Evaluation of the effects of pre-drying treatments and drying methods on the drying kinetics and quality of Tommy Atkin mango slices.

Abstract

Mango is a perishable fruit, harvested once a year during the summer season. Smallholder farmers growing mango experience relatively high post-harvest losses because they receive large volumes of produce at the same time. Drying is a preservation method proven second to cooling in performance. In South Africa, dried mango fetches higher returns compared to canned mango, and mango juice, atchar and jam. Open-Air solar Drying (OAD) is a popular drying method used for producing dried mango. However, this method of drying has setbacks resulting in produce quality loss. Convective Oven Drying (OVD) is a more efficient drying method, however, it has high-energy consumption. In South Africa, there is lack of research on hot-air drying methods, as well as their performance when drying mango fruit. With the current shift in use of renewable energy for drying operations, research is focusing on improving solar drying technologies. These include increasing the drying capacity and reducing the drying time through modifying a greenhouse. Considering the research gap in South Africa on drying technologies and the limitations of open-air solar drying, this study comparatively assessed the performance of three drying methods, namely, (a) Convective Oven Drying (OVD), (b) Open-Air solar drying (OAD) and (c) a Modified Ventilation Solar Drying (MVD). OVD was carried out at a set temperature of 70˚C, OAD and MVD at ambient temperature of 15.55˚C -36.77˚C, at an RH of 22.96%-79% and solar radiation of 317W.m-2- 1016 W.m-2. The MVD improved the ambient conditions to obtain an average maximum temperature of 64.26 ˚C and RH of 17.6%. The drying time was longer for mango slices dried in OAD, MVD and OVD, respectively. The lemon juice pre-treatment did not affect the drying time. The drying time was reduced for 3 mm, as compared to the 6 mm and 9 mm dried mango slices, due to the relatively high drying rates. Drying took place in the falling drying rate period for most mango samples, indicating that diffusion was the driving mechanism in the drying experiments. The effect of the drying methods on the drying kinetics of mango slices (3 mm, 6 mm and 9 mm) as well as the effect of lemon juice pre-treatment was investigated. Non-linear regression analysis was used to assess the empirical model that best fits the experimental moisture ratio data. The quality parameters that were evaluated included the colour, rehydration, sensory properties, changes in mango microstructure and microbial changes. The empirical model that was best fit for the drying data was that of the Midilli et al., followed by Page model because a higher R2, a lower root mean square (RMSE) and a lower chi-square (X2) was obtained from non-linear regression analysis. A quality analysis indicated that colour change (ΔE) was not significantly (P>0.05) affected by pre-treatment, although control samples that were dried in the OAD had a relatively higher colour change, resulting to browning. The rehydration ratio and electronic microscopy (SEM) showed structural changes in dried samples, with thicker mango slices having a relatively lower rehydration ratio, and the SEM scans dominated by cracks and pores, which were much more visible for 9 mm mango slices. Sensory evaluation results indicated that the panellists preferred the flavour and colour of pre-treated 3 mm mango slices compared to thicker control and pre-treated dried mango slices. In addition, the overall acceptability of dried mango was relatively higher for MVD-dried, than OVD- and OAD-dried mango slices, respectively. The fungi and anaerobic bacteria levels found in dried mango slices were higher than the recommended levels of 1x103; however, there were no pathogenic microbes detected in the fresh and dried samples. The study findings show that dried mango is an acceptable produce to consumers, especially in areas like Kwazulu-Natal, were mango is scarce. MVD is a drying method, which is practical and can solve the shortcomings of OAD. This method is a renewable energy alternative to OVD and further improvement is required to increase temperature and ventilation is necessary to reduce drying time.