Grape seed extract (GSE) is one of Japan's natural food additives, and is recommended for use as an antioxidant. The component of interest is polyphenol, mainly proanthocyanidins, which are condensed tannins. A vanillin-hydrochloric acid (HCl) assay is specific for flavan-3-ol. The sensitivity of GSE components to the vanillin-HCl assay differed according to the chemical structure. We applied this assay to the determination of proanthocyanidin content in GSEs, health foods containing GSEs and grape seed oils. The concentration of proanthocyanidins in GSEs and health foods was overestimated in the vanillin-HCl assay when (+)-catechin was used as a standard. However, the concentration of proanthocyanidins in health foods with high protein or a high colored substance content was underestimated. No proanthocyanidins were detected in grape seed oil. Furthermore, we examined the content of low molecular weight compounds in GSE by reversed-phase HPLC. The percentage of low molecular weight compounds, including gallic acid, (+)-catechin, (-)-epicatechin, procyanidins B1, B2 and C was estimated to be 5.5-12.2%(w/w).Key words ---grape seed extract, proanthocyanidins, vanillin-hydrochloric acid assay, reversed-phase HPLC, health food, grape seed oil
INTRODUCTIONGrape seed extract (GSE) is a natural food additive approved in Japan. Its standard usage is as an antioxidant in confectionery or processed fishery foods at a concentration of 0.01-1.0%. In Japan, GSE is used as a health food material rather than as a food additive (by private communication with Mr. Asano at Japan Food Additives Association). The consumption of GSE in Japan is approximately 100000 kg/year.1) The biologically active constituents of GSE are proanthocyanidins (Fig. 1), which represent a variety of polymers of flavan-3-ol, such as catechin and epicatechin [2][3][4][5] and have a strong antioxidative effect in aqueous systems. [6][7][8] Colorimetric procedures such as the vanillinhydrochloric acid (HCl) assay, 9-11) buthanol-HCl assay, 12) and the Folin-Ciocalteau assay 13) have been generally used for the determination of proanthocyanidins in foodstuffs. The vanillin-HCl assay is specific for a narrow range of flavanols and dihydrochalcones that have a single bond at the 2,3 position and free m-oriented hydroxy groups on the B ring.9) The principle of the vanillin-HCl assay is as follows: vanillin is protonated in an acid solution, giving a weak electrophilic carbocation that reacts with the flavonoid ring at the 6 or 8 position. This intermediate compound is dehydrated to give a red colored compound.
11)In this study, we applied the vanillin-HCl assay proposed by Sun et al. 9) to the determination of proanthocyanidin content in GSEs, health foods containing GSE and grape seed oils consumed in Japan. We also determined the content of some low molecular weight compounds by reversed-phase HPLC.
MATERIALS AND METHODSMaterials ---Gallic acid (GA), vanillin and flavone were purchased from Wako Pure Industries, Ltd. (Osaka, Japan). (+)-Catechin (CT) (purit...