The impact of once-a-day milking in New Zealand on milk composition and processing : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, Manawatū, New Zealand
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2024-02-15
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Massey University
Figures 2.1 and 2.2 are reused with permission.
Figures 2.1 and 2.2 are reused with permission.
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Abstract
Almost 10% of farmers in New Zealand have adopted a once-a-day (OAD) milking system, but the impact of OAD milking beyond the major milk components is not well understood. The aim of this thesis was to study the effect of OAD milking on the proximate, protein and mineral composition of milk and the potential implications for processing. Milk was sampled throughout the lactation from individual cows, of the Holstein-Friesian, Holstein-Friesian×Jersey and Jersey breeds, selected and balanced for parity, breeding worth and calving date. This milk was characterised for proximate and protein composition. For processing purposes, the milk was mixed proportional to the respective yields to firstly study the heat-induced changes to milk proteins and secondly the rennet- and acid-induced gelation properties.
It was found that OAD, compared to TAD, milking impacts the protein composition of the milk. This effect was more pronounced in mixed milk samples. On an individual level, only the κ-casein and α-lactalbumin contents increased and decreased, respectively. The breed effect was explored but the sample size was too small to draw firm conclusions. In the mixed milk samples, the κ-casein content, the κ-casein glycosylation degree, and the αs2-casein content were higher, while the αs1-casein, β-lactoglobulin and αlactalbumin contents in the protein fraction were lower in OAD than in TAD milk. The calcium, magnesium and phosphorus concentrations and distribution in the milk were unaffected by the milking frequency. In contrast, the sodium, chloride, and copper contents were lower, and the potassium and iodine contents were higher in OAD milk. Casein micelles were smaller in OAD than in TAD milk. The proportions of κ-casein and αs2-casein in the serum phase were higher in raw and pasteurised OAD than in TAD milk. In UHT milk from a OAD milking system, the degree of whey protein denaturation was lower than in TAD milk. The fate of denatured whey proteins, either aggregation in the serum phase or association with the casein micelle, was affected by the type of heat treatment, the type of protein, milking frequency, and the stage of lactation. Despite differences in milk composition and the heat-induced changes to the proteins in the milk, the gelation properties did not differ between OAD and TAD milk. The gelation pH of acid-induced gelation was higher for OAD than for TAD milk.
The results of this study provided valuable insights in the potential implications of OAD milking for processing properties and product quality.
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dairy science, milking frequency, protein composition, mineral composition, dairy processing