The food expo activity
Objective:
During this activity, the students will discover or rediscover the different techniques and
methods of food preservation that prevent spoilage or slow spoilage.
Background
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.
Procedure
Ahead of time: Three weeks before the activity, get your students to think by asking them
these questions:
1. How can we preserve foods so that they do not become contaminated by microorganisms, in other words, to prevent the micro-organisms from multiplying on
foods or to kill them?
2. Is heat capable of killing micro-organisms such as bacteria?
3. What happens if fruits are left on the kitchen table instead of in the refrigerator?
4. How might we be able to keep foods without putting them in the refrigerator?
5. How is food for astronauts preserved?
6. What is the secret to preserving jams?
Then, present the different food preservation techniques and organize a food fair with
them.
The activity
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.
Questions
1. What is the name of the food preservation technique presented by the team?
2. Was this technique developed to prevent contamination, to stop the growth and
metabolism of micro-organisms or to kill the microorganisms?
3. For which foods is this technique used?
4. Against which micro-organism do you think this technique works best to protect
foods?
Some preservation techniques
• Canning
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
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.
• Sterilization
Sterilized milk is placed at the extremely high temperature of 148.9°C or 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
micro-organisms. Low temperatures, approaching 0°C and below, slow the growth and
metabolism of micro-organisms. 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, micro-organisms 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 vapor. At the end of this process, food and
micro-organisms 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