Effect of pasture and total mixed ration-based diets on the composition of whole milk powder and whole milk digestate

• Significant differences in milk composition between pasture and concentrate diets • Significant variation in fatty acid, amino acid and mineral profiles between diets • Raman spectroscopy effectively differentiates pasture and concentrate-based diets • Limited variation in composition of digestate produced by static digestion The composition of whole milk powders (WMP) produced from the milk of three groups of cows fed diets consisting of perennial ryegrass (GRS), perennial ryegrass/white clover (CLV) or indoor total mixed ration (TMR) was determined. Raw milk from each group was heat treated, homogenised, evaporated and spray-dried to yield WMP. Reconstituted WMPs were also enzymatically digested using the INFOGEST 2.0 method to produce corresponding digestate samples. No significant differences in gross composition were observed between the WMP samples. Significantly higher ( p ˂ 0.05) concentrations of selenium and iodine were present in TMR-derived WMP than those derived from both GRS and CLV. Total fatty acid profiles of milk from each diet were distinct, with each exhibiting important differentiating compounds, such as conjugated linoleic acid (GRS and CLV) and palmitic acid (TMR). Minor differences in total and free amino acid and free fatty acid composition between the diets were also recorded, although the overall effect of digestion was similar between the feeding systems. Raman spectroscopy displayed distinct spectra for each diet based on levels of carotenoids and fatty acid unsaturation. Further significant differences in proteomic profiles were observed between WMP samples from each diet, although differences in the peptidomic profiles of the digested samples were less distinct. This study determined a significant effect of feeding system on the composition of WMP, but significant variation in the composition of digestate between feeding systems could not be conclusively identified by static in-vitro digestion, which may receive further insights using dynamic methods in future work.