Abstract/Summary

The demand for aseptically packed ultra-heat-treated milk (UHT) with a validated long shelf life has been gradually increasing worldwide. However, extracellular enzymes secreted by psychrothrophic bacteria that are present in raw milk can hydrolyse milk protein and cause quality defects in the final UHT product. Therefore, robust means of controlling the microbiological quality of raw milk are required, to reduce incidences of UHT product defects and extend their shelf-life. The aim of this thesis was to develop a raw milk improvement strategy to enhance UHT milk stability that is transferrable into industry practice. UHT processing and storage stability studies were carried out at pilot and commercial scale with raw milk of varying quality. The importance of establishing control measures over psychrothrophic bacteria counts was confirmed. UHT milk produced from raw milk of low microbiological quality destabilised within five weeks at ambient storage conditions. The UHT milk also developed significant levels of free amino nitrogen concentrations that indicated protein hydrolysis was caused by bacterial proteolytic activity. The study established microbiological quality requirements for raw milk that were key to validate the stability of UHT milk, stored at various temperatures (4, 21 and 30°C) over a period of 360 days. Free amino nitrogen values coupled with viscosity and sensory observations were also used to confirm UHT milk acceptability. The incorporation of additional psychrothrophic and thermoduric bacteria counts in raw milk producers’ payment incentives, resulted in the production of premium microbiological quality raw milk by reference and comparison to various guidelines and published data. Historical analysis of Jersey raw milk quality used for UHT milk production provided a significant benchmark to other dairy processors and expanded on the literature for milk produced by this type of breed. The findings of this thesis indicated that UHT milk processors need to further evaluate and monitor raw milk microbiological suitability by focusing on specific bacteria counts, as means of improving UHT milk stability and validation of product shelf-life extensions.