When we read the word “uht” on the milk cardboard, it is the acronym of Ultra high temperatures And it means that the milk was subjected to a pasteurization process in which it was heated to Very high temperatures (usually included between 130 ° C and 160 ° C) for very short times, under ten seconds. This allows to obtain a milk a long conservation, which can be transported very far from the production area or kept for weeks on the shelves of supermarkets without the need for a refrigerator. There are two main methodologies: indirect UHH, in which milk is heated through heat exchangers, and direct UHHT (upperization), which provides for the injection of steam into milk. However, exposure to high temperatures can involve alterations of milk proteins, With consequent potential industrial and qualitative problems, as confirmed by several studies. The same research suggests mitigation strategies such as the addition of compounds stabilizers or accurate control of thermal conditions to avoid unwanted modifications in organoleptic properties (smell, flavor, consistency) of milk.
What the abbreviation “UTH” means and how pasteurization takes place
The acronym UHT is for Ultra high temperatures And it refers to the particular milk pasteurization process, which provides very high temperatures for short times. UHT pasteurization, like any other thermal milk treatment, has as its objective Reduce the microbial charge and guarantee prolonged conservation of the product. On an industrial level, two main methods of treatment are distinguished, which differ in the way the heat is transferred to milk:
- Indirect UHT method: the milk is heated through heat exchangers: in these plants the milk and the heat source (such as steam or hot water) flow into two close sections, separated from metal surfaces: the heat is sold to milk Without the heating source, they come into contact with the food.
- Direct UHT Method (updating): It is injected into water vapor milk, which gives in to the food part of its thermal energy. The steam must strictly derive from drinking water. This direct contact between milk and heat source determines an inevitable passage of water molecules between steam and milk, consequently it is likely that the product at the end of the process is diluted. However, the next cooling phase can take place vacuum, allowing excess water to evaporate. This happens because the vacuum reduces the pressure in the cooling chamber e lowers the water evaporation pointallowing its removal without having to apply high temperatures again.
After cooling, UHT milk is packaged in conditions sterile To avoid possible contaminations (otherwise, a lot of work at all). Multistrate cartoons are often used as those Tetrapàk® (consisting of paper, polyethylene and aluminum) which, thanks to the insulating coverings and the aseptic sealing, allow UHT milk to have an ‘high conservability: if left in a cool and dry place (as well as the dispensation of the house), it can in fact resist up to several months (always paying attention to the expiry date indicated on the cardboard), without there being alterations at the organoleptic level. However, it must be remembered that, once opened, UHT milk comes into contact with the air and becomes vulnerable to microbial contamination, drastically decreasing its shelf life for a couple of days, if kept specifically in the refrigerator.
The UHT process can change the structure of milk proteins
Milk is a complex substance rich in essential nutrients, among which numerous proteins stand out, consisting of long chains of amino acids that fold up to form secondary and tertiary three -dimensional structures of the proteins themselves. These provide indispensable amino acids for the growth and development of the body, supporting for example muscle growth. The main milk proteins are the casein, which represent 80% of the total proteins in cattle and are essential for the transport of calcium; then there are instead the serum proteins, like theserum albumin or the lactoferrine.
In the scientific world it is known that proteins are sensitive to heat: high temperatures can in fact change their three -dimensional structure, causing the breakdown of intramolecular bonds and the loss of their biological function (process known as denaturation of proteins). And this, unfortunately, also occurs during the UHT process, as found by an UFV study (Universidade Federal de Viçosa).
The researchers, through a series of analyzes, confirmed that the milk exposure to as much high temperatures as those of the UHT treatment It can cause protein denaturation. And it does not end here: the high temperatures not only alter the structure of proteins, but cause aggregations between the various proteins giving shape to insoluble structures that can fall.
To better understand the phenomenon ofaggregationyou have to imagine the denatured protein as a ball of thread that unrolls.
When the protein “ball” unrolls, the chain of amino acids of which it is formed exposes parts that in normal conditions would be folded inside: some of them have the natural tendency to attract each other causing the aggregation of nearby proteins, while others are regions hydrophobic that finding himself exposed to the aqueous phase of the milk reject it by becoming insoluble and precipitating on the bottom of the container. This is a great obstacle whether it happens during the industrial process (the precipitates can form incrustations at the entire machinery), whether it takes place in the post-thermal treatment period, altering the consistency of UHT milk through sedimentation or geling.
What are the solutions?
The researchers point out that, although the UHT treatment allows you to obtain long -preservation milk without the use of preservativeshowever, involves critical issues that must be addressed: the authors suggest for example to add to milk Cherlants agents or polymers able to prevent the aggregation and precipitation of proteins, or in any case to carry out accurate checks of temperature and exposure time during treatment.
The latter trick not only prevents unwanted modifications in the consistency of milk, but it can also avoid its possible change of color: high temperatures can induce the famous in milk Maillard reaction between amino acids and sugars, which leads to the formation of compounds such as melanidineresponsible for thebrownness of milk.
