Physicochemical and antioxidative properties of red and black rice varieties from Thailand, China and Sri Lanka

Abstract

Nine red and three black rice varieties from Thailand, China and Sri Lanka were analysed to determine their proximate composition and their physicochemical and antioxidant properties. Four groups of rice varieties with different amylose contents were identified. Cyanidin 3-glucoside and peonoidin 3-glucoside were confirmed as the dominant anthocyanins in black rice varieties with contents ranging from 19.4 to 140.8 mg/100 g DM and 11.1–12.8 mg/100 g DM, respectively. Total phenolic content (TPC) differed significantly between the varieties, but not between the colours. Highest TPC was found in the red Thai rice Bahng Gawk (BG) with 691 FA equivalent mg/100 g DM, which showed as well the highest antioxidant properties. In red varieties, the major phenolic acids in the free form were ferulic, protocatechuic and vanillic acid, whereas in black varieties protocatechuic acid was dominant followed by vanillic and ferulic acid. In the bound form, ferulic acid was predominant in both colours, where contents differed significantly, followed by p-coumaric and vanillic acid. The antioxidative capacity did not differ significantly between both colours but amongst genotypes. Antioxidant capacity of rice varieties ranged within 0.9–8.1 mmol Fe(II)/100 g DM for FRAP and 2.1–12.3 mmol TEAC/100 g DM. DPPH scavenging ability ranged from 13.0% to 76.4% remaining DPPH.

Introduction

Rice is a major cereal crop in the developing world. It is consumed as a staple food by over one-half of the world’s population with approximately 95% of production in Asia (Bhattacharjee, Singhal, & Kulkarni, 2002). Although widely consumed as white rice, there are many special cultivars of rice that contain colour pigments, such as black rice, red rice and brown rice. Their name refer to the kernel colour (black, red or purple) which is formed by deposits of anthocyanins in different layers of the pericarp, seed coat and aleurone (Chaudhary, 2003).

According to FAOSTAT (2009), in 2008 China was the major rice producer with 187.4 Mio tonnes, followed by Thailand with 32.1 Mio tonnes and Sri Lanka with 3.1 Mio tonnes. The latest agronomic data from Thailand indicate an increasing rice export (10.2 Mio tonnes) in 2008 compared to 2007 (9.2 Mio tonnes), of which white non-glutinous rice yielded 40.8%, followed by parboiled rice (26.4%) and Jasmine rice (24.4%). According to agricultural statistics from the Ministry of Agriculture and Cooperatives in Thailand, in 2008 the categories glutinous rice and husked rice accounted together for 4.9% and contain as well coloured rice cultivars. Specific data for coloured rice date back to 2003, where Chaudhary (2003) quoted China as the richest country in black rice resources (62%) followed by Sri Lanka (8.6%), Indonesia (7.2%), India (5.1%), the Philippines (4.3%), Bangladesh (4.1%), and few in Malaysia, Thailand and Myanmar.

Coloured rices are reported as potent sources of antioxidants and encouragements as viable sources of antioxidants for functional foods were made (Yawadio, Tanimori, & Morita, 2007). Of these, red rice gained popularity in Japan as a functional food because of its high polyphenols and anthocyanin content (Itani & Ogawa, 2004). Before the health beneficial effects of pigmented rice emerged, Chaudhary (2003) saw an upcoming demand of black rice as an organic food colouring agent which has been at least partly possible due to the increased production of black rice.

Black rice has a number of nutritional advantages over common rice, such as a higher content of protein, vitamins and minerals, although the latter varies with cultivar and production location (Suzuki, Kimura, Yamagishi, Shinmoto, & Yamaki, 2004). Anthocyanin pigments have been reported to be highly effective in reducing cholesterol levels in the human body (Lee, Kim, Hsieh, & Eun, 2008). Inhibitory effects of extracts of pigmented rice bran on in vitro allergic reactions were determined by Choi, Kang, Koh, Nam, and Friedman (2007). Effects of peonidin, peonidin 3-glucoside and cyanidin 3-glucoside, major anthocyanins extracted from black rice, also exerted an inhibitory effect of cell invasion on various cancer cells (Chen et al., 2006).

Several works were done on rice to either determine the anthocyanin profile (Ichikawa et al., 2001, Ryu et al., 1998, Yawadio et al., 2007), the antioxidant activity of rice (Choi et al., 2007, Shen et al., 2009) or more recently with selected fractions thereof – e.g. rice bran (Chotimarkorn et al., 2008, Lai et al., 2009).

However, characterisation of the rice cultivars in terms of composition of nutrients and phytochemicals, as well as rheological aspects is rather seldom taken into consideration. This work covers these aspects in terms of technological and nutritional sense by elucidating the composition of thirteen different rice cultivars and the relationship between antioxidant activities.