Applicability of Bioluminescence Assay and Aerobic Bacterial Counts for the Evaluation of Raw Milk Quality.

Abstract

Raw milk samples were analyzed to determine the bacteriological quality by the standard plate count (SPC), adenosine-5$\sp\prime$-triphosphate (ATP) assay, preliminary incubation counts, and the psychrotrophic bacteria count (PBC). In a comparison of SPC and ATP content, an overall correlation coefficient of 0.97 was found between the two methods. The correlations between ATP content and SPC of fresh raw milk samples at zero hour and after preliminary incubating these samples at 12.8 and 15.6$\sp\circ$C for 18 hours were.96,.97, and.97, respectively. The regression coefficients for all types of milk samples were highly significant ($P <$.01). High correlation coefficients demonstrated that the ATP measurement in combination with preliminary incubation practices could be a rapid technique for the determination of bacteriological quality of milk at receiving stations and milk processing plants. Median SPC for 175 fresh raw milk samples was 36,000/ml. The bioluminescence technique is reported previously to have a good correlation with SPC in the range of 1.0 $\times$ 10$\sp5$ to 1.0 $\times$ 10$\sp{10}$ bacteria/ml. The commingled milk coming at the receiving stations sometimes have a bacterial count that is less than 1.0 $\times$ 10$\sp{5}$ bacteria/ml. A correlation coefficient of.87 was calculated between the SPC and ATP values for the overall data while lowering the SPC range from 1.0 $\times$ 10$\sp5$ to 1.0 $\times$ 10$\sp4$ bacteria/ml. This range is more practical as a quick screening of raw milk at dairy processing facilities since 81 percent of all raw milk samples fell in this range. An equation (SPC = $-$1642213 + 6751(ATP)) was developed from the combined experimental data which could estimate the SPC of raw milk samples using their ATP content. Another phase of the experiment was to ascertain if the Petrifilm$\sp{\rm TM}$ technique could be effectively used in place of agar method to estimate PBC. The correlation coefficient between the Petrifilm$\sp{\rm TM}$ and the agar plate method for both the fresh and preliminary incubated raw milk samples was.99. A high linearity of relationship shows that one method can be substituted with the other. However, the Petrifilm$\sp{\rm TM}$ technique took longer than the usual time of 25 h to develop red colonies at 21$\sp\circ$C because of the slow reduction of the dye reagent triphenyl tetrazolium chloride (TTC) at this temperature.