Effects of different extraction methods on phenolic compounds and antioxidant activity in Hemerocallis flower

Abstract: Hemerocallis fulva is one of the most popular flowers rich in bioactive substances, such as polyphenols, flavonoids, and alkaloids anthraquinones, particularly on antioxidant, antidepressant and anticancer. The extraction of phenolic compounds and antioxidant properties can also greatly contribute to the popularization of H. flower in the agricultural industry. However, only a few reports were focused on the extraction, forms and antioxidant activities of phenolic compounds in the H. flower. The present study aims to explore the effects of different extraction on the content, composition and antioxidant activity of phenolic compounds in various Hemerocallis flowers. Five kinds of flowers with different colors (yellow, orange, red-orange, jujube red, and bright red) were selected as raw materials. Three processing treatments (ultrasound, enzymolysis-ultrasound, and fermentation-ultrasound) were also utilized to determine the content of total flavonoids, total polyphenols, and scavenging rate of free radicals. Furthermore, high-performance liquid chromatography was adopted to identify the free, conjugated, and bound fractions of phenolic compounds, where the relationship was also established between phenolic compounds and antioxidant activity. Specifically, the types of total flavonoids and polyphenols were as follows: 'Hongyingwu', '63#', 'Jianzhuanglilan', 'Meiguixiniu', and 'Taiyangwu'. The results showed that there was a significant difference (P＜0.05) of antioxidant activity in the different varieties of flowers, where the antioxidant activity was positively correlated with phenolic content. Furthermore, the antioxidant activities of flowers depended mainly on the varieties and extraction. More importantly, the vitamin C equivalents of DPPH (1,1-diphenyl-2-picrylhydrazyl) were 44.32, 40.63, 38.24, 37.64, and 35.60 mg/g, respectively, for the 'Hongyingwu', '63#', 'Jianzhuanglilan', 'Meiguixiniu' and 'Taiyangwu' under the fermentation-ultrasonic processing. The corresponding ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) vitamin C equivalents were 39.74, 36.24, 30.88, 28.38 and 24.88 mg/g, respectively. The vitamin C equivalent of DPPH of 'Hongyingwu' increased from 38.24 mg/g by ultrasonic extraction to 41.49 mg/g by enzymatic ultrasonic extraction and 44.32 mg/g by fermentation-ultrasonic extraction. The vitamin C equivalent of ABTS of 'Hongyingwu' increased from 31.82 mg/g by ultrasonic extraction to 35.12 mg/g by enzymatic ultrasonic extraction and 39.74 mg/g by fermentation-ultrasonic extraction. Among the three fractions, the content of phenolic compounds in the free fraction was the highest, accounting for more than 75% of the total contents. Enzymatic hydrolysis and fermentation significantly (P＜0.05) improved the contents of most phenolic compounds in the free fraction of 'Hongyingwu'. Particularly, the p-Coumaric acid was newly detected, while the free quercetin content after fermentation reached three times that of the original. The Vitamin C equivalent of DPPH and ABTS free radical scavenging rate of free phenols increased from 40.53 and 35.57 mg/g for ultrasonic processing to 48.20 and 47.40 mg/g for fermentation-ultrasonic processing, respectively. Correlation analysis demonstrated that the chlorogenic acid, caffeic acid, quercetin, and rutin in the free fraction were highly correlated with the antioxidant activity, indicating a great contribution to antioxidant activity. The finding can provide a theoretical basis for the development and utilization of H. flower in functional foods and cosmetics.