Héctor A. Iglesias

Water sorption isotherms of foods and foodstuffs : BET or GAB parameters?

Abstract 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.

Some features of the parameter k of the GAB equation as applied to sorption isotherms of selected food materials

Abstract This work reports that the parameter k (factor correcting the properties of the multilayer) in the GAB sorption equation varies from nearly unity to as low as 0·56 for a large variety of food constituents and related materials. Values of k for proteins fall in a relatively narrow range of 0·82–0·88 (average 0·84±0·03 SD), while for starchy materials they fall in the range 0·70–0·77 (average 0·74±0·03 SD).

Adsorption isotherm of amorphous trehalose

The water adsorption isotherm at 25°C of freeze-dried amorphous trehalose and trehalose with the addition of maltodextrin (MD) DE 10.9 was obtained. Moisture uptake as a function of time at various relative humidities (RH) was also measured. At low relative humidities (RH) amorphous trehalose adsorbs moisture but at 44% RH and above the isotherm exhibits a plateau indicating crystallisation (formation of the dihydrate) of amorphous trehalose and this was reflected on the shape of the isotherm. Trehalose crystallisation was delayed in the system containing maltodextrin (50: 50) as compared with pure trehalose.

An alternative to the Guggenheim, Anderson and De Boer model for the mathematical description of moisture sorption isotherms of foods

Abstract The fitting abilities of an isotherm equation originally developed by Ferro Fontan et al. ( J. Food Sci. , 47 , 1590–4, 1982) (FF) was compared with the well-known GAB model. For this purpose literature data of 156 food isotherms (comprising 92 different food products) were examined with both models. The results of the evaluation indicated that the FF equation is an accurate tool for the mathematical description of food isotherms. The FF equation represents accurately the sorption isotherm of 92 different food products (proteins, starchy, cereals and oilseeds, meats, vegetables, miscellaneous) in the range of water activity 0.10–0.90 with only 2–4% average error in the predicted moisture content. In fact, the average error for the 156 isotherms examined was only 3.3%. The FF equation only has three parameters that need determining (as has the GAB model); thus it is proposed as an alternative equation to the widely used GAB model.