Phenolic acid composition and antioxidant properties of aqueous extracts from wheat and sorghum flours and cookies

Abstract

Cereal grains are important food staples and sources of antioxidant phenolics. Dietary phenolics have the potential to reduce oxidative stress and help combat associated conditions like cardiovascular disease. The availability of phenolics from sorghum and wheat products (e.g. cookies) for absorption depends on their fate during processing and during digestion in the gastro-intestinal tract. The effect of simulated GIT pH conditions on total phenolic content (TPC), phenolic acids and antioxidant properties of whole wheat, non-tannin sorghum and condensed tannin sorghum flours and their cookies was determined. The TPC (Folin-Ciocalteu method) of all the samples analysed ranged between 37.0 – 405.3 mg Catechin equivalent / 100 g sample. Extracts from cookies had higher overall TPC than from flours, possibly due to release of bound phenolics or formation of Maillard reaction products (with reducing properties) during baking. Extracts from condensed tannin sorghum samples had higher TPC than the other cereal extracts, possibly due to presence of tannins in the tannin sorghum samples. Phenolic acids were analyzed using High Performance Liquid Chromatography-Mass Spectrometry. p-Hydroxybenzoic, caffeic, p-coumaric and ferulic acids were identified in extracts from the flour and cookie samples. Ferulic acid was the dominant phenolic acid in both wheat and sorghum samples ranging between 109.8 – 1389.4 mg / 100 g sample. ABTS radical scavenging capacity of all the extracts analysed ranged between 0.1 – 4.7 mM Trolox equivalent/ 100 g sample. Condensed tannin sorghum sample extracts had the highest ABTS radical scavenging capacity, possibly due to tannins in the extracts. Metal chelation capacity of all the extracts analysed (ferrous ion chelation assay) ranged between 325.6 – 8424.9 μg EDTA equivalent/ g sample. Wheat sample extracts had the highest metal chelating capacity. Inhibition of copper-mediated LDL oxidation (thiobarbituric acid reactive substances assay) of all the samples analysed ranged between 1.0 – 5.4 μM Trolox equivalent / g sample, with wheat extracts having the highest inhibition. Inhibition of LDL oxidation by the extracts (especially from wheat) may be related to their metal chelation ability as shown by high and positive correlation (r = 0.86) between inhibition of LDL oxidation and Iron Chelating effect (%). There were positive overall correlations between TPC and antioxidant properties (radical scavenging or metal chelation). Antioxidant properties of the extracts may be related to their phenolic contents. Phenolic acids identified in the extracts such as ferulic and caffeic acids may exert antioxidant effects by radical scavenging or metal chelation. The presence of phenolic acids in the GIT pH extracts of the wheat and sorghum cookies shows that they are stable under GIT pH conditions. Overall, this study shows that the cookies have potential as a health promoting ready-to-eat snack to protect against diseases related to oxidative stress such as atherosclerosis.