Effect of Dairy Cow Diets on the Composition and Processing Characteristics of Milk

Milk composition is affected by many factors including stage of lactation, type and level of feed intake, and environmental conditions. Changes in milk composition affect milk processability and the yield and quality of dairy products. The overall aim of this thesis was to investigate the effect of different herbage allowance and feeding systems on seasonal changes in milk composition, biochemical, processing characteristics, or product manufacture and their quality. Effects of reducing daily herbage allowance (DHA) from 15.0 to 11.8 kg dry matter per cow to a spring-calved herd during early lactation (EL; 29-70 days in milk, DIM) on milk composition and processability (e.g., rennet gelation, heat coagulation time) were examined throughout lactation (up to 267 DIM). Reducing DHA led to reductions in milk yield and concentrations of protein during EL; otherwise, it had little effect on milk composition or on the selected processing characteristics in mid- or late lactation. The comparative effects of three different dairy cow diets or feeding systems on the milk composition and its impacts on the quality of Mozzarella cheese and low heat skim milk powder (LHSMP) were also studied: grazing perennial ryegrass (Lolium perenne L.) only (GRO), grazing perennial ryegrass and white clover (Trifolium repens L.) (GRC), and housed indoors offered total mixed ration (TMR). Feeding system affected milk composition and processability to an extent dependent on stage of lactation and year of study. Most notably, GRO-based milk had higher concentrations of protein, casein, Ca, lower concentrations of lactose, I, Cu and Se, and stronger rennet gelation characteristics than TMR milk. Milk from grass based feeding system had a higher mozzarella cheese-yielding capacity than milk from TMR based feeding system, and produced cheese which had lower levels of I, Cu and Se, was more yellow and became more fluid and flowable on heating to 95 °C. Moreover, the use of a novel technique called cavitation rheology (CR) to measure the mechanical properties of Mozzarella cheese, produced from GRO milk, during ageing at 4 °C. The linear modulus (E) obtained from CR was much lower when compared G′ (storage modulus) from shear rheology, but correlated significantly with G′ and firmness of the cheese. Finally, composition and functionality of low-heat skim milk powder produced from three different feeding systems was studied. Compared to TMR milk, LHSMP from GRO milk had a higher protein content and a lower lactose, I, Cu and Se content, a more green-yellow colour, and on reconstitution (10%, w/w; RSM) had better rennet coagulability. RSM from all three feeding systems had similar casein micelle size and hydration, ethanol stability and yoghurt making characteristics. The differences in milk and product characteristics obtained from different feeding systems may provide a foundation for product differentiating parameters suited to different consumers, processors and markets.