Antioxidant and antibacterial properties of endogenous phenolic compounds from commercial mustard products
This study investigated the antioxidant and antimicrobial properties of endogenous phenolic compounds in Oriental (Brassica junceae) and yellow (Sinapis alba) mustard seeds. Phenolics in selected Canadian mustard products (seeds/ powder/ flour) were extracted using Accelerated Solvent Extraction (ASE) and their corresponding sinapate profiles were established through HPLC-DAD analysis. The antioxidant capacity of each extract was assessed by DPPH assay and correlated with the total phenolic content (TPC) measured using the Folin–Ciocalteau method. Sinapine was the major phenolic compound in all the samples analysed, with negligible amounts of sinapic acid. The sinapine content, expressed as sinapic acid equivalents (SAE), ranged from 5.36 × 103 ± 0.66 to 14.44 ± 0.43 × 103 µg SAE/g dry weight of the samples, with the highest in the yellow mustard seed extract and lowest in Oriental mustard powder. The level decreased in the following order: yellow mustard seed > Oriental mustard seed > yellow mustard bran > Oriental mustard bran > yellow mustard powder > Oriental mustard powder. Extracts from yellow mustard seeds had the highest TPC (17.61× 103 ± 1.01 µg SAE/g), while Oriental mustard powder showed the lowest TPC with 4.14 × 103 ± 0.92 µg SAE/g. The DPPH radical scavenging activity of mustard methanolic extracts ranged between 36% and 69%, with the following order for both varieties: ground mustard seed > mustard bran > mustard powder. The antioxidant activities of the extracts correlated with their TPC (correlation coefficients were ≥ 0.72).This study confirmed that Canadian yellow and Oriental mustard varieties and their products are rich sources of endogenous phenolic compounds. The antimicrobial effectiveness of Oriental (1071 ppm sinapine) and yellow (1200 ppm sinapine) mustard seed phenolic extracts, and of sinapic acid standard in two different concentrations (1200, 3000 ppm) against five strains of E. coli O157:H7 (02-0627, 02-0628, 02-0304, 00-3581and non motile 02-1840) and three strains of L. monocytogenes (GLM-3, GLM-4, 2–243) were investigated using minimum inhibitory concentration (MIC) assay. The MICs were determined with a microdilution method using 96-well microplate platforms. The tested concentrations of sinapine and sinapic acid standard had no antibacterial activity against all E. coli O157:H7 and L. monocytogenes strains at 35 ºC and pH 7. The effect of various pre-treatments such as microwave irradiation (20 min, 300 W ) and also 2 h soaking with 70% methanol (with and without acidification) on the sinapates profile and contents of selected defatted mustard products were investigated. Microwave irradiation did not affect the phenolic profile significantly as the sinapine content of yellow mustard bran (6.87× 103 ±0.47 µg SAE/g), yellow mustard powder (19.31× 103 ±0.01 µg SAE/g), Oriental mustard bran (7.28× 103 ±0.06 µg SAE/g), and Oriental mustard powder (12.19× 103 ±2.07 µg SAE/g) were similar to their corresponding untreated samples. However, the soaking process, irrespective of its pH, significantly reduced the sinapine content in all investigated samples. Soaking in acidified 70% methanol resulted in further decreases in the sinapine values to reach 4.73 × 103, 10.82 × 103, 3.25 × 103 and 10.01 × 103 µg SAE/g in yellow mustard bran, yellow mustard powder, Oriental mustard bran and Oriental mustard powder, respectively.