Control of Strength and Stability of Emulsion Gels by a Combination of Long- and Short-Range Interactions Theo B. J. Blijdenstein, ²,‡ Wouter P. G. Hendriks, ² Erik van der Linden, ‡ Ton van Vliet, ²,‡ and George A. van Aken* ,²,§ Wageningen Centre for Food Sciences (WCFS), P.O. Box 557, 6700 AN, Wageningen, The Netherlands, Food Physics Laboratory, Department of Agrotechnology and Food Sciences, Wageningen University, P.O. Box 8129, 6700 EV, Wageningen, The Netherlands, and NIZO food research, P.O. Box 20, 6710 BA, Ede, The Netherlands Received February 20, 2003. In Final Form: May 22, 2003 This paper discusses the change in phase behavior and mechanical properties of oil-in-water emulsion gels brought about by variation of long- and short-range attractive interactions. The model system studied consisted of oil droplets stabilized by the protein -lactoglobulin (-lg). A long-range depletion attraction was obtained by addition of dextran. At short distances, the interaction is dominated by electrostatic repulsion between the adsorbed layers of -lg. This interaction was varied by addition of Ca 2+ ions and by changing the NaCl concentration. Combination of long- and short-range attraction resulted in a substantial decrease in the rate of serum separation and an increase in the emulsion gel modulus at small deformations compared to depletion attraction alone. The flocculation process and the morphology of the flocs were investigated by diffusing wave spectroscopy and confocal scanning laser microscopy. Above a minimum concentration, dextran induced fast depletion flocculation, leading to a network of emulsion droplets. This network quickly collapsed due to gravity. Addition of Ca 2+ ions above a minimum concentration induced slow flocculation, and the flocs creamed before a network was formed. Addition of both dextran and Ca 2+ ions resulted in a two-step mechanism of emulsion gel formation. A network is quickly formed by depletion flocculation and subsequently the bonds between the emulsion droplets are reinforced by Ca 2+ ions. Due to this reinforcement, rearrangements of this network were suppressed resulting in a smaller rate of serum separation. Introduction The stability and mechanical properties of colloidal systems are of technological interest for numerous ap- plications, e.g., drilling mud, paints, inks, cosmetics, and foods. The properties of such materials are to a large extent determined by the interactions between the colloidal particles of which they consist. More specifically, for protein-stabilized emulsions these interactions are de- termined by the interaction between the oil droplets and by the adsorbed protein layer around the droplets. 1 In relatively dilute emulsions (φ < 0.4 wt %) with repulsive droplet-droplet interactions, the droplets are distributed homogeneously over the emulsion and the emulsion behaves as a liquid. The droplets cream due to a density difference between the droplets and the con- tinuous liquid, which results in the formation of a dense creamed layer on top of the emulsion. When the net interaction between the emulsion droplets becomes suf- ficiently attractive to overcome the Brownian motion of the emulsion droplets, they will flocculate. The relative rates of flocculation and creaming determine the structural evolution of an emulsion. If the rate of creaming exceeds the rate of flocculation, small flocs will be formed, which cream faster than individual droplets. This results in faster formation of a creamed layer. In the opposite case, the flocs grow large enough to overlap and form a network. 2,3 Due to collapse of this network, which is similar to a large deformation compression, a serum layer will separate at the bottom of the emulsion. 4 The morphology, stiffness, and strength of these colloidal networks can be tuned by varying the interaction between the particles in the network. Note that the term “strength” is used in relation to a nonlinear (large) deformation or yielding and “stiffness” in relation to linear (small) deformations. For example, a deeper well in the interaction potential curve usually yields a more open structure. 2,5,6 However, also the shape of the interaction potential curve is important, which can be altered, e.g., by combining different types of interactions. As an example, the effect of a secondary energy minimum on the initial stages of flocculation was discussed in a recent paper by Behrens and Borkovec. 7 These authors formulate a two-step flocculation mechanism, in which reversible flocculation into the secondary minimum precedes a slow coagulation into the primary energy minimum. This paper treats the effect of a combination of interac- tions on the phase behavior and gel strength of oil-in- water emulsions, stabilized by the milk protein -lacto- globulin (-lg). A long-range attraction was induced by addition of the polysaccharide dextran, which results in a relatively weak depletion attraction. 8-10 A change in short-range interaction was accomplished by addition of * Corresponding author. Tel: 31-318-659589. Fax: 31-318- 650400. E-mail: aken@nizo.nl. ² Wageningen Centre for Food Sciences. ‡ Wageningen University. § NIZO food research. (1) Dickinson, E. Colloids Surf., B 2001, 20, 197-210. (2) Bremer, L. G. B.; Bijsterbosch, B. H.; Walstra, P.; van Vliet, T. Adv. Colloid Interface Sci. 1993, 46, 117-128. (3) Poon, W. C. K.; Haw, M. D. Adv. Colloid Interface Sci. 1997, 73, 71-126. (4) Blijdenstein, T. B. J.; van Vliet, T.; van der Linden, E.; van Aken, G. A. Food Hydrocolloids 2003, 17, 661-669. (5) Berli, C. L. A.; Quemada, D.; Parker, A. Colloids Surf., A 2003, 215, 201-204. (6) Tuinier, R.; de Kruif, C. G. J. Colloid Interface Sci. 1999, 218, 201-210. (7) Behrens, S. H.; Borkovec, M. J. Colloid Interface Sci. 2002, 225, 460-465. 6657 Langmuir 2003, 19, 6657-6663 10.1021/la0342969 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/15/2003