Two-stage freezing of part baked breads: Application and optimization

Partially baked bread or part baked bread can be frozen to extend its shelf life. Process conditions can influence frozen part baked bread quality, which can be related to excessive drying of the surface at the end of the post-baking chilling and freezing process. This excessive drying might be responsible for crust flaking and/or dull colour of the crust. Build-up of ice below the crust during freezing may also contribute to the flaking of the crust, and hence it can also be considered a quality indicator. A two-stage freezing process was proposed and studied in this work, where air temperature and air velocity undergo a step change between the two stages. A mathematical model developed previously was used to predict the weight loss, ice concentration at the crumb–crust interface and freezing time as functions of the cold air conditions (velocity, temperature and relative humidity) with the objective of identifying optimal process conditions. The results showed that rapid freezing (5 m/s and 233 K) tends to minimize the weight loss and the freezing time. On the other hand, slow freezing (0.5 m/s and 253 K) applied at the beginning of the freezing process tends to minimize the ice content at the crust–crumb interface during the starting period and can be combined with a rapid freezing applied during the second step (0.5 m/s and 233 K).