李辉, 林河通, 林毅雄, 袁芳, 林福兴. 干制荔枝果肉吸附等温线及热力学性质[J]. 农业工程学报, 2014, 30(22): 309-315. DOI: doi:10.3969/j.issn.1002-6819.2014.22.038
    引用本文: 李辉, 林河通, 林毅雄, 袁芳, 林福兴. 干制荔枝果肉吸附等温线及热力学性质[J]. 农业工程学报, 2014, 30(22): 309-315. DOI: doi:10.3969/j.issn.1002-6819.2014.22.038
    Li Hui, Lin Hetong, Lin Yixiong, Yuan Fang, Lin Fuxing. Moisture sorption isotherms and thermodynamic properties of dried litchi pulp[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2014, 30(22): 309-315. DOI: doi:10.3969/j.issn.1002-6819.2014.22.038
    Citation: Li Hui, Lin Hetong, Lin Yixiong, Yuan Fang, Lin Fuxing. Moisture sorption isotherms and thermodynamic properties of dried litchi pulp[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2014, 30(22): 309-315. DOI: doi:10.3969/j.issn.1002-6819.2014.22.038

    干制荔枝果肉吸附等温线及热力学性质

    Moisture sorption isotherms and thermodynamic properties of dried litchi pulp

    • 摘要: 为了解干制"乌叶"荔枝果肉含水率与水分活度、贮藏温度之间的复杂关系,并为干制"乌叶"荔枝果肉贮藏条件的确定提供技术依据,运用吸附原理,在水分活度为0.112~0.976范围内,研究了干制"乌叶"荔枝果肉在20、30和40℃时的水分吸附等温线;采用8种模型对试验数据进行拟合,通过比较模型决定系数(R2)和均方根误差确定用于描述干制"乌叶"荔枝果肉吸附等温线的最适模型;通过不同温度下干制"乌叶"荔枝果肉的吸附等温线数据获得净等量吸附热、焓变、熵变和自由能等热力学性质。结果表明,干制"乌叶"荔枝果肉的水分吸附呈Ⅲ型等温线,在相同水分活度时,平衡含水率随温度的升高而下降。Peleg模型用于描述吸附等温线是较适合的,决定系数为R2为0.9950~0.9979,均方根误差为1.9431~2.7102。热力学性质显示,净等量吸附热随含水率的增加而降低,在较高含水率时趋近于0。焓变与净等量吸附热有相同的值,其范围为0.95~186.98 kJ/mol。熵变随含水率的增加而降低,并没有表现出对温度的依赖性。自由能随含水率和温度的增加而减小。焓-熵补偿理论适用于干制"乌叶"荔枝果肉中的水分吸附过程,此水分吸附过程是焓驱动的。研究结果为干制"乌叶"荔枝果肉的加工和贮藏稳定性提供理论依据。

       

      Abstract: Abstract: In order to reveal the complex relationship among the moisture content, the water activity and the temperature, and to provide technical data for preservation of dried 'Wuye' litchi (Litchi Chinensis Sonn. cv. Wuye), the static gravimetric method was used to determine moisture sorption isotherms of dried 'Wuye' litchi pulp at three temperatures (20, 30, and 40°C) over a range of water activities from 0.112 to 0.976 based on the adsorption theory. The experimental data of dried 'Wuye' litchi pulp were fitted by eight models (BET, Halsey, Henderson, Peleg, Smith, GAB, Modified Henderson, Oswin) available in the literature. The optimal model used to describe the sorption isotherms of dried 'Wuye' litchi pulp was determined by comparing the models' coefficient of determination (R2) and root mean square error. Thermodynamic properties such as net isosteric heat, enthalpy changes, entropy changes, and free energy that provide a deeper understanding of the properties of water and energy requirements associated with sorption process were determined from moisture adsorption isotherm data of dried 'Wuye' litchi pulp at different temperatures. The results showed that the moisture sorption isotherms of dried 'Wuye' litchi pulp exhibited type Ⅲ sigmoid shape. At low and intermediate water activity, the equilibrium moisture content increased slowly with water activity (0.1-0.4), while at high water activity (>0.4) equilibrium moisture content increased rapidly with water activity. The equilibrium moisture content decreased with the increasing temperature at constant water activity. The Peleg model was found to be the best for describing the relationship between the moisture content, the water activity and temperature. The agreement between experiment and predicted values of this model was excellent (R2 ranging from 0.9950-0.9979,root mean square error ranging from 1.9431-2.7102). The thermodynamic properties of dried 'Wuye' litchi pulp showed that net isosteric heat of adsorption decreased with an increasing in moisture constant, and approached to zero at high moisture content. enthalpy changes had the same values as the net isosteric heat ranged from 0.95 to 186.98 kJ/mol. entropy changes values ranged from 1.11 to 587.24 J/(mol·K) at moisture levels ranging from 8%-50% for dried 'Wuye' litchi pulp. These values decreased as moisture content increased and did not show temperature dependence. Free energy ranged from 0.60 to 18.58 kJ/mol for dried 'Wuye' litchi pulp. Free energy became smaller as the moisture content and the temperature increased. The plot of the enthalpy versus entropy showed a linear relationship for dried 'Wuye' litchi pulp with R2 equal to 0.9999, which indicated the existence of compensation. The values of isokinetic temperature and harmonic mean temperature were 319.34 K and 302.93 K respectively. Therefore, the enthalpy-entropy compensation theory was suitable for water adsorption of dried 'Wuye' litchi pulp because isokinetic temperature was significantly different from the value of harmonic mean temperature. Since isokinetic temperature > harmonic mean temperature, the mechanism of the adsorption process of dried 'Wuye' litchi pulp was enthalpy driven. The results provide a theoretical basis for the processing and storage stability of dried 'Wuye' litchi pulp.

       

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