Born in the second half of the nineteenth century by the French chemist Louis Pastuerfrom which it takes its name, the pasteurization it is a process that allows you to drastically reduce the concentration of microorganisms in foods, to increase safety and shelf life. The best known is the pasteurization of milk and consists of heat raw milk to high temperatures (above 60 °C) from half an hour to a few seconds, depending on the process used. The higher the temperature, the shorter the pasteurization time. Thanks to this it is possible to obtain a product microbiologically safeaccording to national directives, but other chemical contaminants are not eliminated, such as pesticidesfor which appropriate controls and preventive measures must be carried out upstream of the entire process.
Pasteurization: from wine to milk a method to improve safety and shelf life
We are in 1863 when the French chemist Louis Pasteur was commissioned by the government of Napoleon III to investigate the problems that afflicted the French wine industry: the bottles imported into other countries, during the journey, underwent a process of deterioration which negatively altered the taste. Pasteur discovered that behind this problem was a microbiological process known as acetic fermentationin which bacteria transformed ethanol into acetic acid, responsible for the souring of wine. In fact, the term “fermentation” is not exactly correct, because it indicates an anaerobic metabolic process (i.e. which occurs in the absence of oxygen). This instead occurs in the presence of oxygen, therefore it is aerobic, but is called this by convention.
The scientist devised a way to kill these microorganisms without however altering the organoleptic properties (consistency, appearance, flavour) of the wine: the product had to be heated to around 60°C/70°C (remember that wine is an aqueous solution, if it is heated to 100°C it starts to boil and part of the solution evaporates) and then cooled: this allowed most of the bacteria responsible for acetic fermentation to be killed. Subsequently, the process was also extended to other foods and was named, in his honour, pasteurisation. The best known is that of milk.
In milk, the main purpose of pasteurization is therefore reduce microorganisms which can affect safety and security shelf life of the food. In fact, this treatment aims to prolong what is defined in the food sector shelf lifei.e. the period during which a product can be stored in optimal conditions without undergoing significant alterations from a hygienic-sanitary, organoleptic and nutritional point of view.
The raw milkthat is, milk that has not undergone any heat treatment, has a shelf life significantly lower to that of pasteurized milk: microorganisms, if not controlled, proliferate disproportionately and through metabolic reactions lead to the formation of acids which lower the pH of the milk and alter its taste, consistency and shelf life.
How is milk pasteurization done?
During the pasteurization process, milk is exposed to high temperatures (over 60°C) for a certain period of time. Based on temperatures and timing, we can distinguish three main types of milk pasteurization:
- LTLT (Low Temperature Long Time): the milk is heated to 63°C for 30 minutes.
- HTST (High Temperature Short Time): here the temperature rises up to 72°C, but the exposure time decreases drastically to 15 seconds.
- UHT (Ultra High Temperature): 100°C are exceeded but for very short times, below 10 seconds.
It should be noted how the heating temperature and process time are interrelated inversely proportionalthat is, as one increases, the other decreases. This means that higher temperatures for a few seconds they allow you to reach a level of microbiological safety which at lower temperatures requires longer exposure times. This balance is crucial to preserve the quality of the finished product and minimize any alterations due to heat.
At an industrial level, these processes take place thanks to named plants pasteurizers. Inside there are some heat exchangerswhich can be, mainly, tubular or with plates: both systems are designed to ensure a thermal energy transfer between a fluid (such as hot water or steam) and milk without the two coming into contact with each other, but simply flowing in two close sections.
Before the actual heat treatment, raw milk comes first filtered and then subjected to preliminary heating to reduce its viscosity and thus facilitate its flow in the system. At the end of pasteurization, however, the milk is rapidly cooled to temperatures between 4°C and 6°C (using cold water or ethylene glycol as a cooling fluid in the heat exchanger), this step is essential to prevent the development of residual thermophilic bacteria that survived the treatment.
Which contaminants are not eliminated by pasteurization
Pasteurization mainly aims to break down i biological contaminants present in milk, but there are also others. In general we can divide them into two large categories:
- Contaminants chemists: pesticides, mycotoxins (toxins produced by fungi) or heavy metals. These substances are not eliminated during the pasteurization process, which is why their presence must be checked upstream, with preventive measures such as monitoring animal diet or regulating and limiting the use of certain chemicals in dairy farming.
- Biological contaminants: bacteria, viruses, yeasts and molds, targets of pasteurization. Particular attention is paid to pathogenic bacteria and to those deteriorating, that is, those that are not pathogenic, but must still be eliminated because they can compromise the quality of the milk, decreasing its shelf life.
However, some clarifications must be made: pasteurization it is not a complete sterilizationbut it is a process that reduces the microbial load significantly without completely eliminating all microorganisms. Some bacteria, such as those belonging to the genus Bacillus, they are also capable of transforming into spores: these are as if they were stronger versions of bacteria, and they are heat resistantwhich means they are not sensitive to heat and consequently they manage to survive pasteurization.
According to Italian regulations (DPR 54/97) thermised bovine milk (i.e. which has received mild heating) and intended for pasteurisation must contain, at 30°C, a aerobic mesophilic germs (most widespread genus of microorganisms) less than 100,000 per ml. Once pasteurized, the milk must give a negative result to the milk test phosphatase (this is an enzyme which, if the process has taken place correctly, has been destroyed by heat) and a positive result in the blood test peroxidase (enzyme more resistant to heat whose presence demonstrates that the temperature necessary to inhibit bacterial growth has been reached but not to compromise the nutritional properties). Only by respecting these parameters is the milk in question retained Safe for food use.
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