Compositional and physical properties of skim milk and whey protein concentrate powders prepared from milk of cows fed perennial ryegrass or total mixed ration

The effect of bovine dietary variations on the composition and physical behaviour of skim milk powder (SMP) and whey protein concentrates (WPC) was investigated. Raw milk was obtained from three groups of 17 cows randomly assigned to perennial ryegrass only (GRS) or total mixed ration (TMR) based diets for a full lactation. This milk was processed into SMP at pilot scale and further processed to produce acid whey, micellar casein whey and sweet whey protein concentrate powders. All samples were analysed for protein and mineral composition. Reconstituted SMP was analysed for heat coagulation time and ionic calcium concentration, and the heat-dependent rheological behaviour of reconstituted WPC and blends of SMP and WPC was also measured. Skim milk powder samples derived from the TMR system were characterised by significantly higher non-protein nitrogen, selenium and iodine and significantly lower ionic calcium contents than those from GRS, and displayed higher heat stability and lower viscosity on heating. Irrespective of diet, there was significant variation between mineral profiles of the WPC sample types. The gel strength of GRS-derived heated WPC powders and the dynamic viscosity of heated mixtures of SMP with WPC were higher than those from TMR, although this difference was less substantial than the differences between WPC types, among which acid WPC displayed significantly lower thermal stability than sweet and micellar casein WPC. The study determined a limited effect of feeding system on the composition and rheological properties of protein ingredients relative to differences between whey streams, and suggests that micellar casein WPC presents the most favourable thermal stability and mineral composition.