## 2 Results and Discussion
### Volatile Compounds in Fermented Rice Samples
It could be produced at high levels during fermentation, possibly due to the large amount of linoleic acid and the high lipoxygenase activity in grains [30]. In addition, furan derivatives could have been generated by thermal degradation during sample preparation in the present study. In particular, sample RTCL16 contained the largest amounts of furans, whereas the amounts of furans did not differ significantly between the other samples. The amounts of aldehydes such as 3-methylbutanal, hexanal, and nonanal were larger than those of heterofermentative LAB. [PERSON] et al. compared the formation of volatile compounds by LAB during the fermentation of rye sourdough with an uninoculated rye sourdough control sample [31]. They reported that aldehydes were more strongly represented in homofermentative than heterofermentative LAB, probably due to the higher alcohol dehydrogenase activity and/or the higher surplus of reductive power (NADH) during the growth of heterofermentative LAB. Hexanal, which contributes a fatty-green odor note, can be readily recognized by its low odor threshold (5 ng/g) in rice [32]. Hexanal mainly derives from the oxygenation of linoleic acid via the sequential actions of lipoxygenase and hydroperoxide lyase [33]. Hexanal is related to further reactions such as the oxidation to hexanoic acid by aldehyde dehydrogenase and the reduction to 1-hexanol by alcohol dehydrogenase during fermentation [34]. Hexan-1-ol was much more abundant than hexanal and hexanoic acid in all of the samples in the present study. The combined amount of hexanal, 1-hexanol, and hexanoic acid was highest in sample RTCL16, followed by sample RTCL3. Nonanal, which can be generated by the decomposition of 9-hydroperoxy-octadecadienoic acid derived from the oxidation of oleic acid [2], was detected only in samples RTCL79 and RTCL31. The amount of its corresponding alcohol, nonan-1-ol, was larger than that of nonanal, as was the case for hexanal and hexan-1-ol. [PERSON] and [PERSON] (2002) found that the concentrations of (_E_)-2-nonanalwere significantly decreased during sourdough fermentation [35]. The activity of short-chain alcohol dehydrogenase of LAB contributes to the reduction of these flavor compounds [35]. 2-Methylbutanal and 3-methylbutanal can be formed enzymatically or nonenzymatically by the Strecker degradation of isoleucine and leucine, respectively. Only two samples in this study (RTCL3 and RTCL16) produced both 2-methylbutanal and 3-methylbutanal. RTCL31 contained only 2-methylbutanal, whereas 3-methylbutanal was only detected in RTJL3 and LPC. leucine produces 3-methylbutanal by Strecker degradation, and this is subsequently reduced to 3-methyl-1-butanol by alcohol dehydrogenase. 3-Methyl-1-butanol was detected in all of the samples, whereas 3-methylbutanal was found only in three samples--RTJL3, LPC, and RTCL3. The ketones detected in fermented rice were methyl ketones including propan-2-one, butan-2-one, 6-methylhept-5-en-2-one, and nonan-2-one. Methyl ketones, which have a characteristic aroma note of ripened cheeses, are formed from fatty acids by enzymatic oxidative decarboxylation (\(\beta\)-oxidation) [2]. Ketones were not detected in sample RTJL4 fermented by _Lactobacillus hilgardii_, while the amount of propan-2-one was largest in sample RTCL16. A particularly interesting finding of the present study was that the amounts of 3-hydroxybutan-2-one and butane-2,3-dione were much larger for the homofermatative/facultatively heterofermatative LAB RTJL3, LPC, RTCL16, RTCL79, and RTCL31 than for the obligatorily heterofermatative LAB KR10, KR7, RTJL4, JFK2-2, JKA1-6, and RTCL9. 3-Hydroxybutan-2-one and butane-2,3-dione can be produced by the metabolism of LAB by glucose, citrate, and aspartic acid [19]. [PERSON] et al. found that _Lac.