Earlier this week, I had an excellent Email question regarding a foodborne outbreak in the United States. You can read more about it here: Multistate Outbreak of Shiga toxin-producing Escherichia coli O121 Infections Linked to Flour
The question was simple enough. How can bacteria live in this environment? After all, one of the three needs for growth – water – is missing.
The premise makes perfect sense yet, once again, bacteria seem to know how to break the rules. Many species can indeed survive drier areas. When it comes to flour, E. coli can survive for over 6 months. If you don’t believe me, check out this report:
One section dealing with bacterial growth in various foods states:
Yersinia pseudotuberculosis, Salmonella enteritidis, and Escherichia coli O157:H7 can survive in flour and infant formula beyond 180 days. Survival in flour was best under refrigerated conditions.
The explanation behind this apparent paradox lies in the nature of the flour itself. While you might not see water, it is still there.
In almost all areas of the world, water exists in the air in the form of humidity. Materials, like foods, may also have water content in the form of vapour pressure. This can be measured and compared to the vapour pressure of distilled water. The result is known as water activity (aw).
Water activity ranges from 0.0 (bone dry) to 1.0 (distilled water). In food safety, water activity is incredibly important as it can determine whether a product is at risk for microbial growth. For fungi, the activity needs to be above 0.7. For yeast, it’s 0.88. For mold, 0.80. As for bacteria, that number is 0.91.
Many food have been tested for their aw and lists now exist. One such example can be found at the FDA:
So, how does this relate to the flour outbreak? If the water activity was above that 0.91 threshold, then the bacteria would not only survive but also grow. If there were no antimicrobial steps in the processing stages, those bacteria would find their way into the food production chain and eventually to your counter.
As to whether water activity played a role in this particular outbreak, only further investigations will tell. There may have been other contamination events during the flour’s food continuum. But when it comes to the question at hand, as long as you have that water activity over 0.91, you can potentially have bacterial growth.