Apparent surface area of selected meal extrudates

We have investigated water vapour adsorption and calculated the specific surface area of meal extrudates. The Brunauer-Emmet and Teller (BET) equation was used to analyse experimental data on water vapour adsorption and to calculate the apparent surface area (S app. ) from both adsorption and from desorption branches of the adsorption isotherm. Values of S app. estimated from adsorption isotherms ranged: from 214 to 294; from 202 to 210; and from 199 to 232 m 2 g -1 for potato, wheat and rice extrudates, respectively. If the desorption isotherms have been used, the ranges of S BET were 327-347, 274-281, and 287-295 m 2 g -1 , respectively. We have observed linear relationship between adsorption- and desorption-apparent surface areas. In the case of rice extrudates the correlation coefficients, R 2 , were very high. Similar linear relationship has been found for potato and wheat extrudates, but correlation coefficients were lower. The surface area, S, is directly proportional to the monolayer capacity N m and the value of apparent surface area obtained from desorption data may be treated as a measure of the amount of strongly bounded water. Therefore the desorption surface area seems to be a better parameter to characterize the solid surface of meal extrudates.