Understanding the impact of New Zealand milk seasonality on dairy product quality : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, New Zealand
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Date
2020
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Massey University
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Abstract
In seasonal calving countries like New Zealand, the seasonal variations in milk composition and properties are particularly pronounced due to the significant impact of the stage of lactation on milk characteristics. This study investigated the seasonal variations in the composition and properties of New Zealand milk, as well as the impact of seasonality on product systems including acid milk gel, yoghurt, whipping cream and UHT milk. In addition, this study also demonstrated the effect of processing on the physicochemical properties of milk and the quality of products.
The study of milk composition and properties over two full milking seasons demonstrated that the seasonal variations in the composition and physicochemical properties of New Zealand milk were largely controlled by the stage of lactation.
Consistent seasonal patterns found include increases in protein and fat and a decrease in
lactose in late-season milk, decreasing proportion of α-Lactalbumin in milk protein during the season and increased glycosylation degree of κ-casein in late season milk.
Both acid milk gels made by the addition of glucono delta-lactone and yoghurts made by bacterial fermentation showed significant decreases in firmness during the late season, despite the standardization of protein and fat content. Standardization was not sufficient in controlling the acid gelation properties of late-season New Zealand milk. The seasonal variation in the glycosylation degree of κ-casein might play an important role in affecting the acid gelation process by altering the electrostatic and hydrophobic
interactions. An investigation into the use of ultrafiltration for standardization demonstrated that a higher proportion of ultrafiltration retentate in the milk improved the overall acid gelation properties of milk.
The seasonal variations in the fatty acid composition and the melting behaviour of milk
fat broadly followed the lactational trend that the high-melting fatty acids (e.g. C16:0) reached the maximum in the mid-season.
The physicochemical changes during the storage of seasonal UHT milk were monitored. The age gelation of UHT milk was least pronounced in the early season. A new hypothesis for gelation mechanism that involves the interaction and sedimentation of κ-CN-depleted casein micelles was proposed.
Description
Table 2.1 is re-used with the publisher's permission.
Keywords
Milk, New Zealand, Composition, Lactation, Dairy products