STRESSES AND CRACKING IN FREEZING PART-BAKED BREAD: A NUMERICAL MODEL

Thermal stresses caused by the volume expansion of water during freezing process of part-baked frozen bread have a direct impact on final bread quality. Crust flaking could be caused by differential strains within the loaf. In order to understand the thermomechanical behaviour of part-baked bread during freezing and to prevent this phenomenon, we modeled the part-baked bread freezing. Thermal stresses during freezing of part-baked bread were simulated with a hookean law, using realistic thermal and mechanical properties. The stress field is assumed to have no influence on heat transfer. Therefore, the temperature field was calculated first, and then the stress field is calculated based on it. It was found that to explain observed cracking patterns, both expansion due to phase change and subsequent thermal contraction must be taken into account. During freezing, the unfrozen core is always under uniform iso¬tropic tensile stress, but that is unlikely to explain the cracking pattern. Cracking during cryogenic freezing is more likely due to tensile tangential stresses caused by thermal contraction, with vitrification at the surface possibly playing a role in crack initiation. The calculations suggest that once a crack forms at the surface it will propagate towards the centre, as has been observed experimentally.