[P003] Poster

12:10 PM - 1:00 PM JST(3:10 AM - 4:00 AM UTC)

[P02-175] Low-Glutelin Rice Fermentation Alters Yeast Nitrogen Response and Enhances Thiol Aroma Formation in Sake

○Taisei Yuyama^1,2, Naoki Akasaka¹, Daisuke Watanabe¹ (1. Nara Institute of Science and Technology (Japan), 2. Iida Co., Ltd. (Japan))

Keywords:

Low-Glutelin Rice,Sake Yeast,4-MMP

Glutelin is the major storage protein in rice, and its content in sake-brewing rice influences the fermentation environment and amino acid composition of the final product. Sake brewed from rice with reduced glutelin content has been reported to contain elevated levels of the thiol compound 4-mercapto-4-methyl-2-pentanone (4-MMP), which imparts characteristic aromas reminiscent of white grape and lychee and has recently attracted attention as a novel aroma component in sake. However, the mechanism underlying 4-MMP formation during sake fermentation remains unclear. In this study, we aimed to identify genes involved in 4-MMP formation by analyzing the transcriptional response of yeast during fermentation with low-glutelin rice. Small-scale sake brewing experiments were conducted using low-glutelin rice and conventional rice, and yeast transcriptomes during fermentation were analyzed by RNA-seq. Comparative transcriptomic analysis revealed that a broad set of genes associated with amino acid biosynthesis and transmembrane transport were upregulated in fermentations using low-glutelin rice compared with conventional rice. Many of these genes are known targets of nitrogen catabolite repression (NCR), a regulatory mechanism that suppresses genes involved in the utilization of less preferred nitrogen sources when preferred nitrogen sources are available. Their upregulation suggests that NCR may be relieved under low-glutelin conditions, thereby promoting the utilization of alternative nitrogen sources and indicating that yeast cells experience a relatively nitrogen-limited environment during fermentation. Among the genes showing increased expression during early fermentation was IRC7, which encodes a β-lyase known to contribute to 4-MMP production in wine fermentation. In contrast, other carbon–sulfur lyase genes potentially involved in thiol release did not show notable increases in expression levels. To directly evaluate the role of Irc7p, an overexpression strain was constructed and subjected to small-scale sake fermentation. This resulted in increased levels of thiol aroma compounds compared with the control, supporting its functional involvement in thiol aroma formation. These results indicate that Irc7p plays a key role in 4-MMP formation in sake brewed from low-glutelin rice. Irc7p catalyzes the cleavage of L-cysteine-S-conjugates to produce volatile thiols, ammonia, and pyruvate, and this reaction is thought to allow yeast cells to obtain nitrogen when readily assimilable nitrogen sources are limited. Consistent with this idea, the upregulation of IRC7 observed in this study may reflect a metabolic response to nitrogen limitation, resulting in enhanced release of thiol aroma compounds. Overall, this study demonstrates that fermentation with low-glutelin rice modulates yeast nitrogen metabolism and identifies IRC7 as a key determinant of thiol aroma formation in sake. These findings provide new insight into the relationship between nitrogen metabolism and aroma production and offer a potential strategy for controlling flavor profiles through modulation of nitrogen metabolism during sake fermentation.