Changes in stability and shelf-life of ultra-high temperature treated milk during long term storage at different temperatures.

Discipline: milk quality; Keywords: food technology, food safety, UHT milk, sensory evaluation, off-flavour, sedimentation, colour, shelf life.                        

In the ultra-high temperature (UHT) process, milk is subjected to high temperatures, above 135 oC for a few seconds, resulting in a product with a shelf-life of several months when stored at ambient temperature. However, chemical and physical changes in the milk can lead to off-flavours, browning, fat separation, sediment formation or gelation during  storage. Important associated factors are processing parameters, storage conditions (time and temperature) and type of packaging. From a consumer perspective, the sensory attributes of the milk including taste and colour as well as visual stability such as fat separation, fat adhesion to the packaging material and sediment formation are of  importance. During storage, fat globules can aggregate and float to the top, resulting in fat separation and fat adhesion to the packaging material. In UHT products, sediment formation and gelation are also well-known problems. Sediment, which is a compact layer adhering at the bottom of the package, consists of aggregates of proteins or protein particles of various sizes. Sediment formation has been shown to increase with storage temperature. In contrast, gelation, consisting of a three-dimensional, voluminous network of proteins, can occur either through enzymatic or non-enzymatic (i.e. physio-chemical) processes.  

Various properties of the unprocessed milk are being used for quality assurance of the raw milk used for UHT treatment. A commonly applied method to predict the heat stability of milk is determination of the heat coagulation time (HCT), i.e. the time it takes for milk to visually coagulate when heated to temperatures above 100 oC. Ethanol stability has been used for over a century as a simple, cheap and quick pass-fail-test to detect low quality raw milk that is not suitable for UHT processing. It has been recommended that raw milk for UHT processing should have an ethanol stability of 74% or higher. Reasons suggested to explain low ethanol stability of the raw milk include low pH caused by acid producing bacteria, salt imbalance, high concentration of ionic calcium or high amount of serum proteins. In addition to the application of HCT and ethanol stability to assess the suitability of raw milk for UHT processing, there are only few studies reporting their use in evaluating changes in UHT milk stability during storage. The aim of the study by Dr M.A Karlsson and colleagues was to investigate the shelf-life of UHT milk by frequent sampling from a commercial and a small pilot scale production site, followed by long-term storage, up to one year after production, to identify the sensory changes limiting the shelf-life of the product when stored at different temperatures. The study was published in Heliyon, Volume 5 of 2019, e02431 with title: Changes in stability and shelf-life of ultra-high temperature treated milk during long term storage at different temperatures. 

In the study, the stability of UHT milk 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.                    

The stability of the UHT milk during storage was in general not affected by production scale (commercial dairy plant or pilot plant). Storage temperature was found to have a major impact on the stability of UHT treated milk during the 52 weeks of storage. A long shelf-life of UHT milk was favoured by cold or ambient storage temperatures, whereas the shelf-life decreased considerably when storage temperature increased. The shelf-life of UHT milk stored at 4 and 20 oC was limited by sediment formation to 3436 weeks followed by a taste deviation at 4052 weeks of storage. The shelf-life of UHT milk stored at 30 and 37 oC was limited by several quality parameters including taste, colour and sediment formation, from storage weeks 1620. Changes in HCT, pH and ethanol stability of the UHT milk suggested that different mechanisms are attributed to the changes in stability at different storage temperatures.