Sequential Pre-Treatment Strategies to Improve Papaya Drying: Impact on Mass Transfer, Morphology, and Rehydration Rate

Papaya cubes were subjected to freezing–thawing, ethanol, and ethanol solution containing ferulic acid and caffeic acid, both individually and in combination, as pre-treatments for convective drying (60°C). Empirical and diffusional models were used to describe the drying process. In addition, energy consumption, firmness, total phenolic compounds (TPC), carotenoids, water adsorption isotherms, morphology, and the rehydration profile of the dehydrated cubes were also evaluated. The results revealed that all pre-treatments reduced drying time, ranging between 810 and 1170 min, and energy consumption (6.80–12.77 kWh/kg). According to the mass transfer models, effective diffusivity (1.37–5.18 × 10⁻¹⁰ m/s²) and convective mass transfer coefficient (1.04–3.11 × 10⁻⁵ m/s) were significantly influenced by process conditions. The combined pre-treatments, particularly sequential freezing–thawing with ferulic acid solution (S8), significantly enhanced the retention of bioactive compounds in the dehydrated papaya cubes, achieving TPC and carotenoid levels 4.2 and 2.8 times higher than the control, respectively. Regarding water adsorption isotherms, the Peleg model showed the best fit (R² > 0.9972, p < 1.76%), being classified as Type II. Scanning electron microscopy (SEM) images of the dried samples revealed the formation of cavities and irregular surfaces due to the pre-treatments, which significantly influenced the increase in rehydration rate, resulting in faster moisture absorption and the restoration of initial texture and volume.