This thesis investigates the effect of bovine feeding system (pasture vs. non-pasture) on the sensory and volatile profiles of raw milk, pasteurised milk, and dairy powders, but also investigates the impact of feeding system on lipid oxidation (LO) in whole milk powder (WMP), and in commercially available WMP, skim milk powder (SMP), and infant milk formula (IMF). The thesis also highlights the benefits of using sophisticated extraction, separation, and identification gas chromatography mass spectrometry techniques (GCMS), including olfactometry to increase our understanding of the source and evolution of aromatic compounds in dairy products, and also how these are further influenced by processing, shelf-life and storage conditions. Chapter 1 provides an updated review of the factors influencing LO in dairy powders, focusing on the combination of instrumental and sensory techniques. A HS-SPME GCMS method was validated in Chapter 2 to quantify 13 VOC associated with LO which was subsequently employed to assess WMP, SMP, and IMF under controlled storage conditions in tandem with sensory analysis in Chapter 3. The WMP and SMP samples remained stable for up to 4 months, but the IMF had concentrations of LO VOC above their odour thresholds immediately after manufacture, which continued to increase during storage, likely related to the addition of poly-unsaturated fatty acids during processing. Hexanal, heptanal and pentanal were correlated with painty, oxidised, cooked, and caramelised odours in all samples. In Chapter 4, the same13 VOC originating from LO were again used to track the VOC profile of WMP derived from GRS, CLV, and TMR feeding systems during storage. Significant variations in the concentrations of 14 fatty acids were observed due to diet. Feeding system was shown to impact both LO stability and sensory properties of WMP. WMP derived from GRS was most susceptible to LO, likely due to increased concentrations of conjugated linoleic acid and α-linolenic acid that oxidise to specific aldehydes. The onset of LO issues was off-set in WMP from TMR during the early stages of storage due to the prevalence of sweet, caramel aroma’s but it rapidly detoriated around 4 months due to increases in aldehyde concentration. Significant differences were also observed in the phytochemical profile of the milk samples derived from pasture and non-pasture-based feeding systems, with white clover enhancing isoflavone content in Chapter 5. Chapter 6 demonstrated that the odour active VOC influencing sensory perception were quite different between pasture and non-pasture feeding systems, with many VOC influencing the perception of raw milk likely present from direct transfer from feed. Heretofore this was not anticipated, as it was assumed that rumen metabolism was the biggest factor influencing the VOC profile of raw milk. Approximately a third of all VOC were aroma active in raw milk derived from GRS and TMR, respectively. Many of the key dietary-derived-odour-active VOC in raw TMR milk, likely arose during the production of the TMR as most were either derived from Maillard reactions generated by thermal treatments. This study has conclusively demonstrated the impact of bovine diet on the volatile and sensory character of milk. It also clearly demonstrates significant differences in milk and dairy products produced with pasture-based feeding systems generated in Ireland, as opposed to concentrate-based feeding systems, more widely practiced globally. The outcomes of this thesis provide insights into: (1) the effect of feeding system on the oxidative stability, aromatic and sensory properties of milk and dairy powders, (2) the identification and source of aroma active VOC in milk and dairy powders, (3) methods used to profile the aroma of milk and dairy powders, and (4) future possibilities and considerations
