Denaturation and aggregation of whey protein have been extensively studied but there is limited knowledge of their effects on processing properties of infant milk formulae (IMF) systems. In this study, the separate effects of denaturation and aggregation of whey protein on the physicochemical characteristics during processing of a model IMF were examined. Whey protein solutions (8%, w / w , protein) were pre-heated for 2 min at 72 or 85 °C, followed by addition of 2.2 mM calcium (Ca) (H-BCa), or at 85 °C after addition of the same level of Ca (H-ACa), to give pre-treated whey protein for inclusion in three model IMF systems, encoded as H-72-BCa, H-85-BCa and H-85-ACa, respectively. Unheated control samples without (UH-C) and with (UH-C-Ca) added Ca were also prepared. Model IMF systems (5.2%, w / w , protein, 60:40 whey protein:casein ratio, pH 6.8) were then prepared incorporating these pre-treated whey protein ingredients and subjected to lab-scale high-temperature short-time (HTST) heating at 85 °C for 2 min; whey protein denaturation was >81.2% in all samples after HTST. Aggregation of whey protein resulted in a significantly ( P