Microbiology of heat-processed canned foods

Lab.7.
Practical Food Microbiology
Microbiology of heat-processed canned foods
Introduction:
Canning is a method of preserving food in which the food is processed, sealed in an airtight
container and heated to a temperature that destroys microorganisms.
The objective in the thermal canning of foods is the destruction of microorganisms also to
inactivate enzymes that cause spoilage.
These products nevertheless undergo microbial spoilage under certain conditions.
Regardless of the safety assured in canned foods, any damage or defective canned products
are a potential public health problem.
Canned food has been involved in enteric infection and food poisoning, including cases of
typhoid, botulism, salmonellosis and staphylococcal poisoning.
Leakage occurs from can defects, punctures, contaminated cooling water sometimes leaks to
the interior through pinholes or poor seams.
A viable mixed microflora of bacterial rods and cocci is indicative of leakage.
Some microorganisms that grow in canned foods, do not produce gas and therefore cause no
abnormal appearance of the can.
Imported canned products may need to be examined at point of entry to the cities.
Spoilage of canned Foods
Swollen cans often indicate a spoiled product.
During spoilage, cans may progress from normal to abnormal appearance, see physical
examination of can.
Spoilage is not the only cause of abnormal cans. Overfilling or denting may also be
responsible.
The incidence of spoilage in canned foods is low, but when it occurs it must be investigated
properly.
The main reasons for spoilage are preprocess spoilage underprocessing, inadequate cooling,
and contamination of the can resulting from leakage through seams.

To determine the type of bacteria involved in spoilage it is important to conceder the
following facts:
1- Type of food involved ( particularly acidity)
2- Type of change in food (acid, gas, purification, blackening...etc)
3- Thermophilic or mesophilic character of microorganisms
Classification of food products according to acidity:
Interpritation of the possible causes of can spoilage

Physical examination of can
Procedure:
Describe internal and external condition of can, including evidence of leakage, swelling,
corrosion, etc.
Open can in an environment that is as aseptic as possible.
Remove large enough portions from center of can.
After removal of inoculum, transfer all remaining contents of cans to sterile closed containers,
and refrigerate at about 4°C.
Use this material for repeat examination if needed and for possible toxicity tests.
Prepare inoculum for microbiological cultivation.
Cultivation
Total count, Coliform count, Thermophiles, Clostridium
Results:
Condition
of can
Can
contents
Odor Appearance Gas pH Culture Diagnose