Phenolic and polyphenolic compounds of wheat (Triticum spp.): extraction and antioxidative properties

Liyanapathirana, Chandrika M. (2005) Phenolic and polyphenolic compounds of wheat (Triticum spp.): extraction and antioxidative properties. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Phenolic and polyphenolic compounds were extracted from wheat (Triticum aestivum L.) in order to assess their biological activity under different in vitro conditions. The effects of primary processing such as milling and pearling were also examined. The antioxidant capacity of wheat was evaluated using a number of in vitro assays based on scavenging of several free radicals and reactive oxygen species (ROS) and control of lipid and DNA oxidation. -- The optimum conditions for the extraction of crude phenolics from whole grain and bran of soft and hard wheats were determined using response surface methodology (RSM). A face-centred cubic design (FCD) was used to investigate the effects of three independent variables, namely solvent composition (%), extraction temperature (°C) and time (min) on the response, that is the total antioxidant capacity (TAC). Although the optimum extraction conditions were established, in the interest of operational costs related to energy consumption, phenolics were generally extracted using 80% aqueous ethanol for prolonged periods at low temperatures. Crude phenolics extracted with 80% aqueous ethanol yielded 75-80% antioxidant activity as compared to that under optimum conditions. The extractability of phenolics from wheat under simulated gastric pH conditions revealed that pH treatment facilitated the extraction of phenolics. In the latter study wheat phenolics of non-treated and treated samples were extracted into water. The antioxidant activity of treated samples was much higher than that of non-treated samples. Phenolics extracted belonged to the free, soluble esters and insoluble-bound fractions. Soluble esters of phenolics and insoluble-bound phenolics were extracted after alkaline hydrolysis of samples. The contribution of bound phenolics to the total phenolic content was significantly higher than that of free and esterified fractions. In the in vitro antioxidant assays, the bound phenolic fraction demonstrated a significantly higher antioxidant capacity than free and esterified phenolics. In all experiments detailed above, two commercial samples of soft (70% Canadian Eastern soft red spring and 30% Canadian Eastern soft white winter) and hard (90% Canadian Western hard red spring and 10% Canadian Eastern hard red winter) wheat mixtures were used. The antioxidant potential of the milling fractions examined decreased in the order of bran > whole grain > flour. Most of the phenolics were concentrated in the bran and these were not available for analysis under normal extraction conditions. -- The effects of primary processing, namely pearling and milling, on the phenolic content and antioxidant capacity were determined using two wheat cultivars, namely CWAD (Canadian Western Amber Durum; Triticum turgidum L. var. durum) and CWHRS (Canadian Western hard red spring; Triticum aestivum L.). After pearling the phenolic content and antioxidant capacity in the grains were significantly decreased. As the degree of pearling was increased there was a gradual decrease in the phenolic content and hence the corresponding antioxidant capacity in the resultant pearled product. The by-products resulting from 10-20% pearling possessed the greatest antioxidant capacity. Among different milling fractions the bran had the highest phenolic contents while the endosperm possessed the lowest and this was also reflected in antioxidant capacity of different milling fractions in the two cultivars. -- The HPLC analysis of commercial wheat mixtures revealed that ferulic acid was the predominant phenolic acid while in the pure cultivars examined sinapic acid was the major phenolic acid. This study demonstrated the importance of bran in the antioxidant activity of wheat, hence consumption of wheat as whole grains may render beneficial health effects.

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