Associative and segregative interactions between gelatin and low-methoxy pectin: Part 2—co-gelation in the presence of Ca

The effect of segregative interactions with gelatin (type B; pI=4.9; 0–10 wt%) on the networks formed by low-methoxy pectin on cooling in the presence of stoichiometric Ca²⁺ at pH 3.9 has been investigated by rheological measurements under low-amplitude oscillatory shear. Samples were prepared and loaded at 85 °C, cooled (1 °C/min) to 5 °C, held for 100 min, and re-heated (1 °C/min) to 85 °C, with measurement of storage and loss moduli (G′ and G″) at 10 rad s⁻¹ and 2% strain. The final values of G′ at 5 °C for mixtures prepared at the same pH without Ca²⁺ were virtually identical to those observed for the same concentrations (0.5–10.0 wt%) of gelatin alone, consistent with the conclusion from the preceding paper that electrostatic (associative) interactions between the two polymers become significant only at pH values below 3.9. Increases in moduli on cooling in the presence of Ca²⁺ occurred in two discrete steps, the first coincident with gelation of calcium pectinate alone and the second with gelation of gelatin. Both processes were fully reversible on heating, but displaced to higher temperature (by 10 °C), as was also observed for the individual components. The magnitude of the changes occurring over the temperature range of the gelatin sol–gel and gel–sol transitions demonstrates that the gelatin component forms a continuous network; survival of gel structure after completion of gelatin melting shows that the calcium pectinate network is also continuous (i.e. that the co-gel is bicontinuous). On progressive incorporation of NaCl (to induce phase separation before, or during, pectin gelation) the second melting process, coincident with loss of calcium pectinate gel structure, was progressively abolished, indicating conversion to a gelatin-continuous network with dispersed particles of calcium pectinate. These qualitative conclusions are supported by quantitative analyses reported in the following paper.     

An unusual manifestation of phase separation in mixtures of disordered gelatin with konjac glucomannan

The behaviour of konjac glucomannan (KGM) and the galactomannans locust bean gum (LBG), guar gum and fenugreek gum at a concentration of 0.2 wt% in mixtures with type B gelatin in the solution state at 40 °C was examined by visual inspection. At gelatin concentrations of ∼2 wt% and above, the mixtures with LBG and guar gum formed cloud-like flocs, attributed to association of unsubstituted “smooth” regions of the mannan backbone in response to segregative interactions with gelatin. Fenugreek gum, which has no such smooth regions, did not form flocs, but gave a dispersion of small, gel-like particles. Mixtures of gelatin with KGM showed the unusual, striking phenomenon of forming a single gel-like lump which remained suspended in the surrounding liquid, neither rising nor sinking. The single lumps formed by mixtures of 0.2 wt% KGM with 5.0 wt% gelatin were studied in detail. The gelatin content, as determined by macro-Kjedahl analysis and comparison of the area of differential scanning calorimetry (DSC) cooling and heating peaks with calibration curves for gelatin alone, was ∼1.35 wt%. Rheological characterisation by low-amplitude oscillatory measurements and creep-recovery experiments showed that, despite its gel-like appearance, the lump had the properties typical of a solution of densely entangled polysaccharide coils, including close Cox–Merz superposition of steady-shear viscosity (η) and complex dynamic viscosity (η*). The concentration of KGM, determined by comparison of the maximum value of η* in the Newtonian region at low frequency with calibration curves for KGM alone, was 3.4 wt%. The composition of the surrounding liquid was obtained by calculations of mass balance, giving values of 5.20 wt% gelatin and 0.024 wt% KGM. The “p-factor” determined as the ratio of water to gelatin in the solution divided by the ratio of water to KGM in the lump was 0.65, well within the range found for other biopolymer mixtures. It is concluded that the gelatin–KGM mixtures show normal liquid–liquid phase separation, but that the phases have exactly the same density, preventing resolution into two liquid layers, and that enthalpically unfavourable interactions are instead minimised by one of the phases assuming spherical geometry.     

Gelation behaviour of gelatin and alginate mixtures

Mixtures of alginate and gelatin were studied by rheology as a function of different parameters, such as temperature, biopolymer concentrations, calcium concentration and ionic strength. In particular conditions, the formation of a mixed gel of alginate and gelatin is obtained. A slow release of calcium ions leads first to an irreversible alginate gel and cooling results in a reversible gelatin gel. Depending on experimental conditions, non-linear behaviours upon gelation of alginate occur and a collapse of alginate gel is directly observable by rheology. These trends are favoured between 35 and 45 °C, by a high total biopolymer concentration or a high calcium concentration and ionic strength. Different mechanisms could be responsible for this collapse, such as a competition between alginate gelation and phase separation in the biopolymer mixture or an over-association of alginate chains at high Ca²⁺ concentration, favoured by the presence of gelatin.     

Effect of pH on the in vitro interactions between bile acids and pectin

 The interactions between pectin and bile acids (BA) were investigated after in vitro incubation at 37°C in the pH range of 5.0 – 8.0, separation of the non-absorbed BA using membrane filtration and their estimation by HPLC and HPTLC. In general, the interactions increased with the degree of esterification and the molecular weight (viscosity) of the pectin. They were reduced if the carboxyl groups in the polysaccharide molecules are arranged in a blockwise manner and in the presence of Ca²⁺. The highest interactions were found for unconjugated BA, followed by the corresponding glycine-conjugates. Taurine-conjugates showed the lowest interaction with pectin. Chenodeoxycholic acid and deoxycholic acid reacted more effectively than the trihydroxy bile acid cholic acid in the in vitro experiments. The results show that structural parameters of both BA and pectin play an important role in the extent of their interaction. Received: 28 November 1996 / Revised version: 20 January 1997