苦瓜片气体射流冲击干燥特性及干燥模型

Drying Characteristics and Model of Bitter Melon Slice in Air-Impingement Jet Dryer

【目的】提高苦瓜片的干制品质、缩短干燥时间,通过研究不同条件下气体射流冲击技术对苦瓜片干燥特性的影响,并根据干燥过程中水分的变化规律确定最适干燥模型。【方法】利用实验室自制气体射流冲击干燥机干燥苦瓜片,探讨不同风温(40、50、60、70和80℃)、风速(9、10、11、12和13 m·s^(-1))和切片厚度(2、3、4、5和6 mm)对物料干燥特性和水分有效扩散系数的影响,计算出干燥活化能。以确定系数(R^2)、卡方(χ~2)及均方根误差(RMSE)为评价指标,并利用Origin 8.0软件将试验所得数据与5个常用的干燥模型进行拟合,筛选出最适干燥模型,建立模型参数与干燥条件之间的关系,并检验干燥模型的预测效果。【结果】苦瓜片的气体射流冲击干燥属于降速干燥,没有明显的恒速干燥阶段。在试验条件下,风温、风速和切片厚度对苦瓜片在气体射流冲击干燥过程中的干燥特性均有一定影响,风温越大、切片厚度越小、风速越大,物料的干燥速率越大,水分比下降越快,干燥所需时间越短,但风速的影响远不如风温和切片厚度明显。通过费克第二定律可以计算出苦瓜片在干燥过程中的水分有效扩散系数,且随着风温、风速和切片厚度的增加而增加,最高的有效扩散系数为2.9668×10^(-9) m^2·s^(-1)。通过阿伦尼乌斯公式可以计算出苦瓜片干燥过程中所需的活化能Ea为29.89 kJ·mol^(-1)。所选的5个模型均具有较高的拟合度(R^2>0.98),都能较好的预测苦瓜片在气体射流冲击干燥过程中水分的变化规律,其中Two term exponential模型具有最大的确定系数R^2(0.99937)、最小的卡方值χ~2(0.00876)和均方根误差RMSE(0.000077),是苦瓜片气体射流冲击干燥的最适模型。【结论】风温、风速和切片厚度对苦瓜片气体射流冲击干燥过程中的干燥曲线、干燥速率曲线和水分扩散系数均有影响,且风温>切片厚度>风速。在风温40—80℃,风速9—13 m·s^(-1),切片厚度2—6 mm范围内,Two term exponential模型的拟合度最高,模型可有效描述苦瓜片在气体射流冲击干燥过程中的水分变化规律。

【Objective】In order to improve the drying quality of the bitter melon slice（BMS）, shorten the drying time, the effects of the air-impingement drying conditions on the drying characteristics of the BMS were studied and the drying kinetics model was established to predict the moisture change in the drying process. 【Method】The BMS were dried by the air-impingement jet dryer made by the authors＇ laboratory. The effects of different air temperatures（40, 50, 60, 70 and 80℃）, air velocities（9, 10, 11, 12 and 13 m·s^-1） and slice thickness（2, 3, 4, 5 and 6 mm） on the drying characteristics of materials and effective moisture diffusion coefficient of water were studied, and the activation energy was calculated. With the R^2, χ~2 and RMSE as the evaluation indexes, the optimum model was screened within the five commonly used dry models fitting the experimental data by Origin 8.0 software, then the relationships between the model parameters and drying conditions was establish and the prediction effect of the optimum drying model was verified.【Result】The air-impingement jet drying of BMS occurred in the falling rate drying period, and there was no constant drying rate stage. Under the experimental condition, air temperature, air velocities and slice thickness all had a certain effect on dry characteristics of BMS in air-impingement jet drying process. With the increase of air temperature and air velocities, the decrease of the slice thickness, the moisture of material decreased more fast and the drying rate was rising, the drying time was short, but the influence of air velocities was less significant than air temperature and slice thickness. The water effective diffusion coefficient of the BMS during drying process could be calculated by Fick＇s second law, and it increased with the increase of air temperature, air velocities and slice thickness. The maximum effective diffusion coefficient was 2.9668×10-9 m2·s^-1. Arrhenius was used to calculate the activation energy of BMS in drying process and the value of Ea was 29.89 k J·mol^-1. The five selected models all had good fitness（R^20.98）, they could predict the moisture change of the BMS in air-impingement jet drying process. Of the five models, the Two term exponential model had the highest coefficient of determination R^2（0.99937）, the lowest chi-square χ~2（0.00876） and root mean square RMSE（0.000077）, and it is the optimum model of the BMS in air-impingement jet drying. 【Conclusion】All the factors including the air temperature, air velocities and slice thickness had influence on the drying curve, the drying rate curve and moisture effective diffusion coefficient, and their influences were in an order of the air temperatureair velocitiesslice thickness. Two term exponential model could properly describe the air-impingement jet drying behavior of BMS and could be used to predict the moisture change of the BMS in air-impingement jet drying process under the condition that the air temperatures between 40 and 80℃, air velocities between 9 and 13 m·s^-1 and slice thickness between 2 and 6 mm.