Protein-Polysaccharide Interactions in Food Colloids

Covalent protein-polysaccharide conjugates are sometimes used to avoid any flocculation and phase separation that is produced with weak non-specific protein-polysaccharide interactions. An example of such conjugates is that produced with globulin-dextran or bovine serum albumin-dextran. These conjugates produce emulsions with smaller droplets and narrower size distribution and they stabilize the emulsion against creaming and coalescence. 

The above interactions are manifested in the variation of viscosity with surfactant concentration. Initially, the viscosity shows an increase with increasing surfactant concentration, reaching a maximum and then decreasing with further increase in surfactant concentration. The maximum is consistent with the for- 

Aqueous solutions of proteins and polysaccharides may exhibit phase separation at finite concentrations. Two types of behavior may be recognized, namely coacer-vation and incompatibility. Complex coacervation involves spontaneous separation into solvent-rich and solvent-depleted phases. The latter contains the protein-poly-saccharide complex that is caused by nonspecific attractive protein-polysaccharide interaction, e.g. opposite charge interaction. Incompatibility is caused by spontaneous separation into two solvent-rich phases, one composed of predominantly protein and the other predominantly polysaccharide. Depending on the interactions, a gel formed from a mixture of two biopolymers may contain a coupled network, an interpene- 

---

Dickinson, E. (1993). Protein-polysaccharide interactions in food colloids. In Dickinson, E., Walstra, P. (Eds). Food Colloids and Polymers Stability and Mechanical Properties, Cambridge, UK Royal Society of Chemistry, pp. 77-93. 

Owing to the diverse chemical nature of functional groups in proteins and polysaccharides, they are prone to a variety of types of molecular interactions, both in bulk aqueous media and at air-water or oil-water interfaces. To a first approximation one may consider an adsorbed layer of biopolymers at the interface as simply a special type of highly concentrated biopolymer solution. Thus, the same variety of interactions that are typically found for biopolymers in a bulk aqueous media also occur in biopolymer adsorbed layers at the interfaces in food colloids. Moreover, these same molecular interactions are also involved in the close encounters between pairs of colloidal particles covered by adsorbed biopolymer layers. In the rest of this chapter we shall briefly remind ourselves of the main basic types of intermolecular interactions readers requiring more detailed background information are directed to other sources (Cantor and Schimmel, 1980 Lehninger, 1982 Israelachvili, 1992 Dickinson, 1998 Finkelstein and Ptitsyn, 2002 McClements, 2005, 2006 Min et al., 2008). 

In general, surface activity behaviour in food colloids is dominated by the proteins and the low-molecular-weight surfactants. The competition between proteins and surfactants determines the composition and properties of adsorbed layers at oil-water and air-water interfaces. In the case of mixtures of proteins with non-surface-active polysaccharides, the resulting surface-activity is usually attributed to the adsorption of protein-polysaccharide complexes. By understanding relationships between the protein-protein, protein-surfactant and protein-polysaccharide interactions and the properties of the resulting adsorbed layers, we can aim to... 

Lin, C. F. 1977. Interaction of sulfated polysaccharides with proteins. In Food Colloids. H.D. Graham (Editor). AVI Publishing Co., Westport, Conn. 

A polysaccharide can be added as a component in a protein system to produce a protein-polysaccharide composite structure. Tolstoguzov (2003) reviewed the main function of protein and polysaccharide in protein-polysaccharide food formulation. Generally, polysaccharides have less surface activity in comparison to proteins. This inferiority is related to their low flexibility and monotonic repetition of the monomer units in the backbone. The low surface activity of polysaccharides results in their inability to form a primary adsorbed layer in the system. The nature of interactions between polysaccharides and adsorbed proteins, as well as their influence on colloid stability, can either stabilize or destabilize the emulsions. Attractive protein-polysaccharide interactions can enhance the emulsion stability by forming a thicker and stronger steric-stabilizing layer. In contrast, the attractive interactions... 

These systems are widely used in many food products. A section will be devoted to the interfedal phenomena in food colloids, in particular their dynamic properties and the competitive adsorption of the various components at the interlace. The interaction between proteins eind polysaccharides in food colloids will be briefly described. This is followed by a section on the interaction between polysaccharides and surfactants. A short review will be given on surfactant association structures, microemulsions and emulsions in food [3]. Finally, the effect of food surfactants on the interfacial and bulk rheology of food emulsions will be briefly described. The formation of aggregation networks and the application of fractal concepts is then considered. This is followed by a section on applications of rheology in studying food texture and mouth feel. 

In the field of food colloids, the use of molecular thermodynamics provides a set of qualitative and quantitative relationships describing fundamental phenomena occurring in the equilibrium state of systems for which the intermolecular interactions of biopolymers (proteins and polysaccharides) play a key role. The phenomena and processes amenable to discussion from the thermodynamic point of view are ... 

Nowadays it is well established that the interactions between different macromolecular ingredients (i.e., protein + protein, polysaccharide + polysaccharide, and protein + polysaccharide) are of great importance in determining the texture and shelf-life of multicomponent food colloids. These interactions affect the structure-forming properties of biopolymers in the bulk and at interfaces thermodynamic activity, self-assembly, sin-face loading, thermodynamic compatibility/incompatibility, phase separation, complexation and rheological behaviour. Therefore, one may infer that a knowledge of the key physico-chemical features of such biopolymer-biopolymer interactions, and their impact on stability properties of food colloids, is essential in order to be able to understand and predict the functional properties of mixed biopolymers in product formulations. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

---