This thesis aimed to characterize plant phenolics with respect to their application as food preservatives. Hereby, an interdisciplinary approach including identification of bioactive principles, optimized recovery, antimicrobial spectra, and modes of action was applied to three promising plants; mango (Mangifera indica L.), Oriental mustard (Brassica juncea L.) and jocote (Spondias purpurea L.).
Antibacterial activity of mango seed kernel extracts has been linked to tannins with various degrees of galloylation. Methods for their rapid analysis using UHPLC-DAD-ESI-MS n and recovery of single gallotannins using high-speed counter-current chromatography (HSCCC) were established. Fractions containing penta-, hexa-, hepta-, octa-, nona-, and deca-O-galloylglucoses were used to investigate antibacterial spectra and mechanism of action of individual compounds.
Minimum inhibitory concentrations (MICs) were determined using a critical dilution assay and it was found that Gram-positive bacteria were more susceptible to gallotannins than were Gram-negative; lactic acid bacteria were resistant to inhibition by gallotannins. Several investigations indicated that iron chelating capacity of gallotannins contributes to their antibacterial activity but is not exclusively responsible for it. Further parameters demonstrated to be involved in the antibacterial activity of gallotannins include the bacterial outer membrane and membrane-bound proteins.
Methanolysis of mango seed kernel extracts, causing degradation of higher gallotannins and resulting in a less complex chromatographic profile, was investigated as a way to enable quantification and therefore standardization of associated bioactivities. Efficacy of methanolyzed mango kernel extracts as natural preservatives in a food matrix was evaluated in an application trial involving the foodborne pathogens E. coli O157 and L. monocytogenes.
A similar interdisciplinary approach was taken to investigate mustard seed meal as a source for antibacterial active phenolic compounds. Sinapic acid and several sinapoyl conjugates were characterized; their inhibitory activity against select microorganisms and optimized recovery were evaluated.
In continuation with investigations on mango fruits, the phenolic profile of jocote peels was characterized using UHPLC-DAD-ESI-MS n. This was the first study identifying phenolic compounds in jocote, confirming the importance of tropical fruits as a source of natural bioactive compounds and enabling chemotaxonomic studies within the Anacardiaceae family.