Tuesday, November 16, 2010

Mushrooms, Mold, and McDonalds: Day 26

By Monica

Time for another McMold update! For those of you who haven’t been following, here’s the experimental design and initial pictures, and a video of the 17 day results.

Let’s get right to it. Here's Day 26. Salted homemade fries. From left to right: dry (no growth), lid (mold), lidded with water (bacteria).

Salted McD's fries. From left to right: dry (no growth), lid (mold), lidded with water (bacteria and mold).

Unsalted homemade fries. From left to right: dry (no growth), lid (mold and bacteria), lidded with water (bacteria).

Unsalted McD's fries. From left to right: dry (no growth), lid (fungi), lidded with water (bacteria).

And here's a closer picture of the burgers. Homemade on left, McD's on right. From top to bottom: dry (no growth on either), lid (mold on homemade, bacteria on McD's), lidded with water (bacteria on homemade, mold on McD's).


Basically, what this experiment has shown is that under very high water activity, the fries have problems cultivating molds but bacteria will grow just fine. At lower water activities , molds will grow just fine. Obviously, none of the fries in the top row are rotting because the water activity is too low for either bacteria or fungi.

This little experiment demonstrates that the salt, at least in the amounts McDonald’s uses, and types of fats or fat content have nothing to do with decomposition, as some food scientists have been claiming in popular news reports. It’s all about the moisture: not the salt, not the fat, and not the preservatives, at least for french fries. The amount of moisture determines whether the fries decompose, and also the type of decomposition: bacterial or fungal.

An interesting study could be set up with many more replicates to determine if there is any difference in species composition of fungi and bacteria in the jars under different water activities. This might be of marginal interest to mycologists, but probably not to the general public, and it would be a lot more work, so I probably won’t do it. There are also personal concerns, such as objections from spouses.


I think it’s possible that the different textures of the buns might have something to do with the microbial growth in these jars, but it would take far more replicates to determine exactly what is going on. None of the burgers are growing copious amounts of mold as the fries are. Ideally, a much bigger experiment could be done testing a variety of different types of buns -- e.g. storebought white, whole grain, organic, and homemade -- under different water activities to find out what sort of microbial growth is fostered. For bread specifically, it seems logical that preservatives might alter the rate of microbial growth, even if they can’t stop microbial growth altogether. Otherwise, why would the food industry use them?


We can conclude that food pretty much rots, whether it comes from McDonald’s or anywhere else. It just needs some moisture to do so.

I am on the verge of discarding the fries, since I don't think there is any more to be learned from them. Stay tuned to see what develops on the burgers and the butter and “not butter”s.

Update: 11/16 Replaced an incorrect photo.

Crossposted from Spark a Synapse.

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