Effect of Water Distribution during Pre-drying on the Microstructure and Texture Properties of Peach Crisps Produced by Hot Air-Vacuum Freeze Drying

In this study, experiments were conducted to investigate the effect of moisture distribution during pre-drying on the microstructure and textural quality of hot air-vacuum freeze dried peach slices. The moisture distribution during the hot air pre-drying process at different temperatures (40, 60 and 80 ℃) and the product temperature during heating were monitored. Three levels of dry-basis moisture content (7, 6 and 5 g/g) were selected as moisture conversion points for each temperature. The color, shrinkage rate, microstructure, pore distribution, textural properties and hygroscopicity of peach crisps were measured. The results showed that drying temperature had a great impact on the moisture distribution during the pre-drying process, but the overall trends of moisture mobility were consistent among the different drying temperatures. The lower the moisture content of the pre-dried sample, the closer the color of the final dried sample to that of the fresh sample. The color of the sample dried at 40 ℃ with a moisture conversion point of 5 g/g was the closest to that of the fresh sample. Drying time had a greater effect on the shrinkage rate than temperature. It took longer to dry peach slices to the same moisture conversion point at 40 ℃ than 60 and 80 ℃. The sample shrank distinctly during both pre-drying and combined drying. There was a significant difference in the pore structure between the freeze-dried and combined dried samples. The sample with a moisture conversion point of 5 g/g had the most heterogeneous pore structure. The average hardness value of the hot air-vacuum freeze dried sample increased by 52.11% compared with that of the freeze-dried sample. The lower the moisture content of the pre-dried sample, the higher the hardness value of the hot air-vacuum freeze dried sample. This study showed that hot air pre-drying can effectively control the crunchiness and hardness of peach crisps. The decrease in the hygroscopicity of the hot air-vacuum freeze dried sample compared with the vacuum freeze dried one may be related to the structure changes during the pre-drying process. In summary, hot air-vacuum freeze drying is conducive to improving the texture quality and storage stability of peach crisps than vacuum freeze drying.