Biopolymer phase separation

Thies, C., Microencapsulation methods based on biopolymer phase separation and gelation phenomena in aqueous media. In Encapsulation Technologies and Delivery Systems for Food Ingredients and Neutraceuticals, Garti, N. and D. J. McClements, Eds., Woodhead Publishing Limited, Philadelphia, PA, 2012, pp. 177-207. 

Poly(VPGVG) (Fig. 6) has been smdied most thoroughly and it was shown that it exhibits an inverse phase transition. The biopolymer undergoes phase separation from solution upon increasing temperature, resulting in a p-spiral structure and simultaneous release of water molecules associated with the polymer chain (Fig. 7). 

It did not give rise to phase separation or precipitation. Similar behavior was observed when other types of polysaccharides were examined [53,54]. By now all the commercially important polysaccharides have been applied to the fabrication of hybrid silica nanocomposites in accordance with Scheme 3.2. What is more, various proteins have been entrapped in silica by the same means. In all instances the THEOS demonstrated good biocompatibility with biopolymers, even though its amount in formulations was sometimes up to 60 wt%. Biopolymer solutions after the precursor admixing remained homogeneous to the point of transition into a gel state. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

Soft Interaction Induced Phase Separation in Biopolymers and Design of New Biomaterials... 

Figure 3.2 Evolution of the microstructure of phase-separated biopolymer emulsion system containing pectin and 0.5 vt% heat-denatured (HD) whey protein isolate (WPI) stabilized oil droplets, (a) Composition 1U 3L (one-to-three mass ratio of upper and lower phases). The large circles are the water droplets (W), while the small circles are the oil droplets (O). This system forms a W2/W1-O/W1 emulsion, where O is oil, Wi is HD-WPI-rich and W2 is pectin-rich, (b) Composition 2U 2L. This system forms an 0/Wi/W2 emulsion, where O is oil, Wi is HD-WPI-rich and W2 is pectin-rich, (c) Composition 3U 1L. This system forms an 0/W]/W2 emulsion, where O is oil, Wi is HD-WPI-rich and W2 is pectin-rich. Reproduced from Kim et al. (2006) with permission.

In practice, from a knowledge of measured values of the osmotic second virial coefficients it is rather easy to calculate the spinodal curve. It is worthy of note here to observe that reciprocal values of m, for biopolymers of rather high molecular weight (> 104 g/mol) are often comparable with the magnitude of A 24. This requires that, as well as values of the osmotic second virial coefficients, the molecular weight should also be taken into account in the prediction of the boundary conditions relating to phase separation. 

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.

Norton I.T., Frith W.J. (2003). Phase separation in mixed biopolymer systems. In Dickinson, E., van Vliet, T. (Eds). Food Colloids, Biopolymers and Materials, Cambridge, UK Royal Society of Chemistry, pp. 282-297. 

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 phenomenon was called simple coacervation by Bungenberg de Jong (1949) in order to distinguish it from complex coacervation where both polymers are concentrated in the same solvent-depleted phase. The phenomenon of simple coacervation in aqueous food biopolymer systems has attracted considerable interest for many years. This is because of the perception of the potential of these phase-separated biopolymer... 

Nowadays it is established that confocal microscopy observation can be a more sensitive method to assess die phase state of mixed biopolymer systems than the traditional centrifugation or viscometric methods (Alves et al., 1999, 2001 Vega et al., 2005). Indeed, microscopy can demonstrate that a system may be already phase-separated at compositions well below the apparent binodal line (as determined by these other methods). The report of Alves et al. (2001) demonstrates the relationship between specific compositional points in the phase diagram (Figure 7.1) and the observed microstructure (Figures 7.2 and 7.3) for water + gelatin + locust bean gum (LBG). The white areas in Figures 7.2 and 7.3 corre-... 

For purposes of illustration in what follows, we consider the cases of various specific biopolymer mixtures demonstrating the roles of different thermodynamic parameters in determining the tendency towards phase separation. The deciding role of a greater positive value of Ay is indicated for mixtures in which there is a small difference in thermodynamic affinities of the two biopolymers for the solvent, AA2 = A - Au. A pair... 

Furthermore, the governing role of the tendency of biopolymers towards self-aggregation (An < 0) in aqueous medium seems to manifest itself in the greater area of phase separation for mixture (2) (pH = 7.0) as compared with mixture (1) (the same biopolymers at pH = 7.8), despite lower values for both y and A42 at pH = 7.0 (Wasserman et al., 1997) ... 

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... 

When a biopolymer mixture is either close to phase separation or lies in the composition space of liquid-liquid coexistence (see Figure 7.6a), the effect of thermodynamically unfavourable interactions is to induce biopolymer multilayer formation at the oil-water interface, as observed for the case of legumin + dextran (Dickinson and Semenova, 1992 Tsapkina et al, 1992). Figure 7.6b shows that there are three concentration regions describing the protein adsorption onto the emulsion droplets. The first one (Cprotein< 0.6 wt%) corresponds to incomplete saturation of the protein adsorption layer. The second concentration region (0.6 wt% < 6 proiem < 6 wt%) represents protein monolayer adsorption (T 2 mg m 2). And the third region (Cprotein > 6 wt%) relates to formation of adsorbed protein multilayers on the emulsion droplets. 

There have been numerous systematic studies of model systems of mixed biopolymers describing the relationship between phase separation and gelation microstructure (Kasapis et al., 1993 Alevisopoulos et al., 1996 Alves et al., 2000 Loren et al., 1999 Bourriot et al., 1999 Loren and Hermansson, 2000 Butler, 2002 Lazaridou and Biliaderis, 2009). 

As a general principle, it is established that the gelation of one (or both) of the biopolymer components retards the process of phase separation and so leads to kinetically trapped microstructures (Bourriot et al., 1999 Butler and Butler-Heppenstall, 2003 Loren and Hermans son, 2000). 

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