Properties of frozen wheat doughs at subzero temperatures

The subzero properties of wheat doughs were measured by dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA) over the temperature range -90 to +40 degrees C and by H-1-nuclear magnetic resonance (NMR) T-2 relaxation over the range -45 to 0 degrees C. The experiments revealed two transitions in the dough: one independent of frequency at -10 degrees C (attributed to ice melting) and one dependent on frequency at -30 degrees C (attributed to a glass transition). The glass transition temperatures measured by DMTA moved to higher temperatures during frozen storage when the optimal water content of dough was used. A reduction in the water content eliminated this phenomenon. A similar effect of water reduction was observed by NMR studies, in which amplitude ratios and decay times were used to calculate the phase transitions. However, the glass transition recorded by NMR was independent of frozen storage with optimal water content. The changes of water state in frozen doughs were studied by ultracentrifugation (the amount of liquid phase) and NMR (freezable water based on liquid amplitude ratios). Frozen storage increased the liquid phase in dough with optimal water content. Thus, ice crystals are growing during frozen storage resulting in the concentration of polymers and a higher glass transition observed by DMTA. The increase of liquid phase during storage was substantially lower when the water content of dough was decreased. Ice crystals' growth can be minimised by reducing water content. The experiments were carried out with four different flours. The measurement of glass transition temperature by DMTA, DETA or NMR did not reveal great differences in doughs made from different hours. The amount of liquid phase was strongly flour dependent. (C) 1998 Academic Press.