Mode of action, interaction and recovery of plant secondary metabolites for potential applications as food preservatives

Abstract

Plants synthesize antimicrobial secondary metabolites that might have potential applications in food and pharmaceutical industries. Waste from food processing including peels, seeds, bark and cereal bran contain high amounts of these compounds. Their recovery from food by-products would be benefitial for the food industry, reducing costs and environmental damage. Phenolic compounds and glycoalkaloids are antimicrobial metabolites that coexist in plants of the Solanaceae family. However, in order to be efficiently utilized, their modes of action need to be completely elucidated. Furthermore, their recovery requires new sustainable and economically viable methods. This thesis partially elucidated the interactions between some of these compounds and their antimicrobial mechanisms of action. Moreover, an environmentally friendly method for their recovery from potato by-products was developed. The investigation of the structure-function relationship of antibacterial phenolic acids showed that their activity is enhanced at lower pH values. The antibacterial activity of hydroxybenzoic acids is more dependent on their hydrophobicity compared to hydroxycinnamic acids. The double bond on the side chain plays an important role on the antibacterial activity of hydroxycinnamic acids. Lactic acid bacteria metabolize phenolic acids likely as a strain-dependent detoxification mechanism. By studying the antifungal activity of potato secondary metabolites it was concluded that resistance to glycoalkaloids varies among fungal strains. Synergistic activity between phenolic acids and glycoalkaloids was found. The pattern of fungal sterols was related to their phylogenetic classification and to their resistance to potato glycoalkaloids. A method for recovery and fractionation of phenolic acids and glycoalkaloids from potato peels using water/ethanol solvents was developed. The crude extract contained mainly phenolic ii acids (chlorogenic, neochlorogenic and caffeic acids) and glycoalkaloids (α-chaconine and α-solanine). Solid phase fractionation allowed high recovery of glycoalkaloids and phenolic compounds. The phenolic acids fraction was free of toxic glycoalkaloids and, therefore, suitable for food applications. Alkaline hydrolysis of the crude extract followed by fractionation increased about five times the recovery of caffeic acid, enhancing the antimicrobial properties of the phenolic acids fraction.