atmospheric and Room-Temperature Plasma Mutagenesis

Aspergillus oryzae 3.042 was mutagenized using atmospheric and room-temperature plasma (ARTP) technology to enhance its salt-tolerant proteases activity. Compared to the starting strain, mutant H8 subjected to 180 s of ARTP treatment exhibited excellent genetic stability (15 generations), growth rate, and significantly increased activities of neutral proteases, alkaline proteases, and aspartyl aminopeptidase during fermentation. Mutant H8 significantly enhanced the contents of 1-5 kDa peptides, aspartic acid, serine, threonine, and cysteine in soy sauce by 16.61, 7.69, 17.30, 8.61, and 45.00%, respectively, but it had no effects on the contents of the other 14 free amino acids (FAAs) due to its slightly enhanced acidic proteases activity. Analyses of transcriptional expressions of salt-tolerant alkaline protease gene (AP, gi: 217809) and aspartyl aminopeptidase gene (AAP, gi: 6165646) indicated that their expression levels were increased by approximately 30 and 27%, respectively. But no mutation was found in the sequences of AP and AAP expression cassettes, suggesting that the increased activities of proteases in mutant H8 should be partially attributed to the increased expression of proteases. ARTP technology showed great potential in enhancing the activities of salt-tolerant proteases from A. oryzae.

Effects of Tetragenococcus halophilus and Candida versatilis on the production of aroma-active and umami-taste compounds during soy sauce fermentation

BACKGROUND

Soy sauce is a well-known condiment worldwide. However, the high salt content in soy sauce contributes to the high intake of sodium salt, which usually results in high blood pressure. High salt soy sauce usually has the better quality (aroma and taste) than low salt. Tetragenococcus halophilus and Candida versatilis are important starters for soy sauce fermentation. It is of urgent need to explore what the effect of these two strains on the aroma- and taste-attributes of soy sauce to achieve high quality fermentation with low salt.

RESULTS

In this study, aroma-active and taste compounds in soy sauce were reviewed and listed. Then, soy sauce fermentation inoculated with different combinations of T. halophilus (at different inoculated time) and C. versatilis were completed. Aroma-active and taste compounds in different samples were quantified. Multivariate analysis was used to analyze these data. The aroma-active compounds which were highly related to the inoculation of T. halophilus and C. versatilis were found. Meanwhile, the addition time of T. halophilus could also be highly related to the production of aroma-active compounds. More importantly, T. halophilus was highly correlated with the production of umami-taste compounds in soy sauce, including aspartic acid, glutamic acid, alanine and N -succinyl-glutamic acid.

CONCLUSION

These results will provide a better understanding of the effects of T. halophilus and C. versatilis on the formation of significant aroma-active and umami-taste constituents in soy sauce. Furthermore, it will be helpful to realize fermentative control of soy sauce with high quality at low salt. © 2020 Society of Chemical Industry.

The arginine deiminase pathway of koji bacteria is involved in ethyl carbamate precursor production in soy sauce

Ethyl carbamate (EC) is a group 2A carcinogen generated from a few precursors in many fermented foods and alcoholic beverages. Citrulline, urea, carbamoyl phosphate, and ethanol are common precursors detected in fermented foods. In this study, citrulline was proved to be the main EC precursor in soy sauce, which was found to be accumulated in moromi mash period and correlated with the utilization of arginine by koji bacteria. Six koji isolates belonging to three genera were identified to be able to accumulate citrulline via the arginine deiminase (ADI) pathway. Among these strains, only Pediococcus acidilactici retained high activities in synthesis and accumulation of citrulline in the presence of high concentration of sodium chloride. These results suggested that P. acidilactici is responsible for the accumulation of citrulline, one of the EC precursors, in the process of soy sauce fermentation.

Dynamics of microbial community during the extremely long-term fermentation process of a traditional soy sauce

BACKGROUND

Soy sauce produced by long-term natural fermentation is a traditional specialty in Asia, with a reputation for superior quality and rich flavour. In this study, both culture-dependent and culture-independent approaches were used to investigate the microbial diversity and community dynamics during an extremely long-term (up to 4 years) natural fermentation of Xianshi Soy Sauce, a national intangible cultural heritage.

RESULTS

Genera of Bacillus, Aspergillus and Cladosporium were detected by both methods above. The relative abundance of the genera Bacillus and Weissella was significantly higher in the late stage than in the early one, while the genera Klebsiella and Shimwellia were opposite (P < 0.05). For microbial community structure, subsequent analyses showed that obvious changes occurred with fermentation time, while there was a fair homogeneousness among samples of the same year, especially during the late fermentation stage.

CONCLUSIONS

The clustering analysis tended to separate the fermented mashes of the 4th year from the earlier stages, suggesting the necessity of the long fermentation period for developing distinctive microbiota and characteristic quality-related compounds. This is the first report to explore the temporal changes in microbial dynamics over a period of 4 years in traditional fermentation of soy sauce, and this work illustrated the importance of isolation of appropriate strains to be used as starter cultures in brewing processes. © 2016 Society of Chemical Industry.

Effect of Soy Sauce on Serum Uric Acid Levels in Hyperuricemic Rats and Identification of Flazin as a Potent Xanthine Oxidase Inhibitor

This is the first report on the ability of soy sauce to effectively reduce the serum uric acid levels and xanthine oxidase (XOD) activities of hyperuricemic rats. Soy sauce was partitioned sequentially into ethyl acetate and water fractions. The ethyl acetate fraction with strong XOD inhibition effect was purified further. On the basis of xanthine oxidase inhibitory (XOI) activity-guided purification, nine compounds including 3,4-dihydroxy ethyl cinnamate, diisobutyl terephthalate, harman, daidzein, flazin, catechol, thymine, genistein, and uracil were obtained. It was the first time that 3,4-dihydroxy ethyl cinnamate and diisobutyl terephthalate had been identified from soy sauce. Flazin with hydroxymethyl furan ketone group at C-1 and carboxyl at C-3 exhibited the strongest XOI activity (IC50 = 0.51 ± 0.05 mM). According to fluorescence quenching and molecular docking experiments, flazin could enter into the catalytic center of XOD to interact with Lys1045, Gln1194, and Arg912 mainly by hydrophobic forces and hydrogen bonds. Flazin, catechol, and genistein not only were potent XOD inhibitors but also held certain antioxidant activities. According to ADME (absorption, distribution, metabolism, and excretion) simulation in silico, flazin had good oral bioavailability in vivo.

Comparative study on the novel umami-active peptides of the whole soybeans and the defatted soybeans fermented soy sauce

BACKGROUND

Two kinds of soy sauce produced via fermentation of the whole soybeans and the defatted soybeans (soy sauce termed 'SSS' and 'SSD', respectively) were subjected to the treatment using aqueous ethanol solutions with high concentrations. Then tasty peptides were separated from SSS and SSD by sensory guided fractionation, using macroporous resin and reverse-phase high-performance liquid chromatography (RP-HPLC), and identified by ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF-MS/MS).

RESULTS

The result showed that umami-active fractions and ≤ 3 kDa peptides were mainly concentrated in the supernatants resulted from the treatment with 60% ethanol and the precipitates resulted from the treatment with 80% ethanol. The contents of ammonia nitrogen, non-salt solids, bitter amino acids, amino acids in peptides and the proportion of ≤ 3 kDa peptides in SSS were higher than those in SSD. Sixteen and fourteen tasty peptides were separated from SSS and SSD, among them five dipeptides (γ-Glu-Glu, Glu-Glu, γ-Glu-Cys, γ-Glu-Leu, Glu-Leu and Ile-Glu) with a glutamic acid residue were identified both in SSS and SSD, which have been reported as umami/kokumi-active peptides in soy sauce. Several peptides identified from SSS (Thr-Gly-Cys, Gly-Leu-Glu, Val-Glu-Ala-Leu and Gly-Gly-Gly-Glu) and SSD (Asp-Arg, Asp-Ala-Glu, Glu-Val-Cys and Gly-Gly-Gly-Glu) are tasty and/or umami-active peptides but have not been reported as tasty peptides in the past.

CONCLUSION

Most of the peptides separated from SSD and SSS could impart an umami-enhancing effect on soy sauce, and the marginally more kokumi and bitterness hydrophobic peptides were found in SSS than SSD. © 2020 Society of Chemical Industry.

Extracellular Proteome Analysis and Flavor Formation During Soy Sauce Fermentation

Aspergillus oryzae is an excellent strain for soy sauce fermentation because of its complicated enzyme system, especially protease. The aim of this study was to investigate the key enzymes and flavors during soy sauce fermentation, and a comparative assessment of extracellular enzymes during various fermentation stages at the proteomic level via iTRAQ analysis is presented. Many important enzymes related to the amino acid and glucose metabolisms participated in the material decomposition under high-salt stress. Dipeptidase, dipeptidyl aminopeptidase, leucine aminopeptidase, aspartic protease pep1, and extracellular metalloproteinase played positive roles during the early stage of soybean mash fermentation, whilst leucine aminopeptidase A and extracellular metalloproteinase NpI were the dominant proteolytic enzymes during the later period of fermentation. At the same time, β-glucosidase and β-xylanase exerted great effects upon glucose metabolism throughout the fermentation process. The results show that protease and amylolytic enzymes are complementary in the formation of flavors such as alcohols, acids, esters, aldehydes, furans, and pyrazines during soy sauce fermentation.

Koji Molds for Japanese Soy Sauce Brewing: Characteristics and Key Enzymes

Soy sauce is a traditional Japanese condiment produced from the fermentation of soybeans, wheat, and salt by three types of microorganisms, namely koji molds, halophilic lactic acid bacteria, and salt-tolerant yeast. The delicate balance between taste, aroma, and color contributes to the characteristic delicious flavor imparted by soy sauce. In soy sauce brewing, protein and starch of the raw materials are hydrolyzed into amino acids and sugars by enzymes derived from koji molds. These enzymatically hydrolyzed products not only directly contribute to the taste but are further metabolized by lactic acid bacteria and yeasts to most of organic acids and aromatic compounds, resulting in its distinctive flavor and aroma. The color of the soy sauce is also due to the chemical reactions between amino acids and sugars during fermentation. Therefore, koji mold, which produces various enzymes for the breakdown of raw materials, is an essential microorganism in soy sauce production and plays an essential role in fermenting the ingredients. In this review, we describe the manufacturing process of Japanese soy sauce, the characteristics of koji molds that are suitable for soy sauce brewing, and the key enzymes produced by koji molds and their roles in the degradation of materials during soy sauce fermentation, focusing on the production of umami taste in soy sauce brewing.

Separation, biochemical characterization and salt-tolerant mechanisms of alkaline protease from Aspergillus oryzae

BACKGROUND

The salt tolerance of proteases secreted by Aspergillus oryzae 3.042 closely relates to the utilization of raw materials and the quality of soy sauce. However, little is known about the salt-tolerant proteases and their salt-tolerant mechanisms.

RESULTS

In this study, we isolated and identified a salt-tolerant alkaline protease (AP, approximately 29 kDa) produced by A. oryzae 3.042. It was considered as a metal-ion-independent serine protease. The optimum and stable pH values were both pH 9.0 and the optimum temperature was 40 °C. Over 20% relative activity of AP remained in the presence of 3.0 mol L^-1 NaCl after 7 days, but its K_m and V_max were only mildly influenced by the presence of 3.0 mol L^-1 NaCl, indicating its outstanding salt tolerance. Furthermore, AP was more stable than non-salt-tolerant protease at high salinity. The salt-tolerant mechanisms of AP could be due to more salt bridges, higher proportion of ordered secondary structures and stronger hydrophobic amino acid residues in the interior.

CONCLUSIONS

The above results are vital for maintaining, activating and/or modulating the activity of AP in high-salt environments. They would also provide theoretical guidance for the modification of AP and the engineering of A. oryzae 3.042 so as to secrete more AP. © 2018 Society of Chemical Industry.

Sensoproteomic Discovery of Taste-Modulating Peptides and Taste Re-engineering of Soy Sauce

Soy sauce, one of the most common Asian fermented foods, exhibits a distinctive savory taste profile. In the present study, targeted quantitation of literature-known taste compounds, calculation of dose-over-threshold factors, and taste re-engineering experiments enabled the identification of 34 key tastants. Following the sensoproteomics approach, 14 umami-, kokumi-, and salt-enhancing peptides were identified for the first time, with intrinsic taste threshold concentrations in the range of 166-939 μmol/L and taste-modulating threshold concentrations ranging from 42 to 420 μmol/L. The lowest taste-modulating threshold concentrations were found for the leucyl peptide LDYY with an umami- and salt-enhancing threshold of 42 μmol/L. Addition of the 14 newly identified peptides to the taste recombinate (aRec_Dipeptides) increased the overall taste intensity and mouthfulness of the recombinate, and comparison with the authentic soy sauce confirmed the identification of all key tastants. Finally, these data as well as the quantitative profiling of several (non)-fermented foods highlight the importance of fermentation with respect to taste formation. On the basis of this knowledge, microorganisms with specific digestion patterns may be used to tailor the taste profile and especially the salt taste sensation of soy sauces.

Solid-phase microextraction Arrow for the volatile organic compounds in soy sauce

A novel solid-phase microextraction Arrow was used to separate volatile organic compounds from soy sauce, and the results were verified by using gas chromatography with mass spectrometry. Solid-phase microextraction Arrow was optimized in terms of three extraction conditions: type of fiber used (polydimethylsiloxane, polyacrylate, carbon wide range/polydimethylsiloxane, and divinylbenzene/polydimethylsiloxane), extraction temperature (40, 50, and 60°C), and extraction time (10, 30, and 60 min). The optimal solid-phase microextraction Arrow conditions were as follows: type of fiber = polyacrylate, extraction time = 60 min, and extraction temperature = 50°C. Under the optimized conditions, the solid-phase microextraction Arrow was compared with conventional solid-phase microextraction to determine extraction yields. The solid-phase microextraction Arrow yielded 6-42-fold higher levels than in solid-phase microextraction for all 21 volatile organic compounds detected in soy sauce due to the larger sorption phase volume. The findings of this study can provide practical guidelines for solid-phase microextraction Arrow applications in food matrixes by providing analytical methods for volatile organic compounds.

Simultaneous Analysis of 3-MCPD and 1,3-DCP in Asian Style Sauces Using QuEChERS Extraction and Gas Chromatography-Triple Quadrupole Mass Spectrometry

Acid hydrolyzed vegetable protein (aHVP) is used for flavoring a wide variety of foods and also in the production of nonfermented soy sauce. During the production of aHVP, chloropropanols including 3-monochloropropane-1,2-diol (3-MCPD) and 1,3 dichloropropane-2-ol (1,3-DCP) can be formed through the reaction of the hydrochloric acid catalyst and residual fat and the reaction of 3-MCPD with acetic acid, respectively. 3-MCPD is a carcinogen, and 1,3-DCP has been classified as a genotoxic carcinogen. The European Union (EU) has set a maximum concentration of 0.02 mg/kg of 3-MCPD in aHVP, and the Food and Drug Administration (FDA) set a guidance limit of 1 mg/kg of 3-MCPD in aHVP. 1,3-DCP is not an approved food additive, and the Joint FAO/WHO Expert Committee on Food Additives (JEFCA) has set a limit at 0.005 mg/kg, which is close to the estimated method detection limit. Currently there are few analytical methods for the simultaneous determination of 3-MCPD and 1,3-DCP without derivatization due to differences in their physical chemical properties and reactivity. A new method was developed using QuEChERS (quick, easy, cheap, effective, rugged, and safe) with direct analysis of the extract without derivatization using gas chromatography-triple quadrupole mass spectrometry (GC-QQQ). Additionally, a market sampling of 60 soy sauce samples was performed in 2015 to determine if concentrations have changed since the FDA limit was set in 2008. The sampling results were compared between the new QuEChERS method and a method using phenylboronic acid (PBA) as a derivatizing agent for 3-MCPD analysis. The concentrations of 3-MCPD detected in soy sauce samples collected in 2015 ( Collapse

Key Words

• 1,3-DCP
• 3-MCPD
• soy sauce

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MESH Headings

• Carcinogens/analysis
• Carcinogens/isolation & purification
• Chemical Fractionation/methods
• Flavoring Agents/analysis
• Food Contamination/analysis
• Gas Chromatography-Mass Spectrometry/methods
• Limit of Detection
• Propane/analogs & derivatives
• Propane/analysis
• Propane/isolation & purification
• alpha-Chlorohydrin/analysis
• alpha-Chlorohydrin/isolation & purification

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Grants

• FD999999 Intramural FDA HHS

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Accumulation of Citrulline by Microbial Arginine Metabolism during Alcoholic Fermentation of Soy Sauce

Citrulline, the major precursor of ethyl carbamate in soy sauce, is an intermediate catabolite of arginine produced by bacteria present in soy sauce moromi mash. Pediococcus acidilactici is responsible for the formation of citrulline during the lactic acid fermentation process of soy sauce. However, citrulline accumulation during the alcoholic fermentation process and the corresponding bacteria involved have not been identified. Salt-tolerant, arginine-utilizing bacteria were isolated from moromi mash during the alcoholic fermentation process. Under normal cultivation conditions, arginine utilization by these strains did not contribute to citrulline accumulation. However, the conversion of arginine to citrulline by these bacteria increased when cultivated during the alcoholic fermentation process. Additionally, the ethanol-enhanced solubility of free fatty acids in moromi mash stimulated the accumulation of citrulline. Staphylococcus exhibited the highest capability in the conversion of arginine to citrulline.

Determination of Key Volatile Compounds Related to Long-Term Fermentation of Soy Sauce

The changes of volatile compounds in soy sauce during long-term fermentation (12 months) were investigated using solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). A total of 144 and 129 compounds were identified in soy sauce with long-term fermentation by SPME and SBSE, respectively. The contents of most compounds, such as acids, aldehydes, benzene and benzene derivatives, esters, lactones, pyrazines, pyrones, and pyrroles, showed a tendency to increase, whereas those of alcohols and ketones decreased according to long-term fermentation. In addition, principal component analysis and partial least squares discriminant analysis were applied to discriminate soy sauce samples according to fermentation periods and determine key volatile compounds related to long-term fermentation. The initial fermentation stages were mainly associated with some alcohols, ketones, and lactones, whereas the later stages were strongly associated with most esters, some phenols, benzene and benzene derivatives, and pyrroles. Moreover, the key volatile compounds associated with long-term fermentation in soy sauce samples were ethyl 3-methylbutanoate (ethyl isovalerate), ethyl pentanoate (ethyl valerate), 1-octen-3-yl acetate, 3-(methylthio)-1-propanol (methionol), ethyl benzoate, ethyl 2-phenylacetate, 1-(1H-pyrrol-2-yl)ethanone (2-acetylpyrrole), and 5-pentyl-2-oxolanone (γ-nonalactone). PRACTICAL APPLICATION: This study investigated changes of volatile compounds in soy sauce during long-term fermentation (12 months) using solid-phase microextraction and stir bar sorptive extraction. In addition, the key volatile compounds associated with long-term fermentation in soy sauce samples were determined. These results may help to predict the effective contributors related to long-term fermentation of soy sauce and improve the quality of soy sauce during long-term fermentation.

Chromohalobacter is a Causing Agent for the Production of Organic Acids and Putrescine during Fermentation of Ganjang, a Korean Traditional Soy Sauce

Ganjang, a Korean traditional fermented soy sauce, is prepared by soaking doenjang-meju (fermented soybeans) in approximately 20% (w/v) solar salt solution. The metabolites and bacterial communities during ganjang fermentation were simultaneously investigated to gain a better understanding of the roles of the microbial population. The bacterial community analysis based on denaturing gradient gel electrophoresis of 16S rRNA gene sequences showed that initially, the genus Cobetia was predominant (0 to 10 d), followed by Bacillus (5 to 74 d), and eventually, Chromohalobacter became predominant until the end of the fermentation process (74 to 374 d). Metabolite analysis using (1)H-NMR showed that carbon compounds, such as fructose, galactose, glucose, and glycerol, probably released from doenjang-meju, increased rapidly during the early fermentation period (0 to 42 d). After removal of doenjang-meju from the ganjang solution (42 d), the initial carbon compounds remained nearly constant without the increase of fermentation products. At this point, Bacillus species, which probably originated from doenjang-meju, were predominant, suggesting that Bacillus is not mainly responsible for ganjang fermentation. Fermentation products including acetate, lactate, α-aminobutyrate, γ-aminobutyrate, and putrescine increased quickly with the rapid decrease of the initial carbon compounds, while Chromohalobacter, probably derived from the solar salts, was predominant. Multivariate redundancy analysis indicated that the Chromohalobacter population was closely correlated with the production of the organic acids and putrescine during the ganjang fermentation. These results may suggest that Chromohalobacter is a causing agent responsible for the production of organic acids and putrescine during ganjang fermentation and that the solar salts, not doenjang-meju, is an important microbial source for ganjang fermentation.

Improving Soy Sauce Aroma Using High Hydrostatic Pressure and the Preliminary Mechanism

Using high hydrostatic pressure (HHP) to treat liquid foods can improve their aroma; however, no information about the effects of HHP on soy sauce aroma has yet been reported. The effects of HHP on the aroma of soy sauce fermented for 30 d were investigated using quantitative descriptive analysis (QDA), SPME-GC-olfactometry/MS, hierarchical cluster analysis (HCA) and principal component analysis (PCA). Results showed that the pressure used during HHP treatment had a greater influence on soy sauce aroma than the duration of HHP. Compared to the control, soy sauce that was treated with HHP at 400 MPa for 30 min (HHP400-30) obtained the highest sensory score (33% higher) by increasing its sour (7%), malty (9%), floral (27%) and caramel-like (47%) aromas, while decreasing its alcoholic (6%), fruity (6%) and smoky (12%) aromas; moreover, the aroma of HHP400-30 soy sauce was comparable with that of soy sauce fermented for 180 d. Further investigation demonstrated that HHP (400 MPa/30 min) enhanced the OAVs of compounds with sour (19%), malty (37%), floral (37%), caramel-like (49%) and other aromas (118%), and lowered the OAVs of compounds with alcoholic (5%), fruity (12%) and smoky (17%) aromas. These results were consistent with the results of the QDA. HHP treatment positively regulated the Maillard, oxidation and hydrolysis reactions in raw soy sauce, which resulted in the improvement and accelerated formation of raw soy sauce aroma. HHP was capable of simultaneously improving raw soy sauce aroma while accelerating its aroma formation, and this could treatment become a new alternative process involved in the production of high-quality soy sauce.

Effect of salt-tolerant yeast of Candida versatilis and Zygosaccharomyces rouxii on the production of biogenic amines during soy sauce fermentation

BACKGROUND

This study aimed to enhance and improve the quality and safety of soy sauce. In the present work, the change of biogenic amines, such as histamine, tyramine, cadaverine, spermidine, was examined by the treatment of Candida versatilis and Zygosaccharomyces rouxii, and the influence of salt-tolerant yeast on biogenic amines was analysed during the whole fermentation process.

RESULTS

The results showed that the content of biogenic amines was elevated after yeast treatment and the content of biogenic amines was influenced by using yeast. The dominating biogenic amine in soy sauce was tyramine. At the end of fermentation, the concentrations of biogenic amines produced by Zygosaccharomyces rouxii and Candida versatilis in the soy mash were 122.71 mg kg(-1) and 69.96 mg kg(-1) .

CONCLUSIONS

The changes of biogenic amines in high-salt liquid soy mash during fermentation process indicated that a variety of biogenic amines were increased in the fermentation ageing period, which may be due to amino acid decarboxylation to form biogenic amines by yeast decarboxylase. The fermentation period of soy sauce should be longer than 5 months because biogenic amines began to decline after this time period.

Formation of Ethyl Carbamate during the Production Process of Cantonese Soy Sauce

The aim of this work was to clarify the formation of ethyl carbamate (EC) and its influence factors throughout the production process of Cantonese soy sauce. The results showed that EC was not detected in the koji-making and early moromi fermentation stages, but started to be generated when pH of the moromi decreased to about 4.9—at the same time, the levels of ethanol, urea and citrulline increased significantly. Most EC was formed during raw soy sauce hot extraction (40.6%) and sterilization (42.9%) stages. The EC content exhibited the highest correlation with ethanol throughout the whole production process (R = 0.97). The simulation soy sauce produced in laboratory led the same conclusion—moreover, the contents of EC, ethanol and citrulline were higher in soy sauce fermented at 30 °C than in soy sauce fermented at 15 °C. Extraction of raw soy sauce by squeezing contributed little to EC formation. Further research showed that citrulline and ethanol led to significant increases in EC levels in raw soy sauce upon heating. These results indicate that ethanol and citrulline are two critical precursors of EC and that EC is mainly formed during the heat treatment stage of soy sauce.

Degradation Mechanism of Soybean Protein B3 Subunit Catalyzed by Prolyl Endopeptidase from Aspergillus niger during Soy Sauce Fermentation

Soy sauce secondary precipitate formed due to the B₃ subunit seriously affects soy sauce's appearance quality. In this study, a prolyl endopeptidase (APE) from Aspergillus niger, which could degrade approximately 50% of the B₃ subunit and increase proline content by 24% in soy sauce, was isolated and identified. The results showed that APE was an acidic salt-tolerant serine protease (62 kDa), which was optimally active at 40 °C and pH 4.0, and retained more than 69% activity in 3 M NaCl solution over 10 days. As a potential substrate of APE, the B₃ subunit contains 10 proline residues. High salinity could not damage the hydrogen bonds, salt bridges, and interior hydrophobicity of APE; thus, the spatial structures and activity of APE in 3 M NaCl solution were stable within 3 days and decreased thereafter. High salinity made the B₃ subunit more rigid and lowered the catalytic activity of APE on the B₃ subunit, hindering complete hydrolysis of the B₃ subunit. This was the first report about the APE capable of degrading the B₃ subunit and reducing the secondary precipitate of soy sauce, providing a new possibility to solve the secondary precipitate of soy sauce.

Simple and sensitive analysis of histamine and tyramine in Japanese soy sauces and their intermediates using the stable isotope dilution HILIC-MS/MS method

We established a simple, sensitive, and reproducible method to analyze the histamine and tyramine levels in Japanese soy sauce and its mash (called moromi) using hydrophilic interaction liquid chromatography with tandem mass spectrometry (HILIC-MS/MS). Histamine and tyramine quantification was performed using their stable isotopes for electrospray ionization-tandem mass spectrometry in the selected reaction monitoring mode. The sample pretreatment process was a simple, one-step liquid-liquid extraction. HILIC separation was performed with a gradient elution of aqueous ammonium formate and acetonitrile. Because of validation tests, the linearity, the accuracies, and precisions were sufficient. The limit of detection and the limit of quantification were 0.09 and 0.29 ppm for histamine and 0.13 and 0.42 ppm for tyramine, respectively. We successfully applied this method to histamine and tyramine determination in four kinds of commercial Japanese soy sauces and also in moromi samples during soy sauce production.

Autolysis of Aspergillus oryzae Mycelium and Effect on Volatile Flavor Compounds of Soy Sauce

The autolyzed mycelia of Aspergillus oryzae are rich in proteins, nucleic acids, sugar, and other biomacromolecules, and are one of the main contributors to the flavor profile of commercially important fermented goods, including soy sauce and miso. We induced autolysis of the mycelia of A. oryzae over 1 to 10 d, and found that the maximum dissolved amounts of total protein and nucleic acid ratio accounted for 28.63% and 88.93%, respectively. The organic acid content, such as citric acid, tartaric acid, succinic acid, lactic acid, and acetic acid, initially increased and then decreased as autolysis progressed, corresponding to changes in pH levels. The main characteristic flavor compounds in soy sauce, namely, ethanol, 2-phenylethanol, and 2-methoxy-4-vinylphenol, were all detected in the autolysate. Subsequently, we tested the effect of adding mycelia of A. oryzae during the fermentation process of soy sauce for 60 d, and found that addition of 1.2‰ A. oryzae mycelia provided the richest flavor. Overall, our findings suggest that compounds found in the autolysate of A. oryzae may promote the flavor compounds of soy sauce, such as alcohols, aldehydes, phenols, and esters.

Simultaneous Quantitation of Advanced Glycation End Products in Soy Sauce and Beer by Liquid Chromatography-Tandem Mass Spectrometry without Ion-Pair Reagents and Derivatization

The aim of this study was to develop a simple and sensitive method to analyze several advanced glycation end products (AGEs) simultaneously using liquid chromatography-tandem mass spectrometry (LC-MS/MS), and to apply this method to the quantitation of AGEs in brown-colored foods. The developed method enabled to separate and quantitate simultaneously seven AGEs, and was applied to the determination of free AGEs contained in various kinds of soy sauce and beer. The major AGEs in soy sauce and beer were N^ε-carboxymethyllysine (CML), N^ε-carboxyethyllysine (CEL), and N^δ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine (MG-H1). Using the developed LC-MS/MS method, recovery test on soy sauce and beer samples showed the recovery values of 85.3-103.9% for CML, 95.9-107.4% for CEL, and 69.5-123.2% for MG-H1. In particular, it is the first report that free CML, CEL, and MG-H1 were present in beer. Furthermore, long-term storage and heating process of soy sauce increased CML and MG-H1.

Analysis of ethanol in soy sauce using electronic nose for halal food certification

The purpose of this study was to analyze ethanol content in soy sauce using mass spectrometry (MS) with electronic nose (e-nose) to determine if MS e-nose can replace gas chromatographic analysis for halal certification. Gas chromatography-flame ionization detector (GC-FID), the standard method of ethanol content, was used to analyze 24 different kinds of soy sauce. Ethanol was detected from 13 soy sauce samples in the concentration range of 0.0004-1.7wt%. The MS e-nose data were analyzed by discriminant function analysis (DFA). Based on an addition method, the results were more than 96.6% accurate when the ethanol concentrations were greater than 0.5%. A high correlation between the first score of the DFA plot and the ethanol concentration was observed. Thus, mass spectrometry based on e-nose is an efficient method for determining ethanol as a primary screening tool for halal certification.

Formation by yeast of 2-furanmethanethiol and ethyl 2-mercaptopropionate aroma compounds in Japanese soy sauce

Two aroma compounds of volatile thiols, 2-furanmethanethiol (2FM) and ethyl 2-mercaptopropionate (ET2MP), were formed in five types of Japanese soy sauce during fermentation by yeast. The concentrations of 2FM and ET2MP in the soy sauce samples increased during alcoholic fermentation. The concentrations of 2FM and ET2MP were higher in the soy sauce fermented by Zygosaccharomyces rouxii than in that fermented by Candida versatilis. The enantiomers of ET2MP were separated by gas chromatography in a capillary column. The average enantiomeric ratio of ET2MP in the soy sauce was approximately 1:1. 2FM was formed by yeast in a medium prepared from cysteine and furfural, and cysteine is considered the key precursor of 2FM by yeast in soy sauce.