Assessment of Raw Milk Microbiological Quality by Adenosine-5'-Triphosphate Content and Preliminary Incubation Bacterial Count.

Abstract

Two hundred raw milk samples were collected and stored in sanitized plastic containers which were transported in ice to the laboratory. Three 100 ml sub-samples were aseptically removed from each container. One sub-sample (Fresh) was analyzed for Standard Plate Count (SPC) and adenosine-5'-triphosphate (ATP) content. The two remaining sub-samples underwent preliminary incubation; one sub-sample was incubated at 12.8(DEGREES)C for 18 h (PI 12.8) and the other at 7.2(DEGREES)C for 48 h (PI 7.2). Following incubation, SPC and ATP content were determined. Correlation of ATP content with SPC-Fresh, SPC-PI 12.8 and SPC-PI 7.2 was 0.72, 0.84 and 0.89 with log(,10) transformed variables. Although linear regression coefficients were significant (P < .01), coefficients of determination indicated that raw milk ATP content was not predictive enough for raw milk bacterial estimates. Storage of tank truck and silo raw milk samples for 5 d at 2.2(DEGREES)C resulted in increases in median SPC, Psychrotrophic Bacteria Count (PBC), Proteolytic Count (PC) and mean tyrosine value (TV). Median SPC-Fresh for both types of samples was 105,000/ml, 88,000/ml for tank truck samples and 220,000/ml for silo samples. Median PBC-Fresh for both types of samples was 46,000/ml, 36,000/ml for tank truck samples and 81,500/ml for silo samples. Mean SPC doubling time was 14.3 h for tank truck samples and 20.6 h for silo samples. Mean PBC doubling time was 16.3 h for tank truck samples and 21.9 h for silo samples. Median PC-Fresh for both types of samples was 6,500/ml, 6,000/ml for tank truck samples and 11,500/ml for silo samples. Changes in raw milk microbiological quality during storage at 2.2(DEGREES)C were investigated by dividing the samples into acceptable and unacceptable groups based on Standard Plate Count. An SPC-Fresh of 100,000/ml, and SPC-PI 12.8 values of 100,000/ml, 150,000/ml, 230,000/ml and 300,000/ml were used to dichotomize the samples. Dichotomization based on SPC-PI 12.8 values yielded acceptable sample groups having consistently lower median microbial counts during storage than did the SPC-Fresh dichotomizing value. Median SPC, PBC and PC of the acceptable sample groups decreased over all sampling days as the SPC-PI 12.8 dichotomizing value decreased.