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 籽用西瓜（citron watermelon）是一种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，具备的营养特性使其成为极具研究价值的对象。因此，本研究旨在表征籽用西瓜种子的理化组成，同时测定其干燥动力学特性，将数学模型拟合至实验数据，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，干燥温度设置为变量，空气流速固定为1.0 m·s⁻¹。随着干燥温度升高，种子的水分含量、水分活度（water activity, a_w）、灰分浓度、总滴定酸度、脂质含量与还原糖含量均显著降低。籽用西瓜种子富含脂质与灰分，糖分浓度与酸度较低；其干燥动力学特性可通过双项模型与扩散近似模型实现极佳拟合，拟合效果次之的Midilli模型与Page模型同样具备可接受的拟合精度。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）型方程实现良好拟合。随着干燥温度升高，焓变与熵变均呈降低趋势，而吉布斯自由能（Gibbs free energy）则逐渐升高。