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Summary
Factory data was collated to provide information on the typical extent of seasonal variation in protein content of milk from the West Gippsland area of Victoria in south-eastern Australia over a 30-month period. Permeate from the ultrafiltration of milk at 10°C and 50°C was used to adjust the protein concentration of milk. Trials were undertaken with early-, mid- and late-lactation milk and the effect of protein standardisation on the composition and storage stability of protein-adjusted UHT-processed milk samples was investigated. The onset of gelation was influenced by seasonal effects, with early-lactation milk gelling faster than latelactation milk. Milk with a lower protein content tended to develop gelation later than milk with higher protein concentrations. Protein standardisation using permeate from milk ultrafiltered at 50°C extended the gelation time to a small extent compared to 10°C UF milk, with slight variations in the calcium content the only discernible difference. The gelation characteristics were different to that normally observed for UHT milk with formation of a curd-like layer on the surface of the sample. It is possible the gelation may be partially attributed to some denatured whey protein/κ-casein complex on the milkfat globule membrane that would be reformed after homogenisation of the heated milk. This complex could initiate further aggregation of protein/fat complexes.
Introduction
It is well recognised that cow's milk is a highly variable commodity. Considerable fluctuation is seen in the total and relative content of major and minor components, especially fat and protein (White and Davies 1958; Phelan et al 1982). These variations are associated with many factors, including breed, geographic region, feed, stage of lactation, condition of the cow and season of the year. Functional and nutritional properties of milk products may vary significantly as a result of these compositional differences (Lucy 1996; Sweetsur and Muir 1982). Until the latter part of this century, the milkfat was the component of greatest economic importance. Milk payment schemes were based on fat content and legislation was introduced to permit adjustment of the milkfat content. This practice has enabled dairy factories to provide milk with a constant milkfat content throughout the year, despite variations in the fat content of the milk supply.
The situation now is that protein is the most highly valued component and...