*Correspondent: Fax: +31 318 650400; e-mail: dekruif@nizo.nl Whey protein aggregates and their interaction with exo-polysaccharides Kees G. de Kruif 1,2 * & Remco Tuinier 2 1 NIZO Food Research, Ede, The Netherlands 2 Van’t Hoff Laboratory of Physical and Colloid Chemistry, Debye Institute, Utrecht University, Utrecht, The Netherlands (Received 17 May 1999; Accepted in revised form 24 June 1999) Summary We made a systematic investigation of the interaction of an exo-polysaccharide (EPS) with whey proteins. The EPS was produced by a lactic acid bacterium Lactococcus lac- tis subsp. cremoris strain NIZO B40. The interaction of the EPS with milk components is of relevance to the consistency of fermented milk products. EPS and whey protein iso- lates (WPI) can be mixed in all proportions. Even at an EPS concentration of 10 g.L -1 no instability is visible. In yoghurt milk the whey proteins are usually denatured. We pre- pared aggregated whey protein colloid particles (AWC particles) which had an average radius of 27  3 nm, as derived from the radius of gyration. The interaction of AWC particles and EPS was studied at a neutral pH and an ionic strength of 0.1 M. At low EPS concentrations the mixture was stable. At higher concentrations the mixtures showed phase separation owing to depletion interactions. In the one-phase region the strength of the segregative interaction can be determined from turbidity, light scattering and neutron scattering measurements. Then, by applying statistical mechanical methods, the position of the phase boundary could be predicted. The viscosity of the mixtures was increased owing to the segregative interactions in the EPS/AWC mixtures. Within the two-phase region the mixtures showed so-called spinodal decomposition, during which different microstructures were formed. Understanding the formation of such structures is of inter- est for the development of new food structures. Keywords Depletion interaction, neutron scattering, polysaccharide, viscosity, whey proteins. Introduction Whey proteins in their native state are small nanometer-sized particles dispersed in a continu- ous phase containing various salt ions and lactose. When used in food systems, the whey proteins are usually denatured owing to thermal treatments. As a result the whey proteins are aggregated, either among themselves or with other food particles e.g. casein micelles or emulsion droplets. Other com- ponents in food systems are polysaccharides (PS) e.g. xanthan, carrageenan or exo-polysaccharides (EPS) from lactic acid bacteria. Usually their func- tion is to thicken the food system. Polysaccharides are therefore referred to as food hydrocolloids or thickeners. In the past several studies have been made on the gel formation of whey proteins in combination with PS. From these studies it is how- ever difficult to determine the strength and the nature of the interaction. Capron et al. (Capron et al., 1999) studied the interaction of -lactoglobulin (-lg) with carrageenan. Here we aim to study the interactions of EPS with whey proteins in systems that are non-gelling. Nevertheless, the addition of polysaccharides to protein solutions clearly International Journal of Food Science and Technology 1999, 34, 487–492 © 1999 Blackwell Science Ltd 487