Microbiology Of Canned Foods

 Foods are processed for convenience and safety. Food processing involves
procedures such as: drying, canning, freezing, and pasteurization. For
example, drying is a process by which water is removed from the product,
causing the product to become dehydrated. Since microorganisms need water
to grow, without moisture, they can’t flourish.

Microbiology Of Canned Foods

Canning is a process where
foods are put into a container and given a high heat treatment to make the
product sterile. The process of canning, be it vegetables, meat, or seafood,
makes food safe because all dangerous microorganisms are destroyed. The
canning process was developed to preserve food safely and for long periods of

History of Canning

The canning process dates back to the late 18th century in France when the
Emperor Napoleon Bonaparte, concerned about keeping his armies fed,
offered a cash prize to whoever could develop a reliable method of food
Nicholas Appert conceived the idea of preserving food in
bottles, like wine. After 15 years of experimentation, he realized if food is
sufficiently heated and sealed in an airtight container, it will not spoil. An
Englishman, Peter Durand, took the process one step farther and developed a
method of sealing food into unbreakable tin containers, which was perfected
by Bryan Dorkin and John Hall, who set up the first commercial canning
factory in England in 1813. 
As more and more of the world was explored, and
as provisioning armies took on greater importance, the demand for canned
foods grew. Thomas Kensett, who emigrated to the United States, established
the first U.S. canning facility for oysters, meats, fruits and vegetables in New
York in 1812. More than 50 years later, Louis Pasteur provided the
explanation for canning’s effectiveness when he was able to demonstrate that
the growth of microorganisms is the cause of food spoilage.

Basic Principal of Canning

The basic principles of canning have not changed dramatically since Nicholas
Appert and Peter Durand developed the process. Heat sufficient to destroy
microorganisms is applied to foods packed into sealed or “airtight” containers.
The canned foods are then heated under steam pressure at temperatures of
The amount of time needed for processing is different for each
food, depending on the food’s acidity, density and ability to transfer heat. For
example, tomatoes require less time than green beans, while corn and
pumpkin require far more time. Processing conditions are chosen to be the
minimum needed to ensure that foods are commercially sterile, but retain the
greatest flavour and nutrition.

Spoilage of Canned Products

Heated canned foods may undergo spoilage either due to chemical or
biological reasons. The most common spoilage of canned foods is the
hydrogen swells produced as a result of action of food acid with the metal can.
Such spoilage occurs mostly due to imperfect tinning and lacquering of
interior of the can used for canning acidic foods. 
Biological spoilage of
canned foods by the microorganism may result either from the survival of the
organisms after the heat treatment or leakage of the container permitting
entrance of the microorganisms, Surviving organisms may be vegetative cells
or spore formers depending upon the heat treatment. Acid foods are processed
at a temperature around 100OC which result in killing of all vegetative cells of
bacteria yeast and molds.

Clostridium botulinum a Major Threat in Canned Products

Growth of the bacterium Clostridium botulinum in canned food may cause
botulism − a deadly form of food poisoning. These bacteria exist either as
spores or as vegetative cells. Botulism is an intoxication that is caused by
the ingestion of a virulent nerve toxin produced by the growth of the gram
positive, obligate anaerobe, spore-former Clostridium botulinum. 
bacterium appears to be a normal inhabitant of the soil, hence is ready
contamination of most foods. The spores can survive harmlessly in soil and
water for many years. When ideal conditions exist for growth, the spores
produce vegetative cells which multiply rapidly and may produce a deadly
toxin within 3 to 4 days of growth in an environment consisting of
• a moist, low-acid food 
• a temperature between 4°C and 49°C 
• less than 2 percent oxygen
It is able to grow in absence of oxygen in a wide variety of foods and in so
doing produces a protein neural toxin, two to three grams (an amount
equivalent to the quantity of salt in the average salt shaker on your table) of
which would be sufficient to kill human being. 
However, the organism will
not grow in the presence of oxygen or nitrate salts and it does not produce the
toxin at a pH below 4.7. Only one strain, which is found associated with
marine organisms, is able to produce the toxin at refrigerator temperature. 
toxin is destroyed by boiling it at 100o
C for 10 to 15 min. However, the spore
requires a temperature of 121o
C for 15 min to kill it. The toxin acts by binding
to nerve junctions and destroying the nerve. The symptoms, which occur
usually within 12 to 36 hours, but which can take up to 8 days to appear,
classically consist of double vision, dizziness, inability to speak, breathe or
Death often occurs due to the inability to breath. The only treatment
is the injection of antitoxin to the several varieties of the toxin. This treatment
is only effective against free toxin, as once the toxin has bound to the nerves
the damage is irreversible. 
The entire canning process is built around
ensuring that all spores of this bacterium contaminating any canned food
are destroyed in the sterilization process. Industry has a sterling record in
that deaths from commercial-botulism are very rare. This is influenced by the
fact that once a product is known to contain botulism toxin none of that
product is ever again purchased by a customer. 
The majority of botulism
poisonings occur in HOME-CANNED FOODS prepared by grandma or
your favourite aunt. A rule of thumb is “READ THE CANNING
DIRECTIONS” and if you think a food might contain the botulism toxin never
tastes even the smallest drop of it!
Some interesting additional information about this disease is: 
• Never feed raw honey to a child under the age of two because the
botulism spores can grow in the immature gut and produce the toxin. This
can not occur in the adult due to our gut micro flora which is absent in
• The botulism toxin is being used to treat certain neurological conditions
where nerves that shouldn’t fire do. In these cases tiny quantities of the
botulism toxin is injected into the nerve, which the toxin kills and cures
the condition. 
• Ducks and chickens often die from botulism poisoning by eating rotting
material in which the bacterium has grown. However, vultures, which as you know, eat disgusting rotten, stinking carrion, are immune to the toxin
through evolution.
Botulinum spores are on most fresh food surfaces because they grow only in the absence of air, they are harmless on fresh foods. Botulinum spores are very hard to destroy at boiling-water temperatures; the higher the canner temperature, the more easily they are destroyed. 
Therefore, all low-acid foods should be sterilized at temperatures of 115°C to 121°C, attainable with pressure canners operated at 10 to 15 PSIG. PSIG means pounds per square inch of pressure as measured by gauge. At temperatures of 115°C to 121°C, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. 
The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in boiling-water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes. 

Leave a Comment