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 ultrasoundassisted 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 m 2 /s) resulting in a 20% decrease in drying time, and thus involving a higher energy saving and preserving heat labile nutrients. PRACTICAL APPLICATIONSThe 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.Pretreatments were structured in combinations of two ultrasonication times (both at 40 kHz): 10 and 30 min; two osmotic solution concentrations: 50 and 70% sucrose in water (% w/w); and four immersion times in osmotic J. DEHGHANNYA, R. GORBANI and B. GHANBARZADEH PLUM DRYING BY ULTRASOUND AND OSMOTIC DEHYDRATION

Convective drying in hot air is still the most popular method applied to reduce the moisture content of fruits and vegetables. Conventional hot-air drying of Mirabelle plum is considered to be a slow and energy intensive process. This is due to the fact that the waxy skin of Mirabelle plum has low permeability to moisture, a fact which results in high shrinkage. The aim of this study was to investigate the effect of ultrasound-assisted osmotic dehydration pretreatment on shrinkage of Mirabelle plum as a function of moisture content with the end goal of optimizing operating conditions that minimize shrinkage of the produce during drying. Results showed that application of ultrasound-assisted osmotic dehydration led to a significant (p < 0.05) decrease in shrinkage (from 76.41 to 64.05%). A linear relation between moisture loss and shrinkage was observed. Results indicated that shrinkage may be easily estimated from changes in moisture content, and independent of the drying rate. Inversely, determination of shrinkage would provide an indirect indication of moisture content.

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Prunes are generally produced by drying fresh plums with a stream of hot air. Changes in various quality indicators, such as bulk density of foodstuffs, have substantial impact on the performance of drying processes. Optimization of bulk density is, therefore, a requirement for development and perpetuity of drying technologies. The aim of this study was to investigate the effect of ultrasound-assisted osmotic dehydration pretreatment on bulk density of Mirabelle plum at various operating conditions (sonication time, concentration of osmotic solution and immersion time in the osmotic solution). Results showed that application of ultrasound-assisted osmotic dehydration, compared to control sample, led to a significant (p \ 0.05) increase in bulk density from 1.33 to 9.90 %. Experimental data of bulk density variations versus dimensionless moisture content were well fitted to an empirical power model with high R 2 .