Ultra-high temperature processing (UHT), ultra-heat treatment, or ultra-pasteurization is a food processing technology that sterilizes liquid food by heating it above 135 °C (275 °F) – the temperature required to kill bacterial endospores – for 2 to 5 seconds. UHT is most commonly used in milk production, but the process is also used for fruit juices, cream, soy milk, yogurt, wine, soups, honey, and stews. UHT milk was first developed in the 1960s and became generally available for consumption in the 1970s.
The heat used during the UHT process can cause Maillard browning and change the taste and smell of dairy products. An alternative process is HTST pasteurization (high temperature/short time), in which the milk is heated to 72 °C (162 °F) for at least 15 seconds.
UHT milk packaged in a sterile container, if not opened, has a typical unrefrigerated shelf life of six to nine months. In contrast, HTST pasteurized milk has a shelf life of about two weeks from processing, or about one week from being put on sale. A significant percentage of milk sold in the US as organic food is UHT treated.
The most commonly applied technique to provide a safe and shelf-stable milk is heat treatment. The first system involving indirect heating with continuous flow (125 °C [257 °F] for 6 min) was manufactured in 1893. In 1912, a continuous-flow, direct-heating method of mixing steam with milk at temperatures of 130 to 140 °C (266 to 284 °F) was patented. However, without commercially available aseptic packaging systems to pack and store the product, such technology was not very useful in itself, and further development was stalled until the 1950s. In 1953, APV pioneered a steam injection technology, involving direct injection of steam through a specially designed nozzle which raises the product temperature instantly, under brand name Uperiser; milk was packaged in sterile cans. In the 1960s APV launched the first commercial steam infusion system under the Palarisator brand name.
In Sweden, Tetra Pak launched tetrahedral paperboard cartons in 1952. They made a commercial breakthrough in the 1960s, after technological advances, combining carton assembling and aseptic packaging technologies, followed by international expansion. In aseptic processing, the product and the package are sterilized separately and then combined and sealed in a sterile atmosphere, in contrast to canning, where product and package are first combined and then sterilized.
The Importation of Milk Act 1983 was passed by the Parliament of the UK following a successful appeal to the European Court of Justice that prevented government from banning the importation of UHT milk.
In June 1993, Parmalat introduced its UHT milk to the United States. In the American market, consumers are uneasy about consuming milk that is not delivered under refrigeration, and reluctant to buy it. To combat this, Parmalat is selling its UHT milk in old-fashioned containers, unnecessarily sold from the refrigerator aisle. UHT milk is also used for many dairy products.
UHT milk has been sold since 2008 on American military bases in Puerto Rico and South Korea due to limited availability of milk supplies and refrigeration. UHT milk gained popularity in Puerto Rico as an alternative to pasteurized milk due to environmental factors. For example, power outages after a hurricane can last up to 2 weeks, during which time pasteurized milk would spoil from lack of refrigeration.
In 2008 the UK government proposed to produce 9 pints out of every 10 as UHT by 2020 which they believed would significantly cut the need for refrigeration, and thus benefit the environment by reducing greenhouse emissions. However the milk industry opposed this, and the proposition was abandoned.
Two studies published in the late 20th century showed that UHT treatment causes proteins contained in the milk to unfold and flatten, and the formerly "buried" sulfhydryl (SH) groups, which are normally masked in the natural protein, cause extremely-cooked or burnt flavours to appear to the human palate. One study reduced the thiol content by immobilizing sulfhydryl oxidase in UHT-heated skim milk and reported, after enzymatic oxidation, an improved flavor. Two US authors prior to heating added the flavonoid compound epicatechin to the milk, and reported a partial reduction of thermally generated aromas.
Ultra-high-temperature processing is performed in complex production plants, which perform several stages of food processing and packaging automatically and in succession:
- Flash heating
- Flash cooling
- Aseptic packaging
In the heating stage, the treated liquid is first pre-heated to a noncritical temperature (70–80 °C [158–176 °F] for milk), and then quickly heated to the temperature required by the process. There are two types of heating technologies: direct, where the product is put in a direct contact with the hot steam, and indirect, where the product and the heating medium remain separated by the equipment's contact surfaces. The main goals of the design, both from product quality and from efficiency standpoints, are to maintain the high product temperature for the shortest period possible, and to ensure that the temperature is evenly distributed throughout.
Direct heating systems
Direct systems have the advantage that the product is held at a high temperature for a shorter period of time, thereby reducing the thermal damage for the sensitive products such as milk. There are two groups of direct systems:
- Injection-based, where the high-pressure steam is injected into the liquid. It allows fast heating and cooling, but is only suitable for some products. As the product comes in contact with the hot nozzle, there is a possibility of local overheating.
- Infusion-based, where the liquid is pumped through a nozzle into a chamber with high-pressure steam at a relatively low concentration, providing a large surface contact area. This method achieves near-instantaneous heating and cooling and even distribution of temperature, avoiding local overheating. It is suitable for liquids of both low and high viscosity.
Indirect heating systems
In indirect systems, the product is heated by a solid heat exchanger similar to those used for pasteurization. However, as higher temperatures are applied, it is necessary to employ higher pressures in order to prevent boiling. There are three types of exchangers in use:
- Plate exchangers,
- Tubular exchangers
- Scraped-surface exchangers.
For higher efficiency, pressurized water or steam is used as the medium for heating the exchangers themselves, accompanied with a regeneration unit which allows reuse of the medium and energy saving.
After heating, the hot product is passed to a holding tube and then to a vacuum chamber, where it suddenly loses the temperature and vaporizes. The process, referred to as flash cooling, reduces the risk of thermal damage, removes some or all of the excess water obtained through the contact with steam, and removes some of the volatile compounds which negatively affect the product quality. Cooling rate and the quantity of water removed is determined by the level of vacuum, which must be carefully calibrated.The process is called flash cooling 
Homogenization is part of the process specifically for milk. Homogenization is a mechanical treatment which results in a reduction of the size, and an increase in the number and total surface area, of fat globules in the milk. This reduces milk's tendency to form cream at the surface, and on contact with containers enhances its stability and makes it more palatable for consumers.
UHT milk has seen large success in much of Europe, where across the continent 7 out of 10 people drink it regularly. In countries with a warmer climate such as Spain, UHT milk is preferred due to the high cost of refrigerated transportation and "inefficient cool cabinets". UHT is less popular in Northern Europe and Scandinavia, particularly in Denmark, Finland, Norway, Sweden, the United Kingdom and Ireland. It is also less popular in Greece, where fresh pasteurized milk is the most popular.
- UHT and pasteurized milk contain the same amount of calcium.
- UHT milk's protein structure is different from that of pasteurized milk, which prevents it from separating in cheese making.
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