Heliyon (Jun 2021)
The kinetics of thin-layer drying and modelling for mango slices and the influence of differing hot-air drying methods on quality
Abstract
This study compared the thin-layer drying kinetics of hot-air methods, namely, convective oven drying (OVD), uncontrolled solar drying (UAD) and modified ventilation greenhouse solar drying (MVD). Additionally, the effects of these drying techniques on colour, rehydration characteristics and microstructure of Tommy Atkin mango slices were investigated. The experiments were conducted on mango slices of three different thicknesses: 3 mm, 6 mm and 9 mm. The drying curves generated from the experimental data revealed that the rate of drying increased with thickness and that a thickness of 3 mm is optimal. It was discovered that increased drying rates resulted in a decrease in the drying time. When 3 mm slices were dried using OVD and MVD, the duration of the drying process was reduced by 85% and 80%, respectively, in comparison to the samples dried under UAD conditions. Lemon juice pre-drying treatment had no significant (p < 0.05) effect on the drying rate or duration of the drying process. Non-linear regression analysis was used to optimise the drying coefficients by fitting the moisture ratio data to eleven suitable thin-layer models. The model parameters developed by Midilli et al. performed the best in terms of predicting the experimental moisture ratio (R2 = 0.9810-0.9981, χ2 = 1.465 × 10−6-3.081 × 10−5 and RMSE = 0.0003-0.0004). Additionally, increasing the slice thickness to 6 mm and 9 mm prolonged the drying times, resulting in significant changes in sample quality, including the total colour (ΔE), rehydration and microstructure. In comparison to OVD- and MVD-dried samples, UAD-dried samples exhibited the greatest colour change and had the highest rehydration ratio values. Also, the surface of the UAD-dried samples developed a more porous structure with distinct cracks. Based on the results, MVD was determined to be a viable alternative method for drying 3 mm mango slices on a large scale.
