Osmotic treatment of fruits and vegetables preceding convective drying may strongly affect properties of the final product (Lewicki and Lukaszuk, 2000; Lewicki and Pawlak, 2003). During osmotic dehydration, many aspects of cell structures are affected such as alteration of cell walls, splitting of the middle lamella, lysis of membranes (plasmalemma and tonoplast), tissue shrinkage (Alvarez et al., 1995) which could strongly influence the transport properties of the product during processing.
All these phenomena cause changes in the macroscopic properties of the sample, such as optical and mechanical properties, which are related to the product appearance and texture, respectively. All these changes greatly affect organoloptic properties of the osmo-dehydrated plant due to solute uptake and leaching of natural acids, color, and flavor compounds out of osmodehydrated plant tissue; as a result, natural composition of the product is modified (Lazarides et al., 1995).
Although compositional changes may have a positive and negative effect on the final product, rehydration of osmotically dried fruit is lower than in the untreated fruit due to the rapid impregnation of a subsurface tissue layer with sugar (Nsonzi and Ramaswamy, 1998a); moreover, if the osmosis takes more time, the rehydration rate would be lower.
Many investigators demonstrated that the quality (color, texture and rehydration capacity) of air, freeze or vacuum- dried fruits and vegetables could be improved by a prior osmotic step (Flink, 1975; Hawkes and Flink, 1978; Lerici et al., 1985; Nsonzi and Ramaswamy, 1998a). There have been numerous research studies on the application of color change during osmotic dehydration.
Impact of osmotic dehydration on color
The color of the products is measured by lightness (L* value), redness or greenness (a* value) and yellowness or blueness (b* value), during or after drying. Falade et al. (2007) reported transparency and that the color of the fruit may alter favorably due to physical and chemical changes during osmotic dehydration.
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They evaluated L*, a*, b* values of osmosed and osmo-oven dried watermelon, and reported that color parameters increase with an increase in osmotic solution concentration. Osmotic dehydration improves fruit quality by stabilizing color parameters and allows less color loss of fruit by enzymatic oxidative browning due to infusion of extensive sugars. In addition, reducing the water activity of samples also decreases the non-enzymatic browning reaction (Krokida et al., 2000).
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