Kinetic study of the convective drying of spearmint

This work presents the kinetic study of the convective drying of Tunisian spearmint in two manners. The traditional protocol which consists in carrying out drying experimental tests at constant temperatures 40–45–50–55 °C and a new protocol which consists in carrying out these tests at variable temperatures in an increasing and decreasing way between 40 °C and 55 °C. All the experimental drying curves show only a falling drying rate period. The main factor in controlling this rate is found to be the drying air temperature. The mode at variable temperatures in an increasing way is more performant and more efficient than the mode at decreasing way or at constant temperature. These experimental curves are fitted to a number of mathematical models. The Midilli–Kucuk drying model is found as the best model which suitably follows the drying curves of spearmint. The characteristic drying curve of spearmint is determined and compared with the modes at variable temperatures. Those at constant temperature and increasing temperature are very close with a correlation coefficient (R2) of 0.8 and Chi-square (χ2) of 0.014, whereas the decreasing mode is a little distant due to the problem of rehumidification at the drying end. Initial moisture has a great influence on a characteristic drying curve.