MOISTURE TRANSFER AND NUTRIENT RETENTION IN GUAVA DURING DRYING AND STORAGE

Abstract

Retention of nutrients in food during processing and storage is nowadays an important issue for food processors. This is mainly due to the change of food consumption pattern, the current market demand, and the regulatory requirement. The present study investigates some of the important aspects involved in both processing and storage of guava, a tropical fruit which among all the fruits ranked second in terms of vitamin C content and, also, has a high amount of fiber. Experiments conducted include studies on equilibrium and kinetics of desorption and resorption of guava, and nutrient retention of guava during drying and storage. Vitamin C is known as the least stable one among all the nutrients, thus it is selected as an index for nutrient retention study. In the equilibrium study, sorption (desorption and resorption) data are obtained with gravimetric method at different temperatures (30°C, 50°C and 70°C) and water activities (aw 0.09 to 0.97), and are fitted with BET, GAB and Halsey sorption isotherm models. Both the GAB and Halsey models give good fittness for desorption and resorption data. The kinetic study of desorption is conducted in a drying rig at different air temperature (40°C, 50°C, 60°C and 70°C) and velocity (0.8m/s, 1.2m/s, 1.6m/s, 2.0m/s, 2.5m/s and 3.0m/s). The results are analyzed with models considering external mass transfer resistances into account. The model which considers both external and internal mass transfer resistances predicts adequately the drying data obtained at different air velocities and air temperatures. The parameters of the model, such as surface mass transfer coefficient (hm) and initial effective diffusion coefficient (Deff,0), are empirically correlated with both the air temperature and velocity, individually. The kinetic study of resorption of dried guava is conducted by gravimetric method at different temperatures (30°C, 40°C and 50°C) and environmental relative humidities (43%RH and 75%RH). The data are analyzed by internal diffusion controlled mechanism. Calculated effective diffusion coefficients (Deff) are correlated with temperature. The Arrhenius equation gives the good fitness. The nutrient (vitamin C) retention study covers both the drying and storage. In the drying at different air velocity (2.0m/s and 2.5m/s) and temperature (40°C, 50°C and 60°C), the maximum loss of vitamin C has been found to be 40%. High temperature and high velocity of air is more effective to retain vitamin C during drying, as this condition will remove the moisture rapidly. In storage, deterioration of vitamin C of dried guava follows pseudo first order reaction under different water activity (aw 0.43, aw 0.75, aw 0.84, and aw 0.97) and temperature (30°C, 40°C and 50°C) conditions. The calculated rate constants are correlated with temperature and water activity. Activation energy and temperature constant are derived as functions of water activity. With the obtained empirical equations, vitamin C retention and shelf life can be predicted.