Volatile Flavor Compounds and Flavor Profiles of Thai Soy Sauce

Chapter 23

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Volatile Flavor Compounds and Flavor Profiles of Thai Soy Sauce Sittiwat Lertsiri,*,1 Pitipong Wanakhachornkrai,1 Apinya Assavanig,1 Siree Chaiseri,2 and Thongchai Suwonsichon3 1Department

of Biotechnology, Faculty of Science, Mahidol University, Bangkok, Thailand 2Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok, Thailand 3Department of Product Development, Faculty of Agro-Industry, Kasetsart University, Bangkok, Thailand *[email protected]

Soy sauce is fermented soybean condiment originally invented in China from the ancient time. Since this product has been spread widely in Asia, each region has developed its own recipe and fermenting process, leading to varieties of product flavor profiles. In this work, the volatile flavor compounds and their relationship with flavor profiles of Thai soy sauce were studied. As a result, 60 odor-active compounds have been characterized by gas chromatography-mass spectrometry (GC-MS) and GC-olfactometry. The odor activities of those compounds were further investigated by approaches of Aroma extract dilution analysis, Dynamic headspace dilution analysis, and Calculation of odor activity value. We concluded that 2-methyl propanal, 1-propanol, 3-methyl-1-butanol, 2,6-dimethyl pyrazine, mathional, 3-methyl butanoic acid, 2-methoxyphenol, 3-hydroxy-2-methyl-4H-pyran-4-one, and 4-hydroxy-2,5-dimethyl-3(2H)-furanone were key aroma compounds of Thai soy sauce. Apart from chemical analytic work, modified quantitative descriptive analysis of 18 Thai soy sauces was conducted to understand their flavor profiles.

© 2010 American Chemical Society Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Soy sauce is a fermented soybean condiment consumed worldwide. This kind of products has been originated in China as first recorded 2700 years ago. The production of soy sauce has been transferred to Asian countries, especially; Japan has long history for soy sauce production and development, for more than 1300 years (1). Soy sauce is made from main raw materials of soybeans, wheat, and brine; and classified into Chinese-type and Japanese-type based on the amount of wheat used (2). In Japanese-type soy sauce, soybeans and wheat are used at equal amount while less wheat or cereal is used in Chinese-type. Thai soy sauce is produced as the Chinese type, in small to medium-scale manufacturers, with production know-how passed from generation to generation. The mixture of cooked soybean and wheat or rice flour (7:1 ratio) is inoculated with koji starter (Aspergillus oryzae). After fermentation (40-48 h), the resulting koji is then immersed in a brine solution with the ratio of koji to brine of 1:3 (w/v). This gives moromi with 18-22% salt concentration. After the moromi fermentation is completed by lactic acid bacteria (Tetragenococcus halophilus) and halotolerant yeast (Zygosaccharomyces rouxii and Candida species), soy sauce is separated, filtered, and pasteurized before bottling (3, 4). The characteristic flavor-aroma formation in the soy sauce depends on the manner of production, raw materials and strains of microorganism used (2). Since this kind of products has been widely consumed in Asia, each region has developed its own recipe and fermenting process, hence varieties of soy sauce flavor characteristics from various origins are evident. Up to now, investigations on volatile constituents of soy sauce in various countries, i.e. Japan (5, 6), Korea (7), Indonesia (8), and China (9) have been reported. Previously, we have indentified 93 volatile compounds in Thai soy sauce by GC-MS with various sample preparation techniques, i.e. dynamic headspace sampling, direct solvent extraction, and vacuum simultaneous steam distillation-solvent extraction, in order to cover wide range of their volatility (10). In this work, we further determined odor activities of volatile compounds being present in Thai soy sauce based on their concentration and gas chromatography-olfactometry methodology. The results from instrumental analysis and sensory descriptive analysis revealed impact of the odor-active compounds and their contribution to flavor profile. The knowledge obtained would be applicable for manufacturing optimization and quality standardization of Thai soy sauce fermentation.

Materials and Methods Materials Thai soy sauces were purchased from local markets in Thailand. Samples were stored in the dark at room temperature. Organic solvents were purchased from Merck (Darmstadt, Germany) and redistilled prior to use. Internal standard and authentic reference compounds were obtained from Aldrich Chemical (St. Louis, MO). Other chemicals were of the best grade available supplied from Merck (Darmstadt, Germany). 376 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Aroma Extract In order to extract wide range of volatile compounds, soy sauce sample (25 g) was saturated with sodium chloride and adjusted to pH 3.0 with hydrochloric acid. Then, the sample was extracted with dichloromethane (3 times x 20 mL). After solvent layer was recovered, the sample was re-adjusted to pH 12.0 with sodium hydroxide solution and was extracted as described above. The solvent layer pool was subjected to vacuum distillation with liquid nitrogen cold trap for 4 h (10). Distillate obtained was frozen overnight to remove water. After concentrating with gentle nitrogen stream to 5 mL, the distillate was dried over anhydrous sodium sulfate and further concentrated to 50 µL prior to gas chromatography-mass spectrometry (GC-MS) and aroma extract dilution analysis (AEDA). Dynamic Headspace (DHS) Analysis Soy sauce sample with internal standard of 2-methyl-3-heptanone at the final concentration of 152.6 ng/g was saturated with sodium chloride prior to DHS and dynamic headspace dilution analysis (DHDA). A sampling tube (15.2 x 1.6 cm i.d.) contained 5 ml of the sample was connected to a Tekmar Dohrmann 3100 purge and trap concentrator (Tekmar, Cincinnati, OH). The sample was preheated for 2 min at 40 °C, and was purged with ultra high-purity helium (50 mL/min) for 20 min to a Tenax TA trap. The trap was then dry-purged for 7 min prior to desorption of volatiles at 220 °C for 2 min. The volatiles desorbed were directly transferred onto gas chromatography-mass spectrometry (GC-MS) via electric pressure controlvolatiles interface with split ratio of 10:1 (10). Gas Chromatography-Olfactometry (GC-O) The system, Agilent 6890 GC (Agilent, USA) with HP-FFAP column (25 m x 0.32 mm i.d. x 0.50 μm film thickness) and a flame ionization detector (FID), was equipped with a sniffing port (ODO II, SGE Incorporation, USA). Effluent from a GC column was split at 1:5 ratio to the FID and sniffing port. Oven temperature was hold at 40 °C for 4 min and rose at 10 °C/min to 200 °C and maintained for 20 min. The carrier gas was ultra high-purity helium at a constant flow of 1.5 mL/min. Sniffing was performed by two panelists. Aromagram indicated odor description, retention time, and odor intensity (i.e. very strong, strong, medium, weak and very weak). Two GC-O experiments, i.e. AEDA (11, 12) and DHDA (13, 14) were performed. For AEDA, the aroma extract were stepwise diluted 1:5 in dichloromethane to obtain dilutions of 5, 25, and 125 folds of the initial aroma extract. Flavor dilution factor (FD value) was defined as the highest dilution at which odorants was detected and represented as log5 FD. DHDA was conducted by combining dynamic headspace acquired GC-O with stepwise reducing purge volume of DHS, i.e. 800 mL (default value), 200 mL, 50 mL, and 12.5 mL as dilution steps. Odor potency of the odorant was determined as minimal purge volume of DHS. 377 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Gas Chromatography-Mass Spectrometry (GC-MS) GC-MS analysis was performed using the Agilent 6890 Plus GC/HP 5973 MSD, equipped with an HP-FFAP column as described above. One microlitre of aroma extract was injected at 230 °C with splitless mode. The GC oven temperature was programmed from 45 °C to 220 °C for HP-FFAP at the rate of 15 °C/min. The initial and final hold times were 2 and 11.40 min, respectively. For DHS-GC-MS, the GC oven temperature was held at 45 °C for 5 min, then programmed from 45 to 180 °C at 10 °C /min, following with 180 °C to 240 °C at 20 °C/min. DHS analysis of each sample was performed in duplicate. The carrier gas was ultra high- purity helium at a constant flow of 1.5 mL/min. Capillary direct-interface temperature was 280 °C. Ionization energy was 70 eV. Mass range was 35 to 450 a.m.u. (3.50 scan/s) for direct injection of the aroma extract or 20 to 350 a.m.u. (4.33 scan/s) for DHS analysis (10). Compounds Identification Positive identification of a component was performed by comparison of its retention index (RI), mass spectrum, and odor quality with an authentic reference compound or data available. Tentatively identified compounds were uniquely identified on the basis of the mass spectra from the Wiley 275.L mass spectral database (Hewlett-Packard Co.). Quantification of Volatile Compounds The standard addition method was used for quantification. Calibration curves were constructed by spiking 50 µL of reference solutions to make final concentration of 625-5000 ng/g and 10 µL of 2-methyl-3-heptanone solution as the internal standard to make final concentration of 152.6 ng/g in 50 g of soy sauce samples. After saturating with sodium chloride, samples were subjected to DHS analysis as mentioned above. Compounds were quantified on the basis of selected ions from MS data recorded in a scan mode (15). The standard curves were obtained with good linearity (r2 > 0.99). The concentration of each volatile compound was then applied to calculate odor activity values (11). Sensory Descriptive Analysis for Flavor Profile of Thai Soy Sauces Flavor profiles of 18 samples of Thai soy sauce were evaluated using modified quantitative descriptive analysis method (16, 17). The analysis embodied a screening criterion, development and defining sensory attributes, training, monitoring and testing the panellists’ accuracy, and quantitative scoring of sample characteristics. Panellists were recruited among members of the Department of Product Development, Faculty of Agro-Industry, at Kasetsart University and trained for 3 months. Soy sauce attributes were developed based on agreement among the panellists. Fourteen sensory attributes derived from consensus were 6 aroma attributes, 4 flavor attributes and 4 taste attributes. The definitions of these attributes were described in Table I. A scoring card was developed with a 15-cm 378 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

line scale marked with directional terms or anchor words (none to extreme) from each end. The samples were also determined their volatile flavor compounds by DHS-GC-MS as mentioned above.

Table I. Definition of Attributes of Sensory Descriptive Analysis Descriptives


Attributes Aroma Sweet

Sweet aroma of palm sugar

Molasses-like

Molasses odor

Alcohol

Pungent odor of alcohol

Soy bean paste-like

Odor of soy bean paste

Sour

Pungent odor of vinegar

Fishy

Fish sauce-like odor

Flavor Sweet

Sweet flavor of cooked soy bean

Sour sauce-like

Red vinegar flavor

Soy bean paste-like

Soy bean paste flavor

Fishy

Fish sauce-like flavor

Taste Sweet

Sweetness of sugar

Salty

Salty taste

Sour

Sour taste of vinegar

Bitterness

Bitter taste

After taste Salty

Left over of salty taste

379 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Table II. Odor-active compounds and their odor activities in aroma extract of three soy sauce samples (A, B, and C)


381


Table III. Odor-active compounds and their odor activities based on DHDA and OAV in two soy sauce samples (A and B)

Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Statistical Analysis Cluster analysis was applied to categorize soy sauce samples based on sensory evaluation. Principal component analysis (PCA) was applied to determine correlation of sensory attributes and peak area of volatile compounds obtained from DHS-GC-MS, based on Pearson correlation matrix with a level of significance at 95%.

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GC-O Experiments and Odor Activity Values We totally found 60 odor-active compounds on aromagrams of aroma extract and DHS obtained from all samples analyzed. From first screening, 29 odor-active compounds were detected in all aroma extract of three samples (Table II). Among these, methional and 2-methyl propanal were not identified in our previous work (10). This was due to the peaks area of these compounds was below target minimal peak area for detection above 105 and 104 counts for direct solvent extraction and DHS, respectively. However, in this work, GC-O revealed occurrence of methional and 2-methyl propanal as important and common odor-active compounds found in all samples tested. By AEDA, the most potent odorants which gave the highest log5 FD factor were methional, 3-methyl butanoic acid, 3,4-dimethyl cyclopentenolone, 2-methoxyphenol, 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF), and 4-vinyl-2-methoxyphenol (Table II). These compounds contributed to brothy, caramel-like, sour or sweaty, and clove-like or spicy notes. These attributes described aroma profile of the aroma extract when it was sniffed directly. By the nature of aroma extract preparation, direct solvent extraction might cause low recovery of highly volatile compounds since they could be lost during sample preparation (10, 18). With this reason, some compounds, i.e. dimethyl sulfide, ethyl propionate, and propanol could only be detected on DHS-GC-O (Table III). AEDA is effective for screening of odor-active compounds; however this technique seemed to disregard matrix effects on volatile release (6). In reality, the odor is perceived when the odor-active compound is released from the food matrix and available for olfactory receptors. Since DHS exploits volatility difference of volatiles and non-volatile sample matrices (10), only the volatiles released and partitioned in a headspace are carried to GC. With this approach, DHS-GC-O enabled us to investigate release of volatile flavor compounds in the present study. However, it is important to remark that DHS is less sensitive to low or semi-volatile compounds, e.g. hydroxyl furanone and hydroxyl pyrone (10). To investigate the impact of individual volatile flavor compounds on the overall aroma profile, DHDA and odor activity value concept were employed. Odor activity value (OAV) was calculated based on ratio of concentration to odor threshold in water since water is the main matrix of soy sauce (6, 11). However, this had to be remarked that other matrices of soy sauce, i.e. amino acids, peptides, carbohydrate, and brown pigment, as well as environmental conditions (pH and ionic strength) also played roles in flavor release. 382 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


From DHS-GC-MS, 27 volatile compounds were detected, whereas 18 of them were odor-active based on DHS-GC-O (Table III). From DHDA, 14 compounds were common to both soy sauce samples tested. 2-Methyl propanal, 1-propanol, 3-methyl-1-butanol, and methional were the most potent odor-active since they were perceived at the lowest minimal purge volume (Table III). According to the odor activity value concept, odorants contribute to the overall aroma if their concentrations exceed their odor threshold in a given matrix or OAV reaches the value of “1” (19). Considering OAVs, minimal purged volume from DHDA, and FD factor from AEDA of each odor-active compound, there were four occurrences of consensus and disagreement among them. Firstly, low volatile compounds, i.e. phenol and 4-ethyl phenol, had quite high FD value but were not odor-active in these two samples since their OAVs was less than 0.01. Secondly, 9 odorants perceived on DHDA agreed with OAVs which was higher than “1”. Thirdly, some compounds, however had OAV less than “1”, i.e. 1-propanol, 2,3-pentanedione, 1,6-dimethyl pyrazine, they were odor-active even at the lowest minimal purge volume. Fourthly, some compounds, such as 2-methyl propionic acid, butanoic acid, and methionol had high FD values and high OAVs (1.8-382.5), on the other hand, they were not detected on DHDA. Moreover, different matrix effects of sample A and B were also observed in case of 2,3-butanedione and 3-hydroxy-2-butanone. Based on DHDA, these compounds had the OAVs in the range of 0.8-13.9 in both samples, but they were odor-active only in sample B. Moreover, some compounds, such as 2-methyl propanal, 2-methyl butanal, and 3-methyl butanal, had low minimal purge volume with high OAV (47.2-2424). They were perceived by the panelists at weak to medium intensity even at high purge volume. This was possibly due to the saturation of the odor-active compound to olfactory receptors. It has been discussed that OAVs should be used primarily to figure the impact of odor-active compounds. However, in some cases, OAV may not be appropriate for ranking of significance of those odor-active compounds in the matrix given (20). From the results mentioned above, all odor-active compounds detected were classified into 5 groups based on their odor attributes; 1) malty, brothy, meaty, sulfury, 2) sweet, flowery, 3) caramel-like, vanilla-like, 4) sour, cheesy, sweaty, 5) clove-like, spicy, woody, 6) burnt, pyrazine-like, cooked rice-like, peanut-like. We concluded that 2-methyl propanal, 1-propanol, 3-methyl-1-butanol, 2,6-dimethyl pyrazine, mathional, 3-methyl butanoic acid, 2-methoxyphenol, 3-hydroxy-2-methyl-4H-pyran-4-one, and HDMF were important key aroma compounds of Thai soy sauce. Among these compounds, mathional, 3-methyl butanoic acid, and HDMF were also reported as key aroma compounds in Japanese soy sauce based on OAV concept (6). Moreover, 3-methyl-1-butanol, 2,6-dimethyl pyrazine, mathional, 2-methoxyphenol, and HDMF were mentioned to be common volatiles found in Chinese soy sauce (9). Sensory Descriptive Analysis Totally, 18 samples of Thai soy sauce were subjected to descriptive analysis and DHS-GC-MS. Firstly, we applied cluster analysis to cluster soy sauce samples tested based on their similarity of sensory attributes. There were 5 main groups of 383 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


samples. In addition, 15 sensory attributes of all samples were then explored by principal component analysis (PCA) aiming to extract the characteristic attributes of each group of Thai soy sauce. As a result, two principal components (PC) could be extracted from the original variables and could explain 57.16 % of the total variance (Figure 1). The sensory attributes contributed to the 1st PC were molasses-like aroma, alcohol aroma, fishy aroma, fishy flavor, and salty taste. This PC explained the variance of all variables up to 32.79 %. Sweet, sour, and bitter tastes were attributed to the 2nd PC which explained the variance at 24.37%. From graphical presentation (Figure 1), each group could be described for its character with the following attributes; group 1: sweet taste; group 2: fishy flavor, salty; group 3: sweet flavor; group 4: fishy aroma, group 5: soybean paste-like aroma and soybean paste-like flavor.

Figure 1. PCA biplot presenting relationships among soy sauce samples and sensory descriptive attributes obtained from 18 soy sauce samples. Sensory attributes with bold letter contributed to the 1st PC, while sensory attributes with italic letter contributed to the 2nd PC. The sensory attributes and samples displayed in the circles were correspondent to cluster analysis. A_aroma; F_flavor; and T_taste. 384 Cadwallader and Chang; Chemistry, Texture, and Flavor of Soy ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


In addition, correlations among sensory attributes and DHS volatile compounds were also observed (Table IV). As expected, sweet aroma attribute correlated with ethyl propionate which exhibited flowery odor. Sweet flavor attribute also correlated with the compound giving flowery, malty, and sweet odors. Sour aroma and sour sauce-like flavor were ascribed by acids. Molasses-like aroma was a complicated attribute showing correlation to varieties of compounds. Alcohol aroma had high correlation coefficient with molasses-like aroma (r = 0.74). Therefore volatile compounds correlated with this attribute were resembled to molasses-like aroma. Surprisingly, methionol which was not odor-active based on DHDA exhibited correlations with various attributes both for aroma and flavor. On the other hand, methional, 2,6-dimethyl pyrazine, and 2-methoxyphenol which were characterized as most potent odor-active compound in both AEDA and DHDA showed correlation to none of aroma and flavor attributes. We explained this occurrence as all samples contained these compounds at the level of substantial odor activity, so that they became saturated to panellists’ perception.

Table IV. Pearson correlation coefficients among sensory attributes and DHS volatile compoundsa Aroma

Attributes Compound

Sweet

Molasses

Alcohol

Sour

Fishy

0.29

0.38

2-Methyl propanal

0.47

0.33

2-Methyl butanal

0.48

0.36

0.30

Ethyl propionate

0.29

0.32

0.36 0.48

0.35

0.33

0.33

1-Propanol 2,3Pentanedione

a

Sour Soy sauce bean paste

0.27

-0.28

0.42

Sweet

Dimethyl sulfide

3-Methyl butanal

0.42

Flavor

0.37

2 and 3-Methyl1-butanol

0.34

0.28

Acetic acid

0.43

0.36

0.46

2 and 3-Methyl butanoic acid

0.33

0.39

0.53

0.31

Methionol

0.38

0.40

0.39

0.30

0.40

Significance level at 95%.


0.36

The present study demonstrated the relationships among volatile compounds, volatile flavor compounds, and flavor profiles of Thai soy sauce through GC-O experiments, odor activity, and sensory analysis. The roles of key aroma compounds were also explained by those relationships.

Acknowledgments


This research project was granted by National Center for Genetic Engineering and Biotechnology (BIOTEC), Thailand (Grant No. BT-B-06-FG-14-4401).

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Volatile Flavor Compounds and Flavor Profiles of Thai Soy Sauce

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