Diffusion

Authored by: Varzakas Theodoros

Food Engineering Handbook

Print publication date:  December  2014
Online publication date:  December  2014

Print ISBN: 9781482261691
eBook ISBN: 9781482261707
Adobe ISBN:

10.1201/b17843-10

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Abstract

Diffusion is a ubiquitous physical process that describes mass transfer purely under the influence of concentration gradients. Molecular diffusion is the transfer of individual molecules through a fluid by means of random individual movements. Diffusion phenomena are particularly significant when they are a limiting step in a sequence of interrelated processes or where diffusion is the principal process taking place. Within the food industry, examples of diffusion-dependent processes include the following: the extraction by leaching of sugar, apple juice, and coffee; vegetable blanching where both diffusion from and into the food may be important such as loss of nutrients by leaching; canning; preservation by salting and pickling; dehydration operations; frying, where the oil uptake and the water loss occur simultaneously; acidification, where the acid is diffused from the solution into the food. In leaching operations, the leachable solids are contained in a framework of insoluble solids, known as the marc, the occluded solution and, in some cases, sparingly soluble solids or solute precursors (Schwartzberg and Chao 1982). Diffusion occurs primarily within the occluded solution but the marc restricts the diffusion process and strongly affects the rate of diffusion. Therefore, when a material is being leached from a dry solid, the solid must first imbibe the solvent, to dissolve the external solute, before diffusion begins (Varzakas 1998a,b; Varzakas et al. 2005a).

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