Osmotic dehydration and other applications

The simultaneous application of vacuum to fruits throughout the entire osmotic dehydration process, or in the first minutes of the treatment or through regular pulsed cycles, was regularly discussed by Fito's group and others. These authors dealt largely with mass transfer kinetics and rates in vacuum osmotic dehydration (Fito, 1994; Fito and Pastor, 1994; Shi and Fito, 1994; Shi et al., 1995; Panades-Ambrosio et al., 1996; Rastogi and Raghavaro, 1996; Castro et al., 1997; Martinez-Monzo et al., 1998b), with microstructural modifications (Barat et al., 1999, 2000), and with composition and physicochemical changes (Chafer et al., 2000; Moreno et al., 2000; Chiralt et al., 2001).

Distance (mm)

Fig. 18.3 Texture analysis profiles of frozen-defrosted 1 cm3 apple cubes, vacuum infused with gelatine and non-infused, representing shearing force ('cuttability') versus cutting distance.

Distance (mm)

Fig. 18.3 Texture analysis profiles of frozen-defrosted 1 cm3 apple cubes, vacuum infused with gelatine and non-infused, representing shearing force ('cuttability') versus cutting distance.

It emerges from these various studies that vacuum application during osmotic treatment has all the more effect because the product is porous. The vacuum accelerates solute exchange towards the matrix thanks to a forced and early penetration of the solution; it is above all favourable to water extraction, as water molecules can migrate more easily in the intercellular pores filled with liquid, leading to higher water loss levels. Generally, pulsed vacuum osmotic dehydration is recommended because of its economical advantages and satisfactory mass transfer improvement.

There was no significant difference in the volume change in fruits at the macroscopic level between atmospheric pressure and vacuum osmotic dehydration caused by the dehydration effect, but cell deformation and cell wall shrinkage were not as important in vacuum treatment because of the absence of gas in the food structure.

In the works listed above, the noticeable quality improvements (pH, water activity, stability, colour, texture, etc.) in fruits treated by 'vacuum osmotic dehydration' are mainly explained by the protective effect of infused solutes or by a larger overall reduction in water content in the products.

Other interesting applications offered by vacuum technology have been proposed in the literature:

• vacuum hydration of dry beans (Sastry et al., 1985): vacuum hydration pretreatments greatly decreased the incidence and severity of splitting in the canned product and accelerated water uptake;

• vacuum infiltration of sodium chloride intopotatopiecesbeforeohmicheating (Wang and Sastry, 1993): this infiltration is especially effective on particles with a thickness of less than 1 cm, modifyingtoasignificantdegreetheelec-trical conductivity of the product;

• designing a bioindicator to check the effectivenessofcontinuousasepticheat treatments of particles in food liquid (Sastry etal.,1988):thebioindicatoris made from mushrooms pieces vacuumimpregnatedwithalginatesolutionand spores of B. stearothermophilus: the bacterialsporesareimmobilisedbyfor-mation of the alginate gel after dipping in a calcium bath;

• vacuum application of browning inhibitors to cut apple and potato (Sapers et al, 1990): ascorbate- or erythorbate-basedinhibitorswereused toprolong colour stability or appearance of fresh cut products stored at 4°C.

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