To reduce the adverse effects of hot air drying, ginger slices were pretreated prior to drying. The effects of 2 pretreatment methods (blanching versus dipping in 0.1% ascorbic acid solution) and drying temperatures (60, 70 and 80 °C) on the drying characteristics and the qualities of the dried ginger slices in terms of total phenolic content (TPC), antioxidant activities (DPPH and ABTS methods) and color were investigated. It was noted that the moisture content for both un-pretreated and pretreated samples decreased more rapidly when drying was performed at higher drying temperatures. However, the drying time of un-pretreated and pretreated samples was not significantly different. The TPC and antioxidant activities of the ginger slices increased markedly after drying. The increase in these values was higher for pretreated samples. The dried ascorbic acid solution dipped sample exhibited the highest values of TPC and antioxidant activity. Moreover, the total color difference of the ascorbic acid solution dipped ginger was slightly lower than the total color difference of the un-pretreated ginger. However, the tested drying temperatures, ranging from 60 to 80 °C, did not affect the final TPC, antioxidant activities, and color of the ginger samples.

The impinging stream drying technique has a high capability for removing water in a very short period. Energy consumption is strongly impacted by several of this type of dryer’s operating parameters, i.e., air temperature, air velocity, particle feed rate, and air recycle percentage. A mathematical model would be useful for finding the optimum condition from just a small number of experiments. A mathematical model at a mesoscopic scale that couples heat and mass transfer between gas and solid particles and tracks moisture or temperature distribution inside a single particle was developed in this study; it was used to simulate the effects of air velocity (15–25 m/s), parboiled paddy feed rate (80–250 kg_{dry solid}/h) and exhaust air recycle level (0–80%) on the specific energy consumption (SEC). The developed model was able to predict the average moisture content, average grain temperature, and SEC accurately. The simulated moisture distribution inside a grain kernel showed that this drying technique, which incorporated 4 min tempering between drying cycles, provided a nearly constant rate of drying even though the moisture content decreased to 0.22–0.25 d.b. (dry basis). The use of higher air temperatures noticeably reduced the total specific energy consumption (SEC_total). The air velocity and parboiled paddy feed rate had synergetic effects on the energy consumption. Drying at lower air velocities consumed lower energy when the parboiled paddy feed rate was lower than 170 kg_{dry solid}/h. At higher parboiled paddy feed rates, ranging from 170–250 kg_{dry solid}/h, however, the use of higher air velocities tended to save more total energy. At 80% of recycled exhaust air, the SEC_total was around 4 MJ/kgH₂O, which saved 40–45% of SEC_total compared with the case of without exhaust air recycle.

Water sorption tests of Roselle (Hibiscus sabdariffa L.) carried out under laboratory conditions are presented together with mathematical analyses of the moisture sorption isotherms (MSI’s). Moisture equilibrium data for adsorption and desorption of water from Roselle powder were investigated at near ambient air temperatures in the range of 5 and 35 °C and water activity (Aw) ranging from 0.11 to 0.97. The manometric method has been used for water sorption tests. Models for MSI’s are exponential equations. Coefficients of determination are 0.998 and 0.996 (for adsorption and desorption at 5 °C, respectively), 0.998 and 0.999 (for adsorption and desorption at 20 °C, respectively), and 0.998 and 0.999 (for adsorption and desorption at 35 °C, respectively). The equilibrium moisture content (EMC) of Roselle samples increased with an increase of Aw at a constant temperature both for adsorption and desorption. Adsorption curve equates to desorption curve at higher temperatures of tests carried out. Critical values of EMC of samples tested corresponding to the Aw equal to 0.6 were between 13.401% moisture content wet basis (MC w.b.) and 15.934% MC (w.b.) for moisture adsorption and desorption, respectively. These values are useful for storing conditions optimisation from point of view microorganisms grow and structural changes analyses. Crystal structure changes were observed during adsorption and desorption in the microscope, too. It was found out glass transition in dependence on the water content of samples tested.