Several microbiological parameters are used to screen for the overall quality of milk. These analyses are associated with milk quality. They also can provide direction for trouble-shooting sanitation and procedural issues on the farm.
One of these measures is the preliminary incubation count, which also is referred to as PI count or PIC. The PIC reflects the presence of a specific group of bacteria called psychrotrophs that grow well under refrigeration temperatures (below 45°F) and are typically Gram negative. The laboratory test involves two steps. First the milk sample is incubated at 55°F for 18 hours to allow the microogranisms that thrive in temperatures cooler than those inside of a cow's body to proliferate. After incubation, step two is determining a standard plate count (SPC).
A comparison of PIC and SPC for the same sample can yield drastically different results. A PIC that is higher than the SPC indicates that contamination is occurring somewhere outside of the udder, such as in the environment or within the milking system. While PIC reflects management practices, a study has indicated that it does not predict the shelf-life or sensory attributes of pasteurized milk (Martin et al., 2011). Because these organisms do better at colder temperatures, their numbers can increase in milk during cold storage before pasteurization (Murphy and Boor, 2008). On-farm management practices including inadequate teat preparation, bulk tank cleaning, and the use of a specific detergent for cleaning were associated with increased PI counts (Elmoslemany et al., 2010).
The laboratory pasteurization count (LPC) is another microbiological parameter for evaluating milk that some cooperatives include in their reports. The sample is heated to 145°F and held at that temperature for 30 minutes to simulate the pasteurization step in a processing plant. The sample is then cooled and plated in the same method as used to determine SPC. While the pasteurization step kills disease-causing pathogens, some bacteria that form heat-resistant spores can survive. This is concerning because spoilage can be caused if these spores grow under refrigeration. The result can be off flavors in the milk and reduced quality of the final product. A recent study from researchers at Cornell University indicated specific management practices that are associated with increased numbers of psychrotolerant, spore-forming bacteria including cows with dirty udders and farms with less than 200 cows (Masiello et al., 2014).
Another microbiological test that producers may be familiar with is the coliform count. Coliform bacteria encompass a broad category of Gram negative rods that ferment lactose. Contamination due to these bacteria is typically from the environment, but theoretically could also be due to coliform mastitis. Coliform counts frequently are indicator that fecal contamination is occurring.
A closer look at bacteria counts may be able to point producers in the right direction for trouble-shooting problems when they arise. Examining the previously mentioned counts in conjunction with one another may help focus an investigation into the source of the problem.
Dirty cows are likely to have dirty udders, providing possible fecal or environmental contamination that can result in an elevated PIC or coliform count in addition to a higher SPC. Careful and thorough udder preparation including adequate coverage and contact time for teat dips are keys to reducing these counts.
Equipment Condition and Cleaning
Carefully examining milking equipment for any condition that would make cleaning difficult such as gaskets, liners (inflations), and tubing that could be cracked, scored, or worn should be routine. Surfaces should be smooth to allow for easy cleaning. Be on the lookout for and promptly remove build-up of deposits and biofilms. Equipment condition and cleaning issues can be reflected by elevated PIC, LPC, and SPC. Sanitation and condition of equipment should be examined if the LPC is elevated but the coliform count is not. However, if both the LPC and coliform count are elevated, further investigation into milking procedures and equipment is needed.
If milk is not cooling within the allowed timeframe, the result could be higher PIC and SPC. Similarly, if lines are not draining properly and milk is sitting in the lines, the PIC and SPC could increase.
A water supply should be tested routinely for coliform bacteria. Another consideration is 'hard' water containing calcium and magnesium that may interfere with the detergents and sanitizers being used and make them less effective (Robillard et al., 2001). Issues with the water supply could be reflected by increased coliform count due to fecal or environmental contamination of the water or increased PIC due to decreased effectiveness of cleaning and sanitizing agents.
If coliform count and SPC are elevated, there could be an issue with coliform mastitis in the herd. The SPC is likely to be greater than the PIC if this is the case.
- Elmoslemany, A.M., Keefe, G.P., Dohoo, I.R., Wichtel, J.J., Stryhn, H., and R.T. Dingwell. 2010. The association between bulk tank milk analysis for raw milk quality and on-farm management practices. Prevent Vet Med 95:32-40.
- Martin, N.H., Ranieri, M.L., Murphy, S.C., Ralyea, R.D., Wiedmann, M., and K.J. Boor. 2011. Results from raw milk microbiological tests do not predict the shelf-life performance of commercially pasteurized fluid milk. J. Dairy Sci. 94:1211-1222.
- Masiello, S.N., Martin, N.H., Watters, R.D., Galton, D.M., Schukken, Y.H., Wiedmann, M., and K.J. Boor. 2014. Identification of dairy farm management practices associated with the presence of psychrotolerant sporeformers in bulk tank milk. J. Dairy Sci. 97:4083-4096.
- Murphy, S.C., and K.J. Boor. 2008. Sources and causes of high bacteria counts in raw milk.
- Robillard, P.D., Sharpe, W.E., and B.R. Swistock. 2001. How to Interpret a Water Analysis Report. Penn State Extension, F103..