Interactions of ε-Polylysine with Carboxymethyl Sweet Potato Starch with an Emphasis on Amino/Carboxyl Molar Ratio

The interaction between epsilon-polylysine (epsilon-PL) and anionic polysaccharides has gained considerable attention recently because of its scientific impact on the stability and appearance of liquid food systems. The purpose of this study was to characterize the interactions between epsilon-PL and carboxymethyl sweet potato starch (CSS) using isothermal titration calorimetry (ITC), electrical charge, and turbidity measurements. The results showed that the interaction between epsilon-PL and CSS was electrostatic and mainly dependent on the molar ratio of amino groups in epsilon-PL to carboxyl groups in CSS. Additionally, the interaction between epsilon-PL and CSS was also associated with pH, degree of substitution (DS) of CSS, and ionic strength of the system. For the interaction of epsilon-PL with high DS (>0.235) CSS, three states of the epsilon-PL/CSS mixture were observed as transparent, turbid, and precipitated with a successive increase in amino/carboxyl molar ratio. Distinguishingly, a transparent mixture could be obtained for CSS with low DS (0.114) at a sufficiently high amino/carboxyl molar ratio. The present study provided basic guidance in designing liquid food systems containing both epsilon-PL and CSS.