Enzymes participating in Sake fermentation.

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  • 清酒醸造と酵素
  • セイシュ ジョウゾウ ト コウソ

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Abstract

α-Amylase, glucoamylase, and acid protease of rice-koji (a culture of Aspergillus oryzae on steamed rice) are the most important enzymes participating in sake fermentation. The parallel fermentation of sake moromi-mash, in which alcoholic fermentation proceeds keeping pace with dissolution of rice starch by amylases, is constructed with rice dissolution by α-amylase, glucose production by glucoamylase, and alcohol production by yeast.<br>Following three equations were established for the rice dissolution, glucose production, and alcohol production rates, respectively: -dS/dt=k⋅Eaα⋅S (1) dG/dt=k1⋅Ege-k2⋅t (2) dA/dt=k3⋅N (3) where S is rice starch, Ea is α-amylase activity, G is glucose, Eg is glucoamylase activity, A is alcohol, N is yeast number, t is time (day), and k, k1, k2, k3, and α are constants.<br>Apart from the rate equations, it was found that acid protease had great effects on the parallel fermentation; promotion of rice dissolution, hence enhancement of glucose production and alcohol fermentation.<br>Concerning the effect of acid protease promoting the rice dissolution by α-amylase and the small value of index α (0.20-0.25) in equation (1), adsorption of α-amylase onto steamed rice was given attention. Steamed rice adsorbed well α-amylase resulting in poor rice digestion; it was called unavailable adsorption. It could be the reason why activity of α-amylase was restricted as shown by small value of index in equation (1). Action of acid protease on steamed rice released the α-amylase adsorption, promoting the efficiency of α-amylase action.<br>Adsorption of α-amylase onto steamed rice was examined variously and it was inferred that α-amylase and steamed rice combined electrostatically mediated by oryzenin, main rice protein. Isoelectric point of α-amylase is pH 3.7 charging negative at pH 4.5 (pH of sake moromi-mash) and that of oryzenin is pH 6.5 charging positive at pH 4.5. The negative charge of α-amylase is owed with carboxyl residues of acidic amino acids in protein molecule and the positive charge of oryzenin will be owed to basic amino acid residues. Acid protease will attack to the basic amino acid residues to change the protein charge.<br>Application of a saccharogenic α-amylase of Bacillus subtilis having an isoelectric point of pH 5.2 was examined. The amylase did not adsorbed onto steamed rice and hence expected to act effectively in sake moromi-mash.

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