冬枣粉吸湿的色泽、热力学特性及糖物质基础分析

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

为充分了解枣粉吸湿过程中的水分吸附特性,本研究采用静态称量法,分别测量枣粉在温度20、30、40℃,水分活度0.112~0.946下的吸湿情况,探究超微粉碎前后枣粉吸湿色泽的变化、吸附等温线、热力学性质及糖物质基础。结果表明,枣粉吸湿后褐变现象严重,其吸附等温线是III型,Peleg模型最适合描述枣粉的吸附等温线(平均相对预测误差E<5%)。净等量吸附热和微分熵随着枣粉平衡水分含量(干基,下同)的增加呈指数降低,且超微粉低于普通粉。普通粉和超微粉的绝对安全水分含量分别为0.2375、0.2235 g/g。普通粉和超微粉的水分吸附过程均满足熵-焓补偿理论,其吉布斯自由能分别是1152.80、1184.22 J/mol,该过程是一个焓驱动的非自发反应。单糖吸湿对照实验结果表明枣粉中吸湿的主要糖种类为果糖。本研究旨在为枣粉的加工工艺优化及贮藏条件的选择提供理论依据。

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(aw)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.