The objective of this study was to synthesize and evaluate the efficacy of antimicrobial waxes to be used as both physical and biological protection to perishable fruits and vegetables. The existing wax materials used in postharvest coating applications do not provide this antimicrobial functionality. One class of such waxes was obtained by covalently linking quaternary ammonium compounds (QACs) featuring alkyl, benzyl, and stearyl ester hydrophobic side groups to the terminal position of a bromo stearyl ester. A second class was obtained by linking these QACs to the pendant hydroxyl group of an aliphatic diamide made of 12-hydroxystearic acid, stearic acid, and ethylene diamine. In total, six distinct structures having three different QAC groups were synthesized. Compounds containing QACs with C8 alkyl groups exhibited potent inhibition toward the growth of both bacteria and fungi. Notably, the complete inhibition of Penicillium italicum and Geotrichum candidum, two fungi detrimental to the postharvest quality of fruits, as well as the complete destruction of viable cells for Gram-positive and Gram-negative bacteria was observed when these organisms were incubated in contact with QAC waxes or dispersed in an aqueous system at a concentration of 1.0 mM. Comparatively, benzalkonium chloride with an alkyl chain length of 10 carbon can completely inhibit Staphylococcus aureus at a concentration of 1.44 mM. The properties of the attached hydrophobic groups appeared to exert a strong influence on antimicrobial activity presumably due to differences in molecular orientation, size, and differences among microbial cellular structures.
Keywords: antimicrobial coatings; bactericidal; fungicide; microbial growth inhibition; postharvest wax; quaternary ammonium compounds.