The cryotolerance in frozen doughs and in water suspensions of bakers' yeast (Saccharomyces cerevisiae) previously grown under various industrial conditions was evaluated on a laboratory scale. Fed-batch cultures were very superior to batch cultures, and strong aeration enhanced cryoresistance in both cases for freezing rates of 1 to 56°C min'-. Loss of cell viability in frozen dough or water was related to the duration of the dissolved-oxygen deficit during fed-batch growth. Strongly aerobic fed-batch cultures grown at a reduced average specific rate (,u = 0.088 h-1 compared with 0.117 h-') also showed greater trehalose synthesis and improved frozen-dough stability. Insufficient aeration (dissolved-oxygen deficit) and lower growth temperature (20°C instead of 30°C) decreased both fed-batch-grown yeast cryoresistance and trehalose content. Although trehalose had a cryoprotective effect in S. cerevisiae, its effect was neutralized by even a momentary lack of excess dissolved oxygen in the fed-batch growth medium.

The relationship between lipid content and tolerance to freezing at-50°C was studied in Saccharomyces cerevisiae grown under batch or fed-batch mode and various aeration and temperature conditions. A higher free-sterol-to-phospholipid ratio as well as higher free sterol and phospholipid contents correlated with the superior cryoresistance in dough or in water of the fed-batch-grown compared with the batch-grown cells. For both growth modes, the presence of excess dissolved oxygen in the culture medium greatly improved yeast cryoresistance and trehalose content (P.

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