The rate of diffusion of water from any material during osmotic dehydration is dependent upon factors such as type of osmotic agent, concentration of the osmotic solution, temperature, the size and geometry of the material, the solution-to-material mass ratio and the level of agitation of the solution. There are several publications which describe the influence of these variables on mass transfer rate (Lerici et al., 1985; RaoultWack, 1989; Raoult-Wack, 1994; Rastogi et al., 1997; Rastogi and Niranjan, 1998; Rastogi et al., 1999; Corzo and Gomez, 2004).
However, the variables mentioned above can be manipulated over a limited range; outside of these ranges, the quality was adversely affected even though mass transfer rates may be enhanced (Rastogi et al., 2002). There are also some techniques which can combine with osmotic dehydration, and have the ability to alter membranes in order to enhance mass transfer rate.
They include: ultrasound (Rodrigues and Fernandes, 2007) high-intensity electric field (Rastogi et al., 1999), or high hydrostatic pressure (Akyol et al., 2006) and microwave (Li and Ramaswamy, 2006c; Azarpazhooh and Ramaswamy, 2010a,b). The choice of process conditions depends on the expected water loss, soluble solids gain, and the sensory properties of the food products.
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Influence of size and shape on the mass transfer
Some research has been done on the influence of size and shape on the mass transfer kinetics. The surface area to volume ratio has been shown to be the influencing factor with higher ratios favoring better osmotic dehydration rates. Islam and Flink (1982) reported that the size and geometry of the food has some influence on the extent of final solute concentration.
Especially during short dehydration times; at such times, dehydration was primarily a transport phenomenon related to surface area. Lerici et al. (1985) compared osmotic drying of apple slices of four different shapes of (i.e slice, stick, ring and cube) and reported that the solids content increased with a decreasing surface.
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