The Cupcake Experiment

For Kayla's second birthday, we had an Eric Carle-themed party featuring, among other things, a Very Very Hungry Caterpillar cake constructed from upside-down cupcakes arranged in a line. The antenna were made of candles, representing my pixie's two glorious years on this earth.

The cupcakes were a big hit, and I decided to make them again a few weeks later. I didn't have time to frost them after baking, so I let them cool in the Teflon-coated baking pan and then just covered the whole thing with aluminum foil and put it in the refrigerator to deal with later.

The next morning I retrieved the cupcakes and uncovered them to frost them. I found tiny holes in the foil.

They were pin-sized and I never would have noticed them if it weren't for the fact that the morning sun happened to be shining right through the window behind the cake pan in a way that lit up the holes brightly enough to catch my eye.

Notice that the holes are only where the foil was touching the cupcakes.


Apparently something in the cupcakes reacted with and corroded the aluminum. But what?

I used a Betty Crocker cake mix. Following is the ingredients list from the box:

Sugar, Enriched Flour Bleached (Wheat Flour, Niacin, Iron, Thiamin Mononitrate, Riboflavin, Folic Acid), Partially Hydrogenated Soybean and/or Cottonseed Oil, Modified Corn Starch, Propylene Glycol Monoesters of Fatty Acids, Baking Soda, Salt, Sodium Aluminum Phosphate, Dextrose, Monocalcium Phosphate, Distilled Monoglycerides, Dicalcium Phosphate, Maltodextrin, Sodium Stearoyl Lactylate, Datem, Monoglycerides, Xanthan Gum, Natural and Artificial Flavor, Aluminum Sulfate, Yellow 5&6, Nonfat Milk.

To that I added water and salted butter.

After doing some "research" (if one can call 10 minutes of Googling "holes in aluminum foil" doing research) it seems that salt is the most likely culprit.

I wondered if I could identify any other factors contributing to this phenomenon. The ones I figured I could most easily investigate were:

1. Temperature: Did refrigerating everything slow down or speed up the corrosion? Did it make a difference at all?
2. Foil side: Aluminum foil has a flat side and shiny side.* Any difference? (See footnote for more on flat vs. shiny aluminum foil.)
3. Cake side: The top of a cupcake is always kind of goopy. I believe this is due to butter & oils rising to the top of the mix during baking. The goopy part of a cupcake must have slightly different properties than the cakey parts. Did this contribute to my holey foil?

To test these three factors, I needed 8 different test subjects:

#	Temperature	Foil Side	Cake Side
1	Room	Shiny	Spongy
2	Cold	Shiny	Spongy
3	Room	Flat	Spongy
4	Cold	Flat	Spongy
5	Room	Shiny	Goopy
6	Cold	Shiny	Goopy
7	Room	Flat	Goopy
8	Cold	Flat	Goopy

To do this, I cut the top off of a cupcake and quartered it. Then I wrapped them each in carefully folded foil so that part of the foil would touch the goopy bit and part would touch the cakey bit.


Two went in the fridge...


...and two stayed at room temperature.

I observed them over the next two days & recorded the results.

Now, before I post the results, anyone care to form a hypothesis?


*FOOTNOTE: Until a few days ago I thought that there was a functional difference between the flat (or dull, or matte) side of a sheet of aluminum foil and the shiny (or glossy) side. Turns out, there isn't. According to ancient wisdom and Wikipedia, the reason one they look different is that during the manufacturing process, two sheets are run through rollers together to get them nice and thin. Mineral oil is used to lubricate the rollers, rendering the "outside" shiny. When the sheets are pulled apart, the inside is dull. Some believe that you should wrap hot food with the reflective side inward to retain heat better, and cold food with the reflective side outward to keep things colder, but for all practical purposes it really doesn't matter.