Moisture adsorption behaviour of semolina and farina

Moisture adsorption isotherms of semolina (from hard wheat) and farina (from soft wheat) were determined at 20, 35, 50 and 60 °C using the isopiestic method. The adsorbed moisture content significantly affected (p < 0.05) product type, temperature and water activity. Moisture up-take accelerated about 0.75 water activity. The experimental sorption data obtained in this study were applied to Halsey, Harkins-Jura, Henderson, Iglesias-Chirife, Smith BET, GAB isotherm equations to predict the sorption behaviour of semolina and farina. Henderson, Halsey and GAB equations were found to be the most suitable model to describe the isothermal water sorption of semolina and farina at 0.1-0.9 water activity range. Monolayer moisture contents (m0) for two products were calculated from BET and GAB equations. The m0 values of both models decreased with increasing temperature. The values of m0 of semolina and farina at 20 °C were 11.8 and 9.16 g/100 g calculated with GAB, respectively. Adsorption isosteric heat (Qs) decreased; the maximum heat of adsorption was obtained in the moisture content 6-7% and was higher for semolina (15.2 kJ/mol) than farina (14.34 kJ/mol). The Qs value quickly decreased with increase in moisture content to approximately 15% and then was plateau on axis of moisture content. Semolina and farina must be storage below 75% relative humidity at 20 °C prevents caking and deterioration because moisture sorption acceleration was increased after 0.75aw. The moisture content of the products in this storage conditions could be approximately 12.5%.