Interactions of Whey Proteins during Heat Treatment of Oil-in-Water Emulsions Formed with Whey Protein Isolate and Hydroxylated Lecithin RAFAEL JIME Ä NEZ-FLORES, † AIQIAN YE, ‡ AND HARJINDER SINGH* ,‡ Riddet Centre, Massey University, Private Bag 11 222, Palmerston North, New Zealand, and Dairy Products Technology Center (DPTC), California Polytechnic State University, San Luis Obispo, California The interactions of proteins during the heat treatment of whey-protein-isolate (WPI)-based oil-in- water emulsions with and without added hydroxylated lecithin were studied by examining the changes in droplet size distribution and the quantity and type of adsorbed and unadsorbed proteins. Heat treatment at 90 °C of WPI emulsions resulted in an increase in total adsorbed protein; unadsorbed -lactoglobulin (-lg) was the main protein interacting with the adsorbed proteins during the first 10 min of heating, but after this time, unadsorbed R-lactalbumin (R-la) also associated with the adsorbed protein. In emulsions containing hydroxylated lecithin, the increase in total adsorbed protein during heat treatment was much lower and the unadsorbed -lg did not appear to interact with the adsorbed proteins during heating. However, the behavior of R-la during heat treatment of these emulsions was similar to that observed in the emulsions containing no hydroxylated lecithin. In the presence of NaCl, the particle size of the emulsion droplets and the quantities of adsorbed protein increased markedly during heating. Emulsions containing hydroxylated lecithin were less sensitive to the addition of NaCl. These results suggest that the binding of hydroxylated lecithin to unfolded monomers or intermediate products of -lg reduces the extent of heat-induced aggregation of -lg and consequently decreases the interactions between unadsorbed -lg and adsorbed protein. This was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of heated whey protein and hydroxylated lecithin solutions. KEYWORDS: Whey protein; hydroxylated lecithin; emulsion; heat treatment INTRODUCTION Whey proteins are widely used as food ingredients because of their good functional and nutritional properties (1). An important functional property of whey proteins is their ability to facilitate the formation and stability of oil-in-water emulsions (2). Whey proteins, which consist principally of -lactoglobulin (-lg), R-lactalbumin (R-la), and bovine serum albumin (BSA), have globular structures; upon heat treatment above 70 °C, these proteins unfold and aggregate (3, 4). In whey-protein-stabilized emulsions, heat treatment may lead to aggregation of the emulsion droplets (2, 5-9). The stability of whey-protein- stabilized emulsions is affected by many factors, including pH, ionic strength, and the presence of emulsifiers such as lecithin. The heat stability of the emulsion decreases with increasing ionic strength in the emulsion (5-7). Hunt and Dalgleish (5) and Kulmyrzaev et al. (7) reported that emulsions made with whey proteins were stable against heating at low and neutral pH and at low ionic strength. Monahan et al. (10) and Sliwinski et al. (8) reported that heating at 75-80 °C caused significant droplet aggregation but that the effect decreased at higher heating temperatures. The authors explained this trend by the predominance of interdroplet interactions, leading to aggregation of the emulsion droplets, on heating in the temperature range of 75-80 °C, whereas intradroplet protein-protein interactions were favored at higher temperatures. In contrast, Euston et al. (11) reported that heat- induced aggregation of droplets in emulsions formed when whey protein concentrate (WPC) was extensive and proceeded more rapidly as the concentration of whey proteins in the emulsion was increased. Removal of nonadsorbed protein or replacement of the serum with a solution of caseinate decreased the rate of aggregation severalfold (11). It was suggested that the nonad- sorbed protein would act as a “glue”, holding the aggregated emulsion droplets together (5). Lecithins are important ingredients in the commercial manu- facture of emulsions. Many studies have reported their surface- active properties (12-15), competition with proteins at oil/water interfaces (16-18), and interactions with proteins (19, 20). Hydroxylated lecithin, a commercially available modified * To whom correspondence should be addressed: Riddet Centre, Massey University, Private Bag 11 222, Palmerston North, New Zealand. Tele- phone: (64) 6 350 4401. Fax: (64) 6 350 5655. E-mail: h.singh@ massey.ac.nz. † California Polytechnic State University. ‡ Massey University. 10.1021/jf0480039 CCC: $30.25 © xxxx American Chemical Society PAGE EST: 7 Published on Web 00/00/0000