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Mechanical Preparation of Pour Plates for Viable Bacterial Counts of Milk Samples

Food and Drug Administration, Division of Microbiology, Cincinnati, Ohio 45226

ABSTRACT

A machine for making bacterial pour plates to determine the viable bacteria counts of milk samples has been developed. Samples are automatically placed on a turntable conveyor which advances the sample under an arm that dips a .01-ml capacity cylinder and a .001-ml capacity loop into the sample three times. then, a divided petri dish is positioned under the dinning arm the milk in the cylinder and loop is flushed with 3 ml of dilution water, agar is pumped into the dish, and the sample and dish are each atutomatically removed from the turntable. The agar is solidified by passing the dish over a cold plate. The machine plates five samples of milk per minute.

Twenty-six milk samples were compared by the standard plate count and machine plate cylinder techniques; 26 additional samples were tested by the standard plate count and machine plate loop methods. Criteria to compare the machine and standard plate methods included experimental error equal to or less than .005 in log₁₀ units, average difference between geometric means less than 9% and interaction factor not to exceed .007 in log₁₀ units. All criteria were satisfied by the mechanical cylinder and loop methods.