山核桃坚果分段变功率微波干燥工艺参数优化

Parameter optimization of hickory nut drying by phased varying power microwave

为了提高山核桃干果品质、缩短干燥时间和降低干燥能耗,以前期微波功率密度、转换点含水率和后期微波功率密度为试验因素,对山核桃坚果分段变功率微波干燥工艺进行了试验研究。通过单因素试验,研究了山核桃坚果微波干燥特性,确定了山核桃坚果微波干燥各因素合适范围。通过三因素五水平的二次回归正交试验,建立了三因素与失水速率、单位质量干燥能耗以及干燥后物料蛋白质保存率、不饱和脂肪酸保存率、感官品质指标综合分值的二次回归数学模型,分析了三因素对各指标影响的显著性。利用多目标非线性优化方法,确定了山核桃坚果分段变功率微波干燥的最佳工艺参数组合,即前期干燥微波功率密度为6.5 k W/kg,转换点含水率为23.4%(干基),后期干燥微波功率密度为3.3 k W/kg。在此条件下,山核桃坚果失水速率为4.072%/min、单位质量干燥能耗为3.467 k W·h/kg、蛋白质保存率为92.15%、不饱和脂肪酸保存率为91.63%、感官品质指标综合分值为35.28分。研究结果为山核桃坚果干燥加工生产提供一定的理论依据。

Hickory（Carya cathayensis Sarg） is an endemic tree species in China, whose nut is worldwide famous for nutrition. The nut is rich in protein and oleic acid, linoleic acid and other unsaturated fatty acids. In addition, the nut has a variety of essential amino acids and mineral elements. Drying is an important but weak link in processing of walnut products. At present, drying method for the nut usually uses traditional hot air drying, which consumes high energy together with low drying efficiency and poor product quality. Microwave drying technology has the several advantages over the traditional method, which include high efficiency, energy saving, sanitation, high product quality and automatic control. It has been widely used in the drying processes of fruits and vegetables. Currently, most of nuts, such as macadamia nuts, cashew nut, Chinese chestnut and pistachio nuts, are dried by using constant power microwave.Although constant power microwave method is easy to operate and control, the treated nut is easy to be over- dried or burned. Furthermore, the uneven moisture diffusion and high speed of heat and mass transfer generated in the late stage of drying process of constant power microwave reduce the original nutrients and the sense quality. Recent researches have indicated that phased varying power microwave drying method can overcome these problems. In order to improve the product quality, shorten the drying process and reduce drying energy consumption, the phased varying power microwave was used to dry hickory nuts. The technical parameters such as pre-microwave power, moisture content after pre- drying and later microwave power were optimized in this experiment. First, the appropriate range of the various factors of the microwave drying of the hickory nuts was determined by using the single factor experimental method.Then, by using the three- factor and five- level quadratic regression orthogonal experimental method, the relationships between the 3 factors and dehydration rate, energy consumption of drying process, protein retention rate, unsaturated fatty acid retention rate and overall score of sensory quality indicator of dried nuts were analyzed. The regression mathematical models based on experimental indices and factors were established and used to analyze the influence degree of 3 factors on drying process. Finally, the optimal process parameter combination was obtained for drying hickory nuts using the phased varying power microwave through multi-objective nonlinear optimization method. These combinations were as follows： pre- drying microwave power was 6.5 k W/kg, water content of conversion point was23.4%（dry basis）, and late drying microwave power was 3.3 k W/kg. By using these optimized conditions, dehydration rate was 4.072%/min, energy consumption was 3.467 k W · h/kg, protein retention rate was 92.15%, unsaturated fatty acid retention rate was 91.63% and overall score for sensory quality indicator was 35.28 for dried hickory nuts. Our results provide theoretical reference for drying production of hickory nuts.