EFFECT OF PRETREATMENT AND DRYING AIR TEMPERATURE ON DRYING KINETICS AND QUALITY CHARACTERISTICS OF FRAFRA POTATO (Solenostemon rotundifolius) SLICES

Abstract

Frafra potato (FP) is an underutilized root tuber crop with great potential to improve food security, but the tubers are prone to discolouration and rot after harvest. This study investigated the effect of different pretreatment and drying air temperature on the drying kinetics, surface temperature uniformity, and quality characteristics of hot-air dried FP slices. Four pretreatment methods (dipping in 5 % w/v potassium metabisulfite solution for 5 minutes, blanching at 100 °C for 3 minutes, a combination of dipping in potassium metabisulfite solution plus blanching, and no pretreatment) and four drying temperatures (40, 50, 60, and 70 °C) were used. Drying of FP slices occurred in the falling rate period, and drying time was affected significantly (p < 0.05) by pretreatment and drying air temperature. The Page model and logarithmic model fitted best with the drying kinetics of FP slices. Blanched samples had higher final water activity and moisture content (0.46 – 0.63 and 6.42 – 9.89 % wb respectively), compared to samples pretreated with dipping in potassium metabisulfite (0.34 – 0.53 and 6.19 – 9.39 % wb), dipping potassium metabisulfite plus blanching (0.46 – 0.49 and 5.22 – 9.01 % wb) and no pretreatment (0.23 – 0.53 and 5.59 – 7.64 % wb). Samples pretreated by dipping in potassium metabisulfite were more desirable in lightness and whiteness index (56.57 – 64.52 and 51.92 – 59.49 respectively) compared to the other pretreated samples at all levels of drying temperature (40 – 70 °C). Pretreatment and drying air temperature significantly (p < 0.05) affected surface temperature uniformity of dried FP slices and the degree of non-uniformity generally decreased with a decrease in drying temperature. Pretreatment and drying of samples also resulted in 9 – 30 % and 27 – iii 71 % reduction in beta-carotene and total phenolic content, respectively, and an increase of 6 – 177 % in antioxidant activity compared to no treatement samples (fresh FP samples). Vitamin C content in blanched samples increased with increase in drying temperature. Samples pretreated with potassium metabisulfite solution had higher phenolic content at all drying temperatures, and phenolic content in these samples increased as drying temperature increased from 40 – 70 °C. The results in this study demonstrate that pretreatment and drying air temperature had different effects on the quality characteristics of dried FP slices. However, for optimum retention of quality, pretreatment by dipping in K2S2O5 combined with drying at 70 °C is recommended. But where K2S2O5 is unavailable, it is better to dry FP without pretreatment.