Abstract

This study investigated the drying behaviour of purple-speckled Cocoyam and the effect of drying temperature (40 °C, 60 °C and 75 °C), slice thickness (4 mm, 7 mm and 10 mm) and pre-treatments (blanching in boiling water for 3 min, blanching in boiling water for 3 min followed by dipping in 0.1 per cent sodium metabisulfite for 5 min) and non-pre-treated slices. The process and quality criteria under consideration included the total drying time, rehydration ratio, colour difference, browning index and specific energy consumption. The drying air velocity was maintained constant at 0.6 m/s while the relative humidity was monitored to average 5 – 10 per cent inside the drying cabinet during the experiments.

The drying process exhibited a 2-stage falling rate behaviour with kinetics best modelled by the Two-term Exponential, Peleg and Midili models. Non-pre-treated Cocoyam slices performed better than pre-treated slices in terms of decreased drying time, total colour change (ΔE), browning index (B.I) and specific energy consumption (Es) with the exception of rehydration ratio (RR) for all factor settings. The pre-treatment process was found to impart irreversible changes to the material that negatively impacted the majority of the quality attributes considered. The combined effect of both pre-treatments and hot air drying should therefore be evaluated to comprehensively take into account the total changes the entire process imparts on the material. Numerical optimisation results revealed that the most suitable drying conditions were at a drying temperature of 75 °C, slice thickness of 4 mm and without pre-treatment, which yielded a composite desirability index of 0.78. These settings resulted in a total drying time of 109 min, a total colour difference of 2.4, browning index of 9.96, rehydration ratio of 0.7 and specific energy consumption of 6119.3 kJ/kg.