Mathematical modeling of thin-layer drying of opuntia ficus-indica peels

Opuntia ficus indica is a popular summer fruit in Tunisia. It is primarily produced in arid and semi-arid regions and has significant economic implications for rural communities. It is typically sold in local markets as a whole fruit or after removing the peels from the edible pulp. Despite their rich composition in nutrients and bioactive compounds, their peels are undervalued. Within this context, the current study focuses on the potential for peel valorization through the dehydration process. This unit operation is required to reduce the initial high moisture content (about 80%) of fresh peels while preserving their bioactive compounds.

For the experiment, fresh Opuntia ficus-indica peels were collected and sliced into rectangular shapes (5 cm x 4.3 cm) before being placed in trays in a hot-air convective dryer. Three different temperatures (55, 65, and 75°C) were examined. Drying kinetics were determined by measuring the variation in wet mass peels at 20-minute intervals until a constant mass of dried peels was obtained.

The drying kinetics were investigated to determine the diffusion coefficient during the falling rate drying stage. The diffusion coefficient increased from 6.3*10^-08 ± 1.38*10^-10 to 7.9*10^-08 ± 9.18*10^-11 m².s^-1 with increasing temperature in the range of 45-75°C. An activation energy value of 24.69 kJ.mol^-1_, for the Opuntia ficus-indica peel drying process, was deduced from the slope of the linear Arrhenius equation (R²=0.97). Moreover, seven mathematical models were applied to the experimental data. The goodness of fit was assessed using R² and reduced chi-square (χ²). The Page and diffusion models best described the convective drying behavior of Opuntia ficus-indica peels.

In conclusion, modeling the drying process of Opuntia ficus-indica peels contributes to reducing the requirement for the laboratory experiments necessary to design suitable equipment and to adjust process parameters according to specific needs, while maintaining regular dried product quality.