Study On Drying And Powdering Technology And Green Powdered Beverage Of Sweet Potato Leaves

Fresh sweet potato leaves were used as material in this thesis. Firstly, The effect of hot water blanching(80,90,100℃,1~3 min), steam blanching(1~3 min), ascorbic acid blanching(0.1, 0.2, 0.5g/100 mL) and high pressure treatment(100~200MPa,10~20min) on the color and biological activity of sweet potato leaves were compared in order to select an appropriate method to the pre-processing of sweet potato leaves. Secondly, hot air drying, freezing drying and spray drying with superfine grinding were used to prepare sweet potato leaves powder, and the effect of different drying method on the physochemical, nutritional and functional properties of sweet potato leaves powder was compared, and principal component analysis was used to analyse and select quality evaluation index of sweet potato leaves powder. Moreover, the sweet potato leaves powder was used as the material to develop green solid beverange combined with boiling granulating technology in order to achieve effective use of sweet potato leaves. The result showed as follow:(1)The experimental result showed that the color, total polyphenol content(TPC), antioxidant capacity and individual phenolic content of raw sweet potato leaves were well preserved after steam blanching, while significant reductions in the POD activity, PPO activity, TPC and antioxidant activity were observed at 80~100℃ blanching groups. Heavy losses in the TPC and antioxidant activity also occurred through ascorbic acid blanching, while the color changed significantly from green to yellow/green. In addition, the samples treated by the ascorbic acid blanching showed a lower loss compared to water blanching. POD and PPO showed a lower activity after pressure treatments and decreased with the pressure and time increasing. The TPC and antioxidant capacity were decreased significantly after high pressure treatment. HPLC analysis showed that the most abundant sweet potato leaf individual phenolic components were 3,5-CQA and 3,4-CQA, which showing greater losses after different treatments; CA, 4-CQA and 3,4,5-CQA were drastically reduced after 80℃ water blanching. In contrast, 5-CQA and 3-CQA were much stable after all treatments except for high pressure treatments.(2)Hot air dried sweet potato leaf powder showed a higher swelling capacity and emulsion stability. Freeze dried sweet potato leaf powder maintained the characteristics of raw material, showing higher total polyphenol content and total flavonoid content. In addition, its emulsion property was significantly better than that of other drying methods; Spray dried sweet potato leaf powder showed a better bulk density, water absorption and holding oil capacity. Furthermore, compared with other drying methods, spray dried sweet potato leaf powder showed an increased total sugar content and amino acid content. The typical physicochemical properties of sweet potato leaf powder obtained by principal component analysis were holding oil capacity, emulsion activity, emulsion stability and water absorbtion index, the typical nutritional and functional properties were glycine, valine and total flavonoid content. Xu 053601 was considered the most suitable cultivar and spray drying was the best method to make sweet potato leaf powder.(3)The best formula to make green powdered beverage of sweet potato leaves were as follows: sweet potato leaf powder prepared by spray drying, maltodextrin 0.50%, lecithin 3.98%, pre-gelatinized starch 10.00%, β- cyclodextrin 2.51%, sugar10%, green tea flavor 0.3 ‰; Then granulated with the following conditions: inlet air temperature 80~88 ℃, 8% maltodextrin as binder. Intermittent sterilization in low temperature(80℃ for 1 h, 25℃ for 24 h, repeat 4 times), Aluminum foil+ PE bag packaging. The product is a kind of nutrient-rich beverage, which has uniform color, fresh sweet potatoes and green tea leaf scent, stable quality and smooth taste.