Mal J Nutr 16(2): 281 - 291, 2010 Antioxidant Activities and Total Phenolic Content of Aqueous Extract of Pleurotus ostreatus (Cultivated Oyster Mushroom) Yim HS 1,2* , Chye FY 2 , Tan CT 1 , Ng YC 1 & Ho CW 1 1 Department of Food Science & Nutrition, Faculty of Applied Sciences, UCSI University 56000 Kuala Lumpur, Malaysia 2 School of Food Science & Nutrition, Universiti Malaysia Sabah Locked Bag 2073, 88999 Kota Kinabalu, Sabah, Malaysia ABSTRACT Pleurotus ostreatus better known as oyster mushroom is widely cultivated and consumed as food in Malaysia. The present study aims to assess the antioxidative potential and total phenolic content of P. ostreatus aqueous extract. The antioxidant activities were evaluated against DPPH and ABTS radical-scavenging activity, ferric-reducing antioxidant power (FRAP) and β-carotene-linoleate bleaching assay, and the Folin-Ciocalteu method for total phenolic content (TPC). The DPPH and ABTS radical-scavenging activity was found to be 63.20% and 87.29% respectively; antioxidant activity using FRAP at 1.45 mM FE/100g and β-carotene- linoleate bleaching assay was 83.51%, while the TPC was found to be 798.55 mg GAE/100g. These antioxidant activities were compared to synthetic antioxidant, BHA and ascorbic acid. Ascorbic acid showed highest scavenging effects on DPPH and ABTS radical, followed by P. ostreatus and BHA (at maximum safety limit). The ferric reducing power of P. ostreatus was significantly higher than BHA and ascorbic acid. The antioxidant activity as assessed in β-carotene-linoleate bleaching assay was found to be higher in BHA compared to P. ostreatus. The aqueous extract of P. ostreatus was found to respond differently in antioxidant assays. The antioxidative activity of the aqueous extract of P. ostreatus correlated with its total phenolic content. Generally, the antioxidant activities of P. ostreatus’ aqueous extract are comparable to that of BHA and ascorbic acid to a certain extent. Keywords: Antioxidant activities, aqueous extract, Pleurotus ostreatus, total phenolic content *Correspondence author: Hip Seng Yim; Email: hsyim@ucsi.edu.my INTRODUCTION Food industries have long used synthetic antioxidants such as butylated hydroxyl anisole (BHA) and butylated hydroxyl toluene (BHT) as preservatives in food products to prevent lipid oxidation which leads to off-odour in lipid-rich food. However, the restricted use of these synthetic antioxidants in food due to their carcinogenic effects has led to increased interest in antioxidant substances from natural resources (Naveena et al., 2008). Natural antioxidant substances are generally considered safe because they are found abundantly in vegetables, fruits and many others of plant origin. Numerous naturally occurring substances have been studied and found to possess antioxidant activities, for example, phenolic compounds