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