J. Agric. Food Chem. 2004, 52, 5183−5186
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Total Phenolics and Antioxidant Activities of Fenugreek, Green Tea, Black Tea, Grape Seed, Ginger, Rosemary, Gotu Kola, and Ginkgo Extracts, Vitamin E, and tert-Butylhydroquinone TAHA M. RABABAH, NAVAM S. HETTIARACHCHY,*
AND
RONNY HORAX
Department of Food Science, University of Arkansas, 2650 North Young Avenue, Fayetteville, Arkansas 72704
The total phenolics and antioxidant activities of fenugreek, green tea, black tea, grape seed, ginger, rosemary, gotu kola, and ginkgo extracts, vitamin E, and tert-butylhydroquinone, were determined. Grape seed and green tea were analyzed for their phenolic constituents using high-performance liquid chromatography. The total phenolics of the plant extracts, determined by the Folin-Ciocalteu method, ranged from 24.8 to 92.5 mg of chlorogenic acid equivalent/g dry material. The antioxidant activities of methanolic extracts determined by conjugated diene measurement of methyl linoleate were 3.4-86.3%. The antioxidant activity of the extracts using chicken fat by an oxidative stability instrument (4.6-10.2 h of induction time) followed a similar trend in antioxidant activity as determined by the Folin-Ciocalteu method. Seven phenolics in grape seed and green tea extracts were identified that ranged from 15.38 to 1158.49 and 18.3 to 1087.02 mg/100 g of extract, respectively. Plant extracts such as green tea and grape seed extracts can be used to retard lipid oxidation in a variety of food products. KEYWORDS: Antioxidant activities; phenolics; induction time; chlorogenic acid; linoleate
INTRODUCTION
Antioxidants can minimize or prevent lipid oxidation in food products (1). Synthetic antioxidants such as tert-butylhydroxytoluene, tert-butylhydroxyanisole, and tert-butylhydroquinone (TBHQ) have been widely used to retard lipid oxidation in foods (2). However, such synthetic antioxidants are not preferred due to toxicological concerns. For this reason, there have been increasing interests in identifying plant extracts to minimize/ retard lipid oxidation in lipid-based food products (3). Most of these natural antioxidants come from fruits, vegetables, spices, grains, and herbs (4). Green tea contains 50% flavonoids such as catechin and epicatechin that contribute to antioxidant activities (5-8). Polymeric tannins and monomeric flavonoids such as catechin and epicatechin in grape seed extracts could be responsible for their higher antioxidant activities (9, 10). Rosemary has been shown to have antioxidant activity in ground pork products (11). The antioxidant activities in rosemary extracts could be due to carnosol, carnosic acid, rosmarinic acid, rosmanol, and rosemaridiphenol phenolic compounds (12). Vitamin E and TBHQ function as oxygen scavengers in fatcontaining foods to prevent lipid oxidation (13). Ginkgo has been reported to have strong antioxidant activities due to flavone glycosides that scavenge free radicals (14). Gingerol, gingerdiol, gingerdione, and other compounds could be responsible for antioxidant activities of ginger (15, 16). The oxidative stability * To whom correspondence should be addressed. Tel: 479-575-4779. Fax: 479-575-6936. Email:
[email protected].
instrument (OSI) (17) and conjugated diene (18) are two main methods used to evaluate antioxidant activities. It is hoped that information on the total phenolics and antioxidant activities of plant extracts and their individual phenolic compounds can be used as criteria to retard or prevent lipid oxidation in a variety of food products. The objectives of this study were to (i) evaluate green tea, black tea, fenugreek, grape seed, gotu kola, rosemary, ginger, and ginkgo extracts, vitamin E, and TBHQ for total phenolics and antioxidant activities and (ii) select plant extracts that demonstrated higher antioxidant activities in objective i and identify phenolics by high-performance liquid chroamtography (HPLC). MATERIALS AND METHODS Materials. Green tea and black tea (Celestial Seasoning, Inc., Boulder, CO) were purchased from a local supermarket. Gotu kola and ginkgo extracts, ginger powder, and fenugreek (Trigonella foemumgraecum) seeds were purchased from Avalon Foods (Fayetteville, AR). Commercial grape seed extract powder was obtained from Mega Natural Inc. (Madera, CA). TBHQ (Tenox 20A, contains 20% TBHQ) was obtained from EaHPLCstman Chemical Company (Kingsport, TN). Rosemary and vitamin E were obtained from Danisco Cultor (United States). The chicken fat was obtained from a local company (Tyson, Fayetteville, AR). The methyl linoleate was purchased from VWR Scientific Products Inc. (Fayetteville, AR). The standard phenolics were purchased from Sigma Chemical Co. (St. Louis, MO). Fenugreek Extract Preparation. The fenugreek extract was prepared by an established procedure in our laboratory. Fenugreek (T. foenumgraecum) seeds were milled in a Cyclotec mill (Cyclotec 1093
10.1021/jf049645z CCC: $27.50 © 2004 American Chemical Society Published on Web 07/07/2004
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Sample Mill, Tecator AB Box 70, Ho¨ganas, Sweden) and passed through 60 mesh. The ground seeds were defatted in hexane (1:4) with agitation (T-Line lab stirrer, Talboys Engineering Corp., Emerson, NJ) at setting 40 for 30 min and centrifuged (IEC, CRU-5000, Internal Equipment Company, Needham Heights, MA) at 2500g for 10 min at 25 °C. The residue was defatted again using the same procedure. The defatted residue was air-dried overnight under a fume hood to remove the residual hexane. The defatted residue was extracted with 60% ethyl alcohol (1:3) with stirring for 30 min and filtered under vacuum. The filtrate was concentrated in a Rotavapor (Buchi 011, Buchi, Switzerland) and dried under a fume hood overnight. The dried samples were freezedried at
