Comparison of in vitro antioxidant activities and bioactive components of green tea extracts by different extraction methods

Abstract

In this study, in vitro antioxidant activities and bioactive components of green tea extracts (GTE) by ultrahigh pressure extraction and conventional extraction methods (microwave extraction, ultrasonic extraction, Soxhlet extraction and heat reflux extraction) were investigated. DPPH radical-scavenging and FTC method were applied to test the antioxidant activities. The bioactive components were determined by chemical methods. The results indicated that the GTE by ultrahigh pressure extraction exhibited the strongest antioxidant activities. The contents of polyphenols and catechins in the GTE by ultrahigh pressure extraction were significantly higher than those by other extraction methods, which was possibly responsible for the higher antioxidant activities of the GTE by ultrahigh pressure extraction. From the results we can draw the conclusion that not only the more bioactive components are obtained but also the extract has better free radical and reactive oxygen species scavenging activities through ultrahigh pressure extraction method. These findings further illustrate that ultrahigh pressure extraction has a bright prospect for extracting active ingredients from plant materials.

Introduction

Tea is a widely popular beverage around the world for centuries. It is estimated that on an average, about 6 g of tea is consumed per day by a 60 kg human being (Chang et al., 2000). Green tea leaves contain polyphenols, caffeine, amino acids, saponins, tannins, etc. In general, dry green tea leaves (Thea sinensis L.) contain about 10–35% (w/w) polyphenols and 2–5% (w/w) caffeine (Xi et al., 2010). Polyphenols include catechines, flavanols, flavanones, phenolic acids, glycosides and the aglycons of plant pigments, etc., which are natural antioxidants that can be used as alternatives to synthetic antioxidants as they are typically less harmful than synthetic ones and appears to have an equivalent effect upon the inhibition of oxidation (Xi et al., 2009). These antioxidants, which inhibit the oxidation of organic molecules, are very important for living systems and their defence against oxidative stress (Chang et al., 2000). Phenolic antioxidants interrupt the propagation of the free radical autoxidation chain by contributing a hydrogen atom from a phenolic hydroxyl group, with the formation of a relatively stable free radical that does not initiate or propagate further oxidation processes (Xi et al., 2009).

Ultrahigh pressure extraction (UPE) as a novel technique is used for extraction of active ingredients from plant materials. High pressure ranging from 100 to 600 MPa is considered as an alternative extraction method, which is proven to be fast and more effective (Prasad et al., 2010). High pressure can improve the mass transfer rate and enhance solvent permeability in cells as well as secondary metabolite diffusion (Ahmed and Ramaswamy, 2006). High pressure can also cause some structural changes in vegetal materials, such as cellular deformation, cellular membrane damage and amino acid denaturation (Xi, 2006). Application of this technique for extraction of bioactive substances from plant materials yields some advantages, such as short extraction time, mild extraction condition, high extraction yield and less impurity (Prasad et al., 2010). Recently ultrahigh pressure technique had been successfully used for extraction of anthocyanins from grape skins (Corrales et al., 2009), ginsenosides from Korean panax ginseng powder (Shin et al., 2010), biologically active compounds from green tea leaves (Xi, 2009, Xi et al., 2009, Xi et al., 2010) and flavonoids and phenolic compounds from Litchi fruit pericarp (Prasad et al., 2009), corilagin from longan fruit pericarp (Prasad et al., 2010) and so on.

Studies have dealt with the methods of extraction of bioactive compounds from green tea, such as microwave-assisted extraction, supercritical carbon dioxide, heat reflux extraction and Soxhlet extraction, ultrasonic extraction and so on (Yuko et al., 1999, Chang et al., 2000, Park et al., 2008). UPE is a novel technique at present, which we had successfully used to extract major catechins and polyphenols from green tea leaves (Xi, 2009, Xi et al., 2009, Xi et al., 2010). However, there are no studies on the antioxidant activities of green tea extracts (GTE) by UPE. In the present study, we evaluated the GTE by UPE and conventional extraction methods (microwave extraction, ultrasonic extraction, Soxhlet extraction and heat reflux extraction) for their in vitro antioxidant activities by DPPH radical-scavenging and ferric thiocyanate (FTC) methods. In order to understand the relationship between bioactive properties and antioxidant activities, the bioactive constituents of the GTE by UPE and conventional extraction methods were analysed. Therefore, the present study is aimed at the investigation of the comparison of the antioxidant activity of GTE obtained by different extraction methods in relationship to their bioactive constituent contents.