Biopolymers incompatibility

Biopolymer incompatibility is a general phenomenon typical of aU polymers. Biopolymer incompatibility occurs even when their monomers would be miscible in all proportions. For instance, sucrose, glucose and other sugars are normally cosoluble in the common solvent, water, while different polysaccharides usually are not miscible. The transition from a mixed solution of monomers to polymers corresponds to the transition from good to limited miscibility. Normally, a slight difference in composition and/or structure is sufficient for incompatibility of macromolecules in common solvent (Tolstoguzov 1991, 2002). Compatibility or miscibility of unlike biopolymers in aqueous solutions has only been exhibited by a few biopolymer pairs (Tolstoguzov 1991). 

Biopolymer incompatibility seems to provide phase-separated liquid and gel-like aqueous systems. In highly volume-occupied food systems aggregation, crystallisation and gelation of biopolymers and their adsorption at oil/water interfaces favour an increase in the free volume, which is accessible for other macromolecules. Denatura-tion of proteins during food processing decreases their solubility and co-solubility of proteins with one another and with polysaccharides and induces phase separation of the system. 

A limited biopolymer incompatibility under the conditions of pH (7) and low biopolymers concentration may contribute to increased hardness and thermal properties of gels. Excluded volume effects favour gelation of hydrocolloids. For incompatible biopolymers in mixed solutions, the rate of gelation is higher and the critical concentration for gelation is lower than for each of them individually.18,19... 

Antipova, A.S., Semenova, M.G. (1995). Effect of sucrose on the thermodynamic incompatibility of different biopolymers. Carbohydrate Polymers, 28, 359-365. 

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 presence of a thermodynamically incompatible polysaccharide in the aqueous phase can enhance the effective protein emulsifying capacity. The greater surface activity of the protein in the mixed biopolymer system facilitates the creation of smaller emulsion droplets, i.e., an increase in total surface area of the freshly prepared emulsion stabilized by the mixture of thermodynamically incompatible biopolymers (see Figure 3.4) (Dickinson and Semenova, 1992 Semenova el al., 1999a Tsapkina et al., 1992 Makri et al., 2005). It should be noted, however, that some hydrocolloids do cause a reduction in the protein emulsifying capacity by reducing the protein adsorption efficiency as a result of viscosity effects. 

Japanese electronics company Sharp has developed technology to blend PLA biopolymers with conventional plastics recovered from scrapped consumer appliances. Petroleum-based plastics are generally incompatible with bioplastics, and blends tend to show inferior properties such as impact strength and heat resistance. Sharp claims to have overcome these problems with a microdispersion technology that dramatically improves the properties of the blended material. The company expects to use such blends in its consumer electronics products by early 2007. 

On mixing solutions of a protein and a polysaccharide, four kinds of mixed solutions can be obtained. Figure 3.1 shows that two single-phase systems (1 and 3) and two-types of biphase systems (2 and 4) can be produced. The two-phase liquid systems 2 and 4 differ in the distribution of biopolymers between the co-existing phases. The biopolymers are concentrated either in the concentrated phase of system 2 because of interbiopolymer complexing, or within separated phases of system 4 because of incompatibility of the biopolymers. The term biopolymer compatibility implies miscibility of different biopolymers on a molecular level. The terms incompatibility or limited thermodynamic compatibility cover both limited miscibility or limited cosolubility of biopolymers (i.e., system 2) and demixing or phase separation... 

For instance, denaturation and partial hydrolysis of proteins oppositely influence their incompatibility with other biopolymers (Tolstoguzov 1991). Most biopolymers are polyelectrolytes. Factors such as pH and salt concentration affect their interactions with one another, with the solvent and their compatibility. For instance, when the pH is shifted to their isoelectric point (lEP), the thermodynamic incompatibility of proteins is usually enhanced by self-association of the protein molecules. Generally, protein-neutral polysaccharide mixtures separate into two phases when the salt concentration exceeds 0.15 M. 

Now, we consider some features of biopolymers in terms of the incompatibility... 

Normally, sufficiently concentrated solutions of biopolymers differing in chemical composition, conformation and affinity for a solvent are immiscible. Biopolymers are usually incompatible at a sufficiently high ionic strength and at pH values above the protein s lEP, where the biopolymers are charged macro-ions. These conditions are typical of most food systems. When the bulk concentration of the biopolymers is below the cosolubility threshold (or the phase separation threshold) the mixed solution of the biopolymers is stable. However, when the bulk biopolymer concentration is increased above this critical level, the mixed solution breaks down into two liquid phases. 

The incompatibility phenomenon relates to both the occupation of a volume of the solution by macromolecules and the weak repulsion between unlike macromolecules. Phase separation in mixed solutions of a large number of biopolymers studied is sensitive to entropy factors given by the excluded volume of the macromolecules. Phase behaviour strongly depends, therefore, on the molecular weight and the conformation of the macromolecules. The excluded volume effect that depends on the size and shape of the macromolecules determines the phase separation threshold, water partition between the phases of WIW emulsions and biopolymer activity in mixed solution (Tolstoguzov 1986, 1991, 1992). 

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