Biopolymers mixed solutions

Cesaro, A., Cuppo, F., Fabri, D., Sussich, F. (1999). Thermodynamic behaviour of mixed biopolymers in solution and in gel phase. Thermochimica Acta, 328, 143-153. 

With a ternary system of type biopolymer/ + biopolymey + solvent, in order to characterize all the different pair interactions, the following heat effects, Q, should be measured in flow mode (Semenova et al., 1991) (i) biopolymer, solution diluted by pure buffer, Q (ii) biopolymey solution diluted by pure buffer, Qp and (iii) mixed (biopolymer, + biopolymey) solution diluted by pure buffer, Qijh. The specific enthalpy of interaction between biopolymer, and biopolymey can then be obtained from... 

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... 

Figure 3.1. Main trends in the behaviour of mixed biopolymer solutions. Schematic representation of the four possible results obtained by mixing solutions of biopolymers a protein and a polysaccharide a protein and another protein a polysaccharide and another polysaccharide.

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). 

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). 

Excluded volume determines space occupancy in biopolymer solutions. Competition between macromolecules for space in a mixed solution determines the phase separation threshold. In a dilute solution of biopolymers, macromolecules hardly interact with one another, individual macromolecules are independent of one another, and biopolymers are cosoluble. The effects of spatial limitations are enhanced by the transition from a dilute mixed solution, to a semi-dilute biopolymer solution where molecules come into contact with one another, interact, compete for the same space, and do not mix in all proportions. 

The binodal branches do not coincide with the phase diagram axes. This means that the biopolymers are limitedly cosoluble. For instance, on mixing a protein solution A and a polysaccharide solution B a mixture of composition C can be obtained. This mixed solution spontaneously breaks down into two liquid phases, phase D and phase E. Phase D is rich in protein and E is rich in polysaccharide. These two liquid phases form a water-in-water (WIW) emulsion. Hie phase volume ratio is estimated by the inverse lever rule. The phase D/phase E volume ratio equals the ratio of the tieline segments EC/CD. Point F represents the phase separation threshold, that is, the minimal critical concentration of biopolymers required for phase separation to occur. 

Constmction of phase diagrams usually starts with the preparation of series of mixed solutions sufficiently differing in bulk biopolymer concentration. Some of them can be single-phase solutions, others biphasic systems. A true equilibrium between the phases is experimentally obtained by mixing or shaking the water-in-water emulsions under different time-temperature conditions. Separation of the phases by centrifuge provides information about both the number and the volume ratio of the system phases. The closer a system composition is to the critical point, the smaller the difference in density is between the phases and the more difficult their separation is by centrifugation. The amount of each biopolymer in each phase can be quantified by various techniques. Estimation of protein concentration by UV absorbance at 280 nm is widely used because of its simplicity and sensitivity. 

Morris (1981) suggested that when the apparent viscosities of two gum dispersions at equal concentrations are substantially different, in order to account for the different viscosity-coneentration relationships for the two gums, the apparent viscosity of mixtures of solutions of two gums be studied by mixing solutions of the two biopolymers with eoneentrations adjusted to give similar viscosities. The steps in this procedure for peetin and LB gum dispersions were (Lopes da Silva et al., 1992) (1) the shear rate—apparent viscosity data of solutions of pectin and LB gum with different eoneentrations were obtained, and (2) the concentrations... 

Figure 6-21 Phase Diagram of a Mixed Solution of Two Biopolymers, for example, Sodium Alginate...

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... 

FRI Frith, W.J., Mixed biopolymer aqueous solutions - phase behaviour and rheology,... 

Figure 3.3 Illustration of the calculation of the phase diagram of a mixed biopolymer solution from the experimentally determined osmotic second virial coefficients. The phase diagram of the ternary system glycinin + pectinate + water (pH = 8.0, 0.3 mol/dm3 NaCl, 0.01 mol/dm3 mercaptoethanol, 25 °C) —, experimental binodal curve —, calculated spinodal curve O, experimental critical point A, calculated critical point O—O, binodal tielines AD, rectilinear diameter,, the threshold of phase separation (defined as the point on the binodal curve corresponding to minimal total concentration of biopolymer components). Reproduced from Semenova et al. (1990) with permission.

The enthalpy change associated with formation of a thermodynamically ideal solution is equal to zero. Therefore any heat change measured in a mixing calorimetry experiment is a direct indicator of the interactions in the system (Prigogine and Defay, 1954). For a simple biopolymer solution, calorimetric measurements can be conveniently made using titra-tion/flow calorimeter equipment. For example, from isothermal titration calorimetry of solutions of bovine P-casein, Portnaya et al. (2006) have determined the association behaviour, the critical micelle concentration (CMC), and the enthalpy of (de)micellization. 

Biopolymers are, of course, poly electrolytes. This means that electrostatic repulsion between them, as well as the contribution of counterions to the total free energy of the system, are to be included amongst the key factors affecting the character of the biopolymer interactions, and hence the stability of mixed biopolymer solutions with respect to phase separation (Antipova and Semenova, 1997 Grinberg and Tolstoguzov, 1997 Polyakov et al., 1997 Semenova, 1996 Wassennan et al., 1997). For... 

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