Microbiological and chemical quality of raw milk in New York State.

Between December 1993 and January 1996, samples of raw milk from bulk tanks were collected by licensed milk haulers at the time of pick-up from 855 randomly selected farms in New York State (representing approximately 10% of all dairy farms in the state). The milk was examined for microbial and chemical qualities. Bacterial numbers were determined by standard plate count, laboratory-pasteurized count, coliform count, heat-resistant spore-forming psychrotroph count, aerobic spore count (mesophilic), rapid psychrotrophic count, and preliminary incubation count. The frequency distributions for these counts are presented. Paired correlation analyses between the microbiological parameters showed low correlations between test results; no correlation coefficients were > 0.8. The four highest positive correlation coefficients were found between standard plate count and rapid psychrotrophic count (0.7685), rapid psychrotrophic count and preliminary incubation count (0.6648), standard plate count and preliminary incubation count (0.5800), and aerobic spore count and laboratory-pasteurized count (0.5393). All other correlation coefficients were < 0.5. Milk freezing points and acid degree values were determined for all samples. Frequency distributions for these results are also presented.

Evaluation of adenosine triphosphate-bioluminescence hygiene monitoring for trouble-shooting fluid milk shelf-life problems.

An ATP-bioluminescence hygiene monitoring system was used to evaluate food contact surfaces in four fluid milk plants experiencing shelf-life problems. Postpasteurization surfaces, including gaskets, pipe fittings, valves, filler parts, and hand-washed items, were evaluated. Swab results, measured in relative light units proportional to total recovered ATP, were compared with results from the standard method of microbiological swab contact for adjacent sites of equal area. Microbiological procedures included standard plate count, coliform count, and Gram-negative bacteria count. Standard plate counts were < 1, 1 to 50, and > 50 cfu in 65, 22, and 13% of swabbed sites of < 100 RLU (relative light units); in 9, 36, and 55% of sites of 100 to 150 RLU; and in 22, 18, and 60% of sites of > 50 RLU, respectively. Thirteen sites were found with standard plate counts > 10,000 cfu per site and identified with the hygiene monitoring system (> 150 RLU). Gram-negative bacteria were the dominant bacterial type in a majority of these samples. Gram-negative bacteria were detected in a total of 22 sites tested; mean counts were 2100 cfu per site for Gram-negative bacteria and 20 cfu per site for coliform bacteria. Although limited to use on accessible sites, the hygiene monitoring system proved to be an effective, rapid tool for identifying the possible sources of postpasteurization contamination in the fluid milk plants evaluated.

Selection of tests for monitoring the bacteriological quality of refrigerated raw milk supplies.

Raw milk samples were collected from 10 producer bulk tanks. Samples were then subdivided so that milks were subsequently stored at 1.7, 4.4, 7.2, and 10.0 degrees C for 24 and 48 h. After storage, samples were analyzed by seven plating methods: standard plate count, psychrotrophic bacterial count, rapid psychrotrophic count, preliminary incubation count, mesophilic plate count, laboratory pasteurized count, and coliform count by violet red bile agar technique. Impedance protocols on a Bactometer Model 123 for total count, psychrotrophic count, mesophilic count, and coliform count were also used to evaluate the bacteriological quality of the milks. Bacterial counts and impedance detection times were analyzed using nonparametric statistics. Impedance protocols for total count and psychrotrophic count were the best indicators of bacteriological quality. Preliminary incubation count was the best of the plating methods. The laboratory pasteurized count performed poorly. Impedance measurements provided information in the shortest time.

Comparison of dry culture medium and conventional plating techniques for enumeration of bacteria in pasteurized fluid milk.

Standard plate counts, psychrotrophic bacterial counts, and coliforms were determined by conventional plating techniques and by Petrifilm TM plates, a dry culture medium, for 48 commercially processed milk samples (24 whole milk and 24 skim milk). The Petrifilm SM plate counts were compared with counts on standard methods agar for the standard plate count, psychrotrophic bacterial count, and rapid psychrotrophic bacterial count. The Petrifilm violet red bile plate counts were compared with counts on violet red bile agar for coliform test with a solid medium and the preliminary incubation method for detection of coliforms. Standard plate counts were determined within 24 h of packaging and after 7, 10, and 14 d of storage at 6.1 degrees C. Psychrotrophic bacterial counts and coliform counts were determined with 24 h of packaging and after 7 d storage. There was a strong linear relationship between Petrifilm SM and standard methods agar plates (excluding counts on samples plated within 24 h of packaging) and for the psychrotrophic bacterial count method. Petrifilm SM had a weak linear relationship with Standard Methods Agar plates for the rapid psychrotrophic bacterial count. Coliform counts determined on Petrifilm violet red bile plates were generally within the same range as counts on violet red bile agar plates. The positive predictive values for the Petrifilm violet red bile plates and violet red bile agar plates were essentially the same for samples plated within 24 h of packaging.