Sorption Isotherm Properties of Date (Phoenix dactylifera L., Mejhoul variety)

The objective of this study is to model the water adsorption isotherms of the dates of the Moroccan variety "Mejhoul" at three temperatures (25 °C, 30 °C and 40 °C) using the static gravimetric method. Experiments have measured the change in water content of samples according to water activity, with maximum values of 32% at 25 °C, 46.4% at 30 °C and 51.2% at 40 °C for water activities close to 95%. The experimental results were adjusted to GAB (Guggenheim, Anderson and Boer), BET (Brunauer, Emmet and Teller) and Langmuir models in order to determine characteristic parameters such as water content of the mono-molecular layer (Xm) and sorption heat isosteric. The GAB model was the most accurate, with a correlation coefficient of 99.5%. The net sorption isosteric heat varies between 37 and 44 kj/mol, decreasing gradually with increasing water content, indicating a reduction in affinity between water and fruit matrix as humidity increases. These results allow a better understanding of the hygroscopic behaviour of Mejhoul dates and provide useful indications for improving drying, conditioning and preservation processes.