Abstract: 【Objective】In order to improve the drying efficiency and drying quality of purple sweet potato (PSP), the effect of different drying methods on moisture lost, value of color difference, anthocyanins, polyphenol and antioxidant of PSP were studied.【Method】The PSP were dried by hot air cross-flow drying, forced air drying, air-impingement jet drying and hypoxia air-impingement jet drying. Firstly, the effect of the three kinds of drying methods including hot air cross-flow drying, forced air drying and air-impingement jet drying on drying curves, drying rate curves and effective moisture diffusion coefficient under the conditions of drying temperature of 70℃, slice thickness of 1.93 mm and microwave pre-treatment for 3 min was studied. Secondly, the effect of four kinds of drying methods including hot air cross-flow drying, forced air drying, air-impingement jet drying and hypoxia air-impingement jet drying on color, total anthocyanins content, total polyphenol content and DPPH• scavenging activity of PSP was analyzed under the conditions of drying temperature of 70℃, slice thickness of 1.93 mm and microwave pre-treatment for 3 min. Furthermore, the influence of four factors including air temperature, air velocity, nozzle distance and slice thickness of hypoxia air-impingement jet drying on the color, total anthocyanins content, total polyphenol content and DPPH• scavenging activity of PSP were studied. 【Result】According to the experimental data, there was a similarity law in water loss of PSP, compared with the most results of food material drying test. Forced air drying, hot air cross-flow drying and air-impingement jet drying of PSP mainly occurred in the falling rate drying period. There was no stage of constant rate drying in the drying process. The drying rate of air-impingement jet drying was 84.04% higher than the forced air drying, and it was also 61.60% higher than hot air cross-flow drying. The moisture content of PSP decreased very fast during the first 40 min of air-impingement drying process. But the moisture content of PSP decreased very slowly in the later drying process. Effective moisture diffusion coefficient of forced air drying, hot air cross-flow drying, air-impingement jet drying was 9.62×10-9, 10.23×10-9, and 15.02×10-9 m2•s-1, respectively. In this research, the total anthocyanins content, total phenols content and DPPH• scavenging activity of the PSP sample was 90.85 mg•100g-1 and 262.14 mg•100g-1, respectively. Compared to the forced air drying, hot air cross-flow drying and air-impingement jet drying, the color was better and the total anthocyanins content, total phenols content and DPPH• scavenging activity were higher by hypoxia air-impingement jet drying. The total color difference, total anthocyanins content, total phenols content and DPPH• scavenging activity of hypoxia air-impingement jet dried PSP was 20.35, 34.79 mg•100g-1, 139.26 mg•100g-1 and 28.49%, respectively. In the experiments of the effect of different hypoxia air-impingement jet drying conditions on quality of dried PSP, with the drying temperature increase and air velocity, nozzle distance and slice thickness decrease, the total anthocyanins content, total phenols content and DPPH• scavenging activity decreased, but the value of color difference reduced. In addition, the highest survival rate of total anthocyanins content, total phenols content and DPPH• scavenging activity of dried PSP was 59.58%, 82.35% and 82.05%, respectively.【Conclusion】The drying efficiency and quality of air-impingement jet drying PSP is higher than the forced air drying and hot air cross-flow drying. In addition, the hypoxia air-impingement jet drying can improve the dried sample quality of PSP based on the normal air-impingement jet drying.

Key words: drying , hypoxia , air-impingement jet , purple sweet potato , antioxidant