清酒醸造にみられる酵母のエタノール耐性獲得機作

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In sake brewing, in the case of seed mash made according to the traditional method (kimoto), spontaneous fermentation by lactic acid bacteria is utilized during the first period to obtain acidic conditions. In kimoto, good cell survival during the resting period is attained compared with that in the other commonly used seed mash, sokujo-moto, to which lactic acid is added artificially, indicating that the kimoto yeast is better able to tolerate ethanol. The acquisition of ethanol tolerance by Saccharomyces cerevisiae has been investigated in several studies. The linoleic acid content of cellular phospholipids (PL) was 30-50% in sokujo-moto, whereas in kimoto it was below 6% and the palmitic and/or stearic acid content was higher, which corresponded to the free fatty acid compositions of the two seed mashes respectively. As well as palmitic acid, linoleic acid was a major component in the free fatty acids of sokujo-moto, whereas the multiplication of lactic acid bacteria in kimoto significantly reduced the ratio of linoleic acid and increased that of palmitic acid by utilizing linoleic acid as the predominant component of the cellular fatty acid. The survival of palmitic acid-enriched cells was better than that of linoleic acid-enriched ones in the presence of ethanol at high concentration, when S. cerevisiae cultures were grown in a growth medium supplemented with palmitic or linoleic acid. Palmitic acid-enriched cells had a lower permeability coefficient, P', than linoleic acid-enriched cells, suggesting that cell viability corresponds with P'. In the main sake mash seeded with sokujo-moto, the linoleic acid content of intracellular PL was less than that in the seed mash, accompanied by an increase in the palmitic acid content. On the other hand, during the fermentation of main mash seeded with kimoto, the linoleic acid content in the PL of the yeast was detectable only in low quantities (5-10%) and the palmitic acid content increased further with decreasing oleic acid content. To gain a better understanding of the effect of growth in the presence of ethanol as well as that in sake mash, both palmitic acid and ethanol (4-8%) were added to the basal medium, resulting in a striking increase in the palmitic acid content in the PL fatty acid composition, compared with that in the absense of ethanol. On the other hand, the addition of linoleic acid to the medium containing ethanol (4-8%) resulted in an increase in oleic acid and corresponding decreases in the other fatty acids, while the linoleic acid levels were maintained. The membrane permeability coefficient P' in the presence of 20% ethanol became smaller with increasing ethanol concentration in the growth medium, and the P' of palmitic acid-enriched cells was smaller than that of linoleic acid-enriched cells. It is supposed that the increase in palmitoyl residues achieved by supplying palmitic acid exogeneously is effective in terms of increasing the hydrophobic interaction. The lower membrane permeability of cells grown in the presence of 8% could not continue without the addition of glucose. The addition of iodoacetamide as an inhibitor in the glycolytic pathway, or stilbestrol as an inhibitor of plasma membrane ATPase resulted in a rapid increase in membrane permeability, indicating that the additional membrane permeability barrier was dependent on membrane ATPase.
1998-03-25

清酒醸造にみられる酵母のエタノール耐性獲得機作

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