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I have noticed some foods, such as quick-prepare ravioli, specifically state not to cover the pot when warming them. Why should the pot not be covered?

I am especially interested in knowing when I can break this rule. Specifically, we heat food on an underpowered electric stove. Covering the pot of water seems likely to raise the temperature of the water, which the directions for the ravioli state should be boiling. Should I leave the pot uncovered as the directions state, or should I cover the pot to raise the temperature of the water that extra little bit closer towards boiling?

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    I am amused to notice that this is the first SE question that I've ever tagged pot. – dotancohen Oct 19 '14 at 10:37
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    If your stove is underpowered, you may wish to look at techniques which use less water, such as this well researched one which I've used to great effect. Water has a very high specific heat and takes a lot of energy to bring to a boil. – Schwern Oct 19 '14 at 18:38
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    Most things need a light or vigorous simmer for X mins rather than boiled. I find anyway. It's fiddly getting a normal stove to simmer liquid in a pan with a lid on without getting foam everywhere, or the food ruined. Rice recipes and packets always state "Leave lid on", but I never cook rice with the lid on, and get perfect rice. These things are only guidelines, and often their guidelines are as useless as the "serving suggestion" on the front. They're also often more aimed at reducing potential food poisoning, than perfectly cooked food. – James Oct 19 '14 at 19:15
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    @Schwern: Thank you, that was a great read. Interesting enough to get me interested in cooking, in fact. I think that I've got some experimenting to do. Do you realize what you've done? – dotancohen Oct 19 '14 at 19:15
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    Stick around. Total Food Nerds R Us. – Jolenealaska Oct 19 '14 at 19:51
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By quick-prepare ravioli, you mean without sauce, right?

It's fine to (and you should) cover the pot to bring the water to a boil. Incidentally, it's best to start with cold water from the tap, hot tap water will likely be more contaminated, possibly with lead. Once the water is boiling, add the salt, then the pasta. Adding the pasta will bring down the temperature of the water, so you can briefly put the lid back on to more quickly bring the water back up to the boil.

Once the water is boiling again, it's recommended that you keep it uncovered because pasta water is very prone to foaming up and boiling over. It's a royal pain when that happens, so think before covering pasta while it is boiling. If your pot is big enough and you watch carefully, you can save some energy by cooking with the lid on, just be careful because boil-overs happen quickly. Obviously, it's good to have a stockpot with a glass lid for that.

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EDIT: Now that I've covered the conventional wisdom, I'd also encourage you to read this: A to Pasta: is simmering equivalent to roiling boil?, especially the Serious Eats link. I admit, I haven't done it (set in my ways, I suppose), but considering your set-up, you might find the method works better for you.

  • Thank you, you anticipate my tangent questions as well! In fact the ravioli was without sauce. – dotancohen Oct 19 '14 at 11:14
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    Yes, avoid the hot tap. But it's more efficient to boil the cold water in an electric kettle than on the hob. Actually, what I do is put a little water in the pot and put it on the heat, then boil the rest in the kettle. That means that I'm not pouring boiling water into a cold pan, cooling it down again. You quickly get used to how much water to put in the pot so it comes to the boil at about the same time as the kettle. – David Richerby Oct 19 '14 at 20:30
  • Why is hot tap water contaminated? – Petah Oct 23 '14 at 21:00
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    @Petah Because hot water more easily dissolves nasties from inside the pipes, and hot water tanks get nasty over time. The only things you should use hot water from the tap for are cleaning and hygiene tasks like hand washing. – Jolenealaska Oct 24 '14 at 0:45
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While not relevant to ravioli, many other foods will have a different outcome simply because of the amount of water remaining in the food - removing the lid will result in higher evaporation but keeping the lid will keep the moisture inside, "steaming" the food from above.

Depending on the recipe it may easily be that one or the other is the desired result, and the opposite way produces unwanted results.

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    This case is more likely when they call for adding a specific amount of liquid, and either intend for it all to absorb, or to make the final sauce. If they don't specify the amount of water, and call for draining, it's more likely some other reason. (in this case, I think Jolene is right with the issue of foaming / boiling over). – Joe Oct 19 '14 at 16:21
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They probably recommend cooking uncovered because pasta water easily boils over: particles of starch come off the pasta and form a foam. However, this foam can be very much reduced by adding a small amount of oil to the water – say about a teaspoon. As long as you watch it carefully, you can then put the lid back on (maybe crack it open slightly) and turn the heat down to the minimum level that maintains a boil. But do watch carefully as it might still boil over.

Many people add a little oil to their pasta water, usually claiming that it stops the pasta sticking together. Indeed, I've seen a Michelin-starred chef make exactly that claim on YouTube. However, oil hates water so it doesn't stick to or get absorbed by the wet pasta while it's cooking, which means it can't reduce sticking. It's actually there to reduce foaming.

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