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Sampling and Enumeration
One method of creating a sample for enumeration is to take a small amount of food (roughly 10 grams) and dilute with a sterile diluent which is not harmful to bacteria. The sample and diluent should be macerated, or blended, so that microbes are released. Alternatively, the surface of a food item can be rinsed with a diluent if microbes are only present on the surface. The sample is often split into three and applied to petri-dishes of agar in three different methods. This is because each method differs in sensitivity. One sample is spread evenly onto agar, one is poured onto agar and the last sample is spiralled across the surface. The petri-dishes are incubated at various temperatures until bacterial colonies appear. A high temperature of 37 degrees Celsius results in colonies of bacteria appearing more quickly than lower temperatures. Counting the colonies provides a measure of microbial presence, which is then represented as the number of colonies per gram.
Dried Fruit and Vegetables
The ICMSF guidelines on food sampling for microbiological analysis recommends that dried fruits and vegetables are tested for E. Coli, primarily to test for compliance with Good Manufacturing Practice, or GMP. Microbial growth is possible within tissues during the drying process; samples should not be obtained by rinsing the surface of the fruit but from a macerated tissue sample.
Grains, Flour and Nuts
Cereals should be tested for moulds. During enumeration moulds require five days incubation before there are sufficient levels present for counting. The ICMSF recommend that coconuts be tested for salmonella and that other tree nuts are tested for E.Coli as a way of testing GMP compliance via maceration of a sample with a diluent.
Dried Pasta Products
Dried pasta is formed by mixing flour and eggs at a 30 percent moisture level and is often dried slowly. This is why it should be tested for the presence of salmonella and Staphylococcus aureus. This is done by macerating a small sample of pasta with a diluent and incubating at 37 degrees.
Spices and Herbs
Herbs and spices also often naturally contain naturally occurring antimicrobial chemicals. The diluent used to create samples for enumeration should contain compounds which neutralise these chemicals. Different parts of plants are used as spices and are prepared in a variety of ways which results in spices commonly being infected with bacteria or moulds but most predominantly spore forming bacteria.