Many of us keep UHT milk in a cupboard for emergency situations (or for cooking, when we prefer to use it at but room temperature), while fresh milk is purchased often, kept always in the fridge and consumed fast. Where does the difference rely in its treatment and which consequences does it have on its properties?
If you are 40 years or older, you may recall those days when the milkman went to each house to deliver it in bottles. Perhaps you remember the smell of fresh milk and the creamy taste it had. You might also remember how your mother used to boil it before she allowed you to drink it, to kill all the "bugs" that it could contain.
Today this has changed a lot. Although the above may sound very romantic, there are now many more controls and systems to ensure that the product comes to our home in perfect conditions. In addition, we do not need to buy it every day, or even every week. We can buy it once a month and store it, because it will not expire. All this is possible thanks to UHT which is a process through which milk undergoes thermal treatment.
The heat treatment is simply a way of boiling the milk to a certain temperature for a certain time to ensure destruction of pathogens (which can cause disease) and the destruction of microorganisms (which might cause changes in the taste and smell) . It's the same thing your mom used to do at home, but under controlled (and regulated) conditions.
The temperature and time are two parameters that are closely related and must be taken into account in the heat treatment of any food. To ensure destruction of microorganisms, a very high temperature should ideally be used, but keeping in mind that raising the temperature too much can cause a change in the food’s nutritional properties. Lactose for example can undergo caramelization. In order to destroy microorganisms without altering the essential characteristics of the milk, we must find the right time-temperature combination so that the larger the time, the lower the temperature (and vice versa).
Thus, the difference between pasteurized and sterilized milk relies in the temperature that is used for sanitization. Temperatures below 100°C (212ºF) result in it being categorized as “fresh milk”, while temperatures above 100-135ºC (212-275ºF) lead the milk to be labelled as UHT (Ultra High Temperature). These treatments not only apply to dairy products, but are used in a wide variety of foods such as beer, canned food etc..
Let us briefly go through what each one of them involves.
Pasteurization ("fresh milk")
As already mentioned, pasteurization is carried out at temperatures below 100°C (212ºF). This ensures destruction of pathogenic microorganisms, but does not destroy all microorganisms. That's why pasteurized milk is always found in the fridge areas of supermarkets, and must always be kept at refrigerator temperatures (usually for no more than a week).
The legislation allows two different pasteurization treatments:
- a low pasteurization treatment, which involves heating the raw product to 63°C (145ºF) for 30 minutes. This treatment is hardly used because it is very slow and also causes many modifications.
- the High Temperature Short Time treatment (H.T.S.T.), which is commonly used to pasteurize the milk, typically heating at a temperature of 72°C (162ºF) for 15-20 seconds.
These treatments are carried out with a pasteurizer, which is a heater that can reach hot temperatures quite fast.
Sterilization (long life milk)
In the sterilization treatment, the milk undergoes temperatures above 100-135°C (212-275ºF), which ensures the destruction of pathogenic microorganisms and spore forms. They usually use one of the following combinations of temperature and time:
- Classical method: temperature of 110-120 ° C for 15-20 minutes. It is used for what is called "sterilized milk", which is the one that undergoes heat treatment after packaging.
- Method UHT (Ultra High Temperature): 135 ° C for 2-8 seconds. This method is used in most of the long life milks found in supermarkets (UHT milk).
This is a relatively new method, because in the past it presented technical difficulties, such as heating and then cooling the milk fast or keeping it in a sterile, appropriate container. The first problem was solved by applying steam and pressure variations, while the second problem was overcome when tetra brik was invented.
Differences in the way we keep it at home
Different combinations of temperature and time used in pasteurized milk and UHT milk respectively, determine the characteristics of each. Among the most notable are the following:
- Pasteurized milk must be kept cold at all times, while UHT milk can be stored at room temperature until opening.
- Pasteurized milk has an expiration date of four days from its packaging, while UHT milk can be kept outside the fridge for months.
The organoleptic characteristics of pasteurized milk are more similar to those of raw milk. UHT milk, because it has been treated at higher temperatures, undergoes further transformations (some milk compounds are degrades while others are formed, changing the odor, taste and color of milk).
What nutritional changes are irreversibly applied with the UHT treatment?
- Whey protein denaturation
- Whey protein-casein micelle interactions
- Maillard reactions
- Fat globule composition changes
- Vitamin content changes (20-30% loss in vitamins B1 and B12; significant decrease in vitamin C and folic acid)
- Lactose is converted to lactulose when heated at UHT temperatures
- Protein digestibility stays the same
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