Frost formation modeling during the storage of frozen vegetables

The quality of frozen vegetables is affected by temperature fluctuations during storage, when weight loss and frost formation are generally observed. In order to predict frost formation during storage of frozen vegetables, a numerical model consisting of a cylindrical container filled with frozen carrot slices was developed. It was exposed to external temperature fluctuations.

The parameters of the model were identified by experimental measurements or found in the literature. The model was validated using a set of independent experimental results that showed good agreement with the simulated predictions. The model is able to predict air velocities, air and product temperatures and local frost formation. It was used to show that the most important parameters affecting total frost formation are the effective mass diffusivity and the convective heat coefficient within the container.

In the frozen food industry, in particular in the vegetable sector, following freezing, products are stored in bulk in box pallets at low temperatures in cold rooms. Storage The storage duration can be from a few months up to one year, before being packed prior to being commercialized. During long term storage in cold rooms, frozen products are exposed to air temperature fluctuations that exert an important effect on product quality. Consequently, losses for the frozen food industry can be expected due to poor temperature management.

During storage, pallet boxes are heated or cooled by air, so the product and air temperatures inside the pallet are very different from one position to another. Close to the external walls, products and the surrounding air, the temperature changes more rapidly than in the case of products and the air located in the core of the pallet due to thermal inertia. Water from the warmer product is sublimated, transported by the air and finally is deposited on the cooler frozen surfaces where frost formation occurs.

Despite the importance of frost formation during storage of frozen food, few studies have dealt with the modeling of coupled phenomena: airflow, heat transfer and mass transfer for bulk-stored frozen food porous media under conditions of natural convection.

A simple model of heat and mass transfer in a porous media and under conditions of free convection was developed. The model reliably predicts the product temperatures for different temperature cycles. The model is a good tool to predict regions in the container in which frost formation is likely to occur. The frost formation rate is lower than in the experiment, but the order of magnitude is correct. Some of the identified parameters could be adjusted in order to obtain better results in terms of frost formation.

Through a sensitivity study, the importance of different parameters in the predicted frost formation was analyzed, and it was concluded that the parameters which exert the greatest influence are the effective mass diffusivity and convective heat transfer coefficient.