## 3 Results and Discussion
### Aroma Volatile Compounds of Soy Sauces  
Volatile compounds in soy sauces were analyzed by SPME-GC-MS. According to their chemical structure, 39 volatile compounds were detected in different samples including 10 acids, 8 alcohols, 6 aldehydes, 5 esters, 5 ketones, and 2 pyrazines, furan, phenol, and sulfur-containing compounds. Their relative concentration is shown in Figure 3.  
Among the volatile components, the ethanol content was the highest in all product samples: 3.99 \(\pm\) 0.27 \({}^{\text{a}}\) g/kg in KS_C, 6.94 \(\pm\)0.57 \({}^{\text{c}}\) in KS_O, 4.61 \(\pm\) 0.17 \({}^{\text{b}}\) for organic tamari, and 7.21 \(\pm\) 1.90 \({}^{\text{c}}\) for standard tamari sauce. The ethanol content was comparable to the results of koikuchi sauces [61] determined by the same method with an internal standard, where it amounted to 4.08 \(\upmu\)g/kg. The presence of ethanol is a characteristic quality marker for natural koikuchi soy sauces as the effect of alcoholic fermentation. A larger share of wheat results in a larger pool of carbohydrates, which are hydrolyzed into saccharides by the enzymatic activity of active microflora and then fermented by yeast into alcohol. Despite tamari sauce being produced with a low proportion of wheat flour, and thus expected to contain less ethanol content, the analyzed samples were characterized with a higher content of ethanol compared to koikuchi. The relative contents of these volatile compounds in different treatments are analyzed in the heat map in Figure 4; however, ethanol was excluded due to its high concentration. In general, the presence of alcoholic compounds in soy sauces (the effect of yeast and lactic acid bacteria fermentation) results in alcoholic and malty notes. However, 1-octen-3-ol gives a mushroom-like fragrance [62]. The malty effect depends also on the presence of short-chain, branched aldehydes, like 2-methylbutanal and 3-methylbutanal. The relative content of 3-methylbutanal in all soy sauces was low (0.973 \(\div\) 6.01 mg/kg; Figure 4). Another volatile compound with a large relative content (Figure 4, red color) was acetic acid (up to 774 mg/kg), which represents a sour taste. It is worth mentioning that producers sometimes add acetic acid for preserving, and then label the proper information. The content of 3-methyl-1-butanol (malty, rancid odor attributes) differs between standard and organic sauces, and significantly so for the koikuchi style [26]. While the general profile of these sauces was similar, tamari sauces differed depending on the origin of the ingredients. For the fruity note, the presence of the ester is necessary, which can additionally mask the unpleasant odors from the metabolism of sulfur compounds (Met) [63]. The presence of ethyl carbamate in all soy sauce samples was observed (4.47 \(\div\) 7.32 mg/kg). This genotoxic and carcinogenic compound can accumulate in soy sauces during thermal processes [64]. Technologies are being developed to reduce the concentration of ethyl carbamate via optimization of the moromi fermentation step [39], charcoal filtration [65], and the use of low precursor-secreting or high precursor decomposition bacteria [66; 67; 68]. The presence of pyrazines may indicate the roasted (2-ethyl-6-methylpyrazine) and burnt (2,3,5-trimethylpyrazine) [61] aromas, and their concentrations were 9.82-17.92 and 3.01 \(\div\) 11.84 mg/kg, respectively. Among volatile compounds, [69] suggested that the most important aroma compounds in koikuchi shoyu are 3-methyl butanal, sotolome, 4-HEMF, 2-methyl butanal, methional, ethanol, and ethyl 2-methyl propanoate. Another study [2], which examined the key aroma compounds in different types of Japanese soy sauces, revealed 25 key aroma compounds, in which methional and sotolone appear in all types. In our studies, we did not compare the presence of sotolome or furano(one)s carrying the caramel-like fragrance.