Characterization of cold-set gels produced from heated emulsions stabilized by whey protein Aiqian Ye a, b, * , Steve Taylor a a Fonterra Research Centre, Palmerston North, New Zealand b Riddet Institute, Massey University, Palmerston North 4442, New Zealand article info Article history: Received 18 February 2009 Received in revised form 3 June 2009 Accepted 10 June 2009 abstract This paper reports the cold gelation of preheated emulsions stabilized by whey protein, in contrast to, in previous reports, the cold gelation of emulsions formed with preheated whey protein polymers. Emul- sions formed with different concentrations of whey protein isolate (WPI) and milk fat were heated at 90  C for 30 min at low ionic strength and neutral pH. The stable preheated emulsions formed gels through acidification or the addition of CaCl 2 at room temperature. The storage modulus (G 0 ) of the acid- induced gels increased with increasing preheat temperature, decreasing size of the emulsion droplets and increasing fat content. The adsorbed protein denatures and aggregates at the surface of the emulsion droplets during heat treatment, providing the initial step for subsequent formation of the cold-set emulsion gels, suggesting that these preheated emulsion droplets coated by whey protein constitute the structural units responsible for the three-dimensional gel network. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Whey proteins are widely used as food ingredients because of their good functional and nutritional properties (Morr & Ha, 1993). An important functional property of whey proteins is their ability to facilitate the formation and stability of oil-in-water emulsions (Dickinson, 2001). Whey proteins, which consist principally of b-lactoglobulin, a-lactalbumin and bovine serum albumin, have globular structures. Upon heat treatment above 70  C, these proteins unfold and aggregate (Kinsella & Whitehead, 1989; Singh & Creamer, 1991). In whey-protein-stabilized emulsions, heat treatment may lead to aggregation of the emulsion droplets (Demetriades, Coup- land, & McClements, 1997; Dickinson, 2001; Dickinson & Parkinson, 2004; Hunt & Dalgleish, 1995; Kulmyrzaev, Bryant, & McClements, 2000; Sliwinski, Roubos, Zoet, van Boekel, & Wouters, 2003). The stability of whey-protein-stabilized emulsions is affected by many factors, including pH, ionic strength and the presence of emulsifiers such as lecithin (Dickinson, 2001; Hunt & Dalgleish, 1995; Ye & Singh, 2000). Hunt and Dalgleish (1995) and Kulmyrzaev et al. (2000) reported that emulsions made with whey proteins were stable against heating at low and neutral pH, and at low ionic strength. However, Monahan, McClements, and German (1996) and Sliwinski et al. (2003) reported that heating at 75–80  C caused significant droplet aggregation; however, the effect decreased at higher heating temperatures. The authors explained this trend by the predominance of inter-droplet interactions leading to aggre- gation of the emulsion droplets on heating in the temperature range 75–80  C, whereas intra-droplet protein–protein interactions were favoured at higher temperatures. In contrast, Euston, Finnigan, and Hirst (2000) reported that heat-induced aggregation of droplets in emulsions formed with whey protein concentrate (WPC) was extensive and proceeded more rapidly as the concentration of whey proteins in the emulsion was increased. Removal of non-adsorbed protein or replacement of serum with a solution of caseinate decreased the rate of aggregation several-fold (Euston et al., 2000). It was suggested that the non-adsorbed protein would act as a ‘‘glue’’, holding the aggregated emulsion droplets together (Hunt & Dalgleish, 1995). Gelation is an important functional property of whey protein (Morr & Ha, 1993), and contributes to the appearance, water-holding capacity and texture of food products. Whey protein gels can be formed in different ways (Doi, 1993). The most common way to obtain a whey protein gel is by heat treatment to induce gelation (Doi, 1993). Whey protein gels can also be prepared through a cold gelation process (Barbut & Foegeding, 1993). The cold gelation process involves two steps: (1) the denaturation and aggregation of whey protein by heat treatment of a protein solution; (2) the formation of a gel from the denatured and aggregated protein solution without heating through lowering the pH of the solution to a pH close to the isoelectric point of the protein or the addition of Ca 2þ to the solu- tion. The protein concentration during heat treatment must be * Corresponding author. Tel.: þ64 6 350 5072; fax: þ64 6 350 5655. E-mail address: a.m.ye@massey.ac.nz (A. Ye). Contents lists available at ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj 0958-6946/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2009.06.003 International Dairy Journal 19 (2009) 721–727