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 (T_g ) of the yeast decreased with increase in water activity (a_w ), and a_w at which glass transition occurs at 25 °C was determined as the critical a_w (a_wc ). From mechanical relaxation measurements at 25 °C, the yeast exhibited a large mechanical relaxation above the a_wc , and the degree of mechanical relaxation increased gradually with increasing a_w . This behavior corresponded to a gradual increase in molecular mobility with increasing a_w in the rubbery liquid state. Freezable water was observed from a_w ≥0.810, and the proportion of freezable water increased with increasing a_w . Examination of the effect of a_w on the residual biological activity of yeast samples stored at 25 °C for 30 days revealed maximum residual biological activity at a_w = 0.225 to 0.432. In the lower a_w range, the residual biological activity decreased because of oxidation of lipids. In the higher a_w range, the residual biological activity decreased gradually with increasing a_w . 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 a_w ≥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 T_g . As a result, the yeast cells are stabilized maximally only at the a_wc . PRACTICAL APPLICATION: Although the growth rate of yeast cells becomes negligible below a certain a_w , the biological activity of dry yeast decreases gradually during storage. The fact that dry yeast can be maximally stabilized at the a_wc 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.

Keywords: differential scanning calorimetry; freezable water; glass transition; water activity.