In a recent column I soothed a reader's anxiety at discovering that his wife's lasagna sauce was strong enough to eat holes in metal. Specifically, it ate holes in the aluminum foil that had been covering some leftover lasagna in the refrigerator.
I reassured him that all tomato sauces, not just his wife's, will do the same because tomatoes are acidic and aluminum is a metal that is easily attacked by acids. That was true as far as it went, but I missed the boat by not thinking about it a little deeper. It turns out that there is a much more interesting reason why lasagna sauce devours aluminum.
Lois Nicholson, of McLean, a former high school chemistry and physics teacher, reminded me politely that another process is going on besides the simple dissolving of a metal by an acid.
The fact is that tomato sauce can eat holes in the aluminum foil covering a leftover container only if the container is made of metal--not glass or plastic. (The original reader didn't tell me what his container was made of.) When aluminum metal is in simultaneous contact with a different metal and an electrical conductor such as tomato sauce (you knew, of course, that tomato sauce conducts electricity, didn't you?), the combination of the three materials actually constitutes an electric battery. Yes, an honest-to-goodness electric battery. An electrical (more accurately an electrolytic) process, not a simple chemical one, is what chews up the foil. While it would be difficult, not to mention messy, to run your Walkman on lasagna power, it could in principle be done.
How it works
Let's say you put your leftover lasagna into a bowl made of stainless steel, which is, of course, mostly iron. Now, iron atoms and the atoms of many other metals hold onto their electrons (you know that atoms contain electrons, right?) much more tightly than aluminum atoms hold onto theirs. So if given an opportunity, the iron atoms in the bowl will grab electrons away from the aluminum atoms in the foil. The sauce provides that opportunity by offering a conductive path through which the electrons can get from the aluminum to the iron.
When you cover your steel bowl with aluminum foil and the foil happens to be touching some of the sauce, the foil will lose electrons through the sauce to the steel bowl. But as any chemist will tell you, an aluminum atom that has lost electrons is no longer an atom of metallic aluminum; it is an atom of an aluminum compound that is capable of dissolving in the sauce--and that's where the acid comes in. In techspeak, the aluminum has been oxidized to an acid-soluble compound. So after it sits a day or so in the fridge, you're going to see that the aluminum foil has been eaten away only where it has been in contact with the sauce.
If the lasagna had been put into a glass, ceramic or plastic bowl, none of this would have happened because these materials don't conduct electricity and don't suck up electrons from other atoms. (You'll have to either take my word for that or sign up for Food 202.)
Lois Nicholson, no-nonsense experimentalist that she is, checked all this out on her kitchen counter. "Since I had no lasagna," she writes, "ketchup was substituted as the [electrical conductor]. I put a tablespoon or so of ketchup in each of three bowls--stainless steel, plastic and ceramic. Then I laid a strip of aluminum foil across each blob of ketchup, making sure the foil also made good contact with the bowl. Sure enough, after two days the foil in the stainless-steel bowl was eaten away wherever it touched the ketchup, while the foil in the other two bowls appeared unchanged."
Do I have the greatest readers, or what?
Tomato sauce wisdom
There are a few practical morals to this story.
First of all, your leftover sauce (it doesn't have to be tomato sauce; it can be any acidic sauce such as one that contains a wine reduction) can be kept in any kind of container you wish and covered with anything you wish. But if it's in a stainless-steel or copper bowl covered with aluminum foil, just make sure the foil isn't in contact with the sauce.
Second, don't hesitate to use those aluminum lasagna pans sold in supermarkets and housewares departments. They're cheap, disposable and work just fine. Even if you cover them with aluminum foil, it's just aluminum against aluminum; no two different metals, so no electrolytic corrosion.
Third, I know you've been waiting for me to tell you what happens to any aluminum that might dissolve out of the foil if you use a stainless-steel bowl. Does it go into the sauce? Yes. Will it affect the flavor? Not noticeably. Is it still safe to eat the sauce? From everything I've read, yes. There is no scientific evidence for the widely held suspicion that eating aluminum causes Alzheimer's disease.
Now for extra credit, as we professors like to say, what would happen if you covered your aluminum pan of leftover lasagna with stainless-steel foil and some of it touched the sauce?
This will be on the final.
Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking questions to email@example.com.