Sorption equilibrium moisture and isosteric heat of Chinese wheat bran products added to rice to increase its dietary fibre content

Abstract

More and more people concern about supplementing dietary food with wheat bran to increase fibre content, but there has been little study of the sorption isotherms and isosteric heats of wheat bran fibre products, which are important to the quality and storage durability of such commodities. This study collected equilibrium moisture content (EMC) and equilibrium relative humidity (ERH) data on six Chinese wheat bran products via the static gravimetric method and analysed their sorption isosteric heats. Results showed that all six wheat bran products had sigmoidal isotherms. The data were best fitted by polynomial, modified GAB, modified Oswin, and modified Halsey models. The relative safe moisture contents of the six wheat bran products were 12.20%–13.86% wet basis (w.b.). The heat of vaporization of wheat bran products approached the latent heat of pure water at around a moisture content (MC) of 22.5% and approximately 2450 kJ/kg. A process of extrusion and ultrafine grinding reduced the solid surface area and sorption isosteric heats of the monolayer, multilayer and condensed water regions in the wheat bran products. In the temperature range of 10–37 °C, the EMC of sorption in monolayer, multilayer and condensed water regions decreased with increasing temperature. The milled rice to which 1%–3% 200-mesh wheat bran product was added maintained its texture when made into cooked rice and its thermomechanical properties when made into rice dough. After processing by extrusion and ultrafine grinding, wheat bran products have a lower solid surface area and lower monolayer water content (M_m) and sorption isosteric heat values. Some 3% 200-mesh wheat bran product can be added to cooked rice for increasing its fibre content and making it more nutritious.