By Nicole Martin, PhD

Post-pasteurization contamination of fluid milk is a major contributor to premature spoilage. The microorganisms often responsible for this contamination are Pseudomonas and other Gram-negative bacteria. When introduced into fluid milk these Gram-negative bacteria grow rapidly at refrigeration temperatures and cause off-odors and flavors as well as body defect such as coagulation. In order to reduce post-pasteurization contamination, and thereby premature product spoilage, fluid milk processors need to be able to detect Gram-negative bacteria in fluid milk products. The standard method to detect Gram-negative bacteria in fluid milk uses Crystal Violet Tetrazolium Agar (CVTA), a traditional microbiological agar. While this method has been shown to detect Pseudomonas and other relevant Gram-negative bacteria causing post-pasteurization contamination in fluid milk, it is not accessible to many processors, especially those without the capabilities to make sterilized microbiological media in-house. Recently, the Milk Quality Improvement Program investigated the use of a modified Coliform Petrifilm method to detect Gram-negative bacteria in fluid milk. The results of this study “Short communication: Coliform Petrifilm as an alternative method for detecting total gram-negative bacteria in fluid milk” were published in the Journal of Dairy Science and showed that using Coliform Petrifilm incubated at 32°C for 48h prior to evaluation for any colony formation is an acceptable alternative to using CVTA to detect Gram-negative bacteria in fluid milk. This method is easily adoptable as many fluid milk processors already use Coliform Petrifilm and will allow for better detection of post-pasteurization contamination. Processors who wish to implement this method can read the full paper by contacting Nicole Martin at  nhw6@cornell.edu.