The ultra-heat treatment (UHT) process kills all the pathogenic microbes in milk. This process, invented in the 1960s, was widely adopted in the food industry by the 1970s. UHT can extend the shelf life of milk to up to nine months. This process makes milk so stable that it is not necessary to refrigerate UHT milk until it is opened. This milk can produce high-quality milk foam and is suitable for latte art, but it has a strong, cooked flavor that many people find unpleasant. UHT is defined as ‘heating to at least 138°C for 2 s’ (Lee et al 2017). At this high temperature, the milk undergoes the Maillard reaction. This is the same reaction that contributes to the brown colour and caramel flavour of coffee. In UHT milk you won’t see browning, but the process can impart a burnt, sulphurous character to milky coffee.
UHT milk packaged in a Tetra Tak
Heat-Induced Deterioration of Milk
Exposing milk to high temperatures can cause the milk fat globule (MFG) membrane proteins to unfold. When the buried sulfhydryl groups, protected inside the membrane surrounding the milk fat globule, are brought to the outer surfaces, they produce a strongly sulphurous, cooked smell (Hoffmann and van Mil 1997). Then the sulfhydryl and disulphide, found in the protein fraction, are released (Janolino and Swaisgood 1987). These compounds are classed as thiols, which impart a strong and usually bad smell to which humans are extremely sensitive. (The smelly mercaptans added to natural gas are thiols. The spray of the skunk is made up of low-molecular-weight thiols.)
UHT treatment machine, with a tubular heat exchanger design
In a tubular heat exchange UHT machine like the one pictured above, milk is preheated to 90°C as it travels through the pipes, then very quickly taken up to sterilisation temperature of around 138°C for at least two seconds,