不同温度热风预干燥对热风-真空冷冻联合干燥龙眼果干品质的影响

Effect of Hot Air Pre-drying under Different Temperatures on Longan Quality by the Combined Hot Air-vacuum Freezing Drying

摘要:

本研究分析了不同温度热风预干燥对热风-真空冷冻联合干燥龙眼果干品质的影响，建立龙眼果干联合干燥工艺。通过测定80 ℃、100 ℃、120 ℃热风预干燥过程中龙眼果肉的水分含量、褐变度、体积密度、总糖、多糖含量及单位能耗确定了不同温度下联合干燥过程中的最佳水分转换点。同时比较了热风干燥，真空冷冻干燥及不同温度条件下热风-真空冷冻联合干燥等干燥方式下龙眼果干的硬度，色泽变化及感官评价的差异。结果表明，新鲜龙眼100 ℃热风预干燥3 h为最佳的水分转换点，此时龙眼的水分含量为67.63%，褐变度为4.89 /g·DW，体积密度为1.10 g/mL，总糖含量为2.11 mg/g·DW，多糖含量为1.94 mg/g·DW，单位能耗为16.74 MJ/kg，将热风预干燥龙眼果干真空冷冻干燥50 h，水分含量达到7%，此时热风-真空冷冻联合干燥的产品在硬度、色泽及感官评价上都优于热风干燥的产品，更接近于真空冷冻干燥的产品。热风-真空冷冻联合干燥是一种有利于提高龙眼果干品质、高效节能的干燥方式，为高品质龙眼果干的工业化应用提供理论指导。

Abstract:

Effect of hot air pre-drying under different temperatures on longan quality was analyzed, and the combined drying process of longan was established. Three different temperatures of hot air drying 80 ℃, 100 ℃, 120 ℃ were investigated to analyze the moisture content, browning degree, bulk density, total sugar content, polysaccharides content and unit energy consumption of longan, and the optimum water conversion point under different temperatures in drying process was also determined. In addition, the hardness, color feature and sensory evaluation of longan were also analyze by three different drying methods (hot air drying, vacuum freeze drying and hot air-vacuum freeze drying). Results showed that the longan dried with hot air for 3 h at 100 ℃ had the best water conversion point. The moisture content was 67.6%, browning degree was 4.89/g·DW, volume density was 1.10 g/mL, total sugar content was 2.11 mg/g·DW, polysaccharides content was 1.94 mg/g·DW, and unit energy consumption was 16.74 MJ/kg. After pre-drying, longan was dried with vacuum freeze-drying for 50 h, and the moisture content of dried longan was about 7%. Furthermore, the hardness, color feature and sensory evaluation of the dried longan with hot air-vacuum freeze drying were better than those of the dried longan with hot air drying, and more similar to the sample with vacuum freeze drying. Therefore, hot air-vacuum freeze drying method could effectively improve the quality of longan, and achieve energy saving for promoting the utilization of high quality longan in food industry.