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.

Microencapsulation of olive mill wastewater in Saccharomyces cerevisiae cells by spray drying and in vitro bioaccessibility of phenolic compounds

“…Due to the high T g (162 °C) reported for DE of 17–21, 49 it was possible to reduce the stickiness problem encountered when YCs were applied as the carrier material. Because of the low T g values of baker's dry yeast, between 53.7 and 35.7 °C when yeast samples had an a w value between 0.112 and 0.323, and moisture content of 5.6–7.14%, 50 the yeast cells by themselves could not be enough to increase T g of OMW. In the studies, encapsulating bioactive compounds such as ethanolic extracts of grape pomace, 23 pumpkin peel, 27 and flavor oils 24 into yeast cells, the spray-drying yield was not given, therefore we could not make a comparison with our study.…”

Effects of Trehalose Content and Water Activity on the Fracture Properties of Deep-fried Wheat Flour Particles and Freeze-dried Porous Waxy Corn Starch Solids

“…As a typical parameter, ∆ F (the force difference between initial compression and compression after 3 min) was evaluated from the isothermal mechanical relaxation curves as described in previous studies. 5) 27) 28) …”

Why Does Cronobacter sakazakii Survive for a Long Time in Dry Environments? Contribution of the Glass Transition of Dried Bacterial Cells

“…The T g of the a w -adjusted C. sakazakii cells was determined by thermal rheological analysis (TRA) as described in previous studies ( 37 – 40 ) and as shown in Fig. 5 .…”