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Chinese wheat flours, moisture sorption isotherm, isosteric heat, Mixolab pasting behaviour
Data on equilibrium moisture content (EMC) for six Chinese wheat flour varieties were collected by a gravimetric method at 11-96% equilibrium relative humidity (ERH) and a temperature range of 10-35 °C. Six models were fitted to the sorption data, with the modified Guggenheim Anderson deBoer equation, modified Henderson equation (MHE), and a polynomial equation being the best fits. At a constant ERH, the EMC was negatively correlated with temperature, whereas there was a strong effect of temperature on the sorption isotherms of the wheat flours. Initially, the isosteric heats of adsorption for the wheat flours decreased rapidly with increasing sample moisture content (MC); however, when MC was higher than 15% of the wet basis, further increases in MC caused a slight decrease in heat adsorption values. The heat of vaporisation of the wheat flours approached the latent heat of pure water at a moisture content of ~17.5% wet basis, which was ~2,500 kJ/kg. The isosteric heat of sorption values of the wheat flours predicted by the modified Chung-Pfost equation (MCPE) and MHE model negatively correlated with temperature. At 70% ERH, the safe-storage MC of the wheat flours were 14.71 and 13.88% wet basis at 25 and 35 °C, respectively. Among the six varieties of wheat flours, dumpling flour had significantly higher peak and conclusion temperatures of gelatinisation, and solvent retention capacity (water and lactate) than Gaojin flour, but the latter had a higher peak enthalpy of gelatinisation than dumpling flour. Mixolab pasting analysis at constant hydration further showed that Gaojin four had significantly higher protein weakening and starch setback, but less dough development time and stability time, and lower amylase activity than dumpling flour. These may explain why Gaojin flour has higher moisture sorption isotherms than dumpling flour at the studied temperature range
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