Hot Air Drying Optimization and Modelling of Peach Slices

Peach (Prunus persica) is a highly perishable fruit with short shelf-life and susceptible to mechanical damage during harvest and post-harvest operations such as sorting, processing, packaging, and transport. Therefore, converting peaches into dehydrated products will not only reduce their post-harvest losses but also retain their nutritional and sensory qualities. Drying is widely used in food industry, to prolong the shelf life, improve the food stability and reduce the quality changes. In this context, drying combined with a pretreatment (chemical or physical) is an efficient method for reducing physico-chemical and nutritional changes and for improving preservation. In this study, the influence of hot air drying temperature and an innovative dipping solution (carbohydrate/salt diluted solution of trehalose, sucrose and NaCl) on drying kinetics, quality parameters (i.e. colour, shrinkage, antioxidant activity), volatile aroma compounds (VOCs) and rehydration behavior of peach slabs was investigated. Drying experiments were conducted in a convective drier at 45, 50, 55 and 60 °C with a fixed air velocity of 2.3 m/s. Peach fruits were subjected to GC/MS analysis to identify the volatile organic compounds (VOCs), extracted using a SPME fiber of divinylbenzene/carboxen/polydimethylsiloxane. Afterwards, all dried slabs were rehydrated in distilled water at 30 °C. The obtained results revealed that pre-treatment solution not only affected the drying kinetics of peach slabs, it also improved the quality attributes. The combination of pre-treatment and higher drying temperature (60°C) exhibited shorter drying time, better color retention, less shrinkage, and higher rehydration capacity. The major VOCs of fresh peach were: benzaldehyde (553 μg/kg d.b.); 4-decalactone (293 μg/kg d.b.); (E)-2-hexenal (131 μg/kg d.b.), cis-3-hexenylacetate (129 μg/kg d.b.); isopentanoic acid (87 μg/kg d.b.) and 2-phenylethylalcohol (75 μg/kg d.b.). Anyway, the contribution of the substances to the odor note of the fruit depended not only in the quantities of the compounds, but also mainly on their perception threshold. VOCs with odor activity value (OAV) > 1, considered to contribute to the aroma of the peach, were: 4-decalactone (OAV=2.92); benzaldehyde (OAV=1.73); (E)-2-hexenal (OAV=1.60) and 2-phenylethylalcohol (OAV=1.21) All the compounds with the odorous impact of the peaches were significantly reduced after drying, modifying fruits odorous profiles. On the other hand, several new compounds were formed, because of various chemical transformations, including the Maillard reaction. The dried samples compared to fresh peaches showed an increase in aldehydes, ketones and especially terpenes and acids. The highest concentration of these compounds was higher in TR samples. In particular, the concentration of nonanal, described as having partially green aromas, with citrus characteristics, had in all dried samples OAV > 1, with maximum values in TR 50°C (OAV=2.43). Furthermore, a diffusion model including the effect of shrinkage was developed to describe the drying process of both untreated and treated peaches. The experimental data of moisture ratio during drying were well predicted by the diffusion model developed. The calculated values of effective diffusivity increased with increasing drying temperature. Moreover, the treated peach slices had higher moisture diffusivity values in comparison with the untreated peach samples. The calculated values of activation energy indicated that a little less energy is required for the drying of treated peach samples respect to untreated. This behaviour showed that the pre-treatment aids moisture diffusion and evaporation and thus reduces slightly the energy involved in the drying process. In conclusion, overall results showed that for treated peach slabs drying conditions of 60°C air temperature are the optimum for the product quality in terms of drying kinetics, colour, shrinkage, rehydration behaviour and aroma.