Impact of emulsifier-polysaccharide interactions on the stability and rheology of stabilised oil-in-water emulsions

In the present study, the interactions between negatively charged sodium stearoyl 2-lactylate (SSL) and positively charged chitosan were studied in solution and on oil-water interfaces. Phase diagrams of the SSL/chitosan systems were obtained at both pH 4.0 and 5.7 by preparing concentrated solutions and mixing them at different ratios. Optical microscopy revealed that the complexes formed at pH=4.0 were smaller than those formed at pH=5.7 and that the size of the complexes decreased as the SSL/chitosan ratio was reduced. Emulsions (10% w/w oil) were prepared at pH=5.7 by using the complexes of a constant SSL concentration (0.4% w/w) and at different SSL/chitosan weight ratios. The droplet size of the emulsions that were formed with the complexes was greater than those containing only SSL. Using complexes of low SSL/chitosan ratio as emulsifying agent resulted in the production of emulsions with enhanced viscosity as determined by steady shear rheometry. However, none of the samples showed a gel-like behaviour since in all cases the G’ (storage modulus) was lower that G” (loss modulus). The presence of polysaccharide at the interface resulted in an increased stability of the emulsions subjected to environmental stresses, such as heat treatment and a freeze thawing process.