Pasta cooking water
This has been bothering me for some time. Many recipes advise reserving pasta cooking water to loosen up a sauce. Why is pasta cooking water superior to plain old water? Does the pasta starch leach into the cooking water & have some positive effect? It can't be a question of flavor -- pasta doesn't have much flavor.
Yes. The starch in the pasta water can help bind the sauce. It is also helpful to have hot water to loosen the sauce if it is a little too thick.
Quite right. Another thing to note is that apparently a lot of recipes are adapted from restaurants kitchens - and the pasta water you find there is quite different then what you have accessible in your kitchen. Simply because they have boiled dozens, if not a hundred, plates of pasta in the same pot, hence more starch. That said I do think that using the water does help bind the sauce and the heat is probably a factor.
The original comment has been removed
Pasta cooking water has starch and salt. It helps to emulsify the sauce and glaze the pasta properly if you finish cooking the pasta in the sauce and add the water and reduce.