Moisture Sorption Isotherms of Foods: Experimental Methodology, Mathematical Analysis, and Practical Applications

摘要

Knowing the moisture content of a product is insufficient to predict its stability, making it necessary to also know its water activity (a_w), a thermodynamic property describing the interactions between water molecules and the food matrix. Moisture sorption isotherms, i.e., the relationship between moisture content and a_w at constant pressure and temperature describing the sorption process of water molecules into a specific material, are useful when identifying optimal food dehydration and storage conditions. Moisture sorption properties affect physicochemical and biological phenomena such as enzymatic degradation, microbial activity, food microstructure, sensory quality deterioration, nutrient losses, and other changes limiting the shelf life of food products. Some of these phenomena are associated with water mobility, which is also related with the phase transitions from a "glass" or amorphous to a "rubbery" state. Glass transition is a second order phase transition associated with time, temperature, and moisture content. When fresh foods are dried, water removal leaves behind an amorphous material. A desirable final product moisture level is one that corresponds to a glass transition temperature (T_g) higher than the product storage temperature. Therefore, knowing T_g helps in setting the food storage and/or process conditions required to retain textural properties and to predict the shelf life of low and intermediate moisture content foods. The aim of this work is to present information on food moisture sorption isotherms and their relationship with product composition and temperature. Advantages and disadvantages of experimental methods available to obtain sorption isotherms are reviewed, equations describing experimental data are then discussed, and finally, practical applications to shelf life estimations due to moisture gain or loss are presented.