清酒醸造における脂質の動向と香気エステル生成への影響

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Rice is principal raw material of sake, and its quality affects sake manufacturing; peripheral layers of brown rice grain contain large amounts of lipids which are considered to be undersirable for sake quality, but the details are not clearly elucidated.In sake brewing, rice is polished before use, one of the main purposes of the process is to remove the undersirable lipids. Although content of ether extractable lipid (I), mainly triglyceride (TG), decreases most rapidly with decrease of polishing ratio, content of fat by hydrolysis (II), main component of polished rice lipids, remains nearly constant, 0.6%. Palmitic (C_<16 : 0>), oleic (C_<18 : 1>) and linoleic acid (C_<18 : 2>) are the main fatty acids component of I, and the ratio of the total amounts of saturated fatty acids (SFA) to those of unsaturated ones (S/U) shows a tendency to increase with decrease of the polishing ratio; the tendency is especially remarkable in TG fraction. in II, C_<16 : 0> is predominant and a little change is observed on the composition of fatty acids with the polishing ratio.During steaming process, TG of I decreases to 30-50%, of the original amount. The free fatty acids (FFA) produced by hydrolysis of glycerides evaporate successively with steam during the process, particularly a large amounts of unsaturated fatty acids (UFA) are removed, and consequently the value of S/U increases. It is also proved that stimulative hydrolysis of glycerides by steeping of rice in lipase solution cases removal of more greater amounts of UFA. Rice-koji for sake brewing contains 0.31 to 0.56% of I and 0.53 to 0.67% of II. TG, FFA and monoglycerides (MG) are the main components of I, and fatty acids of these lipids are rich in UFA such as C_<18 : 2> and C_<18 : 1>. The ratio of UFA in I changes with the rice-koji making conditions such as aeration and temprature progress. Aeration has the greater effect. Rice-koji preparation under limited aeration causes a marked decrease in the production of C_<18 : 2> with a corresponding increase of C_<18 : 1>.As the fermentation of sake mash proceed, steamed rice and rice-koji are digested and solid fraction in mash decreased, and about 58% of II in raw materials becomes extractable with ether. Because of low solubility of lipid in water, a large proportion of lipids remains in the decreasing solid fraction. A small amount of lipid in liberated is liquid fraction (liberated lipid : III), which increases according to the increase of alcohol concentration in mash and reaches about 500 ppm of the liquid fraction at the final stage of fermentation. Main components of III and I of the solid fraction are FFA, TG and ethyl esters of fatty acids (EtOR).II of solid fraction is mostly consisted of FFA.Since EtOR is not found in raw materials, it seems to be formed by yeast during fermentation. Each fraction of FFA, TG and EtOR is shown to have characteristically different fatty acid composition, respectively. The value of S/U increases during fermentation. Since it is shown that the yeast cultured under the alcoholic fermentation condition synthesis a large quantity of stearic acid (C_<18 : 0>), most of this acid found in III is conosidered to be formed by yeast during the fermentation.The formation of esters such as ethyl acetate (EtOAc) and isoamyl acetate (iAmOAc) found in a medium are highly dependent on the fatty acid composition; SFA and their derivatives added to the medium promote the formation of the esters by various yeast strains, while UFA and their derivatives strongly supress their formation. These effects of fatty acid on the formation of esters have been proved by many pilot plants and full scale sake brewing. These esters largely contribute the excellent flavor of sake and a sake rich in these esters in usually prefered as a good sake in many sensory contests.The fatty acid added to the medium is intactly incorporated into yeast cellular lipids such as TG and p
社団法人日本生物工学会の論文
1985-03-25

清酒醸造における脂質の動向と香気エステル生成への影響

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