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Protein–polysaccharide intramolecular soluble complexes are proved to have superior emulsifying properties in stabilizing PUFAs-based emulsions.

Texture, flavour, nutrition and function are among the important attributes required for modern foods. They all depend on rational design of complex food structures. In particular, the research and development of functional foods nowadays should not only aim at the chemical and bioactive aspects of individual functional ingredients, but also explore the physical and structural aspects of complex foods that deliver and enhance food functions. The complex and highly ordered structures of foods are achieved by understanding and controlling molecular interaction and assembly of food ingredients (Fig. 1). As food structuring agents, food hydrocolloids, mainly referring to protein and polysaccharides, play key roles in the design of food structures that underpin food functions.There are different types of molecular interactions between protein and polysaccharide. Electrostatic interaction could lead to the formation of molecular complexes when protein and polysaccharide are oppositely charged. By means of multiple techniques such as zeta potentiometry, static/dynamic light scattering and turbidimetry, phase diagram and structural transitions of protein/polysaccharide can be established (1, 2), as is shown in Fig. 2. The phase diagram encompasses five regions where protein/polysaccharide complexes of different structures are formed: (I) individual soluble molecules; (II) the intramolecular soluble complex; (III) the intermolecular soluble complex; (IV) the intermolecular insoluble complex and (V) individual soluble molecules (2).Protein/polysaccharide complexes incorporate the surface-activity arising from protein and the interfacial stability arising from polysaccharide, and thus exhibit excellent foaming and emulsifying properties. Our recent study established a relationship between the phase diagram and the stability of emulsions stabilized by protein/polysaccharide complexes (3, 4). Region II (the intramolecular soluble complex) and the polysaccharide-excessive part of Region III (the intermolecular soluble complex) were found to produce emulsions with excellent stability. The other regions however were found to form emulsions with either reversible or irreversible flocculations. The intramolecular soluble complex was also found to have potential in physically and chemically stabilizing polyunsaturated fatty acids that are susceptible to oxidation. Summary Consumers are becoming increasingly fastidious in demanding food products with improved quality and functionality. This largely relies on rational design of food structures. As the two key food ingredients, protein and polysaccharides play important roles in food structuring. The combination of protein and polysaccharide provides rich opportunities for food structure and function designs through molecular interaction and assembly. This paper provides a brief review on the formation and characterization of protein/polysaccharide electrostatic complexes and their applications in stabilizing oil-in-water emulsions, particularly those containing polyunsaturated fatt...