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H. A. Becker. Copyright 1960 by the American Association of Cereal Chemists, Inc. The absorption of liquid water by the wheat kernel proceeds by a heterogeneous mechanism. There is a very rapid initial absorption in which the pericarp is saturated by capillary imbibition. The average moisture gain due to this absorption is 0.045 g. per g., dry basis, at 22 C., but decreases with increasing temperature. Subsequent absorption is directly proportional to the square root of the period of immersion and is described by the diffusion equation [equation] where 1 - M[overbar] is the relative moisture gain and S/V is the surface-to-volume ratio. The average diffusion coefficient is given by the Arrhenius relation D = 30 exp.(-12300/RT). The activation energy, 12.3 kcal. per mole, is identical with that previously found for vacuum drying, but the diffusion coefficient itself is only one-fifth to one-third as large. The effective moisture content at the surface of an immersed wheat kernel is 0.75 g. per g. and is identical with the critical moisture content in drying. |
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