UHT is short for Ultra Heat Treatment, or Ultra High Temperature processing.
What is UHT milk – and how is it different from other types of milk?
When we talk about UHT milk, we refer to milk that has been processed at high temperature (at least 135°C). By rapidly heating milk to this temperature, keeping it there for a few seconds, and then quickly cooling it down, any microorganisms in the raw milk are killed off. Packaging UHT milk in aseptic containers leads to a shelf life of several months, and the milk doesn’t have to refrigerated until the package is opened. UHT milk is also referred to as long-life milk.
Fresh milk usually refers to pasteurized milk, which means it has been heated to 72-74°C for about 15-20 seconds. Fresh milk needs to be distributed and stored under chilled conditions.
Raw milk is what the name suggests – straight from the cow; milk that hasn’t been processed. Drinking raw milk without boiling it first can be dangerous, as it may contain bacteria that can pose health risks.
ESL milk, or Extended Shelf Life milk, lasts longer than fresh milk. It generally has a shelf life of about a month. It can be said to fill the gap between pasteurized and UHT milk. It requires a chilled distribution chain.
Like any other area, the UHT universe uses its own terms, abbreviations and definitions. Here, we guide you through some of some of the concepts you are likely to come across.
Direct heating mixes culinary steam with the product. The direct systems are divided into Steam injection systems and Steam infusion systems. This method is very efficient for rapid heating, but is also somewhat challenging as the demands on the quality of steam is high.
Indirect heating is more common. It uses a partition between the product and heating medium. The heating is not as fast as for direct heating. Indirect systems are based on plate heat exchangers, tubular heat exchangers or scraped surface heat exchangers.
According to the WHO/FAO, commercial sterility of low-acid food is defined as follows:
“Commercial sterility means the absence of microorganisms capable of growing in the food at normal non-refrigerated conditions at which the food is likely to be held during manufacture, distribution and storage.”
When applying a sterilisation procedure – such as heat – not all bacterial spores are killed at the same time. A certain number will be killed within a given unit of time. This principle is independent of the sterilisation temperature. When the number of surviving spores is plotted against their logarithmic reduction, we obtain a straight line, commonly known as “the logarithmic order of death”.
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Microbiology in UHT
This is a broad area, which can be defined as the study of living organisms of microscopic size, including bacteria, fungi and algae. The UHT process eliminates all these bacterial spores and microorganisms.
Ambient food products are processed in a way that allows storage in room temperature. Ambient food packaging means the food is packed in pasteurized and sterilized packs, which increases shelf life.
Aseptic filling and packaging
All microorganisms are killed by the heating treatment. The milk is then packed while being protected from light, oxygen, air and entrance of microorganisms, which keeps it safe for months without any need for cooling or preservatives. Besides milk, many other food products are available in aseptic packaging – for example, desserts, protein drinks, baby food, juices, soups and sauces.
In container sterilisation
The original form of sterilisation - a process from the early 1800s that can be seen a predecessor to UHT. It is still in use and usually involves heating the milk in containers at 115-120°C for 20-30 minutes.
This refers to expiry date. The shelf life is the maximum time a manufacturer recommends their food product is stored before it is consumed. UHT products have a shelf life of several months in unopened packages. Shelf life of UHT milk is not limited by growth of microorganisms, but by physical, chemical and enzymatic changes in the milk, such as browning reactions and cream separation.
Aseptic Intermediate Cleaning
A method that reduces downtime and that is useful in plants with very long production runs. A 30-minute AIC can be done whenever it is necessary to remove fouling without losing aseptic conditions.
The CIP programme must be tailored to and optimised for the different operating conditions in different plants. For UHT plants, the CIP cycle may include pre-rinsing, caustic cleaning, hot-water rinsing, acid cleaning and final rinsing. The cycle takes approximately 70-90 minutes and is normally carried out after each production run.
Fouling is the deposit of proteins and minerals that forms on equipment surfaces during the heat treatment. In UHT treatment, the fouling is usually mineral-rich. There are two main types, low-temperature fouling (type A, <100°C) and high-temperature fouling (type B, >120°C). Type A is soft and voluminous, while type B has a hard surface.
ditives and alternatives
Lactose and lactose-free products
Lactose is a type of milk sugar, which some people have difficulties digesting. Manufacturers add an enzyme called lactase to break down the lactose. Lactose-free products are a growing segment.
A reaction between sugar and protein, taking place when food is heated. In the case of UHT milk, the Maillard reaction gives the milk a slightly sweeter and more caramelised taste. No unhealthy compounds are produced.
Heat load indicators
Lactulose, furosine and whey proteins such as β-lactoglobulin can be used as heat load indicators: chemical markers to control and check the heat treatment intensity.
Thanks to the ultra-heat treatment and the aseptic packaging, there is no need to add preservatives to UHT products.
When it comes to the main nutrients in milk – vitamins (A, B-complex, D and E), minerals (calcium, potassium, magnesium and phosphorus) and proteins, the nutritional value is the same in UHT and pasteurized milk.
Prolonging of the time it takes for creaming to occur, which is achieved by making the fat globules of the milk smaller. The process does not impact nutritional value. In fact, by evenly distributing cream content, homogenisation actually makes the milk easier to digest.
Whole, semi-skimmed and skimmed milk
The fat content of the milk. Whole milk has approximately 3-4% milk fat, while skimmed milk is practically fat-free. Semi-skimmed is in between at around 2% milk fat.