The aim of the present work is to solve the dilemma about the differences between the values of the monolayer and the energy parameters obtained by the regression of water sorption data by foods and foodstuffs using the Brunauer, Emmett and Teller (BET) two-parameter isotherm or the Guggenheim, Anderson and de Boer (GAB) three-parameter isotherm. It is shown that the GAB values are more general and have more physical meaning, and that the two BET parameters can be calculated in terms of the three GAB-parameters. Furthermore, the marked dependency of the BET constants on the regression range as well as the typical upswing at higher water activities observed in the so-called BET plots are explained. It is also shown that the rough agreement early reported by L. Pauling, J. Am. Chem. Soc. 67 (1945) 555-557 between monolayer values and number of polar groups in the aminoacid side chain in several proteins is enhanced if the former are evaluated by means of the GAB sorption equation.
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