Examining the impacts of a ‘typical’ Irish pasture-based system on the composition and quality of milk and dairy products

The overall aim of this thesis was to investigate the impact of a ‘typical’ Irish high pasture allowance bovine feeding system (grass; GRS) on the composition, nutritional quality and techno-functional properties of milk and commonly manufactured dairy products (butter, cheese and whole milk powder) compared to that of the more conventional medium (partial mixed ration; PMR) and low (total mixed ration; TMR) pasture allowance feeding systems. Milk from the TMR herd, collected across 38 weeks of lactation, had highest proportions of protein, lactose and total solids, PMR had highest free amino acid content and somatic cell count as well as lowest fat content and GRS had the highest pH and lowest somatic cell count (Chapter 2). The most notable impact of increasing pasture allowance was the significant increase of health promoting fatty acid (FA) such as conjugated linoleic acid, omega-3 FA and unsaturated FA in milk, butter, cheese and whole milk powder (Chapters 2-5). The higher proportions of total unsaturated FA as a result of GRS feeding was strongly correlated with a reduction of butter hardness and crystallisation temperatures (Chapter 3) and the enthalpy of melting of whole milk powder (Chapter 4). Biomarker models developed utilising the FA profiles of each of these products were capable of distinguishing between high, medium and low pasture derived products (Chapters 2-5). Increasing pasture allowance in the feeding system also increased the ‘yellow’ colour (b*-value) of the milk, butter, cheese and whole milk powder (Chapters 2-5). This colour difference was correlated with Raman bands 1,163 and 1,525 cm-1 associated with the carotenoid content of butter (Chapter 3). Despite milk fat and protein standardisation, pasture allowance and stage of lactation significantly altered the gross composition of whole milk powder as a result of the unstandardized milk components (Chapter 4). The proportions of calcium, copper, zinc and citric acid were also lower in GRS powders compared to TMR and PMR. The type-A HCT curve of rehydrated TMR powders (3.4% protein) had a peak maxima and trough minima occurring 0.1 pH earlier than both GRS and PMR (Chapter 4). The proportions of individual casein proteins including para-κ-casein, αs1-casein and αs2-casein in cheese differed significantly with diet and stage of lactation while individual peptide proportions derived from β-casein and κ-casein exhibited a stage of lactation effect (Chapter 5). This had a significant impact on the textural attributes of cheese including the hardness, gumminess and chewiness (Chapter 5). Overall this research provides the necessary scientific proof points which will be associated with 1.7 million tonnes of Irish ‘grass-fed‘ dairy products exported to 130 markets around the world every year.