133 Determination of the Germ Count of Aerobic and facultatively Anaerobic, Mesophilic Bacteria (Plate Count Method) in Cereals, Cereal Products, and Animal Feed

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Methods Type: Generic Methods
Key data
Number: 133
Analyte: Aerobic and facultatively Anaerobic, Mesophilic Bacteria (Plate Count Method)
Matrix: Cereals, Cereal Products, and Animal Feed
Year of Approval: 1980
Scope: Feedstuffs and cereals or cereal products for the production of feedstuffs.
Principle: It is not possible to carry out a direct count of the bacteria in or on the product to be examined. For this reason an indirect method must be used to detect them. The product is first mixed with a sterile physiological dilution fluid, in order to separate and suspend the bacteria (8.1.), if necessary after grinding the product with grinding apparatus according to 6.9.1. or 6.9.2. A series of tenfold dilutions is made from this initial suspension (8.2.). Aliquot parts of these dilution stages are transferred to petri dishes (8.3.) and mixed with a culture medium which at first is molten (8.4.). When the agar has solidified, the individual bacterial cells are fixed and can multiply and form colonies in these positions during incubation (8.5.). The number of colonies is determined (9.1.) and described as "number of bacteria per g sample" (9.2.).

The accuracy of the method depends on how far one is successful in completely separating all the bacterial cells from the substrate, in avoiding damage to the cells during the necessary manipulations, and obtaining an even distribution of the cells in the culture medium. Greater accuracy is obtained if the count is determined on several subsamples (three, or preferably five).

Definition

Bacterial count means the number of those aerobic and facultatively anaerobic mesophilic bacteria which become visible as colonies on or below the surface of the culture substrate, when an aliquot of a suspension of the product to be examined has been transferred to a bacterial nutrient medium, according to 5.2., and incubated aerobically at 30 °C for 5 days (9.1.).

 

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