ISOTHERMS AND ISOSTERIC HEAT OF DESORPTION OF WATERMELON FRUIT

In this paper, a new unpublished study on the watermelon fruit is presented. It
concerns the experimental determination of the water desorption isotherms at three different
temperatures, using the dynamic method requiring the Dynamic Vapor System device (DVS).
In order to select the appropriate desorption curve equation, 11 different models available in
the literature were applied to the experimental data. The models were compared using the
correlation coefficient, the standard error, the reduced chi-squared, the mean bias error and
the root mean square error; they were predicted by a non-linear regression analysis using the
Curve Expert software. The effect of the temperature on the constants of the found equation is
also examined. Finally, the desorption isosteric heat of watermelon is determined using the
Clausius-Clapeyron relation.
Contrary to long times taken in the static method, the results show that 4.5 days are widely
sufficient to reach the equilibrium via the DVS device. In addition, the obtained curves are of
the type III and the Henderson model is the most appropriate model representing them. In the
interval 0.05 - 0.45 kg of water/kg d.m of equilibrium moisture, the values of the isosteric
heat vary from 65.02 to 113.25 kJ/mol. Finally, the equation giving the isosteric heat of
desorption v.s. the equilibrium moisture content is obtained.