Conventional air drying of cocoyam is a slow and energy-intensive process because cocoyam waxy skin has low permeability to moisture. The overall aim of this study was to assess the effect of drying temperature, slice thickness, ultrasound pretreatment with distilled water (UDW) and ultrasound with osmotic dehydration (UOD) at different ultrasonic time on the drying kinetics and some physicochemical properties of cocoyam during convective hot air drying. Fresh cocoyam tubers were sliced into 2mm, 4mm and 6mm thickness and dried at 40, 50, 60 70, 80 and 90 for determining the effects of temperature and slice thickness on the drying kinetics and some physicochemical properties. The effect of ultrasound pretreatment method and time was investigated by slicing the cocoyam into 4mm thickness and pretreated at ultrasound frequency of 20 kHz at an output power of 600W for 10, 20 and 30 min for UDW and UOD. The salt concentration used for the osmotic dehydration was 23%w/v. The ultrasound pretreated samples were further dried at 50, 60 and 70 in a convective hot-air drying oven and weights were recorded at 30 min interval until constant mass was obtained. The reduction in slice thickness (6-2 mm) and the increase in temperature (40-90) showed a significant decrease in the drying time and increase in drying rate. The sample with 2mm slice thickness dried at 90 had the highest drying rate and the shortest drying time. The effective moisture diffusivity and the mass transfer coefficient were also increased as the temperature increased. Ultrasound pretreatment method also showed significant reduction in the drying time for UDW (25%) and UOD (46%) compared to the untreated sample at 70.
The UOD samples also enhanced the colour and retained most of the nutrient (carotenoid, vitamin C and proximate composition) while reducing the anti-nutrient (oxalate). The UOD samples had the lowest activation energy (10.697 x 103kJ), the highest moisture diffusivity3.782×10-10m2/s and mass transfer coefficient (2.006 × 10-8m/s) among the untreated and UDW sample. Wang and Singh, Page and Peleg models were found to be the most fitted models for the drying characteristics of cocoyam for untreated, UDW and UOD samples respectively. Ultrasound pretreatment technology is a potential non-thermal process that can be incorporated in the convective drying of cocoyam for more energy efficient drying and to achieve a product with high nutritional quality.