Antioxidant phenolics of different barley (Hordeum vulgarae L.) cultivars

Waisooriya Mudiyanselage, Terrence Madhujith (2007) Antioxidant phenolics of different barley (Hordeum vulgarae L.) cultivars. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The extracts from six barley cultivars (Hordeum vulgarae L.) grown in the Canadian prairies, namely AC Metcalfe, Falcon, Phoenix, Tercel, Tyto, and Peregrine, were evaluated for their potential antioxidant, antiradical, and antiproliferative efficacies by various chemical and biological methods. Antioxidative compounds were solvent-extracted after optimizing extraction parameters (solvent composition, extraction temperature, and time) using response surface methodology (RSM). In addition, the distribution of antioxidative constituents in the barley grain was investigated by separately pearling two barley cultivars (Falcon and AC Metcalfe) into seven fractions (F1-F7) in a layer-wise fashion up to 50% of the kernel weight. Antioxidative efficacy of the layers and the remaining kernel (pearled grain) was evaluated using chemical and biological methods. Furthermore, phenolic compounds present in the six barley samples were separated into free, soluble conjugates, and insoluble-bound fractions using alkaline hydrolysis and the antioxidative potential of each of the fractions was also investigated. -- The optimum conditions for extraction of antioxidative components from barley extracts were 80% aqueous methanol, 60°C, and 40 min as determined by RSM. Total phenolic content (TPC) of whole kernel extracts ranged from 13.58 to 22.93 and 0.81 to 1.38 mg ferulic acid equivalents per gram lyophilizate and defatted material, respectively. The order of TPC was Peregrine > AC Metcalfe > Falcon > Tyto > Phoenix > Tercel. Total antioxidant capacity (TAC), as measured by Trolox equivalent antioxidant capacity (TEAC), ranged from 3.74 to 6.82 μmol Trolox equivalents per gram defatted material. Antioxidant and antiradical efficacies of barley extracts were evaluated using different assays including 2,2-diphenyl-1-picrylhydrazyl (DPPH), superoxide, oxygen radical absorbance capacity (ORACFL), hydroxyl radical averting capacity (HORACFL), and photochemiluminescence (PCL). All six barley extracts showed significant antioxidant and antiradical activities although the order of their activity changed from assay to assay. Barley extracts exhibited substantial metal chelation activity as measured by 2,2 '- bipyridyl competition assay. Evaluating the whole barley extracts using a number of model systems such as β-carotene/linoleate, bulk stripped corn oil as well as accelerated oxidation study, using Rancimat®, further revealed that the whole barley extract possessed strong antioxidant activity. Whole grain extracts exhibited strong inhibitory effects against copper-induced human LDL cholesterol oxidation as well as peroxyl and hydroxyl radical-induced DNA double strand scission. Barley extracts also exhibited substantial antiproliferative effect against growth of Caco-2 human colorectal adenocarcinoma cells in a concentration-dependent manner. -- In the second phase of the study involving fractionation, it was revealed that anti oxidative constituents were mainly located in the outer 9% (w/w) of the kernel. TPC of Falcon and AC Metcalfe fractions ranged from 0.51 to 6.26 and 0.17 to 4.16 as ferulic acid equivalents per gram defatted material. TAC, as measured by TEAC, varied from 0.45 to 60 and 0.69 to 56 μmol Trolox equivalents per gram defatted material, respectively. Antioxidant efficacy gradually diminished from the outer layers to the inner layers. In general, antioxidant efficacy determined by other methods followed a similar trend to that of TAC. -- In the last phase of the study involving hydrolysis of phenolic compounds, it was revealed that a greater proportion of antioxidative compounds were present in insoluble-bound form followed by soluble conjugate form. It was further revealed that the antioxidant and antiradical efficacies of insoluble bound phenolic compounds were the highest followed by soluble conjugates and free phenolic. The predominant phenolic acid detected in barley extracts was ferulic acid. The other phenolic acids detected were vanillic, caffeic, p-coumaric and sinapic acids.

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