Associative and segregative interactions between gelatin and low-methoxy pectin: Part 1. Associative interactions in the absence of Ca 2+

Abstract

The effect of pH and mixing ratio on associative (electrostatic) interactions between gelatin (type B; pI=4.9) and low-methoxy pectin (DE 31.1) has been studied by differential scanning calorimetry (DSC), turbidity, and rheological measurements under low-amplitude oscillatory shear. All three techniques showed two thermal transitions for mixtures prepared below pH ∼3.5, one coincident with formation and melting of gelatin helices (below ∼35 °C) and the other centred at higher temperature (∼50 °C). The second transition was abolished by addition of salt (0.1 M NaCl), and is attributed to electrostatic association between the two polymers. Rheological measurements of network strength (storage modulus, G′) were consistent with conversion from a gelatin network augmented by additional crosslinking through pectin, to a (much weaker) pectic acid network augmented by additional crosslinking through gelatin as the relative proportion of pectin was increased. As observed in previous studies, compositions close to charge balance gave dense flocs on mixing. Mixtures prepared with either component in substantial (electrical) excess, however, remained homogeneous, suggesting solubilisation of the electrostatic complex by surplus charge. For these homogeneous preparations, the magnitude of the DSC endotherm corresponding to dissociation of the electrostatic complex remained roughly constant at ∼8 J/g of gelatin, and the magnitude of the first transition (below 35 °C) also remained roughly constant at a value close to that observed for gelatin alone (Δ H≈30 J/g). For compositions around charge balance, however, there was a massive increase in both endotherms, attributed to additional non-covalent interactions within flocculated particles. The lowest pH at which no significant electrostatic association could be detected by any of the investigative techniques was 3.9, which was therefore used in the study of segregative interactions in calcium pectinate–gelatin co-gels described in the following paper.