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Kt ki Hkg

Using equation 78 and noting that PA = H$A and P% = Hca gives fusion of gases (02, C02, C2H4, etc.), water vapor, and volatile aroma components through one or more layers of polymeric films. The complex physical structure and chemical composition of foods, which may vary even within the same food and may change during processing or storage (10), makes the direct application of mass transfer and diffusion principles to food processing difficult. In this section some applications of mass transfer and diffusion principles are presented.

Diffusion in Gases

There are few cases of direct application of this principle to food processing, because convection effects are usually dominant. Corrections to the diffusivity are often necessary to account for such effects. The bulk flow of gases (convection) is caused by differences in total pressure and temperature created during processing. In many cases, mechanical equipment such as agitators, fans, or blowers are used to increase convection and, therefore, the transfer rate. Typical examples of diffusion in gases are (1) the evaporation of liquids from containers (e.g., benzene during cleaning of glass containers or hydrogen peroxide during sterilization of plastic containers for use in aseptic processing and packaging); (2) the escape of volatile-aroma food components from open storage tanks, mixing equipment, or concentration vessels; and (3) the loss of carbon dioxide from carbonated beverages.

Diffusion in Liquids

The phenomenon of diffusion in liquids is common in food processing, and its applications are extensive. A typical example is the extraction of sucrose from sugarcane or sugar beets (11), whereas other examples include the extraction of caffeine from coffee beans and edible oil from soybeans. Distillation, another application of diffusion in liquids, is used to remove or recover volatile components from foods (12), to produce ethanol and alcoholic beverages from fermentation liquids, and to purify solvents (10).

Comparison of equation 78 with equation 85 gives

so that the overall mass transfer coefficient of the liquid phase can be related to that of the gas phase by Henry's constant.

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