Thomas K C, Ingledew W M
Department of Applied Microbiology and Food Science, University of Saskatchewan, Saskatoon, Canada.
Appl Environ Microbiol. 1990 Jul;56(7):2046-50. doi: 10.1128/aem.56.7.2046-2050.1990.
Although wheat mashes contain only growth-limiting amounts of free amino nitrogen, fermentations by active dry yeast (Saccharomyces cerevisiae) were completed (all fermentable sugars consumed) in 8 days at 20 degrees C even when the mash contained 35 g of dissolved solids per 100 ml. Supplementing wheat mashes with yeast extract, Casamino Acids, or a single amino acid such as glutamic acid stimulated growth of the yeast and reduced the fermentation time. With 0.9% yeast extract as the supplement, the fermentation time was reduced from 8 to 3 days, and a final ethanol yield of 17.1% (vol/vol) was achieved. Free amino nitrogen derived in situ through the hydrolysis of wheat proteins by a protease could substitute for the exogenous nitrogen source. Studies indicated, however, that exogenously added glycine (although readily taken up by the yeast) reduced the cell yield and prolonged the fermentation time. The results suggested that there are qualitative differences among amino acids with regard to their suitability to serve as nitrogen sources for the growth of yeast. The complete utilization of carbohydrates in wheat mashes containing very little free amino nitrogen presumably resulted because they had the "right" kind of amino acids.
尽管小麦醪液中仅含有有限量的游离氨基氮以限制生长，但即使醪液中每100毫升含有35克溶解固体，在20摄氏度下，活性干酵母（酿酒酵母）发酵仍能在8天内完成（所有可发酵糖被消耗）。用酵母提取物、酪蛋白氨基酸或单一氨基酸（如谷氨酸）补充小麦醪液可刺激酵母生长并缩短发酵时间。以0.9%的酵母提取物作为补充剂时，发酵时间从8天缩短至3天，最终乙醇产量达到17.1%（体积/体积）。通过蛋白酶水解小麦蛋白原位产生的游离氨基氮可替代外源氮源。然而，研究表明，外源添加甘氨酸（尽管酵母很容易吸收）会降低细胞产量并延长发酵时间。结果表明，氨基酸在作为酵母生长氮源的适宜性方面存在质的差异。含有极少游离氨基氮的小麦醪液中碳水化合物的完全利用大概是因为它们含有“合适”种类的氨基酸。
