Diffusion and the Potato

A raw potato is a great source of vitamins and minerals, and a particularly rich source of potassium and protein. However, it is well known that processed or cooked vegetables of any variety contain a much lower source of nutrients than raw vegetables. The key to the decrease lies in the diffusion of the nutrients out of the vegetable during processing or cooking.

Gekas, Íste, and Lamberg (1993) of the University of Lund in Sweden, have performed experiments consisting of the diffusion of heat sensitive nutrients, such as glucose, fructose, citric acid, potassium, calcium, and magnesium, from the tissue of the potato. The loss of these nutrients primarily occur during industrial processing that consists of blanching raw potato in order to modify the texture of the final product. However, little consideration is usually given to the effects of the blanching step on the nutrients contained in the potato tissue. Due to the thermal processes involved, the blanching promotes the diffusion of nutrient solute out of the potato tissue.

The experiment performed by Gekas, Íste, and Lamberg consisted of thin cylindrical slices of potato, that, due to the high symmetry of the slice, was modeled by a one dimensional solution of Fick's second law, just as any other non-steady state diffusion problem. Numerous examples of this type of problem may be found in almost any text book on materials science and engineering, such as Callister, or in text books strictly devoted to thermal processes, and in the example problems section of this Web site.

The results achieved by Gekas, Íste, and Lamberg in their experiments on diffusion of nutrients in potato tissue show that nutrient movement is hindered by the structure of the tissue, in the same basic way that atomic structure hinders diffusion in metals. However, similar to other diffusion processes, the hindrance decreases with an increase in temperature. Also, as different materials have different diffusion coefficients, the nutrients in the potato also have different diffusion coefficients, apparently related to the structure of the nutrient, whether molecular or ionic. The difference of the diffusion coefficients for the several different types of nutrients is analogous to different diffusion coefficients for metal or gas atoms in alloys.

Please see Gekas, Íste, and Lamberg, Diffusion in Heated Potato Tissue, Journal of Food Science, July-August, 1993, v58, n4, pp827-831.

and Callister, William D., Jr., Materials Science and Engineering: an Introduction, John Wiley & Sons, 1994, pp90-105.

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