Water sorption and physical properties of locust bean gum–pectin–starch composite gel dried using different drying methods

Locust bean gum–pectin–starch gel was dried with four different drying methods: air, vacuum, freeze and microwave vacuum drying to a moisture content of about 3–5% on wet basis. The effect of the drying method on water sorption properties of dried gel was evaluated at 25 °C. Three different equations proposed in the literature (GAB, Oswin, Hasley) were used to fit the sorption data. All gave the best fit to the experimental data with a correlation coefficient of 0.99. The porosity of the dried gels varied with different drying methods and ranged between 68% and 87%. Freeze-dried gel developed the highest porosity, whereas the lowest porosity was obtained from air drying method. Air and microwave vacuum-dried gels had narrow pore size distribution where as freeze and vacuum-dried gels had wider pore size distribution. Also, the freeze-dried gels had pore structural collapse and low bulk density. Isoteric heat of sorption, which is an indicator of state of water absorbed by the solid material, was calculated for each drying methods using the Clausius–Clapeyron equation.