6 2 0.25 s 0 -I 1 0.21 B P L 2 0.15 r 0 0.1 COMMUNICATIONS L -c In a recent article (York 1981) water sorption and &sorption isotherms of gelatin, maize starch and maize starch : barbital mixtures were obtained and analysed according to an approach developed by Young & Nelson (1967a). Young & Nelson attempted to relate equilibrium water vapour sorption and desorption with biological materials to relative humidity by assuming three basic mechanisms of water uptake into a biological cell: (1) a monomolecular layer of water on the outer surface of the cell; (2) multimolecular layers of water on top of the first layer; and (3) water absorbed within the cell. The approach used to derive appropriate equations was essentially the same as that used to derive the Langmuir and BET equations for vapour adsorption (Brunauer et a1 1938) with an additional contribution by the absorbed water. In this regard, an important stipulation in this model is that water entry into and exit from the biological cell is determined by the amount of water bound to the outer cell surface. Young & Nelson concluded that the hysteresis observed in moisture sorption and desorption curves with such materials arises because absorbed moisture within the cell cannot be removed during desorption until sufficient water from the monolayer has been removed from the surface. Two equations were derived to describe total water sorption, M,, and desorption, Md. where: and M, = A(9 + a) + B$ Md = A(6 + a) + BB(RH,,,)(2) The exact definition of each parameter isdescribed by Young & Nelson (1967a), so it is sufficient to point out Table 1. Various constants obtained for water sorption by wheat and maize starch from the Young & Nelson model.+Material A B E Reference Wheat 0,053 0.067 0. I I Young& Nelson (1967) Wheat 0,050 0.067 0.10 Thisstudy Maize starch 0412 0.12 0.15 York(1981) Maize starch 0.015 0.11 0.14 Thisstudy t Simultaneous multiple regrerional anal sis of sorption and deso tion data. correlation coeIficients in this stu& are all no less than 0 . 3 * Correspondence. WI that A(O + a) represents the contributions of monolayer and multilayer sorption, while the second term in each equation represents the absorbed water uptake. A third parameter which determines the values of 9 and (Y directly, and 9 indirectly, is E. whereThe parameter, E, is analogous to the C term in the BET equation, where q, is the sorption energy of molecules bound to the surface of the cell and qL is the energy of water condensation with itself, reflecting the energy associated with multilayer sorption.
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