For years I used my mom’s way of cooking pasta. Putting a tablespoon or so of oil in the pot of water prevents the foamy boil-over. Doing it tonight with spaghetti, I realized I don’t know why this is.
The oil isn’t enough to cover the surface of the water, so it can’t (can it?) be a simple matter of a “thicker skin” on the water.
Anyone know why this little trick works so well?
I was taught that adding the oil prevents the pasta from clumping and sticking together.
And, yes if the heat is up too high it will boil over. I speak from experience.
Oh, it can spread out far enough. I have a book (not with me, though. Dammit) entitled “How to Dunk A Doughnut”, which addresses the oil on water thing. At least, I think it’s this book. Apparently, some old person put some oil on the water, and it changed how it looked for a very large area. Of course, I have no way to cite this, but I’m sure someone smart can use this as a jumping-off point.
Oil on water spreads out to nearly a one molecule thickness if there’s enough surface area. One tablespoon is more than plenty to cover any pot.
Not according to Alton Brown. 
At the very least, he didn’t find enough oil actually sticking to pasta in a controlled experiment to make that probable.
Try cooking pasta with no oil. Provided you give it the occaisional stir, etc, I bet you will see no difference, I stopped using oil ages ago. You can skip the salt, too. Just use a decent quantity of water.
There is absolutely no need to add oil to your cooking pasta. Ask any professional italian cook and they’ll tell you the same.
This reminds me of the the whole checking if the pasta is done by throwing it and seeing if it sticks nonsense too.
Some pressure cooker recipes recommend adding oil to rice and legume recipes to prevent foaming (which can clog the valve stem, which makes the cooker to 'splode.)
I haven’t tried it, but maybe it’s the same principle?
WAG: Foam comes from starch, and the oil gets in between the starch molecules?
As a methodology for testing pasta I would have to agree with you. As to whether it works or not, I am not so sure. Intuitively, it would seem to have some sense behind it.
Trying to get back to the original post, I always thought that a little bit of oil added to the water can fight boil-over… though as duckster pointed out, the effect is not strong enough to be said to PREVENT it.
The key, I believe, is in the word ‘foamy’. If the level of the water isn’t extremely close to the lip of the pan, then boilover is dependent on building up a foam of water bubbles from the process of boiling. Oil does something to the surface tension of water that makes up these bubbles, popping them and letting the boiled vapors within out. This starts to build holes in the foamy layer, and any foam that rises very high will fall over into the holes and get attacked by the oil.
Does that make any sense?? It’s not scientifically rigorous I know. (I can’t remember if “the cookbook decoder” comments on this issue – I remember that that book is a great one for anyone who’s curious about the scientific explanation of what’s going on during cooking various foods.)
That’s essentially correct, chrisk. Foam are bubbles of air held together by water. If you reduce the surface tension, bubbles are less likely to form, and there are fewer of them. You can prove this easily by pouring a little oil on a pot of pasta that is bubbling over; the bubbles will dissipate quickly.
I agree with you on the oil, but you shoul alwasy add salt to pasta water. it does two things:
Salt will raise the boiling point of the water. It will boil hotter, and thus cook the posata faster.
It’s the only chance you have to season the pasta. Pasta is, by it’s nature, bland. Adding salt to the water will cause some of the salt to go into the pasta, making it taste better.
Oil decreases the surface tension of the water. Water has a high surface tension to begin with, which is why it foams so easily. Foam is essentially just a lot of long lived bubbles.
Want a neat experiment? Next time you get a really foamy beer from a keg, dip your clean finger in the beer. Nothing much happens. Now wipe your finger along the crease where the side of your nose meets your face. Dip the finger in your beer again - the foam will vanish quickly. Your skin oils lowered the surface tension of the beer, which destroyed the bubbles. Now you have room to put more beer in the cup.
Ah, college.
While salt water does boil at a higher temperature than fresh water, if you do the math, the amounts used in cooking are completley negligable. It would take something like a pound of salt in a quart of water to raise the boiling temp one degree, or some similarly ridiculous amount. I forget where I saw it, but I have seen the equations worked out.
OK, so I’m zero for one. I was also taught that you can check the doneness of pasta by periodically fishing out a piece and breaking it in half. Once the inner whiteness is gone and it is all a uniform color, the pasta is done.
Thanks, chrisk. That’s what I was looking for. I meant the “foam-over”. While cooking last night I left the pasta boiling at the highest heat and was able to leave it unattended for 5 mins. Without oil, I have to almost constantly stir the pot to keep it from foaming over.
So are we nearing agreement that it’s the oil weakening the water tension in the foam?
Thanks again for the help everyone.
Well this is not throwing it at the fridge and seeing if it sticks, which is what I have heard and seen people do.
Any chef will tell you that the best way to find out if your pasta is done is… (Drum Roll)… to taste it!
That’s right. Taste one of the little suckers (after blowing on it a bit, of course), and when the pasta is al dente (has a little bite to it), it’s done! Unless you’re going to finish it off on a skillet with sauce and other yummy things (some dishes call for this) in which case you shoudl fish it out when it’s a little less done.