Changes occuring in yeasted wheat flour doughs during cryogenic freezing and frozen storage

Typescript (photocopy).During this study, an apparatus was designed and constructed to allow for cryogenic freezing of 10 gram loaves of yeasted wheat flour dough. Dough loaves were frozen by direct contact freezing with boiling Freon compounds. Their boiling temperatures were -10°C, -20°C, -30°C and -40°C. Heat transfer across the surface of the dough exposed to Freon was estimated from the mass vaporization rate of Freon. After freezing the dough loaves were stored in frozen storage for periods of time up to 3 months. When removed from frozen storage, the samples were slowly thawed. Some samples were subsequently baked and their loaf volumes determined by a rapeseed displacement test. Other samples were used to determine the microbial population and used for microscopic (SEM) examination. Two dimensional finite difference solutions of the heat equation were employed to predict temperature distribution of the dough loaves as they were being frozen. These yielded solutions which predicted center temperatures during freezing that were 5-12.5% higher than experimentally measured temperatures. It was found that freezing temperatures of approximately -20°C produced the least change in microbial population and loaf volume after 3 months of frozen storage. Temperatures of -40°C produced the greatest changes after the same period of storage. Microbial population was reduced in 2 ways. Initially, there was a step-like reduction in the viable cell count as a result of freezing. The magnitude of the reduction was dependent upon the freezing temperature. Secondly, during frozen storage, there was a gradual decrease in viable yeast cell count over the 3 month test period. Dynamic force-deformation testing of unyeasted wheat flour dough samples revealed no change in texture as a result of freezing and thawing. SEM examination showed no observable changes in the micro-structure upon freezing and thawing. This suggests that changes in the gas producing and retaining ability of the dough are chemical in nature