Antioxidant capacity in Ginkgo biloba

Abstract

The antioxidant capacity, AOC, of Ginkgo biloba nuts was determined after various periods of cooking. The AOC was based on the ability of the sample to scavenge 2,2′-azino-bis-(3-ethlybenzthiazoline-6-sulfonic acid) free radical ABTS+. The heat-stable antioxidant components present in the nuts accounted for 60% AOC. Thus, around 40% of the AOC was lost when the nuts were cooked for 4 h. After an initial fall, related to a loss in vitamin C, the AOC remained constant. During heating, the non-ascorbic acid AOC compounds were transferred from the nuts to the leachate. A gradual formation of non-ascorbic acid antioxidants was observed in the nuts. The effect of cooking on AOC in the Ginkgo nuts was compared with Ginkgo biloba standardised leaf extracts, dried leaves extracts and Ginkgo commercial capsules.

Introduction

There are considerable debates on the therapeutic and nutritional effects of Ginkgo biloba nuts versus the standardised leaf extract. The promotional literature for the latter purports or implies that Ginkgo leaf extract has been used for thousand of years in China. However, until the last 20–30 years, Ginkgo leaves were rarely used in traditional Chinese herbal medicine for therapeutic purposes. The nuts have been traditionally consumed and their use was first mentioned in herbals in the Yuan dynasty [1280–1368 AD], published in 1350 AD Del Tredici, 1991, Fosters and Chongxi, 1992). Recent research suggests that Ginkgo biloba nuts do in fact have a health giving property (Anonymous, 1997).

Ginkgo biloba nuts are in season from the middle to the end of winter. Ginkgo nuts are consumed with the intention that they possess health benefits. The nuts contain a range of phytochemicals which are reported to possess anti-cancer activities, can treat neurological dysfunction when tested both in vivo and in vitro (Youdim & Joseph, 2001), and aid in disease prevention.

An extract from the leaves of Ginkgo biloba, used in the treatment of cerebral insufficiency, was developed in the 1960s (Maurer, Ihl, Dierks, & Frolich, 1997) and designated EGb 761. This preparation is an acetone/water (60:40) extract from the dried leaves (Drieu, 1986). The main active ingredients of medicinal Ginkgo leaf extracts include 22–27% flavonoid glycosides, 5–7% terpene lactones, and less than 5 μg/g Ginkgolic acids (DeFeudis, 1998). The flavonoid glycosides were determined by high-performance liquid chromatography as quercetin and kaempferol including isorhamnetic, and calculated as acyl flavonol glycosides with molar masses of 756.7 (quercetin glycosides) and 740.7 (kaempferol glycosides). The terpene lactones include 2.8–3.4% Ginkgolides A, B and C and 2.6–3.2% bilobalide.

Ginkgo biloba standardised leaf extract, Egb 761, supplied as a dry powder, has received much attention in recent decades (DeFeudis, 1998, Packer, 1999). Many researchers suggested that the leaf extracts possess medicinal properties, and the active ingredients of the leaves are widely reported (DeFeudis, 1998, Packer, 1999). Ginkgo leaves have attracted much attention as agents for improving circulation, particularly cerebral circulation, which may lead to improved mental function (Kleijnen and Knipschild, 1992a, Oken et al., 1998, Wadsworth and Koop, 2001). Many researches have been conducted on the role of the extract in the treatment of diseases involving free radicals and oxidative damage (Bridi et al., 2001, Pitchumoni and Doraiswamy, 1998). Although antioxidant action has been attributed to this extract, the mechanism of the multiple principles involved in this pharmacological activity has not been completely established. The pharmacokinetics of the Ginkgo extracts has been studied in experimental animals and in humans (Hofferberth, 1994, Kanowski et al., 1996, Kleijnen and Knipschild, 1992b, Le Bars et al., 1997).

In Europe and, to a lesser degree, in the United States, Ginkgo leaf extracts are promoted in the health food market and/or drug stores as popular dietary supplements in the form of capsules, tablets or liquid herbal concentrates. In contrast, the nuts are only presented in the form of food. It seems that the therapeutic effects of the nuts and the leaves are different. Consumers' preference is usually to consume products in the form of food rather than tablet/capsule, a method of ingestion usually associated with feeling unwell (Anonymous, 2001).

Antioxidants are increasingly being recognised as important health promoters in conditions such as cardiovascular problems, many forms of cancer and even aging (Packer, 1999). The antioxidants are able to reduce the effect of free radicals formed in the body either due to exposure to environmental pollutants or because the bodies' own defense mechanisms are reduced in dealing with the natural production of these compounds. Although Ginkgo nuts are suspected to possess antioxidant property, there is limited information on the constituents responsible for the property. No studies have been conducted on the antioxidant capacity (AOC) of Ginkgo biloba nuts when consumed as a food product. In view of the popularity of Ginkgo nuts in Southeast Asia, a study of their AOC is considered important.

In this investigation, the total AOC of Ginkgo biloba nuts both in their raw state and following their preparation as a traditional sweet dessert was determined. Ginkgo biloba nuts were subjected to a variety of conditions to simulate food product preparation. As Ginkgo nuts consumed in this region are excessively boiled to obtain the required softness, investigations were carried out to determine the AOC over a range of cooking time.