Sugar, osmotic dehydration

The process involves placing the solid food (whole or in pieces) into solutions of high sugar or salt concentration. Le Maguer (1988), Raoult-Wack (1994), Fito and Chiralt (1997), Behsnilian and Spiess (1998), Spiess and Behsnilian (1998), Lazarides et al. (1999), and Torreggiani and Bertolo (2002) have reviewed the basic principles, modeling and control, and specific applications of osmotic dehydration on fruit and vegetables. Additionally, the most recent research advances in this field can be obtained from the European-founded network on osmotic treatments (FAIR, 1998). 

During osmotic dehydration of apple, pumpkin, and carrot in sugar solution at 30 °C, the rate of water loss was 5-10 times higher than the rate of solid gain and depended on advancement of the dewatering process (Kowalska and Lenart, 2001). Under the same dewatering conditions, pumpkin and carrot reached smaller water contents than apple (Figure 3). 

An empirical equation derived based on osmotic dehydration of apple slices could predict rate of osmosis F, that is, percentage of dehydration of any given fruit slices of specific size with time T, given the concentration of sugar (% B) and the temperature as follows [59] ... 

T.R.A. Magee, A.A. Hassaballah, and W.R. Murphy, Internal mass transfer during osmotic dehydration of apple slices in sugar solution, Irish J. Food Sci. TechnoL, 7(2) 147 (1983). 

K. Videv, S. Tanchev, R.C. Sharma, and V.K. Joshi, Effect of sugar syrup concentration and temperature on the rate of osmotic dehydration of apples, /. Food Sci. TechnoL (India), 27(5) 307 (1990). 

The models based on the irreversible process thermodynamics show that the cell membrane (plasma lemma) represents the major resistance to mass transfer. This is contradicted by findings of Raoult-Wack et al. [46-48], who showed that membranes are not necessary for osmotic dehydration and merely diffusive properties of the material are responsible for high water flux with only marginal sugar penetration. These authors suggest the following mechanism. 

The role of intercellular space and capillary flow in osmotic dehydration was well documented by Hto et al. [51-53]. On this basis the nondiffusional mass transfer model was developed incorporating hydrodynamic mechanism (HDM). Studies done by the same group [19,54] showed that long time of osmotic process is needed to obtain a fully developed water and sugar concentration profiles. A model based on the advancing disturbance front (ADF) was proposed that allows prediction of sample concentration during osmotic dehydration. 

Solutions of sugars are mostly used to dehydrate fruits and glycerol, starch syrup, and sodium chloride are used for vegetables [62,73,91,99]. Sucrose is the most frequently used substance [17,65,100-104]. The control of pH of sucrose solution is recommended for banana slices osmotic dehydration [105]. It was also shown that control of pH of sucrose solution affects the course of osmotic dehydration of apple and carrot [106]. Addition of ascorbic acid to sugar solution is practiced to minimize browning of fruit pieces during osmotic process [72]. Sucrose can be substituted in part by lactose [15]. 

It has been found that the addition of low molecular weight substances such as sodium chloride, malic acid, lactic acid, and hydrochloric acid in concentrations of l%-5% to sugars or starch syrups improves the process of osmotic dehydration. In general, they promote removal of water from the material. Calcium chloride and malic acid were added to sucrose to improve the texture of osmosed apples [130]. 

FIGURE 32.8 The effect of sugar syrup concentration on the course of osmotic dehydration of apples at 50°C (solid line = 70°Bx dashed line = 60°Bx dotted line= 50°Bx). (Adapted from Farkas, D.F. and Lazar, M.E., Food TechnoL, 23, 688,1969.)... 

R. Giangiacomo, D. Torreggiani, and E. Abbo, Osmotic dehydration of fruit. Part I. Sugars exchange between fruit and extracting syrups, J. Food Pmcess. Pres., 77 183 (1987). 

A. Paijoko, S.M. Rahman, K.A. Buckle, and C.O. Perera, Osmotic dehydration kinetics of pineapple wedges nsing palm sugar, LWT, 29 452 (1996). 

A.M. Sereno, R. Moreira, and E. Martinez, Mass transfer coefficients during osmotic dehydration of apple in single and combined aqueous solutions of sugar and salt, J. Food Eng., 47 43 (2001). 

Warczok X, Gierszewska M., Kujawski W, Guellet C. (2007), Application of osmotic membrane distillation for reconcentration of sugar solutions from osmotic dehydration, 6 ep. Purif. Technol, 57,425-429. 

FIGURE 28.8 The effect of sugar syrup concentration on the course of osmotic dehydration of apples at 50°C (70°Bx ... 

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