Clientele: 9 to 12 years old
Type of resource
Experiment to do in class.
Printable version of this page
Protocole - The food expo (PDF 41 Ko)
The students will discover or rediscover the different techniques and methods of food preservation that prevent or slow spoilage.
Description of the activity
Food preservation has always been of concern to humans. In ancient Egypt as in modern times, human beings have developed food preservation techniques.
These preservation techniques have been developed to prevent contamination, inhibit microbial growth and metabolism or to kill microorganisms.
Some preservation techniques
At the start of the 19th century, the Frenchman Nicolas Appert invented canning, which remains one of the most commonly used preservation method. Stored at room temperature, the contents of cans remain effectively stable for at least a year. Food is introduced into containers that are then sealed and submerged in boiling water (100 to 120°C). This process does not guarantee a sterile product, but it kills the anaerobic bacterium Clostridium botulinum which forms spores that can produce a deadly toxin and causes botulism.
Pasteurization, invented in the 19th century by Louis Pasteur, was initially used to make drinking milk safe. Today it is used for the preservation of other drinks such as fruit juices and of foods such as pickles. Pasteurization consists of heating the product to 62.8°C for thirty minutes or to 71.1°C for fifteen seconds, then rapidly cooling it down. This time-temperature relationship has been shown to destroy two pathogenic bacteria that can be found in milk: Mycobacterium tuberculosis which causes tuberculosis and Coxiella burnetii which causes Q fever.
Sterilized milk is placed at the extremely high temperature of 148.9°C for one or two seconds. This milk does not develop a cooked taste, its nutritional properties are preserved, it requires no refrigeration and it can be stored indefinitely.
- Low temperatures
Refrigerating or freezing foods, even if the temperature is extremely low, cannot kill microorganisms. Low temperatures, approaching 0°C and below, slow the growth and metabolism of microorganisms. Frozen prepared meals (they use temperatures below -32°C to avoid the formation of ice crystals) are more and more popular. However, as soon as food is thawed, microorganisms resume their growth, and this is why it is strongly recommended that once a frozen food item is thawed, it should not be refrozen.
- Lyophilisation or freeze-drying
Food is first frozen and then placed into a powerful vacuum (where there is no air) so that the ice is transformed directly into water vapour. At the end of this process, food and microorganisms are dehydrated (examples: food for aquarium fish, freeze-dried meals for astronauts and excursionists). Unlike heat-drying, this process changes neither the shape nor the properties of the food, which may thus be rehydrated.
- Addition of sugar or high osmotic pressure
Water is removed from microbial cells when the cells are placed into solutions that contain large quantities of sugar. This stops the microorganisms from growing but does not kill them (examples: jams and condensed milk). Yeasts and moulds are more resistant to elevated osmotic pressure than bacteria are. This is why jams and jellies that have been exposed to air will more often become contaminated by microscopic fungi than by bacteria. Condensed milk is in part preserved by a high concentration of lactose and sucrose, which also removes the water from microbial cells.
There are other food preservation techniques such as food packaging and the use of chemical additives (for example, the addition of organic acids to bread, of nitrites and nitrates to cold cuts and the substances produced during the production of certain foods such as sauerkraut).
Steps of the activity
Ahead of time :
Three weeks before the activity, get your students to think by asking them these questions :
- How can we preserve foods so that they do not become contaminated by microorganisms, in other words, to prevent the microorganisms from multiplying on foods or to kill them?
- Is heat capable of killing microorganisms such as bacteria?
- What happens if fruits are left on the kitchen table instead of in the refrigerator?
- How might we be able to keep foods without putting them in the refrigerator?
- How is food for astronauts preserved?
- What is the secret to preserving jams?
Then, present the different food preservation techniques and organize a food fair with them.
During a food expo, teams of four students will each present a food preservation technique. Each team must bring to class foods that are preserved using this technique and explain it in the context of an oral presentation or a kiosk that they man.
What is the name of the food preservation technique presented by the team?
Was this technique developed to prevent contamination, to stop the growth and metabolism of microorganisms or to kill the microorganisms?
For which foods is this technique used?
Against which microorganism do you think this technique works best to protect foods?
For links with education programs, consult the document Protocole - The food expo (PDF 41 Ko)