Effects of gliadin and glutenin on the hygroscopicity of freeze-dried apple powders

Wheat gluten addition in freeze-dried apple powders can effectively prevent their undesirable moisture adsorption and caking during long-term storage, but the working mechanism of wheat gluten had not been expounded. Therefore, such anti-hygroscopicity effects were systematically investigated from the perspective of wheat gluten major components: gliadin and glutenin. Herein, moisture adsorption curve/isotherm, morphology, and moisture migration law of the protein-added apple powders were analyzed at varied storage humidities. Results showed that Peleg, GAB, and Ferro-Fontan models could describe the moisture adsorption process of gliadin-added and glutenin-added freeze-dried apple powder. By comparing the model fitting results, it was found that the fitting degree of moisture adsorption isotherm of the sample increased with the increase of water activity, and the imitative effect of the Ferro-Fontan model was the best. According to the result of the fitting prediction, the equilibrium moisture content of glutenin-added apple powder was 4.7% lower than that of gliadin-added apple powder at 25°C and 75% relative humidity (RH). Type III moisture adsorption isotherms were observed for gliadin-added apple powder, while that of glutenin-added apple powder was type II. In addition, the gliadin-added apple powder demonstrated better fluidity and lower water migration when the relative humidity (RH) of the environment was lower than 58%. Once above this RH value, the protecting effect of glutenin was more obvious. These findings not only elucidate the anti-hygroscopic mechanism of wheat gluten in the processing of apple powder, but also provide a new idea for improving the quality of apple powder and the development of new anti-hygroscopic agents.