基于玻璃化转变理论的超微枣粉动态吸湿机制研究

Ultramicro jujube powder (UJP) is one of the most important products of jujube. However, the quality deterioration of UJP which caused by moisture absorption and agglomeration has not been solved yet. Although the hygroscopicity of jujube power was studied before, the major sugar crystallized and moisture adsorption mechanism of UJP were not clear. It was shown that there were relationships between sugar crystallization and glass transition temperature (Tg ). Hence, in the present study, based on glass transition theory, state diagram of UJP was established by studying monolayer moisture content, Tg and sticky temperature; the sugar crystallization kinetics of UJP were established based on changes of sugar crystallization degree under different temperatures and times; the relationships between sugar crystallization and water activity and Tg were identified, the major sugar which occurred crystallization was determined; dynamic moisture adsorption mechanism of UJP was revealed by dynamics (kinetics model) and thermodynamics study (enthalpy and entropy complementary theory). The objective of present study would provide scientific theoretical basis for solving the moisture absorption problem and keeping the quality of UJP.

超微枣粉是枣的主要加工产品形式之一。然而，在其存贮过程中因吸湿、结块等原因引发的产品质量劣变仍未得到有效解决。其中，导致超微枣粉吸湿的结晶化糖单体鉴别以及超微枣粉的吸湿机制尚待明确。前期研究表明，糖的结晶化与玻璃化转变温度密切相关。因此，本项目基于玻璃化转变理论，通过研究超微枣粉的单分子层含水率、玻璃化转变温度及粘结温度，构建超微枣粉状态图；研究超微枣粉在不同温度下糖结晶化程度的变化，构建动力学模型，结合状态图揭示水分活度、玻璃化转变温度与糖结晶化的关系，明确导致超微枣粉吸湿的主要糖种类；并从动力学和热力学角度系统研究超微枣粉水分吸附机制，依托动力学模型及熵-焓互补理论揭示其动态吸湿机理。本研究对解决超微枣粉吸湿问题及保持产品贮藏期间品质具有重要的理论价值和实际意义。

超微枣粉是枣的主要加工产品形式之一。然而，在其存贮过程中因吸湿、结块等原因引发的产品质量劣变仍未得到有效解决。其中，导致超微枣粉吸湿的糖物质基础以及超微枣粉的吸湿机制尚待明确。前期研究表明，枣粉吸湿结块与其玻璃化转变温度密切相关。因此，（1）本项目基于水分活度及玻璃化转变理论，采用BET模型对超微枣粉水分吸附等温线拟合确定了其单分子层含水率；采用Gordon-Taylor方程对不同水分活度下超微枣粉的玻璃化转变温度变化进行拟合，并通过BET及Gordon-Taylor拟合曲线构建枣粉状态图。结果表明：在25°C的条件下，当超微枣粉的干基含水率低于0.0566g/g时可避免玻璃化转变的发生。（2）采用HPLC对超微枣粉中的小分子单糖组分进行测定，表明葡萄糖、果糖及蔗糖是主要的单体糖组分，通过水分吸附实验，确定了引起枣粉吸湿的主要糖单体组分为果糖；通过XRD对其结晶化程度进行检测，结果表明枣粉吸湿后仍主要处于无定形态结构，但是其结晶化的比例有所提高。（3）从动力学及热力学角度揭示其吸湿作用机制；采用7种模型对枣粉的吸附特性进行分析，根据模型拟合的相关系数、卡方及均方根误差，确定BET模型为最适模型；通过热力学分析可知净等量吸附热和微分熵均随着水分含量的增加而降低，且两者呈线性正相关；通过熵焓互补理论的分析得到，枣粉的水分吸附过程主要为熵驱动。本项目研究对于解决超微枣粉吸湿问题及保持产品贮藏期间品质具有重要的理论价值和实际意义。