Zoom in on microorganisms

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© Nicole Catellier, Cinémanima inc.
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© Nicole Catellier, Cinémanima inc.

Food preservation : a horror story for microorganisms

For centuries, man has been fighting problems of food preservation.

Some pathogenic microorganisms can develop in food and be subsequently ingested by humans. To avoid consumption of spoiled food, numerous techniques have been developed to preserve the freshness of food longer and ensure a better control of its quality.

Food preservation methods, old and current, were established according to one or several of the following principles:

  • Prevent contamination
  • Inhibit microbial growth and metabolism (microbiostatic activity)
  • Kill the microorganisms (microbiocide activity)

Here are seven preservation techniques that rely on one or the other of these principles.

1. Aseptic manipulation and treatment

The various stages of food processing and packaging are potential sources of contamination. Some foods can be sterilized, put into sterile containers, and sealed under aseptic conditions. These products can then be stored at room temperature.

2. High temperatures

  • Canning 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 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.
  • Sterilization 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.


4. Dehydration

  • Hot Applying heat to food evaporates the water it contains; this inhibits the growth of microorganisms but does not, however, kill them.
  • Cold The food is first frozen, then a powerful vacuum is applied so that the ice is converted directly to water vapor. This leaves the food and the microorganisms dehydrated. Contrary to dehydration using heat, this process does not alter the shape or the properties of the food that can thus be reconstituted.

5. High osmotic pressure

  • Addition of sugar Water is removed from microbial cells when they are placed in solutions containing large quantities of sugar. This inhibits the microorganisms’ growth but does not kill them. Yeasts and fungi are more resistant than bacteria to high osmotic pressure. This is why it is more common to see jellies and jams contaminated with microscopic fungi rather than bacteria after being exposed to air. Condensed milk is also preserved due to high concentrations of lactose and sucrose, which also remove water from microbial cells.
  • Addition of salt Water is also removed from microbial cells when they are placed in solutions containing large quantities of salt. The salting of food, in addition to improving its flavor, prevents the microorganisms from reproducing.

6. Chemical additives

  • Addition of organic acids Only a few organic acids are legally accepted as food preservatives. Sorbic and propionic acids are used to inhibit the growth of fungi in bread.
  • Nitrites and nitrates Nitrites and nitrates are added to meat to preserve their color and inhibit the growth of anaerobic bacteria. This practice is considered controversial because of the possible role of nitrates and nitrites as mutagenic or carcinogenic agents.
  • Substances produced during manufacturing In some cases, chemical preservatives are produced during the manufacturing of the product or during the preservation process. Sauerkraut and pickles, which are prepared by fermentation, are thus preserved by acetic (vinegar), lactic, and propionic acids produced by the fermentiscible microorganisms. Smoking food generates antibacterial compounds that penetrate and preserve the meat.

7. Irradiation

Food that is "radiating freshness"

Irradiation is a relatively new method of food preservation. It consists of exposing the food to the direct action of electromagnetic radiation, which increases its shelf life. The irradiation lasts only the time it takes to expose the food to the rays and it does not render the products radioactive. This preservation process is gaining in popularity because it could help us with one of our greatest global challenges, i.e., to feed a world population that is constantly growing. By adequate food preservation, we could effectively reduce the loss of perishables, as well as lower the incidence of serious diseases caused by microorganisms such as toxoplasmosis, salmonellosis, listeriosis, chlolera, etc.

  • Nonionizing Nonionizing radiation uses ultraviolet rays. If the exposure time and the intensity are sufficient, the microorganisms present in the food product exposed to the UV will be killed. However, this type of radiation has a limited power of penetration and it thus only used for controlling surface microorganisms, such as in bakeries or meat processing rooms, for example.
  • Ionizing Ionizing radiation can use Cobalt-60, a source of gamma rays, which are similar in nature to light but at much higher energy levels. The rays destroy some bonds in the DNA of the microorganisms, which can no longer reproduce and then die.