Surfactant-polysaccharide complexes

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... 

Gurov, A.N., Nuss, P.V. (1986). Protein-polysaccharide complexes as surfactants. Nah-rung, 30, 349-353. 

Emulsion Formation. An oil-in-water emulsion is produced by homogenizing an oil phase and an aqueous phase together (see above). This emulsion is usually stabilized by an emulsifier that adsorbs to the oil-water interface, thereby facilitating droplet formation and retarding droplet aggregation. Typically, this emulsifier is either a small molecule surfactant (e.g. lecithin. Tween, Spans, etc) or a biopolymer (e.g. protein, polysaccharide, or protein-polysaccharide complex). In addition, a material that will... 

In the past [4-6] it was common to characterize amphiphiles according to their major performance in food systems (1) emulsification and stabilization, (2) protein interactions, (3) polysaccharide complexation, (4) aeration, and (5) crystal structure modification of fats. Such classifications correlate the surfactant chemical structure to its interaction (chemical or physical) with substrates such as fats, polysaccharides, and proteins. It was confirmed fhat certain surfactants interact molecularly with macromolecules, forming complexes and/or hybrids, and alter the macromolecular behavior at the interface. Such activity is an important new contribution of cosurfactants to the surface performance of other surfactants [7]. Such interactions are sometimes a very important contribution of amphiphiles to food systems. 

Let us consider now the case of a specific ionic polysaccharide. The unique properties of complexes of the cationic chitosan with non-ionic sorbitan esters provides an interesting example. Grant and co-workers (2006) have established that mixtures of chitosan and surfactant form emulsion-like solutions and/or creams, where the surfactant component is present as droplets or micelle-like particles and the chitosan solution acts as the system s continuous phase. It was established that the length and the degree of saturation of the surfactant hydrocarbon chain have a significant impact on the development of the chitosan-surfactant complexes. Moreover, an optimal distance between the chitosan s protonated amine groups is required for effective interactions to occur between the polysaccharide and the sorbitan esters. 

Figure 8.15 Cartoon showing how proteins, polysaccharides and surfactants (emulsifiers) might be distributed at the triglyceride-water interface. Inter-facial complexation in vivo between adsorbed protein and charged polysaccharide in the gastrointestinal tract could affect digestion of protein and fat by forming structures that inhibit the accessibility and activity of enzymes (proteases and lipases). Reproduced from Dickinson (2008) with permission.

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

Figure 4.17 shows key aspects of phase behavior of the system water (H20)/Ci4 tiimethylammonium bromide/hyaluronan (an anionic polysaccharide) as a function of added sodium bromide (NaBr) (Lindman and Thalberg, 1993). With no added salt, a region with two liquid phases is seen for a range of surfactant polymer ratios. One phase is rich in both polymer and surfactant. The phase separation stems from complexation with a low net charge on each polymer chain, as discussed above. As salt is added, the eharges on both surfactant and... 

The above dependence on pH of the precipitation of acidic polyelectrolytes with cationic surfactant clearly affords means of separating mixtures of such polymers based on their acid strength. In an interesting development, it was shown (115) that the method can be applied to the separation of neutral polysaccharides by working at sufficiently high pH to ionize their hydroxyl groups. A more convenient method, however, to develop polyanionic character and hence precipitability with added cationic surfactants is to form the borate complex of the polysaccharide by simple addition of borate ion (116). 

Anionic polysaccharides respond in similar fashion to surfactants. They are relatively unaffected by anionic surfactants like sodium or ammonium lauryl sulfate. On the other hand, they form strong ionic complexes with cationic surfactants like dodecyltrimethylam-monium chloride, even at cationic surfactant concentrations below the critical micelle concentration (cmc), or concentration at which the surfactant molecules form micelles in solution (92,93). The behavior of polyelectrolytes in the presence of surfactants is summarized in Chapter 5 and has been reviewed (94). 

The addition of cationic surfactant to an anionic polysaccharide solution shows its greatest effect near the surfactant cmc. As the surfactant concentration increases beyond the cmc, the polysaccharide may go back into solution or it may form an insoluble complex, flocculate, and settle out of solution. In general, as the polysaccharide s anionic charge increases, so does its interaction with cationic surfactants. As an example, alginic acid binds more strongly to cationic surfactants than carboxymethylcellulose. 

The behavior of anionic polysaccharides with nonionic and amphoteric surfactants (betaines) is more complex. For example, nonionic surfactants can form charge transfer complexes with highly charged polysaccharides. Amphoteric surfactants can have various charges and various degrees of interaction with anionic polysaccharides depending on the... 

Amphoteric polysaccharides are challenging to formulate they can be soluble when cationic or anionic, but insoluble in their zwitterionic form. These polyglycans can also have complex chemistries in the presence of salts and surfactants. Salts can modify the pH at which amphoteric polymers become zwitterionic. Surfactant behavior is more complex. When the amphoteric polymer is cationic, it is incompatible with anionic surfactants... 

Bouyer E, Mekhloufi G, Rosflio V, Grossiord JL, Agnely F. Proteins, polysaccharides, and their complexes used as stabilizers for emulsions alternatives to synthetic surfactants in the pharmaceutical field Int J Pharm. 2012 436 359 78. 

Bouyer E, Mekhloufi G, Rosilio V, Grossiord J-L, Agnely F (2012) Proteins, polysaccharides, and their complexes used as stabilizers for emulsions alternatives to synthetic surfactants in the pharmaceutical field Int J Pharma 436 359-378 Bradley EL, Castle L, Chaudhry Q (2011) Applications of nanomaterials in food packaging with a consideration of opportunities for developing countries. Trends Food Sci Technol doi 10.1016/ j.tifs.2011.01.002 (in press)... 

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