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.

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 region where the complexes were still soluble and at low ionic strength, the DLS radius measured in the WP/EPS B40 mixture was smaller than the coil size in the EPS B40 solution but the apparent molar mass was increased. The increase of the molecular mass was attributed to the complexation of WP on the EPS B40 chain, which, at low salt, induced a reduction of the intramolecular repulsion and led to the compaction of the polysaccharide. Also, the ratio of protein to polysaccharide was varied in order to get more insight into the dynamics, the structure, and the apparent stoichiometry of the EPS B40/WP complexes. The results illustrated that phase separation was a consequence of charge neutralization of the complexes and that the apparent stoichiometry of the complexes depends on the order of mixing of the compounds. In time, the complexes rearranged to form neutralized complexes and free EPS B40. The concept of cooperative binding was highlighted in the case studied.

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A novel exopolysaccharide (EPS) produced by Lactobacillus sake 0-1 (CBS 532.92) has been isolated and characterized. When the strain was grown on glucose, the produced EPS contained glucose and rhamnose in a molar ratio of 3:2 and the average molecular mass distribution (M m) was determined at 6 ؋ 10 6 Da. At a concentration of 1%, the 0-1 EPS had better viscosifying properties than xanthan gum when measured over a range of shear rates from 0 to 300 s ؊1 , while shear-thinning properties were comparable. Rheological data and anion-exchange chromatography suggested the presence of a negatively charged group in the EPS. Physiological parameters for optimal production of EPS were determined in batch fermentation experiments. Maximum EPS production was 1.40 g ⅐ liter ؊1 , which was obtained when L. sake 0-1 had been grown anaerobically at 20؇C and pH 5.8. When cultured at lower temperatures, the EPS production per gram of biomass increased from 600 mg at 20؇C to 700 mg at 10؇C but the growth rate in the exponential phase decreased from 0.16 to 0.03 g ⅐ liter ؊1 ⅐ h ؊1. EPS production started in the early growth phase and stopped when the culture reached the stationary phase. Growing the 0-1 strain on different energy sources such as glucose, galactose, mannose, fructose, lactose, and sucrose did not alter the composition of the EPS produced.

The exopolysaccharide produced by Lactobacillus sake 0-1 in a semi-defined medium was found to have an average molecular mass of 6 x 10(6) Da and a composition of D-glucose, L-rhamnose, and sn-glycerol 3-phosphate (3:2:1). The polysaccharide is partially O-acetylated. By means of partial acid hydrolysis, O-deacetylation, deglycerophosphorylation, methylation analysis, and 1D/2D NMR (1H, 13C, and 31P) studies the polysaccharide was shown to be composed of repeating units with the following structure: [formula: see text]