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

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Abstract
A machine for making bacterial pour plates to determine the viable bacteria counts of milk samples has been devel- oped. 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 dipping ann, 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 automatically removed from the turn- table. 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 com- pared 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 in- eluded experimental error equal to or less than .005 in log10 units, average dif- ference between geometric means less than 9%, and interaction factor not to ex- ceed .007 in Iog~0 units. All criteria were satisfied by the mechanical cylinder and loop methods.