Stability of ultra-high temperature treated milk : the effect of raw milk quality, storage temperature and storage time

Abstract: The composition and properties of raw milk are important for the stability of dairy products, especially for products with long shelf-life, e.g. ultra-high temperature (UHT) treated milk. In the UHT process, milk is subjected to temperatures above 135 °C for a few seconds, resulting in a product that can be stored for several months at ambient temperature.This thesis investigated how natural variations in the composition and properties of raw milk affected the stability of UHT milk during long-term storage at different temperatures. For this purpose, samples of unprocessed raw milk as well as the resulting UHT milk were obtained every month for one year from a commercial dairy plant. Additionally, in a full factorial experiment, milk was modified with respect to levels of calcium, citrate and urea to investigate the effect on the stability of the resulting UHT milk.The quality traits analysed in the raw milk included composition, bacterial counts, enzymatic activity, colour and measures of stability, i.e. heat coagulation time (HCT) and alcohol test. During storage of UHT milk, sensory attributes and stability measures were analysed. Multivariate statistics was used to identify batches of milk showing similarities with respect to total variation.There were only small natural variations in the composition and properties of raw milk. Elevated calcium content, and associated decrease in pH, had a strong negative effect on the stability of UHT milk. Higher concentrations of citrate and urea in milk did not affect the stability compared with unmodified reference UHT milk.Storage temperature correlated with colour and pH, whereas storage time correlated with creaming, sedimentation and HCT. Cold (4 °C) or ambient (20 °C) storage gave the longest shelf-life, limited by sedimentation and deviating taste. In UHT milk stored at 30 or 37 °C, the shelf-life was limited by several parameters, including colour, taste and sedimentation. Changes in the stability of UHT milk during storage are suggested to be explained by known mechanismKnowledge of how the composition and properties of raw milk affect the stability of the resulting product can aid the dairy industry to more accurately predict the shelf-life of UHT milk. Better understanding of how storage conditions affect the shelf-life of UHT milk can provide useful guidance to consumers on appropriate storage temperature.