Strategies to control and inhibit the flocculation of protein-stabilized oil-in-water emulsions

Flocculation is a ubiquitous phenomenon affecting the creaming, appearance and rheology of protein-stabilized oil-in-water emulsions. This article reviews some of the main strategies that are available to investigators to prevent and control flocculation under different conditions. For dispersed systems of spherical particles with polymer-coated surfaces, we outline how the application of the basic physico-chemical principles of colloid stability provides useful practical guidance. Emphasis is placed on generic factors that affect the nature and strength of the interdroplet interactions, including effects of non-adsorbed polymers and protein-based particles. For illustrative and comparative purposes, we focus attention on the stability behaviour of model emulsions prepared with two contrasting food protein ingredients, sodium caseinate and soy protein isolate. Some additional strategic insight is inferred from empirical shelf-life data for alcoholic caseinate-stabilized emulsions of composition similar to commercial cream liqueurs. Amongst the numerous mechanistic and practical influences that determine the state of flocculation of a protein-stabilized emulsion, several factors emerge as being of particular importance: protein/oil ratio, emulsification conditions, protein solubility, protein denaturation, pH conditions, ionic strength, and calcium ion content. In addition, there is one further strategy that is consistently successful in improving emulsion stability with respect to flocculation over a wide range of solution conditions: the conversion of a food protein into a covalent protein-polysaccharide conjugate.