While I couldn't find an exact answer as to "why (or how) sauces thicken upon standing", I did find a couple tips for thickening sauces:
- Most sauces benefit from some simmering before you thicken them. Simmering them slowly allows them to reduce, which means excess moisture will evaporate and what's left will be more concentrated in flavor. Sometimes this is enough to thicken a sauce on its own, particularly with cream sauces or marinara and other sauces made from pureed vegetables.
- Reducing sauces can be tricky, though, because the amount of salt and other seasonings will also concentrate, so make sure to purposefully underseason everything and correct it only at the very end.
- Butter is the best thickener to use on very powerfully flavored liquids such as wine and concentrated pan juices. Butter softens the powerful flavors and smooths out the texture. Whisk in partially softened cubes of butter over very low heat, tasting as you go, and serve these sauces in tiny amounts. Don't heat them, or they'll separate.
*eSource: eHow
*Photo credit: thebittenword.com - Flickr
