Rapid freezing of food in an Open Air Cycle Freezer

摘要

An open air cycle freezer with a capacity of 15kW was installed at the Ancoopesca factory, Italy. This was used to gain freezing data for cylinders of surimi which were then used in a mathematical model to predict freezing times over a range of freezing conditions, from -100 to -30°C, that could be achieved in the freezer. This paper uses a method of combining mathematical modelling and empirical data developed by Gigiel and Ketteringham to produce cost effective design data for cooling processes. The thermal properties were calculated by the model from the composition of the surimi and the initial freezing point. The initial freezing point was measured by freezing 3 samples of surimi and thawing them, the thawing plateau being used to measure of the initial freezing point. The initial freezing point and the composition were entered into the model to predict the thermal properties at temperatures between -80 and +30°C. The surface heat transfer coefficients were measured in the freezer using aluminium cylinders. The weight loss from the surimi was measured during freezing and used to estimate the mass transfer coefficient. The predicted freezing times were not significantly different from the measured freezing times. The predicted freezing time for surimi in air at -73°C was 680s, a difference of 42s or 6% from the freezing time measured. Freezing in the open air cycle freezer was faster than freezing in conventional freezers and comes close to the freezing times that could be obtained using liquid nitrogen.