Structural studies on unpackaged foods during their freezing and storage

Abstract

During the freezing and frozen storage of unpackaged foods, their surface is exposed to mass transfer with the environment. Basically, ice sublimes and forms a dry, porous layer. This fact alters the sensory characteristics of the products and originates an important quality loss. In this work, a mathematical model was used to predict the thickness of the dehydrated layer, taking into account the influence of operating conditions and food characteristics. Based on the predictions of the numerical model, 2 regression equations were proposed to calculate the size of the dehydrated layer after freezing and storage, having the operating conditions and food properties as parameters. Besides, the frozen storage of the products was studied over different time periods (1, 2, and 3 mo) using beef cylinders and slices, as well as minced beef balls and hamburgers. The dimension of the dehydrated layer and the induced changes in the food surface structure were determined by image analysis and environmental scanning electron microscopy (ESEM). The experiments determined an increase in the depth of the dehydrated layer when storage times are longer, which could be adequately related to storage conditions through the predictions from the numerical model and the regression equations.