Comparative Analysis of Different Vacuum Freeze Drying Processes on Quality of Osmanthus fragrans
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Abstract:
To obtain high quality dried Osmanthus fragrans flowers, the eutectic point and co-melting point of flowers were measured byelectrical resistance method. The effects of material thickness, pre freezing temperature and time, partition heating program on the freeze-drying efficiency of O. fragrans flowers were analyzed. And the rehydration ratio, active components and microstructure of O. fragrans flowers dried by vacuum freeze of different processes and hot air were compared. The results showed that the eutectic point was -19 ℃ and the co-melting point was -17 ℃. When the pre-freezing temperature was maintained at -25 ℃ for 2.5 h, the pre-freezing requirement reached. The material thickness and partition heating program in the drying stage were important factors affecting the drying efficiency. Compared with hot-air drying treatment, vacuum freeze-drying maintained good tissue shape with the rehydration ratio 4.16 ~ 4.56 g/g. The total flavonoid content in vacuum freeze-drying ranging from 35.90 to 38.42 mg/g·DW and the contents of aroma active compounds were all significantly higher than hot-air dried O. fragrans flowers. The effects of different vacuum freeze-drying processes on the content of total flavonoid and aroma active compounds in O. fragrans flowers were not significant. The rehydration ratio of fast heating vacuum freeze-drying O. fragrans flowers was 4.16 g/g, significantly higher than that of hot-air dried flowers 2.98 g/g, but lower than the slower heating vacuum freeze-dried flowers. Microstructure analysis showed that the slower the heating process, the smoother the petal surface. High quality of the dried O. fragrans flowers could be obtained using partition heating program at - 10 ℃ for 1 h, 5 ℃ for 1 h, 20 ℃ for 1 h, and 35 ℃ for several hours. This study laid a foundation for the storage and preservation of fresh flowers, and the production and utilization of high-quality dried O. fragrans.
