Preparation and structural characteristics of composite alginate/casein emulsion gels: A microscopy and rheology study

Abstract

Composite polysaccharide-protein emulsion gels are ubiquitous in real food systems, but the underlying formation mechanism has not been well understood. In this study, binary alginate/casein systems were selected as a template to prepare four types of composite gels, where the casein concentration was 8.0% (w/v) and the alginate (Alg) concentration varied (0.2, 0.4 & 0.75%, w/v). Based on the composite gel systems, the Alg/casein emulsion gels with 20% oil phase fraction (v/v) were further prepared by dispersing emulsified oil droplets into the binary polymer systems, followed by adding glucono-δ-lactone (GDL) to induce gelation. It was found that irrespective of the polymer types that were used to prepare and/or stabilize emulsions, the oil droplets were always located into the casein-dominant regions. At low Alg concentrations, the emulsion gels featured a casein-dominant network with small Alg-enriched liquid droplets entrapped inside; with increasing Alg addition, the emulsion gels tended to converse into a bicontinuous phase structure, as in the case of composite Alg/casein gel matrices (without oil droplets). Depending on emulsion preparation strategies, the incorporation of oil droplets also affected the phase separation rate and extent between the Alg and the caseins. Moreover, with increasing Alg concentration, the gelation time of the composite emulsion gels was shortened, possibly because a higher concentration of Alg exerted a more pronounced effect on promoting casein aggregation. Overall, our study is anticipated to not only give an insight into the formation of composite polysaccharide-protein emulsion gels, but also to offer a methodology for developing novel emulsion-gel related foods.