黄秋葵水分吸附热力学性质与状态图研究

Moisture adsorption thermodynamic properties and state diagram of okra

研究了黄秋葵在不同温度和水分活度(a_w)范围内的吸附等温线。探讨了黄秋葵的热力学性质,测定了黄秋葵的玻璃化转变温度(Tg),建立了黄秋葵的状态图。结果表明:黄秋葵的水分吸附等温线为Ⅲ型,平衡干基含水率随温度升高而降低,随a_w增加而增加。GAB模型为描述黄秋葵水分吸附特性的最适模型。黄秋葵的净等量吸附热、微分熵与积分焓随含水率增加而降低。积分熵为负值,且随着含水率增加而升高。扩张压力随a_w增加而升高。黄秋葵的水分吸附过程遵循熵焓互补理论,其过程为熵驱动、自发过程。黄秋葵的Tg随含水率增加而降低。根据状态图,得到黄秋葵最大冷冻浓缩溶液时的玻璃化转变温度为-61.14℃,对应的溶质含量为0.7263 g/g(即非冻结水含量为0.2737 g/g)。

Okra is studied on the adsorption isotherm within different temperature and water activity（aw）. Discussed the thermodynamic properties of okra, measured the glass transition temperature（Tg） of and established state diagram of okra. Results： showed that the moisture adsorption isotherm of okra samples followed type Ⅲ isotherm. The equilibrium moisture content in dry basis decreased with increasing temperature and increased with increasing aw. The optimal model for describing the moisture absorption property of okra was GAB model. The net isosteric of adsorption heat, integral and differential entropy enthalpy decreased with increasing of moisture content. Integral entropy was negative, and increased with the increase of moisture content. Expansion pressure increased with the increase of aw. The moisture adsorption process of okra in line with the entropy enthalpy complementary theory and the process is entropy driven and spontaneous process. The glass transition temperature of okra decreased with increasing moisture content. The state diagram revealed the glass transition with maximum freeze concentration condition as -61.14 ℃ and characteristics solid content of 0.7263 g/g（viz. un-freezable water of 0.2737 g/g）.