Identification and antioxidant properties of polyphenols in lotus seed epicarp at different ripening stages

Highlights

• The contents of PLSE at three different ripening stages were different.

• Catechin, epicatechin, hyperoside, and isoquercitrin were identified in PLSE.

• PLSE had good scavenging abilities on DPPH and ABTS⁺ radicals.

Abstract

In this study, polyphenols from lotus seed epicarp (PLSE) at three different ripening stages were purified by column chromatography and identified by RP-HPLC and HPLC–ESI-MS². The antioxidant activities of PLSE were also investigated. We found that the contents of PLSE at the green ripening stage, half ripening stage and full ripening stage are 13.08%, 10.95% and 6.73% respectively. The levels of catechin, epicatechin, hyperoside, and isoquercitrin in PLSE at the three different ripening stages were different. Moreover, the amounts of catechin and epicatechin decreased, while the contents of hyperoside and isoquercitrin increased as the seed ripened. We found that PLSE at three different ripening stages had good scavenging abilities on DPPH and ABTS⁺ radicals. However, the scavenging ability decreased with maturation. Our results may be valuable with regard to the utilization of lotus seed epicarp as a functional food material.

Introduction

Lotus (Nelumbo nucifera Gaertn) is an important aquatic economic crop which has been cultivated for over 3000 years in China (Guo, Wang, Duan, Du, & Wang, 2010). Today, lotus is widely cultivated and consumed in Asia, Oceania and America (Qi & Zhou, 2013). Almost all lotus tissue is used as a vegetable or in traditional medicinal herbs (Chen et al., 2012b). It has been reported that lotus contains abundant functional components including polysaccharide, polyphenols, flavonols, procyanidins and alkaloids. These bioactive components are related to lotus’s pharmacological activities, such as anti-obesity, anticancer, anti-inflammatory, anti-aging, and anti-cardiovascular, and so on (Huang et al., 2010, Deng et al., 2013).

Lotus seed epicarp is one of the inedible parts of lotus, which grows into black (full ripening stage) from green (green ripening stage) during ripening (Fig. 1a). In recent years, researchers have paid increasing attention to the analysis of bioactive components in lotus seed epicarp at the green ripening stage which has also been referred to as fresh lotus seed epicarp. Kredy et al. (2010) first identified six glycosylated and one aglycone flavonol and studied their antioxidant activities. It was reported that 20 kinds of flavonols from fresh lotus seed epicarp have been identified including myricetin 3-O-galactoside, myricetin 3-O-glucuronide, isorhamnetin 3-O-glucuronide and free aglycone diometin (3′,5,7-trihydroxy-4′-methoxyflavone) (Chen et al., 2012a). Qi and Zhou (2013) added fresh lotus seed epicarp extracts into Chinese Cantonese Sausage and showed that the extracts were non-toxic and effective in inhibiting preadipocyte differentiation. So far, there is no report about bioactive components in lotus seed epicarp at the full ripening stage. Lotus seed epicarp at the full ripening stage is referred to as aging lotus seed epicarp, which has the advantage of large production. However, most of aging lotus seed epicarps are discarded and only a few aging lotus seed epicarps is used as culture medium for edible mushroom.

Previously, we have detected polyphenols in lotus seed epicarp at different ripening stages. Polyphenols are a large and diverse group of phytochemicals in plants (Harborne & Williams, 2000). It was reported that polyphenols had strong antioxidant activities associated with their ability to scavenge free radicals, break radical chain reactions and chelate metals (Kaisoon, Siriamornpun, Weerapreeyakul, & Meeso, 2011). Today, polyphenols are considered to be the most important dietary antioxidants due to their high antioxidant capacity and presence in plant foods (Kardum et al., 2014). Therefore, determination of specific polyphenols in lotus seed epicarp may be valuable for the development of functional foods derived from lotus. The aim of this study was to purify and identify PLSE at different ripening stages and to determine the antioxidant activities of PLSE, including reducing power, ABTS⁺ radical scavenging activity and DPPH radical scavenging activity.