Temperature-Dependent Moisture Sorption and Hysteresis of Corn Extrudates Enriched with Rose Wastewater Ultrafiltration Retentate

The moisture sorption isotherms of corn extrudates fortified with ultrafiltration (UF) concentrate derived from rose wastewater (RWW) were examined. Two formulations containing 4 g (sample S4) and 11 g (sample S11) of retentate, respectively, per 100 g of semolina were prepared and subsequently extruded. Adsorption and desorption isotherms of both samples were determined at 10 °C, 25 °C and 40 °C using the static gravimetric method within a water activity range of 0.11–0.85. The experimental data of equilibrium moisture content (EMC) were fitted using modified GAB, modified Oswin, and modified Halsey equations. The monolayer moisture content of the samples was determined using the BET model. The results showed that the sorption isotherms exhibited a typical sigmoidal (Type II) shape and a pronounced hysteresis effect between adsorption and desorption. The hysteresis effect decreased with increasing temperature. Based on the combined evaluation of error analysis and residual distribution, the modified Oswin model provided the most balanced overall performance and was determined as suitable for the description of the sorption isotherms of the extrudates studied. The effect of UF-retentate level on EMC was not uniform and depended on temperature, water activity, and sorption direction. The monolayer moisture content of the extrudates varied from 4.08 to 8.39% d.b. The higher retentate level was generally associated with higher monolayer moisture values. Packaging and storage at the theoretically estimated range of water activities from 0.09 to 0.24 may be expected to maintain EMCs close to the monolayer.