Evaluation of Electrostatic Interaction between Lysolecithin and Chitosan in Two-Layer Tuna Oil Emulsions by Nuclear Magnetic Resonance (NMR) Spectroscopy

Abstract

The main objective of this work was to investigate the electrostatic interaction between lysolecithin and chitosan in two-layer tuna oil-in-water emulsions using nuclear magnetic resonance (NMR) spectroscopy. The influence of chitosan concentration on the stability and properties of these emulsions was also evaluated. The 5 wt% tuna oil one-layer emulsion (lysolecithin-stabilized oil droplets without chitosan) and two-layer emulsions (lysolecithin-chitosan stabilized oil droplets) containing 5 wt% tuna oil, 1 wt% lysolecithin and various chitosan concentrations (0.025–0.40 wt%) were prepared. The one-dimensional (1D) 31P and 1H NMR spectra of emulsions were then recorded at 25 °C. The results showed that addition of chitosan affected the stability and properties of lysolecithin-stabilized one-layer emulsions. The 31P NMR peak of the choline head group on lysolecithin molecules disappeared when chitosan was added at concentrations above neutralization concentration (> 0.05 wt%). The 1H NMR peak intensity monitoring free amino groups (−NH3+) of chitosan showed a strong positive linear relationship to the chitosan concentration with a high correlation coefficient (R2 ≈ 0.99). This 1H NMR peak in emulsions could not be detected for chitosan in emulsions lower than saturation concentration (< 0.15 wt%). These phenomena indicate an electrostatic interaction between lysolecithin and chitosan at droplet surface in emulsion and were consistent with the results from zeta-potential measurements. The T2* relaxation time of the choline head group (N-(CH3)3) signal of lysolecithin also confirmed that lysolecithin-chitosan electrostatic interaction occurs at the surface of oil droplets in two-layer emulsions. The results suggest that NMR spectroscopy can be used as an alternative method for monitoring the electrostatic interaction between surfactant and oppositely charged electrolytes or biopolymers in two-layer emulsions.