Heat and Mass Transfer

The calculation of heat transfer rates during blanching requires a knowledge of the geometry of the vegetable piece, the blanching conditions, including the initial food temperature and blanch medium temperature, and the thermal properties of the vegetable. For unsteady-state heat conduction, numerical solutions to Fourier's general law are available for the central temperature history of the three elementary shapes; an infinite slab, an infinite cylinder, and a sphere (70). Thermokinetic models for enzyme inactivation have also been studied using unsteady-state heat conduction procedures (71).

The two contributions to the total resistance to mass transfer in the blanching of vegetables are the surface resistance due to convection and the internal resistance due to mass diffusion. These may be represented by Fick's first and second laws, together with a mass balance at the interface. When there is sufficient agitation of the blanching liquid, the surface resistance becomes small, and it can be assumed that the total resistance is due to only the internal resistance. Then only the solution to Fick's second law is required, and this is available for the infinite slab, infinite cylinder, and sphere, with the mean concentration obtained after integration with respect to position as a function of time, again in nondimensionalized form (72).

These solutions were originally developed for drying applications over a wide range of concentrations; however, in the case of blanching vegetables, the concentrations are much smaller and near to zero. The solutions have been recalculated for smaller increments in this range of concentrations using the first ten terms for the three series (6). Where the apparent diffusion coefficient for a solute is known at various temperatures it is possible to predict the changes in concentration that will occur under defined conditions of blanching (73,74).

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