Evaluation of antioxidant activity in pulp of 35 litchi varieties

Abstract：【Objective】Litchi (Litchi chinensis Sonn.) has been cultivated in China for more than 2 300 years, and its cultivated area and yield have always ranked first in the world. After a long period of natural selection and artificial cultivation, a large number of ecotypes and local varieties of litchi have been reserved in China, with abundant germplasm resources. To explore and study phenolic substances and their antioxidant capacity in fruits and vegetables has become a common concern in food, medicine and other fields. Litchi has been regarded as a good nourishing produce since ancient times. Its pulp is rich in polyphenols, vitamin C and other functional nutrients, with high antioxidant capacity. At present, litchi is rich in polyphenols, vitamin C and other functional nutrients, but the research on its functional nutrition is mainly focused on the by-products such as pericarp and core, and the evaluation of nutritional function of fruits with different litchi varieties is less systematic at present. The main purpose of this study was to evaluate the functional quality of litchi germplasm resources and provide theoretical basisfor the selection of antioxidant germplasm.【Methods】The contents of polyphenol, flavone and vitamin C in the pulp of 35 litchi varieties were compared, the differences in antioxidant capacity of each variety were analyzed from the following three aspects: Ferric Ion Reducing Antioxidant Power, DPPH Scavenging Activity and Oxygen Radical Absorbance Capacity, and the correlation analysis and cluster analysis were used to compare and classify the antioxidant capacity of each variety.【Results】(1) The contents of polyphenol, flavonoid and vitamin C were significantly different among different litchi varieties (p ＜ 0.01). Free phenols mainly existed in free form, the contents of free phenols, binding phenols and total phenols were 64.92-173.64, 2.01-23.45 and 69.74-186.22 mg·100 g-1 with the coefficient of variation being 21.47% , 43.43% and 21.36% , respectively; and the content of free phenols was 81.5%-97.76% of total phenols. The total flavone content was 73.44-331.52 mg · 100 g-1 , with a coefficient of variation of 40.15%; vitamin C content was 6.00-56.33 mg · 100 g-1 with a coefficient of variation of 38.39%; DPPH clearance capacity was 71.78%-99.61% with a coefficient of variation of 7.63%; FRAP was 1.18-10.10 μmol·g-1 with a coefficient of variation of 45.29%; ORAC was 0.97-39.72 μmol· g- 1 with a coefficient of variation of 47.77%. (2) The correlation analysis showed that the contents of free polyphenol, total phenol and total flavone in pulp were very significantly positively correlated with FRAP and ORAC (p ＜ 0.01), vitamin C content was positively correlated with FRAP (p ＜ 0.05), but not with DPPH and ORAC (p ＞ 0.05). The correlation coefficient showed: free polyphenols ＞ total phenols ＞ total flavonoids ＞ ORAC ＞ FRAP ＞binding polyphenols ＞ vitamin C ＞ DPPH clearance capacity. (3)According to the contents of functional substances and the level of antioxidant activity, 35 litchi varieties were divided into four categories by cluster analysis. The first category included seven varieties, i.e.,‘Feizixiao’‘Lvsha’‘Hongxiuqiu’‘Jingganghongnuo’‘Lingfengnuo’‘Jinbaoyin’and‘Zhuangyuanhong’, and the feature of this category was of the highest contents of total polyphenols, free polyphenols, total flavonoids and the highest level of antioxidant activity of FRAP and ORAC, but lower vitamin C content and DPPH scavenging ability. The second category included 15 varieties, such as‘Diwangnuo’‘Xiyuangualv’‘Huaizhi’‘Chouzhinuomici’‘Guiwei’‘Tianyan’‘Heizhenzhu’‘Shuijingqiu’‘Nuomici’‘Feicui’‘Miaozhongnuo’‘Caomeili’‘Xianpoguo’‘Xianjinfeng’and‘Jizuinuo’, which was characterized by high contents of total polyphenols, free polyphenols, binding polyphenols, total flavonoids and strong level of DPPH scavenging ability and oxygen radical absorbance capacity, but low reduction ability of iron ions and the lowest content of vitamin C. The third category included six varieties, containing‘Yujinqiu’‘Baila’‘Yanzhihong’‘Xuehuaizi’‘Beiyuanlv’and‘Lingshanxiangli’, which was characterized by the lower contents of total polyphenols, free polyphenols , binding polyphenols and lower level of Oxygen Radical Absorbance Capacity, and the lowest level of DPPH scavenging capacity and reduction ability of iron ions. The fourth category included seven varieties, such as‘Ziniangxi’‘Xiangmili’‘Sanyuehong’‘Baitangyin’‘Hexiachuan’‘Shuidong’and‘Heiye’, which was characterized by the lowest content of total polyphenols, free polyphenols, combined polyphenols, total flavonoids and the lowest level of ORAC, but high level of FRAP and the highest level of DPPH scavenging capacity and vitamin C content.【Conclusion】There were significant differences in antioxidant capacity among different litchi varieties (p < 0.01). Polyphenols and flavonoids were important antioxidants in litchi pulp, while vitamin C was not closely related to the antioxidant capacity of litchi pulp.‘Feizixiao’‘Lvsha’‘Hongxiuqiu’‘JingganghongNuo’‘Lingfengnuo’‘Jinbaoyin’and‘Zhuangyuanhong’were the varieties with high contents of polyphenolsand flavonoids as well as the strongest antioxidant activity, while‘Ziniangxi’‘Xiangmili’‘Sanyuehong’‘Bait-angyin’‘Xiahechuan’‘Shuidong’and‘Heiye’were the varieties with the lowest polyphenol and flavone contents as well as the lowest antioxidant capicity (ORAC). These studies can provide on only the basis for further study on the functional activity of litchi polyphenols, but also the direction for the application of functional components of different litchi varieties and the selection of new litchi varieties with high antioxidant activity.