Interbiopolymer complexing between β-lactoglobulin and low- and high-methylated pectin measured by potentiometric titration and ultrafiltration

The interactions between proteins and polysaccharides are of great interest to the food industry. Few studies have been carried out on diluted beta-lactoglobulin/pectin systems and none has provided a complete investigation on the nature of the interactions involved. The two main objectives of this study were to determine the conditions for the formation of complexes and to identify the type of interactions involved in beta-lg/low-methylated (LM) and beta-lg/high-methylated (HM) pectin systems. The pH values leading to the formation of soluble complexes (pH(c)) were determined using potentiometric titrations. The nature of interactions was determined using sodium chloride, urea and different temperatures. The extent of complex formation was quantified by filtering beta-lg/LM- and beta-lg/HM-pectin solutions, at pH values between 4.5 and 7.0, through a 100 kDa molecular weight cut-off (MWCO) membrane. The pore size of the membrane was used to retrieve the non-complexed protein in the permeate. These proteins were measured using the bicinchoninic acid protein assay (BCA). The amount of beta-lg complexed was greater with LM-than with HM-pectin. At pH 4.5 in deionised water and at a protein/polysaccharide (w/w) ratio of 4:1, about 96% of the beta-lg complexed with LM-pectin, whereas only 78% complexed with HM-pectin. The destabilising effect of sodium chloride, urea and temperature has demonstrated that interactions in the systems are mainly caused by electrostatic forces and, to a lesser extent, hydrogen bonding. (C) 2002 Elsevier Science Ltd. All rights reserved.