Antioxidant activity and phenolic content of phenolic rich fractions obtained from black cumin (Nigella sativa) seedcake

Abstract

The antioxidant activities of crude methanolic extract (CME) and its fractions using ethyl acetate (EAF), hexane (HF) and water (WF) of black cumin seedcake were investigated. DPPH radical scavenging activity, β-carotene–linoleate bleaching, and inhibition of corn oil oxidation were used to evaluate the antioxidant capacity. The total phenolics were found to be 78.8, 27.8, 32.1 and 12.1 mg gallic acid equivalents (GAE)/g in EAF, CME, WF and HF, respectively. The CME and EAF exhibited the highest DPPH followed by WF and HF. The extract/fractions showed high effect on reducing the oxidation of β-carotene. The effect of extract/fractions on the oxidative stability of corn oil at 70 °C was tested in the dark and compared with butylated hydroxyanisole (BHA). The oil peroxide and anisidine values were generally lower with addition of PRFs in comparison to a control. The predominant phenolic compounds identified by HPLC–DAD in CME and WF of black cumin seedcake were hydroxybenzoic, syringic and p-cumaric acids.

Introduction

Epidemiological and in vitro studies strongly suggest that foods containing phytochemicals such as phenolic compounds have potential protective effects against many diseases. Therefore, they may be used as antimutagenic, antibacterial, antiviral and anti-inflammatory agents (Senevirathne et al., 2006). There is increasing evidence that consumption of a variety of phenolic compounds present in natural foods may lower the risk of serious health disorders because of the antioxidant activity of these compounds (Hertog et al., 1993, Surh, 2002, Surh et al., 1999). When added to foods, antioxidants minimize rancidity, retard the formation of toxic oxidation products, maintain nutritional quality and increase shelf life (Jadhav, Nimbalkar, Kulkarni, & Madhavi, 1996). The antioxidant activity of extracts of several plants including their leaves, bark and roots (Kubola and Siriamornpun, 2008, Mariod et al., 2008), fruits and seeds (Liyana-Pathirana et al., 2006, Malencic et al., 2008), tree nuts oils (Miraliakbari & Shahidi, 2008) and seedcake (Mariod et al., 2006, Matthaüs, 2002) has been extensively studied.

Seeds of black cumin (Nigella sativa L.) are used as a spice in cooking and in a wide traditional medicinal uses, the seed volatile oil and its main active constituent, thymoquinone, are extensively reported to exhibited protective effect against many diseases depending on its high antioxidant activity (El-Dakhakhny et al., 2000, Nagi and Mansour, 2000). Peroxide value (PV) is often used as an indicator for the initial stages of oxidation (Gray, 1978). The anisidine value is a more meaningful test for the assessment of the oils quality during heating than the peroxide value. Measurement of the content of conjugated dienes (at 234 nm) and trienes (at 268 nm) is employed to assess the oxidative stability of vegetable oils (St Angelo, Ory, & Brown, 1975).

High performance liquid chromatography (HPLC) with diode array detection (DAD) is an indispensable tool for the provisional identification of the main phenolic structures present in foods (Chirinos et al., 2009).

Several studies on black cumin seeds (Nagi & Mansour, 2000) and shoots and roots (Bourgou et al., 2008) have been reported recently but there are no relevant studies on antioxidant activity of black cumin seedcake. Therefore, the purpose of the present study was to investigate phenolic compounds of black cumin seedcake extract/fractions and to evaluate their antioxidant activity (AOA) by using different in vitro methods. The different extracts were dissolved in few amount of methanol and applied to corn oil at levels of 0.25% and 0.5% to examine their antioxidative activity; the development of the peroxide value (PV), anisidine value (AV) during oxidation of corn oil was evaluated at 70 °C for 72 h.