Antimicrobial properties of phenolic compounds from sorghum

Abstract

Sorghum grains contain phenolic compounds that have been shown to have many favourable effects. In this study the levels of phenolic compounds in condensed tannin and condensed tannin-free sorghums have been determined and the antimicrobial activity of phenolic extracts from bran fractions of the respective sorghums has been further evaluated against pathogenic bacteria Bacillus cereus ATCC 1178, Escherichia coli ATCC 25922 and Listeria monocytogenes ATCC 7644. Defatted bran fractions prepared from a condensed tannin sorghum variety (red) and a condensed tannin-free sorghum variety (white) were analysed for their content of total phenols and condensed tannins. Total phenols were determined using the Folin-Ciocalteu method and condensed tannins with the vanillin-HCL method. Total phenols and condensed tannins of the bran fractions were extracted with aqueous acetone (75 % v/v) and acidified methanol (1% HCL v/v in methanol) respectively, using a bran-to-solvent ratio of 1:4 (w/v). Red sorghum bran contained a higher amount of total phenols and condensed tannins (33.18 mg tannic acid equivalent/g and 117.98 mg catechin equivalent/g of the bran fractions, respectively) than white sorghum bran (6.81 mg tannic acid equivalent/g and 8.52 mg catechin equivalent/g of the bran fractions, respectively). Freeze-dried sorghum crude phenolic extracts (CPE) obtained from defatted bran fractions of condensed tannin and condensed tannin-free sorghum varieties were evaluated for their antimicrobial activities against Bacillus cereus ATCC 1178, Escherichia coli ATCC 25922 and Listeria monocytogenes ATCC 7644 pathogenic bacteria. The extracts were tested at 1, 2, 4 and 20 % concentrations (w/v) in methanol using the paper disc diffusion method and absolute methanol was used as a control. The condensed tannin-free sorghum CPE at concentrations 1, 2 and 4 % had no inhibitory effects on the bacteria tested but was effective against Gram-positive bacteria, B. cereus ATCC 1178 and L. monocytogenes ATCC 7644 at a concentration of 20 %. The condensed tannin sorghum CPE was effective against B. cereus ATCC 1178 and L. monocytogenes ATCC 7644 at concentrations 1 , 2 , 4 and 20 %. None of the tested sorghum extracts inhibited the Gram-negative bacteria, E. coli ATCC 25922. Phenolic extracts from condensed tannin sorghum may be used as antimicrobial agents to prevent the growth of Gram-positive bacteria, B. cereus ATCC 1178 and L. monocytogenes ATCC 7644.