令和6年度 研究成果

TAKAHASHI Kei, KOHNO Hiromi, and OKUDA Masaki

Background
The quantity and composition of rice proteins play a crucial role in determining taste quality of sake, Japanese rice wine. However, the spatial distribution of proteins within rice grains, especially in endosperm tissue, and the differences between rice varieties remain unclear.
Results
Here, we analyzed the crude protein contents and composition ratios of table (Nipponbare and Koshihikari) and genuine sake rice varieties (Yamadanishiki, Gohyakumangoku, Dewasansan, Dewanosato, and Yumenokaori) to elucidate their spatial distribution within the Japonica rice grain endosperm. Seven sake rice varieties were polished over five harvest years using a brewer’s rice-polishing machine. We obtained fractions at 90%–70% (the outermost endosperm fraction), 70%–50%, 50%–30%, and 30%–0% (the central region of the endosperm fraction). Yamadanishiki and Dewanosato exhibited considerably lower crude protein contents than the other cultivars. After applying SDS-PAGE, the protein composition, comprising glutelin/total protein (G/TP), prolamin/TP (P/TP), and G/P ratios of these fractions was determined. In white rice (at a 90% rice-polishing ratio), the average ratio of the major protein composition was G/TP 41%, P/TP 21%, and G/P ratios of 1.97. Gohyakumangoku and Yamadanishiki had higher G/TP ratio, while Dewanosato had a lower value. Despite having lower crude protein contents, Yamadanishiki and Dewanosato exhibited significantly varying G/TP ratios. The G/TP ratio markedly varied among rice varieties, particularly in the rice grains’ central region. The 50%–30% fraction had the highest P/TP ratio among all tested rice varieties, suggesting spatial differences in P/TP within rice grains. Koshihikari had the lowest P/TP ratio. In addition, the 50%–30% fraction had the lowest G/P ratio among all tested rice varieties, with Gohyakumangoku having the highest G/P ratio. Dewanosato had the lowest G/P value, and this value significantly differed from that of Yamadanishiki in the 30%–0% fraction.
Conclusions
We found substantial differences in protein composition within distinct spatial regions of rice grains, and larger differences among rice varieties were observed in the rice grain's central region.

岸本徹、久常有里

アルコール飲料と料理の相性に関する議論では、魚料理と一緒に飲むアルコールの種類によっては、生臭さを感じることがある。しかし、日本酒はこの生臭みを効果的に抑制し、料理との相性を高めることが報告されている。本研究では官能評価を行い、ビールにスルメを合わせると魚由来の金属臭が目立つことを確認した。一方、日本酒とスルメを併用すると、金属臭が効果的に抑制されることが官能評価で確認された。この金属臭の原因物質は、既報の(Z)-1,5-オクタジエン-3-オンと同定された。アルコール飲料中の(Z)-1,5-オクタジエン-3-オンの1ng/L(ppt)以下の定量分析を行った。SBSE抽出、PFBOA誘導体化、NCIイオン化、GC/MS/MS分析を組み合わせた高感度で合理的な定量法を確立した。日本酒、ビール、スルメを混合した場合の(Z)-1,5-オクタジエン-3-オンの生成量を測定したところ、日本酒の方が有意にレベルが低く、平均生成量はビールより7.1倍低かった。本研究は、清酒の生臭さの一因とされる「金属臭」の抑制を成分レベルで初めて実証し、「食品との相性」についての知見を提供するものである。

SHIBATA Yusuke, YAMADA Tasuku, IKEDA Yuriko, KANAI Muneyoshi, FUJII Tsutomu, AKAO Takeshi, GOSHIMA Tetsuya, ISOGAI Atsuko, and TAKAHASHI Toshinari

Hineka is a type of off-flavor of sake and is attributed to the presence of several compounds, including a major one called dimethyl trisulfide (DMTS). The production of the main precursor of DMTS involves yeast methionine salvage pathway. The DMTS-producing potential (DMTS-pp) of sake brewed using the Km67 strain, a non-Kyokai sake yeast, is lower than that of sake brewed using Kyokai yeast; however, the detailed mechanism is unclear. We focused on S-adenosyl-methionine (SAM) and aimed to elucidate the mechanism that prevents DMTS production in sake brewed using the Km67 strain. We revealed that SAM is involved in DMTS production in sake, and that the conversion of SAM to the DMTS precursor occurs through an enzymatic reaction rather than a chemical reaction. Based on previous reports on ADO1 and MDE1 genes, sake brewing tests were performed using the Km67 Δmde1, Δado1, and Δmde1Δado1 strains. A comparison of the SAM content of pressed sake cakes and DMTS-pp of sake produced using the Km67 Δado1 strain showed an increase in both SAM content and DMTS-pp compared to those produced using the parent strain. However, the Km67 Δmde1Δado1 strain showed little increase in DMTS-pp compared to the Km67 Δmde1 strain, despite an increase in SAM content. These results suggest that SAM accumulation in yeast plays a role in the production of DMTS in sake through the methionine salvage pathway. Moreover, the low SAM-accumulation characteristic of the Km67 strain contributes to low DMTS production in sake.