When I’m on the road for business and need a quick bite, my $5.69 value meal usually comes with a side order of guilt. Sure, it’s heavy on the calories, but it’s more than that. Our post “Super Size Me” society now suspects that fast food is superfood, soaked in chemicals and additives that make it taste/look good and last forever on the shelf. The connection between these theories is compelling; fast = artificial = long shelf life = profitable = unnatural = unhealthy = cheap for the consumer. Furthermore, these theories aren’t without merit. Just one example: this fascinating NYT article on the ground beef industry (and incidentally one more reason I shop at Costco).
Still, a well-intentioned nutritionist from Canada should probably have checked a few facts before going public with her “zombie-cheeseburger” research (the latest in a long line of “fast food never rots” stories). Apparently, Melanie Hesketh bought a McDonalds cheeseburger (no pickles or mustard) and left it unwrapped on her kitchen counter for a whole year, and nothing happened. No deterioration from mold or bacteria at all. Ms. Hesketh is quoted as saying “obviously it makes me wonder why we choose to eat food like this when even bacteria won’t eat it,”.
Well, there’s a reason for that, and (no big surprise here) it has everything to do with water activity. Let’s start with mold. For mold to grow, it needs a water activity of above 0.7. That means that if the burger is placed on a kitchen counter, and the room humidity is below 0.7 aw, the burger will lose water until it reaches equilibrium. This means that Melanie’s cheeseburger likely had a final water activity of about 0.5 or 0.6, a common range for room humidity. At this level, even a cheeseburger covered in mold spores won’t grow anything because the water activity is too low. Ditto for the cardboard flavored patty, and the greasy cheese slice. All of these are materials that lose moisture relatively fast.
Bacteria are even less likely to proliferate on the stale burger than mold – they require at least a water activity of 0.87 to grow. True, the inside of the burger is likely to stay “wet” for days or weeks as it sits on the counter top, but the inside is the spot least likely to harbor bacteria that could grow. The burger has been seared on the grill to at least 165 degrees F, so there’s no bacteria there. Conversely, if the interior of the burger had been ravaged by bacteria, that would mean the hamburger itself was contaminated – undoubtedly a much bigger problem than just some stale bread.
The easiest way to show that the McD’s burger does mold is to just put it in a plastic bag. While researching the “fast food doesn’t rot” theory on the internet, I ran across this thorough analysis from J. Kenji Lopez-Alt on the question. Kenji uses the plastic bag, and mold grows. Very well done and nicely written, even with no mention of water activity.
To sum up in two words: myth busted. Fast food isn’t usually healthy, but it’s not because a cheeseburger can last for a year on the kitchen counter.