Analysis of Color and Thermodynamic Properties and Sugar Substance Basis of Moisture Absorption by Winter Jujube Powder

To fully understand the moisture adsorption characteristics of jujube powder, in this study, the moisture absorption of jujube powder at 20, 30 and 40 ℃ and water activities (a_w) from 0.112 to 0.946 was measured by the static-gravimetric method. The color changes of ordinary and superfine jujube powder after moisture adsorption were investigated, and moisture adsorption isotherms and thermodynamic properties were determined. Finally, the contribution of sugars in superfine jujube powder to its hygroscopicity was evaluated. The results showed that a serious browning phenomenon appeared after moisture adsorption of jujube powder, and its absorption isotherm belonged to type Ⅲ. Peleg model was found to be the most suitable for describing the moisture adsorption isotherm of jujube powder (mean relative percentage deviation modulus (E) < 5%). The net isosteric heat of adsorption and differential entropy decreased exponentially with increasing equilibrium moisture content (on a dry basis) of jujube powder, and the equilibrium moisture content of the superfine powder was lower than that of the ordinary powder. The absolute safe moisture contents of the ordinary powder and superfine powder were 0.2375 and 0.2235 g/g, respectively. The moisture adsorption process of both powders conformed to the entropy-enthalpy compensation theory, being enthalpy-driven and non-spontaneous with Gibbs free energy of 1152.80 and 1184.22 J/mol, respectively. According to the results of the sugar hygroscopicity experiment, fructose mainly contributed to the hygroscopicity of jujube powder. The results of this study will provide a theoretical reference for the optimization of the processing technology and the selection of storage conditions for jujube powder.