Antioxidant properties of Phyllanthus amarus extracts as affected by different drying methods

Abstract

The total phenolic content (TPC) and antioxidant activity of fresh and dried Phyllanthus amarus plant materials were evaluated using the Folin-Ciocalteau method, 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity and ferric reducing antioxidant power (FRAP) assays. Different drying treatments led to significant reduction (P<0.05) in antioxidant properties of P. amarus methanolic extracts, with microwave drying causing the highest decrease in TPC and antioxidant activity exhibited by the reduction in both radical scavenging activity and FRAP. On the other hand, boiling water extracts appeared to exhibit significantly stronger antioxidant potentials (P<0.05) even in dried plant materials due to greater solubility of compounds, breakdown of cellular constituents as well as hydrolysis of tannins. Its strong free radical scavenging activity suggests that it has great potential in the food industry as functional food ingredient.

Introduction

Free radicals and other reactive species present in the body can be generated both endogenously and exogenously. Oxidative damages caused by free radicals to living cells mediate the pathogenesis of many chronic diseases, such as atherosclerosis, Parkinson's disease, Alzheimer's disease, stroke, arthritis, chronic inflammatory diseases, cancers and other degenerative diseases (Halliwell & Grootveld, 1987). Under normal circumstances, the free radicals generated in the body can be removed by the body's natural antioxidant defenses, e.g. glutathione peroxidase, catalase, and superoxide dismutase (Aruoma, 1994). However, endogenous antioxidant defenses are not completely efficient and therefore, dietary antioxidants are required to diminish the cumulative effects of oxidative damage due to excess ROS that remained in our system. Antioxidants are not only needed by our body to combat ROS but are also important as food additives. They can be either synthetic or naturally occurring. Synthetic antioxidants possess carcinogenic activity, which leads to a need for the replacement of synthetic antioxidant with naturally occurring ones (Madsen & Bertelsen, 1995). Numerous natural antioxidants have already been isolated from different varieties of plant materials such as leafy vegetables, fruits, seeds, cereals, and algae (Pokorny, 1991). Natural antioxidants tend to be safer and they also possess anti-viral, anti-inflammatory, anti-cancer, anti-mutagenic, anti-tumour, and hepatoprotective properties.

The aerial parts of the herb Phyllanthus amarus Schum & Thonn. (Euphorbiaceae) have been widely used in folk medicine in India and other tropical countries for the treatment of various diseases and disorders, such as jaundice, constipation, diarrhea, kidney ailments, ringworm, ulcers, malaria, genito-urinary infections, hemorrhoids and gonorrhea (Unander, Webster, & Blumberg, 1991). P. amarus extracts were found to possess anti-viral property against hepatitis B virus (HBV) (Thyagarajan, Subramanian, Thirunalasundari, Venkateswaran, & Blumberg, 1988; Yeh et al., 1993). Its extract has been shown to inhibit DNA polymerase activity of HBV and suppress its mRNA transcription, translation and thus its replication (Blumberg, Millman, Venkeateswaran, & Thyagarajan, 1990; Lee et al., 1996; Ott, Thyagarajan, & Gupta, 1997). P. amarus extract also possessed anti-tumour, anti-carcinogenic and anti-inflammatory properties (Kassuya, Silvestre, Rehder, & Calixto, 2003; Kiemer, Hartung, Huber, & Vollmar, 2003; Rajeshkumar et al., 2002). P. amarus extracts potently inhibit HIV-1 replication, HIV reverse transcriptase, HIV replication in cell culture and block virus uptake (Notka, Meier, & Wagner, 2003). Its extract also blocked HIV-1 attachment to its primary cellular receptor CD4 and the HIV-1 enzymes integrase, reverse transcriptase and protease (Notka, Meier, & Wagner, 2004).

Some herbs or crops are perishable in their fresh state and may deteriorate within a few days after harvest. One way to preserve these plant products is to dry them in order to conserve their desirable qualities, reduce storage volume and to extend their shelf life. Drying functions to inactivate the enzymes polyphenol oxidases and can either be performed by traditional sun drying, microwave drying or oven drying. However, enzymatic and/or non-enzymatic processes that may occur during drying fresh plant tissues may lead to significant changes in the composition of phytochemicals (Capecka, Mareczeek, & Leja, 2005). Generally these processes cause negative attribute to the final food product, but studies by Nicoli, Anese, and Parpinel (1999) showed that the overall antioxidant properties of certain foods may instead be enhanced due to improvement of the antioxidant properties of naturally occurring antioxidants and the formation of Maillard reaction products (MRPs). Kumaran and Karunakaran (2007) reported that methanol extracts of powdered air-dried P. amarus (collected in India) showed high antioxidant activities; however, information regarding its antioxidant properties as affected by various drying methods and its aqueous extracts were not available. Therefore, our objective of this paper is to evaluate the effects of various drying methods on the total phenol contents and antioxidant properties of methanolic and aqueous extracts of the aerial parts of P. amarus.