Effect of Ultrasound-Assisted Osmotic Dehydration Pretreatment on Drying Kinetics and Effective Moisture Diffusivity of Mirabelle Plum

Abstract

Conventional air drying of plum is a slow and energy-intensive process because of its waxy skin having low permeability to moisture. Ultrasound application in drying foodstuffs aims at reducing the time needed to process and improve the quality characteristics of dried product. In this study, the effect of ultrasound-assisted osmotic dehydration pretreatment on drying kinetics and effective moisture diffusivity of Mirabelle plum during drying was examined. Pretreatments including ultrasonication time at two levels (10 and 30 min), osmotic solution concentration at two levels (50 and 70% sucrose) and immersion time in osmotic solution at four levels (60, 120, 180 and 240 min) were performed. Results showed that application of ultrasound-assisted osmotic dehydration pretreatment can lead to a significant increase in effective moisture diffusivity (from 5.84 × 10−9 to 7.36 × 10−9 m2/s) resulting in a 20% decrease in drying time, and thus involving a higher energy saving and preserving heat labile nutrients. Practical Applications The Mirabelle plum is appreciated by consumers for its health benefits. Drying is among the most popular methods to prolong its shelf life. The Mirabelle plum's skin consists of an underlying amorphous wax layer adjacent to the cuticle proper, together with crystalline granules of wax protruding from the surface. The waxy skin has a low permeability to moisture that increases drying time leading to possible quality deteriorations. Therefore, any pretreatment for plum drying processes that decreases drying time by reducing the initial moisture content and preserves the prune (dried plum) quality is of considerable interest. Methodologies such as ultrasound-assisted osmotic dehydration can be implemented as an alternative pretreatment to reduce drying time and, consequently, processing costs. This study provides information on the effects of operating conditions (sonication time, concentration of osmotic solution and immersion time in the osmotic solution) on drying kinetics, total processing time and effective moisture diffusivity.