Effect of protein content on the physical stability and microstructure of a model infant formula

The effect of protein content (6.68–11.88 g protein 100 g−1 powder) on the microstructure and physical stability of dry infant formula was investigated at relative humidities (RH) of 0 and 54.4%. Time-dependent lactose crystallisation of powders occurred at lower water contents as protein level decreased. Surface composition of powders stored at 0% RH were significantly (P < 0.05) different to their bulk composition, with fat extensively covering the surface of powder particles. Increasing RH to 54.4% increased surface lactose levels due to lactose crystallisation. Reducing protein level increased levels of free fat in powders stored at 0% RH for 14 days. Glass transition temperatures of powders decreased only at the lowest protein level studied (6.68 g 100 g−1). Reducing the protein content of infant formulae resulted in altered powder particle surface composition, free fat levels and time-dependent lactose crystallisation properties.