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I've noticed that many Italian chefs add to their pasta sauces some of the water they used to cook their pasta. What is the purpose of this?

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Is that text from a recipe you have found? Your question structure is confusing. –  Katey HW Aug 19 '11 at 13:55
    
Looks as if the question is in the title, and the answer is in the body. –  takrl Aug 19 '11 at 13:56
    
Hi David - we encourage members to answer their own questions if they can, but please note that there are separate spaces for questions and answers. I'm removing the question text - feel free to add it again as an answer. You can grab the original text from the revision history. –  Aaronut Aug 19 '11 at 14:45

10 Answers 10

Adding the pasta water does thicken the sauce & help with acidity. I make a pasta dish my family loves & once in a while it doesn't turn out right. I finally tracked down the reason for this. Every time I forget to add the pasta water, it comes out wet. With it, it's smooth, creamy & all the pasta has soaked up the sauce.

I cook my sauce down & then add a small amount of the water (about 1/3 cup) then cook it a little longer to let it thicken back up. It doesn't take but a minute or two to get it back where it was. I also make my pasta with a little less water. The past water just makes the sauce creamy and luscious.

Give it a try both ways. I'll bet you'll see & taste the difference.

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Everyone seems off base on this. The simple reason to add water is to cut down the acidy taste. My mother (Sicilian) always used 2 cans of puree and 2 cans of paste. She added 2 - 3 cans of water and the sauce never tasted acidy at all.

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Lots of sauces made without pasta water don't taste acidic either, so there must be other reasons people use it. And diluting sauce to reduce acidity is going to dilute other flavors too. (And this is specifically pasta water - your answer makes it sound like she was just adding plain water, to compensate for using so much paste.) –  Jefromi Jan 12 at 23:56

Pasta water does thicken the sauce... you don't pour in huge amounts, just add bits here or there to increase flavour and to thicken with starch. The main benefit of this is that the sauce will bind with the pasta. If you pour in a huge amount, it will drown your sauce.

Source: I learned to cook pasta sauces in a Roman kitchen.

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This is just wrong. The starch in pasta water is already cooked and gelatinized. It can't get any thicker than it already is by cooking it more. It will only thicken the sauce if your sauce is already essentially a water consistency. –  Aaronut Jul 25 at 16:03

Many people build it into to recipes as they feel it changes the mouth feel of a sauce. Some chefs refer to it as adding silkiness, or a creamy finish. This is subjective of course, but i think it does add something to the body of a sauce.

On top of this it's already seasoned and hot so is good to "loosen" a sauce that has been simmering for a while (as other users have stated). I certainly don't think it thickens a sauce (the starch content isn't that high), but it tastes thicker... somehow.

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The starch in the cooking water acts as an emulsifier, so for a dish like spaghetti, aglio e olio it will make a more luscious sauce.

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Harold McGee did a great piece on the amount of water used to cook pasta that discusses this topic.

http://www.nytimes.com/2009/02/25/dining/25curi.html

Basically, if you're going to do it at home, use less water, and you'll have a more concentrated, flavorful liquid, which you can use like stock in a sauce (not saying it is stock, but it has a lot of flavor). It's obviously starchy and salty, but it will have a good amount of wheaty flavor.

I find the water from whole wheat pasta also has a lot more flavor than white pasta.

Probably the most famous dish that I'm aware of that makes use of this technique is Cacio e Pepe, a Roman pasta dish where the sauce is made from olive oil, pasta water, pecorino romano and/or parmigiano reggiano, and of course a good amount of black pepper. Sometimes butter is used as well. It's a pretty cheap meal, but very comforting. Sometimes I make a vegan version with olive oil, earth balance, fresh garlic and some nutritional yeast (fiancee is allergic to dairy).

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In a restaurant you will cook a lot of pasta in the same pot of water over the course of the night. This water will end up having quite a bit of starch. Using a bit of this starch water will help the sauce adhere to the pasta.

At home the resulting water isn't going to contain as much starch, but it will still help. You wont see quite the same effect as a professional kitchen.

I'm not sure which professional chef said it, but he said that it would be great if they bottled this starchy pasta water for home use.

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I think it was Bill Buford in "Heat" –  Chris Cudmore Aug 25 '11 at 18:30
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You can get similar results with a lower water to pasta ratio (less water). –  paul Aug 26 '11 at 4:32

I believe the primary reason is that the pasta water is already hot. When you need to thin your sauce on short notice, you add hot pasta water and it will not cool down your sauce. Secondary benefits are:

  1. The pasta water has nice salinity, so you're not diluting the salinity level of the sauce. This assumes you salted your pasta water. You did, right?
  2. There is some starch in the pasta water. However, since the pasta water has already achieved a high temperature, any thickening benefit would already be achieved. If your pasta water is anything like any I've ever seen, it's just about as runny as any other water. So this benefit is probably negligible. Perhaps it has more of an effect once the pasta water added to the sauce has reduced.
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Pasta water from restaurants can become very 'muddy' according to McGee. Look at my answer here. –  BaffledCook Aug 25 '11 at 18:41
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-1 You're missing the main reason. If you cook your pasta in a reasonable amount of water (i.e. enough that there's not a lot of extra by the end) there's so much starch that it'll be really thick once it comes to room temperature, and it helps the sauce stick to the pasta. –  Jefromi Feb 10 '13 at 18:13

I thought it gives either the pasta or whatever you add it to flavor. Once you cook the pasta in it, it has the starch and the salt that might enhance the dish more when you let it steam and absorb into the food.

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Adding water will thin a sauce, but the starch in the water does help it cling to the pasta, and adds some body to the sauce.

Another key step is to finish cooking the pasta IN the sauce (in a skillet, usually) before serving, allowing the starchy pasta to absorb the sauce more completely.

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I don't believe this. This is primarily water, and won't thicken the sauce. I believe it's to THIN the sauce, and the water just happens to be handy. No down vote, but I think you've repeated a myth. –  Chris Cudmore Aug 19 '11 at 18:28
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@chris, why don't you try it? I'd been having a bit of trouble with this, but I realized it was because I was boiling my pasta in too much water. When you boil the pasta in a smaller amount of water, you find that the water will have a much higher concentration of starch. This starch can be really critical to a cheese-based sauce coming together properly. With a really watery water (for lack of a better term) you just end up with clumps of cheese and (separately) water. –  Ray Oct 8 '11 at 12:17

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