On the water desorption of fruits: a case study of watermelon (Citrullus Lanatus)

Studies on drying of the watermelon seeds or rinds are available, but not on drying of the fruit flesh. Data on the sorption of watermelon fruit (Citrullus Lanatus), which are useful and important when identifying optimal food drying and storage conditions, are also not available. Therefore, the main goal of this study is to investigate the water desorption behavior of watermelon using the dynamic method requiring the automated dynamic vapor sorption (DVS) technique based on a humidity-generating device. The moisture desorption isotherms are determined at three different temperatures (20°C, 30°C and 40°C) over a relative humidity range of 0-80%. In addition, 11 different models are applied to the experimental data in order to select the appropriate desorption curve equation. The models are compared using the correlation coefficient, the coefficient of determination, the standard error, the reduced chi-squared, the mean bias error and the root mean square error; they are predicted by a non-linear regression analysis using the Curve Expert software and MS Excel computer program. The effect of the temperature on the constants of the found equation is also examined. Besides, 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 best fits the experimental data. In the interval 0.05-0.45 kg water.kg dry matter-1 of equilibrium moisture, the values of the isosteric heat vary from 65.02 to 113.25 kJ mol-1. Finally, the equation giving the isosteric heat of desorption v.s. the equilibrium moisture content is obtained.