Yokoigawa Kumio, Sato Machiko, Soda Kenji
Department of Life Science, Faculty of Integrated Arts and Sciences, Tokushima University, Tokushima, Japan.
J Biosci Bioeng. 2006 Sep;102(3):215-9. doi: 10.1263/jbb.102.215.
We examined the effect of poly-gamma-glutamate (PGA) on the freeze-tolerance of four types of commercial bakers' yeast (freeze-tolerant, osmotic-tolerant, low-temperature-sensitive, and ordinary bakers' yeasts). The survival ratio of ordinary bakers' yeast cells frozen at -30 degrees C for 3 d in a medium (0.5% yeast extract, 0.5% peptone, and 2% glucose: YPD medium) was improved by adding more than 1% PGA to the medium; the survival ratio increased from about 10% to more than 70%. All PGA preparations, which differed in average molecular mass (50, 2,000, 4,000, 6,000, 8,000, and 10,000 kDa), showed a similar cryoprotecive effect on the cells. Similar results were also obtained with other types of bakers' yeast, sake yeast and beer yeast. When the four types of bakers' yeast cell were frozen at -30 degrees C for 3 d in dough supplemented with more than 1% PGA, the cells (after freezing and thawing) showed higher leavening ability than those frozen in dough without PGA, irrespective of the molecular mass of PGA. Thus, PGA appears to protect bakers' yeast from lethal freeze injury, leading to a high leavening ability after freezing and thawing. PGA did not decrease the original leavening ability of the bakers' yeast, and was not decomposed by the yeast cells. PGA suppressed the decrease in leavening ability during a prolonged fermentation time, probably because PGA adsorbed inhibitory metabolites accumulated in the dough. PGA could prove useful for improving the freeze-tolerance of bakers' yeast by its addition to dough.
我们研究了聚γ-谷氨酸（PGA）对四种市售面包酵母（耐冻酵母、耐渗透酵母、低温敏感酵母和普通面包酵母）抗冻性的影响。在培养基（0.5%酵母提取物、0.5%蛋白胨和2%葡萄糖：YPD培养基）中，于-30℃冷冻3天的普通面包酵母细胞的存活率，通过在培养基中添加超过1%的PGA得到提高；存活率从约10%提高到超过70%。所有平均分子量不同（50、2000、4000、6000、8000和10000 kDa）的PGA制剂对细胞均表现出类似的冷冻保护作用。在其他类型的面包酵母、清酒酵母和啤酒酵母中也获得了类似结果。当四种类型的面包酵母细胞在添加了超过1% PGA的面团中于-30℃冷冻3天时，无论PGA的分子量如何，冷冻和解冻后的细胞发酵能力均高于在无PGA面团中冷冻的细胞。因此，PGA似乎能保护面包酵母免受致命的冷冻损伤，从而在冷冻和解冻后具有较高的发酵能力。PGA不会降低面包酵母的原始发酵能力，也不会被酵母细胞分解。PGA抑制了长时间发酵过程中发酵能力的下降，这可能是因为PGA吸附了面团中积累的抑制性代谢产物。通过在面团中添加PGA，其可能有助于提高面包酵母的抗冻性。
