Effects of different pretreatments on thin-layer drying kinetics, vitamin A retention and rehydration of orange-fleshed sweet potato slices

Abstract

Orange-fleshed sweet potato (OFSP) is a bio-fortified sweet potato with a high beta-carotene content. OFSP deteriorates rapidly due to its high moisture content. Conventional air drying is one of the most common methods employed but has the disadvantage of low rates of moisture removal, hence the longer drying time, which affects the quality of the final product. Pretreatments prevent quality deterioration and hasten the drying rate. This study investigated the effects of different pretreatments on thin-layer drying kinetics and the quality of OFSP slices. Three-mm-thick OFSP slices were pretreated using three different pretreatments: lemon juice, salt solution, and hot water blanching (HWB). Pretreated slices were dried in a solar dryer. Data collection (weight, temperature, relative humidity, and solar radiation) was done at 1-hour intervals. Four mathematical models (Page, Logarithmic, Modified Page, and Henderson and Pabis) were fitted to the experimental data. Also, the quality parameters of vitamin A and rehydration were investigated. The dryer inside temperatures and relative humidity ranged from 26.93–44.53 oC and 36.87%–65.30%, respectively. The moisture content of fresh OFSP slices for both treated and untreated samples was found to be in the range of 291.55–302.24% (dry basis, db), which reduced to 25.25–35.25% (db) after drying. The drying time ranged between 11 and 13 hours. The page model was found to be the best model for untreated samples, with R2 of 0.9948, RMSE of 0.0151, and χ2 of 0.0097, while the logarithmic model best described the drying behaviour of all the pretreated samples because it had the highest values of R2 and the lowest values of RMSE and χ2. Lemon juice-pretreated samples had the best quality parameters (vitamin A retention and rehydration ratio). A maximum rehydration ratio of 1.6765 was obtained for samples pretreated with lemon juice, and a minimum rehydration ratio of 1.1390 was obtained from HWB-pretreted samples. The results, therefore, indicate that lemon juice can be used as a pretreatment for thin-layer drying of OFSP slices. Pretreatments help minimise postharvest losses associated with OFSP, thus contributing to solving food and nutrition insecurity challenges. Keywords: Orange-fleshed sweet potatoes, bio-fortified, beta-carotene, pretreatments