Color changes during dehydration of onion (Allium cepa L) and garlic (Allium sativum L)

During dehydration, food materials undergo color changes which influence the final quality of the dry product. The kinetics of this phenomenon, due to enzymatic and non-enzymatic browning, has to be known in order to optimize the drying process. The objective of this work was to study changes in color of onion and garlic as a function of temperature and water content. A Hunter Tristimulus Colorimeter was used to measure variations in color (Delta E) of puree and slices. The study was carried out on samples kept at constant temperature and humidity and during drying experiments. Moisture content was varied from raw material up to practically bone dry product and the temperature ranged from 45 to 75 degrees C. Rate of deterioration, expressed as changes in Delta E by unit time, increases with temperature overtaking minimum values for raw material and nearly dry product and reaching a maximum found that color changes can be represented by a pseudo first-order kinetics, the reaction rate constant depending on moisture content and temperature. Effect of temperature follows an Arrhenius-type equation. Both the frequency factor and the activation energy increase with water content according to potential and linear functions. Model parameters for different products were obtained by non linear regression of experimental data.