Mathematical modeling and thermodynamic properties in the drying of citron watermelon seeds

ABSTRACT Citron watermelon is an agricultural product of excellent economic potential. Its seeds are widely used for oil extraction, serving as an energy source, showing nutritional characteristics that make them a suitable product to be studied. Thus, the objective was to characterize citron watermelon seeds regarding their physicochemical composition, in addition to determining drying kinetics, fitting mathematical models to the data, and determining the effective diffusivity coefficients and thermodynamic properties. The seeds were dried in a convective dryer, varying the drying temperature, with air velocity of 1.0 m s-1. With the increase in drying temperature, there were reductions in moisture content, water activity (aw), ash concentration, total titratable acidity, lipids and reducing sugar. Citron watermelon seeds are rich in lipids and ash, have low sugar concentration and low acidity; their drying kinetics was very well described by the Two Terms and Approximation of Diffusion models, followed by the models of Midilli and Page, which resulted in acceptable fits. Effective diffusivity accompanied the increase in drying temperature, and this behavior was well fitted by an Arrhenius-type equation. Enthalpy and entropy variations were reduced with drying temperature, with increments in Gibbs free energy.

摘要 香橼西瓜（Citron watermelon）是极具经济潜力的农产品，其种子广泛用于榨油，可作为能量来源，自身具备的营养特性使其成为极具研究价值的对象。因此，本研究旨在对香橼西瓜种子开展理化特性表征，同时测定其干燥动力学曲线，将实验数据拟合至数学模型，并确定有效扩散系数与热力学性质。
实验采用对流干燥机对种子进行干燥处理，以干燥温度为变量，空气流速设定为1.0 m·s⁻¹。结果显示，随着干燥温度升高，种子的水分含量、水分活度（a_w）、灰分含量、总可滴定酸度、脂质与还原糖含量均出现下降。
香橼西瓜种子富含脂质与灰分，糖含量与酸度较低；其干燥动力学曲线可通过二项式（Two Terms）与扩散近似（Approximation of Diffusion）模型实现极佳拟合，其次为Midilli模型与Page模型，拟合效果均处于可接受范围。有效扩散系数随干燥温度升高而增大，该变化规律可通过阿伦尼乌斯（Arrhenius）型方程进行良好拟合。随着干燥温度升高，焓变与熵变均有所降低，而吉布斯自由能（Gibbs free energy）则呈升高趋势。