# Distinctive Formation of Volatile Compounds in
## 2 Results and Discussion
### Volatile Compounds Based on Metabolic Pathways in Fermented Rice According to Each Microbe
They come from the degradation of specific amino acids, such as sulfur-containing (methionine and cysteine), branched-chain (leucine, valine, and isoleucine), and aromatic ones (phenylalanine, tyrosine, and tryptophan). In this study, branched-chain volatiles, except for 3-methylbutan-1-ol, were detected only in the fermented samples. In particular, levels of 2-methylbutan, 3-methylbutanal, 3-methylbutanal-ol, and 3-methylbutanoic acid were greatly increased in AOR, while much lower levels of these compounds were produced by LPR and LFR compared to other samples. This result might be due to the absence of certain enzymes, such as transaminase, decarboxylylase, and hydrogenase, or weaker activities of these enzymes in lactic acid bacteria than what is found in other microbes. With regard to products of benzenes, the fold change of benzaldehyde was decreased or not found in all samples studied. It could be assumed that benzaldehyde can be converted to other benzenes, such as styrene, 2-phenylethanol, ethyl 2-phenylacetate, 1-phenylethanone, toluene, and ethylbenzene. In particular, 2-phenylethanol with a rose-like odor is commonly found in alcoholic beverages, such as makegeolli and sake [6; 23]. 2-Phenylethanol can be converted to 2-phenylacetaldehyde and ethyl 2-phenylacetate by phenylacetaldehyde reductase and alcohol acetyltransferase, respectively [24]. In this study, the levels of 2-phenylacetaldehyde were greater in AOR than those of benzoic compounds, while that of 2-phenylethanol was higher in ROR, SCR and SFR. Regarding other benzoic volatile compounds, 4-ethenyl-2-methoxyphenol, 1-phenylethanone and styrene can be converted from cinnamic acid which is derived from phenylalanine degradation [25]. A compound called 4-ethenyl-2-methoxyphenol, which is known as the main volatile compound of rice and has a smoky odor, was the only phenolic volatile compound found in this study [26]. The formation of 4-ethenyl-2-methoxyphenol is characterized by a two-step process. Firstly, cinnmic acid is converted to hydroxycinnamic acid by esterase. Then, cinnamate decarboxylase catalyzes its transformation to 4-ethenyl-2-methoxyphenol [27]. In ROS, the formation of these benzoic volatiles was more activated compared to other samples, while that of styrene was decreased and 1-phenylethanone was not detected in AOR. The only sulfur-containing volatile compound in this study, (methyldisulfanyl)methane, could be produced from methionine. Methionine is degraded to methanethiol by methionine-\(\gamma\)-lyase, and then oxidized to (methyldisulfanyl)methane [22]. This compound, which had been reported as an off-odorant in yakju (Korean rice liquor) [28], was detected only in AOR, SFR, and LFR in this study. During fermentation, lipids can be hydrolyzed to glycerol and fatty acids by lipase. The fatty acid metabolic pathway includes both fatty acids and their derivatives. There are various volatile compounds derived from fatty acids, including furans, lactones, and volatile compounds derived from fatty acids (aldehydes, acids, ketones, and esters). \(\beta\)-Oxidation, which is a main fatty acid breakdown pathway, converts fatty acids into two carbon segments (acetyl-CoA), then, further enzymatic activities, such as decarboxylation, oxidation, reduction, and so on, catalyze the formation of aldehydes, alcohols, ketones, acids, and esters [29]. Esters derived from fatty acids, such as ethyl propanoate, ethyl butanoate, ethyl tetradcanoate, and ethyl hexadecanoate, were only detected in AOR and ROR. These volatile compounds can be formed by esterification, combining fatty acids and ethanol, which is catalyzed by esterase [29]. A marked content of ethanol was observed in AOR, ROR, and SCR, while fatty acid esters were not found in SCR. It might be assumed that the different activity of esterase of the individual microbes could affect the formation of fatty acid esters. In addition, higher numbers of ketones were detected in SFR than in other samples in this study.