Aggregation kinetics of heated whey protein-stabilised emulsions: effect of low-molecular weight emulsi®ers Stephen R. Euston * , Suzanne R. Finnigan, Robyn L. Hirst Food Science Section, New Zealand Dairy Research Institute, Private Bag 11029, Palmerston North, New Zealand Received 1 February 2000; revised 7 April 2000; accepted 2 February 2001 Abstract The heat-induced aggregation of emulsions made with mixtures of whey protein and varied molar ratios of water-soluble or oil-soluble low molecular weight surfactants was followed using laser light scattering. At low molar ratio the water-soluble surfactants Tween-60 and phosphatidylcholine increased the aggregation rate, whereas at higher molar ratios the aggregation rate decreased. For oil-soluble surfactants glycerol monostearate and Span-60) the aggregation rate showed an increase at high molar ratio. Differences in the rate of heat-induced aggregation are explained in terms of possible changes in the conformation of adsorbed whey protein, observed differences in the protein surface coverage, and the formation of protein-surfactant complexes. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Emulsion; Heat stability; Whey protein concentrate; Aggregation; Surfactants 1. Introduction Low molecular weight emulsi®ers are an important func- tional ingredient in many dairy food emulsions. Their use and function in dairy foods has been the subject of a recent review Euston, 1997). In dairy products emulsi®ers appear to have an affect through two main mechanisms, namely the effect that they have on adsorbed protein molecules, and the effect they have on fat crystallization Euston, 1997). In products such as whipped cream and ice-cream formation of a stable product is only achieved by destabilization of the initial oil-in-water emulsion, which means fat droplets are able to adsorb to and stabilize air bubbles during the whip- ping process. The destabilization of the emulsion cannot occur in the absence of emulsi®ers, either naturally occur- ring e.g. phospholipids, monoglycerides), or arti®cial e.g. Tweens), as the emulsions are stable to coalescence. Low molecular weight emulsi®ers are able to displace protein from the fat globule surface and lower emulsion stability towards coalescence. A second function of emulsi®ers that is important in the whipping of cream and also in the formation of structure in whipped toppings is the effect that emulsi®ers have on crys- tallization and crystal structure in emulsion droplets Euston, 1997). Emulsi®ers such as monoglycerides and Tweens are known to affect both the rate of crystallization and the morphology of the crystals Barfod, Krog, Larsen, & Bucheim, 1991; Euston, 1997). Fat crystals have been shown to be important in the destabilization of oil-in- water emulsions Boode & Walstra, 1993) and contribute to the stabilization of air bubbles in whipped cream by promoting partial coalescence of the fat droplets Euston, 1997). The phenomenon of protein displacement, or competitive adsorption, by emulsi®ers has been researched extensively Dickinson & McClements, 1995). The extent to which protein is displaced depends on the emulsi®er concentration and type Dickinson, Iveson, & Tanai, 1993; Euston, Singh, Munro, & Dalgleish, 1995), and environmental conditions such as temperature Dickinson & Tanai, 1992). In general, oil soluble emulsi®ers are less ef®cient at displacing protein than water soluble ones Dickinson et al., 1993; Euston et al., 1995), and lead to less protein displacement. The effect of temperature is more complex. Whipping cream and ice- cream emulsion must be cooled or aged) at about 48C prior to whipping Euston, 1997). This is because emulsi®ers displace more protein when they are cooled to this tempera- ture, as adsorbed layers of these molecules undergo complex phase changes, resulting in expulsion of water and protein from the emulsi®er layer Berger, 1990). Emulsi®ers can also have a more subtle effect on the adsorbed protein layer around emulsion droplets. It has been Food Hydrocolloids 15 2001) 253±262 0268-005X/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0268-005X01)00022-4 www.elsevier.com/locate/foodhyd * Corresponding author: Biological Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK. E-mail address: s.r.euston@hw_ac.uk S.R. Euston).