3. Results and discussion
Fig. 1 illustrates the evolution of the air–water surface tension versus time measured by drop tensiometer for the albumin solution and the albumin solution in presence of the various poly- saccharides in acidic conditions. While xanthan leads to a moderate increase in surface tension, carrageenan and guar gum both contribute to a decrease of the overall surface tension, with carra- geenan giving the stronger decrease. In the figure it is also shown the evolution of surface tension by further reducing the pH to 3 in the case of carrageenan: the plateau value of surface tension decreases to a value as low as 26 mN/m. Because the isoelectric point (IP) of the albumin is 4.5, at both pH 4 and 3 the protein is positively charged, guar gum is neutral and xanthan is nearly entirely protonated (pH < pK ¼ 4.8) and thus, also essentially neutral (Garti & Benichou, 2004); being carrageenan the only negatively charged polysaccharide. Since the number of positive residues in the protein increases with decreasing pH, also the electrostatic interactions between carrageenan and albumin are expected to be increased by lowering pH; thus, the decrease in surface tension is likely to be greatly affected by the formation of coacervates between proteins and polysaccharides. The presence of
coacervates in the system contribute to interface rigidity rather than on the interfacial tension.
As a further confirmation that electrostatic interactions play a major role on the establishment of surface properties, Fig. 2 shows the same time-evolution for surface tension stabilized by the same protein–polysaccharide systems, this time at pH of 7.5, where the protein is negatively charged and the carrageenan, xanthan, and guar gum are respectively negatively, negatively and neutrally charged. Thus at this conditions, repulsive inter- actions, hydrophobic interactions and hydrogen bonding inter- actions are the only possible types of interactions for the protein and the polysaccharides, whereas attractive electrostatic inter- actions are inhibited. The resulting plateau moduli of the surface tension is either unaffected by the presence of the polysaccharide (for the carrageenan case), or even increased (xanthan and guar gum).
An additional insight on the interfacial properties of the protein/ polysaccharide systems can be gained by comparing the interfacial elasticity of the interfaces obtained under different conditions. This is expressed by the interfacial dilatational modulus as (Lucassen- Reynders, 1981, chaps. 5–6).
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