Abstract
In the ultra-high temperature (UHT) process, milk is subject to temperatures above 135 °C for few seconds giving a product with a shelf-life of several months. The raw milk quality, UHT process and storage conditions affect the stability. In this study, the stability of UHT milk produced in an indirect system was evaluated by studying changes in taste, colour, fat separation, fat adhesion to the package, sedimentation, gelation, heat coagulation time, pH and ethanol stability during storage for up to one year at different temperatures. UHT milk stored at 4 and 20 °C had the longest shelf-life of 34–36 weeks, limited by sediment formation. Storage at 30 and 37 °C considerably decreased the shelf-life of UHT milk to 16–20 weeks, whereby changes in sediment formation, taste and colour were the limiting factors. Our results suggest that the changes observed at the different storage temperatures can be explained by different known mechanisms.
Highlights
In the ultra-high temperature (UHT) process, milk is subjected to high temperatures, above 135 C for a few seconds, resulting in a product with a shelf-life of several months when stored at ambient temperature
From storage week 16, every second batch of UHT milk produced at the dairy plant was subject to a sensory evaluation
The stability of UHT milk during storage was in general not affected by production scale
Summary
In the ultra-high temperature (UHT) process, milk is subjected to high temperatures, above 135 C for a few seconds, resulting in a product with a shelf-life of several months when stored at ambient temperature. The sensory perception and appreciation of milk varies greatly among consumers (Walstra et al, 2006) and in milk is mainly affected by the fat, protein and lactose content, as well as the manufacturing process (Oupadissakoon et al, 2009; Schiano et al, 2017). Upon long-term storage of UHT milk, the main processes suggested to affect the taste and colour are proteolytic, lipolytic, oxidative and Maillard reaction, initiated by the intense heat treatment of UHT milk (van Boekel, 1998; Walstra et al, 2006). In a previous study by Hansen et al (1980), an untrained panel tested UHT milk heated to 149 C for 3.4 s and found the taste to slightly improve when the milk was stored refrigerated at 4 C for 24 weeks compared to storage at 40 C. It was reported that the taste of the UHT milk deteriorated slightly with increased storage temperature, but it was still considered acceptable at 20 weeks of storage
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