Kinetic modelling of quality deterioration in onions during drying and storage

The kinetics of non-enzymatic browning and thiolsulphinate loss in onion slices during drying at different temperatures (50, 60, 70, 75 °C) and air velocities (0.6, 1.0, 1.2, 1.5 m/s) were studied, the quality losses in dried onions during storage at different temperatures (20, 30, 45 °C) and different water activities (0.332, 0.432, 0.570) were also studied. Non-enzymatic browning was found to follow a zero order reaction while thiolsulphinate loss followed a second order reaction during drying and storage. Temperature dependence of the reaction rates could be described by the Arrhenius equation and the reaction rates changed with moisture content and water activity. Empirical equations were derived describing these relationships. Kinetic models were developed which described the quality losses in onions during drying and storage as a function of temperature, moisture content and water activity. Acceptable agreement was determined between experimental and predicted values.