ORIGINAL RESEARCH The effect of protein profile and preheating on denaturation of whey proteins and development of viscosity in milk protein beverages during heat treatment CLODAGH M KELLEHER, 1,2 TUGCE AYDOGDU, 1,2 KEVIN M MURPHY, 1 JAMES A O'MAHONY, 2 ALAN L KELLY, 2 DONAL J O'CALLAGHAN 1 and NOEL A MCCARTHY 1 * 1 Food Chemistry and Technology Department, Teagasc Food Research Centre and 2 School of Food and Nutritional Sciences, University College Cork, Cork, Ireland The effect of preheat temperature (63 or 77 °C for 30 s; final heat 120 °C for 30 s) and casein to whey protein ratio on the physical characteristics of 3.3%, w/w, dairy protein beverages was inves- tigated. Dispersions preheated at 77 °C had lower viscosity than dispersions preheated at 63 °C. Casein-containing dispersions had significantly lower levels of a-lactalbumin denaturation than whey protein-only dispersions. A higher proportion of casein improved the thermal stability of pro- tein dispersions. Overall, alteration of preheat temperature and casein to whey protein ratio can influence dairy beverage quality, with increasing levels of casein reducing physical changes due to heat treatment. Keywords Skim milk, Whey protein, Casein, Heat treatment, Viscosity, Protein denaturation. INTRODUCTION Commercial production of milk, and its deriva- tives, requires effective heat treatment to ensure it is rendered microbiologically safe and physi- cally stable for consumers. Therefore, the heat stability of milk and the impact of thermal pro- cessing on its physical characteristics are impor- tant considerations for dairy processors (Lewis and Deeth 2009). Factors such as processing parameters, protein concentration and profile, pH, ionic strength and mineral composition can all significantly affect the heat stability and physical characteristics of the product (Singh, 2004; Singh et al. 2015). Heat treatment of dairy products at temperatures greater than 70°C results in denaturation and aggregation of heat- labile whey proteins (Anema and Li, 2003; Che- vallier et al. 2016; Joyce et al. 2017). This denaturation involves the unfolding of whey proteins to expose reactive functional groups, such as the free thiol groups in b-lactoglobulin (b-lg). Subsequently, these functional groups can readily react with other denatured whey pro- teins, caseins or j-casein (j-CN) at the casein micelle surface, to form whey protein aggre- gates, whey protein–casein aggregates or whey protein/j-CN complexes, respectively (Anema and Li 2003; Donato and Guyomarc’h 2009). The exact composition, structure and reactivity of the aggregates formed in heat-treated milk systems are determined by the type of proteins available. The casein to whey protein ratio (CN: WP) can have a significant impact on the type of heat-induced changes, such as particle size, viscosity and heat stability observed in dairy products (Beaulieu et al. 1999; Singh 2004; Donato and Guyomarc’h 2009; Brodkorb et al. 2016). For example, for infant milk formula (IMF), with a total protein content of 1.5%, w/ w, heat stability was found to be significantly affected by the CN:WP ratio (McSweeney et al. 2004). Manipulation of the CN:WP ratio can alter the kinetics of protein denaturation reac- tions (Anema et al. 2006) where the addition of casein can improve heat stability, due to casein– *Author for correspondence. E-mail: noel.mccarthy@teagasc.ie © 2020 Society of Dairy Technology Vol 70 International Journal of Dairy Technology 1 doi: 10.1111/1471-0307.12679