Cationic Antimicrobial (ε-Polylysine)-Anionic Polysaccharide (Pectin) Interactions: Influence of Polymer Charge on Physical Stability and Antimicrobial Efficacy

The cationic biopolymer epsilon-polylysine (epsilon-PL) is a potent food-grade antimicrobial that is highly effective against a range of food pathogens and spoilage organisms. In compositionally complex systems such as foods and beverages, cationic epsilon-PL molecules may associate with anionic substances, leading to increased turbidity, sediment formation, and reduced antimicrobial activity. This study therefore characterized the interactions between cationic epsilon-PL and anionic pectins with different degrees of esterification (DE) and then investigated the influence of these interactions on the antimicrobial efficacy of epsilon-PL. The nature of the interactions was characterized using isothermal titration calorimetry (ITC), microelectrophoresis (ME), and turbidity measurements. High (DE 61%), medium (DE 51%), and low (DE 42%) methoxyl pectins interacted with epsilon-PL molecules through electrostatic forces, forming either soluble or insoluble complexes with various electrical charges, depending on the relative mass ratio of pectin and epsilon-PL. The interaction of pectin with epsilon-PL increased as the negative charge density on the pectin molecules increased, that is, with decreasing DE. The antimicrobial efficacy of epsilon-PL against two acid-resistant spoilage yeasts (Zygosaccharomyces bailii and Saccharomyces cerevisiae) decreased progressively in the presence of increasing levels of all three pectins. Nevertheless, the low DE pectin decreased the antimicrobial efficacy of epsilon-PL much more dramatically, likely due to strong electrostatic binding of epsilon-PL onto low DE pectin molecules reducing its interaction with anionic microbe surfaces. This study provides knowledge that will facilitate the rational application of epsilon-PL as an antimicrobial in complex food systems.