When Milk Goes Bad
By: Rachel Evanowski and Nicole Martin
When milk goes bad, sometimes it goes very, very bad. Such was the case when a
US fluid milk processor received a consumer complaint about one of their products turning gray. That’s certainly not a call you want to get before your morning cup of joe. Thankfully the processor reached out to the Milk Quality Improvement Program for assistance in determining what was causing this unique product spoilage. Microbiological analysis revealed that the product was contaminated with a pigment producing Pseudomonas (Pseudomonas azotoformans). Pseudomonas is known to cause a number of color defects in dairy products, including an incident when fresh mozzarella made in Germany turned blue due to a contamination with Pseudomonas fluorescens and caused international headlines (http://www.bbc.com/news/10359001). While these bacteria do not cause illness, color defects are readily detected and detested by consumers, who may assume the product has been adulterated with chemicals.
Due to the heat sensitive nature of Pseudomonas, their presence in processed dairy products is an indicator of post-processing contamination (PPC). In fluid milk this contamination is commonly associated with ineffective cleaning and sanitation practices, inadequate preventative maintenance programs and cross-contamination. Resolving PPC issues may sometimes prove challenging and in these cases source tracking methods, for example, pulsed field gel electrophoresis, that have been used for decades in foodborne disease outbreak tracking may provide a useful tool for dairy processors to find and resolve PPC. The Milk Quality Improvement Program offers a variety of services to assist dairy processors with tracking and eliminating PPC.
For more information on defects caused by Pseudomonas see the following publications:
