Effects of glass transition and hydration on the biological stability of dry yeast.

Abstract

The purpose of this study was to determine the effects of glass transition and hydration on the storage stability of baker's dry yeast (Saccharomyces cerevisiae). The glass transition temperature (Tg ) of the yeast decreased with increase in water activity (aw ), and aw at which glass transition occurs at 25 °C was determined as the critical aw (awc ). From mechanical relaxation measurements at 25 °C, the yeast exhibited a large mechanical relaxation above the awc , and the degree of mechanical relaxation increased gradually with increasing aw . This behavior corresponded to a gradual increase in molecular mobility with increasing aw in the rubbery liquid state. Freezable water was observed from aw ≥0.810, and the proportion of freezable water increased with increasing aw . Examination of the effect of aw on the residual biological activity of yeast samples stored at 25 °C for 30 days revealed maximum residual biological activity at aw =0.225 to 0.432. In the lower aw range, the residual biological activity decreased because of oxidation of lipids. In the higher aw range, the residual biological activity decreased gradually with increasing aw . The yeast samples maintained a relatively high residual biological activity, because they could maintain relatively low molecular mobility even in the rubbery liquid state, as suggested by their mechanical relaxation behavior. At aw ≥0.809, residual activity decreased to a negligible value. This could be explained by the appearance of secondary hydrate water (freezable water). Hydrate water protects yeast cells from lipid oxidation but reduces the Tg . As a result, the yeast cells are stabilized maximally only at the awc . PRACTICAL APPLICATION: Although the growth rate of yeast cells becomes negligible below a certain aw , the biological activity of dry yeast decreases gradually during storage. The fact that dry yeast can be maximally stabilized at the awc is practically useful as a criterion for controlling storage stability. In addition, it was found that a remarkable reduction in the molecular mobility, which is otherwise ordinarily increased due to the glass-to-rubber transition, is prevented in yeast. It is possible that the crystallization of amorphous sugar can be prevented by yeast extract. The suggested effect is expected to result in enhanced quality of carbohydrate-based foods.