The effect of ohmic heating on nutrient loss diffusion

Studies40,42 have shown that compared with conventional heating, ohmic heating enhanced diffusion of charged species between solid particles and the surrounding liquid, which could have some impact on loss of nutrients from solid particles to carrier liquid. This becomes undesirable only if the carrier liquid is not to be consumed together with the solid particles.

Figure 19.3 shows that the transfer of betanin dye between beetroot and the surrounding fluid increases linearly with applied electric fields. One explanation for the differences in this phenomenon between the ohmic heating and conventional heating is 'electroosmosis,'43 which results in increased transport through the cell membrane.

Another mechanism, 'electroporation', may also be responsible for enhanced diffusion between plant tissues and the surrounding liquid. When an electric field is applied across a membrane, it causes an induced membrane potential. When the induced membrane potential reaches a critical level, membrane ruptures occur, resulting in the formation of pores in the cell membrane,44,45 and consequently increased permeability.

On the other hand, ohmic heating is superior over conventional heating in the case of blanching of plant tissues such as vegetables.46 The loss of soluble solids in water blanching of vegetables affects both the quality and nutrition of the products. In addition, blanching water containing a large amount of soluble solids

Betanin Cell

400 800 1200 1600

Electric field (V/cm)

Fig. 19.3 Diffusion of betanin dye between solid beetroot and surrounding fluid as a function of applied electric field.42

400 800 1200 1600

Electric field (V/cm)

2000

Fig. 19.3 Diffusion of betanin dye between solid beetroot and surrounding fluid as a function of applied electric field.42

cannot be discharged without proper treatment. Mizrahi46 compared hot water blanching and ohmic heating blanching. Hot water blanching was carried out by placing sliced or diced beet into boiling water and taking water samples every 30 seconds; blanching by ohmic heating was done by immersing whole, sliced or diced beets in an aqueous salt solution and passing an AC voltage through the medium. Betanine and betalamic acid concentration in the samples were determined. Solute leaching with both methods followed a similar pattern, and was proportional to the surface to volume ratio and the square root of the process time. By removing the need for dicing and shortening the process time, ohmic heating blanching considerably reduced by one order of magnitude the loss of solutes during blanching of vegetables.

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