Sinapic acid derivatives in defatted Oriental mustard (Brassica juncea L.) seed meal extracts using UHPLC-DAD-ESI-MSn and identification of compounds with antibacterial activity

This study identified phenolic compounds from mustard seed meal and characterized their antibacterial activity. Phenolic compounds were extracted from defatted Oriental mustard (Brassica juncea L.) seed meal and characterized using ultra-high-performance liquid chromatography with diode array and electrospray ionization-mass spectrometric detection (UHPLC-DAD-ESI-MSn ). Sinapic acid and several sinapoyl conjugates were identified based on retention time, UV spectra, MS fragmentation pattern, and by comparison with the authentic sinapic acid reference substance. The crude extract and a purified phenolic fraction exhibited selective antibacterial effects against Gram-negative and Gram-positive spoilage bacteria including Staphylococcus aureus and Listeria monocytogenes; Lactobacillus plantarum was resistant. After alkaline hydrolysis, only sinapic acid could be detected, enabling quantification with the authentic reference substance. Alkaline hydrolysis released 2.66 ± 0.00 mg sinapic acid g−1 dry matter defatted mustard seed meal. Minimum inhibitory concentrations of the hydrolyzed extract against Bacillus subtilis, Escherichia coli, L. monocytogenes, Pseudomonas fluorescens, and S. aureus were 0.1 g L−1 or less. Growth of L. plantarum remained unaffected. Sinapic acid and sinapoyl esters are generally found in members of the Brassicaceae family. Methods for their fast identification will be useful in chemotaxonomic studies. The release of sinapic acid after alkaline hydrolysis not only allows for the quantification using the reference substances but also facilitates the standardization of the antibacterial activity of plant extracts for use as food preservative.