Study On The Nutritional Components Of Sweet Potato Leaves And The Antioxidant Activity Of Its Polyphenols

Sweet potato (Ipomoea batatas L.) leaves, aboveground parts of sweet potato, had a high value ofnutritious, which have the potential to be a new type of leaf vegetable. Polyphenols are the mostabundant bioactive substances in sweet potato leaves, and sweet potato leaf polyphenols have strongbioactivities and deserved to be researched further. At the present study, the basic nutritious of sweetpotato leaves of many different cultivars were determined systematically, and the macroporous resinmethod was used to purify polyphenols from sweet potato leaves and the processing parameters wereoptimized; The antioxidant activities and stabilities of purified products were studied. The aim of thisresearch is to provide data support for the development of sweet potato leaves as a new resource of leafvegetables and data references for its polyphenols used as a natural antioxidant.The nutritional compositions of40sweet potato leaf cultivars were determined. The correlationsbetween antioxidant activity and nutrients were determined to find the main antioxidants. The crudeprotein, crude fiber and ash contents ranged between16.69-31.08,9.15-14.26and7.39-14.66g/100gdry weight (DW), respectively. The correlation coefficient between antioxidant activity and totalpolyphenol content was the highest (0.7589, p <0.0001), indicating that polyphenols are importantantioxidants in sweet potato leaves. Sweet potato leaves, which contain several nutrients and bioactivecompounds, deserved to be developed and applied in many fields.The processing parameters to purify polyphenols from sweet potato leaves by AB-8macroporousresins were optimized, and the purified products were analyzed qualitatively and quantitatively byHPLC. The optimal processing parameters were as follows: initial polyphenol concentrations2.0mgCAE/mL, pH3.0, ethanol desorption solution concentration70%(v/v), and flow rate of injecting andeluting1BV/h. At the optimum condition,5BV sample solution can be adsorbed by AB-8resin columnand just3BV70%(v/v) ethanol solution was need to thoroughly desorb the polyphenols from resins, theadsorption amount and desorption ratio at the optimal parameter were26.8mg CAE/g and90.9%,respectively. HPLC result suggested that polyphenols from sweet potato leaves were mainly composedby8phenolic constituents, and the contents of three di-caffeoylquinic acids were relatively higher thanothers.The antioxidant activities of sweet potato leaf polyphenols of Yuzi No.7and Ximeng No.1wereevaluated by several methods. Both of the two cultivars can effectively scavenge the DPPH,·OH.The·O-2scavenging activity of Yuzi No.7at the concentration of20μg/mL was almost3.1,5.9and9.6times of ascorbic acid, tea polyphenols and grape seed polyphenols, respectively and its oxygen radicalabsorbance capacity was2.8,1.3and1.3times of trolox, tea polyphenols and grape seed polyphenols,respectively. In conclusion the sweet potato leaf polyphenols possess strong antioxidant activities andhave the potential to be used as natural antioxidant.The stabilities of sweet potato leaf polyphenols were assessed by determining the retention ratio oftotal polyphenols content (TPC) and antioxidant activities (AA) in different processing and storageconditions. The retention ratios of TPC and AA of sweet potato leaf polyphenols were above90%after heat treated at50and65°C; while the retention ratios of AA was60%after sweet potato leafpolyphenols were heat treated at90and100°C for90min. Sweet potato leaf polyphenols in pH5solution were more stable, while the degradation of poly-caffeoylquinic acids were greater in pH3and7solution than in pH5solution; The retention ratios of TPC and AA of sweet potato leaf polyphenolswere above80%stored with light, while the content of poly-caffeoylquinic acids in the sample storedwith light were significantly lower than that in the sample stored without light. The sample stored at-18and4°C were relatively more stable than that stored at25°C. In conclusion, low temperature heattreatment had little effects on the stabilities of sweet potato leaf polyphenols; sweet potato leafpolyphenols suit to be applied in a solution system with pH value approximately at5and tend to bestored at low temperature without light.