2. High temperatures
At the beginning of the 19th century, the Frenchman Nicolas Appert invented canning, also called appertisation, a preservation method that is still one of the most widely used today. Stored at room temperature, the contents of the can are stable for at least a year. The food is placed in the containers, which are sealed and immersed in boiling water (100 to 120°C). This process does not guarantee a sterile product but it kills the anaerobic bacteria Clostridium botulinum
capable of forming spores that produce a deadly toxin.
Pasteurization, invented in the 19th century by Louis Pasteur, was first used to ensure that milk could be consumed safely. It is still used today for the preservation of other beverages and foods. The process involves heating the product continuously at 62.8°C for 30 minutes or 71.1°C for 15 seconds, then cooling it rapidly. This time-temperature relationship was determined to kill the bacteria Mycobacterium tuberculosis
(which causes tuberculosis) et Coxiella burnetii
(which causes Q fever), two pathogenic microorganisms that can be found in milk.
The milk is sterilized by being kept at an extremely high temperature of 148.9°C for 1 or 2 seconds. The milk does not develop a “cooked” flavor, its nutritional quality is preserved, it requires no refrigeration, and can be stored indefinitely.
3. Low temperatures
Refrigerating or freezing food, even if the temperature is extremely cold, cannot kill microorganisms. Temperatures approaching 0°C and lower delay the growth and the metabolism of microorganisms. In fact, prepared frozen food (a temperature of -32°C is used to avoid the formation of ice crystals) is increasingly popular. However, as soon as the food is defrosted, the microorganisms start to grow again, which is why it is highly recommended to never refreeze food which has been previously frozen and then thawed out.
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