EFFECT OF PRETREATMENT METHODS ON THE THIN LAYER DRYING CHARACTERISTICS OF THREE POTATOVARIETIES

Abstract

Sweet Potatoesis the seventh most important food crop deteriorate rapidly once it is harvested due to physiological changes, changes in temperature and mechanical damage.This study investigated the effects of pretreatment, temperature and drying air velocity on thin layer drying characteristics and mathematical modelling of sweet potato (Ipomoea batatasL.) The experiment was conducted at four different temperatures i.e. 50, 60, 70 and 80°C and air velocity 1.5 and 2.0m/s using a convective cabinet dryer. Three varieties of sweet potato tubers namely Orange fleshed, yellow skin white flesh and yellow skin cream flesh were purchased and prepared into slices of approximately 3mm thickness. They were pre-conditioned with 5% sodium metabisulphite and 1% citric acid, blanched and spread on a stainlesssteel tray prior to drying in a cabinet dryer and the open sun. The experimental data of moisture ratio of the sweet potato slices were fitted with thirteen mathematical models, and the effective diffusion coefficients were calculated. The results showed that temperature, velocity and pretreatment influenced the drying process significantly. Sodium metabisulphite enhanced the drying process better. The Hiiet. al., Model showed the best fit to experimental drying data for pretreatment, temperature and air velocity considered in this study for cabinet dryer and the open sun except for the samples blanched before drying at 2.0 m/s air velocity in the cabinet dryer Where Midiliet al., has the best goodness of fit which was found to be the most satisfactory for samples blanched and pretreated with citric acid before dying in the cabinet dryer. However, the Midiliet al. model has the highest value of coefficient of determination, R2 (0.9906 - 0.9966) and lower values of Standard error of estimate, SEE (0.0241 – 0.0400), root mean square error RMSE (0.0209 0.0346) and chi-square χ2 (0.0006 – 0.0016) for the orange flesh potato pretreated with blanching and citric acid at in the open sun drying method and Hiiet al. shows the best fit for the orange flesh potato treated with sodium metabisulpite and untreated sample with highest value of coefficient of determination, R2 (0.9904 - 0.9997) and lower values of Standard error of estimate, SEE (0.0241 – 0.0400), root mean square error RMSE (0.0057 0.0346) and chi-square χ2 (0.0000- – 0.0018) rangesThe effective moisture diffusivity (Deff) of the dried sweet potato varieties increased from 4.47 x 10-10 to 12.14 x 10-10 m2/s, 4.32 x 10-10 to 9.46 x 10-10 m2/s and 4.23 x 10-10 to 10.25 x 10-10 m2/s for orange flesh potato, yellow skin – white flesh potato and yellow skin – cream flesh potato respectively as the drying air temperature of a thin-layer dryer increased from 50 to 80 0Cand 1.5 – 2.0 m/s.The activation energy of diffusion(Ea) was calculated and was found in the range of 9.49–31.69 kJ/mol. Further studies should be considered to check the influence of the pretreatment on the chemical composition of the dried product and the effect of different packing material on the shelf life of the dried products.