Coacervation of proteins and anionic polysaccharides is both of practical and theoretical interest. From a large body of literature, it seems that the phase separation is mainly entropically driven, and may most probably be attributed to the delocalisation of the counter ions of the protein and the polysaccharide. The protein and polysaccharide appear to form complexes in solution, which can be viewed as new colloidal entities. These complex particles are neutral and exhibit an attractive interaction, which leads to a phase separation of the gas-liquid type in which a (very) dilute colloidal phase coexists with a very concentrated colloidal phase. In the case of strong poly-acids, usually, a precipitate is formed rather than a liquid coacervate phase. The structure of the concentrated polymer phase seems to resemble a continuous polymer phase in which the protein can diffuse around, as well as the individual polysaccharide molecules. Time scales of diffusion vary from milliseconds to days depending on the strength of the interaction. From a rheological point of view, the concentrated phase is much more viscous than elastic and the rheology resembles the behaviour of a (viscous) concentrated particle dispersion. Theoretical developments are limited probably due to the difficulty to describe the (correlated) charge distribution in the system. There is a strong interest in coacervates for the use of encapsulation. For the same reason, much attention is given to replacing the traditional gelatin by milk and plant proteins.

Complex coacervation of proteins and anionic polysaccharides

Microbial exopolysaccharides are biothickeners that can be added to a wide variety of food products, where they serve as viscosifying, stabilizing, emulsifying or gelling agents. Numerous exopolysaccharides with different composition, size and structure are synthesized by lactic acid bacteria. The heteropolysaccharides from both mesophilic and thermophilic lactic acid bacteria have received renewed interest recently. Structural analysis combined with rheological studies revealed that there is considerable variation among the different exopolysaccharides; some of them exhibit remarkable thickening and shear-thinning properties and display high intrinsic viscosities. Hence, several slime-producing lactic acid bacterium strains and their biopolymers have interesting functional and technological properties, which may be exploited towards different products, in particular, natural fermented milks. However, information on the biosynthesis, molecular organization and fermentation conditions is rather scarce, and the kinetics of exopolysaccharide formation are poorly described. Moreover, the production of exopolysaccharides is low and often unstable, and their downstream processing is difficult. This review particularly deals with microbiological, biochemical and technological aspects of heteropolysaccharides from, and their production by, lactic acid bacteria. The chemical composition and structure, the biosynthesis, genetics and molecular organization, the nutritional and physiological aspects, the process technology, and both food additive and in situ applications (in particular in yogurt) of heterotype exopolysaccharides from lactic acid bacteria are described. Where appropriate, suggestions are made for strain improvement, enhanced productivities and advanced modification and production processes (involving enzyme and/or fermentation technology) that may contribute to the economic soundness of applications with this promising group of biomolecules.

Heteropolysaccharides from lactic acid bacteria.

Protein/polysaccharide complexes and coacervates in food systems

The field of polyelectrolye–protein complexes is now benefiting from the convergence of three themes whose disciplinary distinctions formerly produced fragmentation. Theory and simulations, previously restricted to isotropic models of colloids and polyelectrolytes, are now taking into account the heterogeneity of charges in both macroions and providing far more realistic depictions of proteins. Applications of protein–polyelectrolyte systems have been strongly influenced by the opportunities presented by multilayer assembly, and the dichotomy between semi-synthetic systems and the protein–biopolyelectrolyte cognate pairs of biochemical interest has been giving way to less parochial recognition of universal effects. The junction of these disciplines, along with the development of many new methods of investigation, form the subject of this review focused on recent developments and their foundations.

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Polyelectrolyte-protein complexes

Recent advances on protein–polysaccharide electrostatic complex and coacervates' formation, structure and properties are presented. The first section emphasises on the thermodynamics of complexes formation, the contribution of enthalpic and entropic effects is specifically discussed. Then, their structure is described at different length scales and recent findings on the phase-ordering dynamics of complex and coacervates formation are covered. Finally, the most relevant functional properties of protein–polysaccharide complexes and coacervates for food applications are described.

Protein–polysaccharide complexes and coacervates

The formation of electrostatic complexes of whey protein (WP) and a nongelling carrageenan (CG) was investigated as a function of pH, ionic strength, temperature, and protein-to-polysaccharide (Pr:Ps) ratio. On lowering the pH, the formation of soluble WP/CG complexes was initiated at pH(c) and insoluble complexes at pH(phi), below which precipitation occurred. The values of the transition pH varied as a function of the ionic strength. It was shown that at [NaCl] = 45 mM, the value of pH(phi) was the highest, showing that the presence of monovalent ions was favorable to the formation of complexes by screening the residual negative charges of the CG. When CaCl(2) was added to the mixtures, complexes of WP/CG were formed up to pH 8 via calcium bridging. The electrostatic nature of the primary interaction was confirmed from the slight effect of temperature on the pH(phi). Increasing the Pr:Ps ratio led to an increase of the pH(phi) until a ratio of 30:1 (w/w), at which saturation of the CG chain seemed to be reached. The behavior of WP/CG complexes was investigated at a low Pr:Ps ratio, when the biopolymers were mixed directly at low pH. It resulted in an increase of the pH of the mixture, as compared to the initial pH of the separate WP and CG solutions. The pH increase was accompanied by a decrease in conductivity. The trapping of protons inside the complex probably resulted from a residual negative charge on the CG. If NaCl was present in the mixture, the complex took up the Na(+) ions instead of the H(+) ions.

Complexation of whey proteins with carrageenan.

Whey proteins (WPs) and the exopolysaccharide B40 (EPS B40) form electrostatic complexes under specific conditions. EPS B40 is a natural thickener in yogurt-like products. It is a phosphated polysaccharide and thus has a strong polyelectrolyte character. When the WP and the EPS B40 were mixed at pH values near or below the isoelectric point (pI) of the protein, soluble complexes were formed at pHc and phase separation took place below pHφ. The formation and the structure of those complexes were studied by various methods, including turbidity, dynamic and static light scattering (DLS and SLS), and viscosity measurements. The results showed that the strength of the interaction was strongly pH- and salt-dependent. The ζ-potential of the protein at pHc and pHφ was linearly dependent on the square root of the ionic strength (√I), showing the electrostatic nature of the interaction. Light scattering and viscosity measurements provided new results on the behavior of the complexes at the molecular level. In the r...

Complex Formation of Whey Proteins: Exocellular Polysaccharide EPS B40

Complex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific pH window between 2.5 and 4.8. After phase separation, a concentrated polymer (also called coacervate) phase was obtained, whose viscoelastic properties were studied at various pH values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0, from 0.3 to 30 s−1 and a shear thinning above 30 s−1, indicating a structural change. Hysteresis in the flow curve was measured at the pH values at which the electrostatic interactions were the strongest. Hysteresis was due to a slow structural rearrangement of the coacervate phase and, with time, the initial viscosity was completely recovered, showing that structural changes were reversible. In frequency sweep experiments, the values of the viscous modulus G″ were up to ten times higher than the values of the elastic modulus G′, indicating the highly viscous character of the coacervates. pH 4.0 appeared to be the pH at wh...

Gerard W. Robijn

Papers

Complexation of whey proteins with carrageenan.

Complex Formation of Whey Proteins: Exocellular Polysaccharide EPS B40

Rheological properties of whey protein/gum arabic coacervates

Determination of the structure of the exopolysaccharide produced by Lactobacillus sake 0–1

Structural characterization of the exopolysaccharide produced by Lactobacillus acidophilus LMG9433.