Nancy asks via Twitter:
@thefoodgeek you have prob already covered it, but making a white sauce w/o lumps? we started warming up the milk prior to adding it, helps- nancy mulrine (@ancym) June 29, 2012
When you are creating the kind of sauce that becomes lumpy, you are putting a thickening powder into a liquid. With the most common powders that home cooks use for thickening, you need to bring them up to a boiling temperature before they exert all of their thickening force, though they can still bind weekly together at lower temperatures. We're talking about starches, the two most common of which are cornstarch and flour. Mostly we think about floured sauces becoming lumpy, but it happens with cornstarch as well.
The way starches thicken a sauce is by soaking up water and using the heat to loosen their molecular structure. A starch is a tightly-packed string of sugars in a long, long chain. With the energy from the heat and the extra water, those long chains go from being curled up in a tiny ball to spread throughout whatever liquid they are currently boiling in. This causes a molecular net to form, which traps all of the water in the pockets. If all goes well, everything is evenly distributed and everyone is happy.
Unfortunately, it's easy for things to go terribly wrong, because starches don't take long to spring into action. So if you take your boiling sauce liquid and just dump your starch into to, the outside of that starch is going to expand immediately, and all around the starch it will be extra thick, keeping the powder inside from ever touching the liquid. If you break the ball apart, the process repeats itself over and over again. It is disheartening.
The answer is to cause the starches to separate from each other before they go into the boiling water. There are four common methods to accomplish this:
- Sifting. You put the starch in a sifter and release it into the boiling liquid while you stir. Depending on your sifting device, this may require 3 hands.
- Slurry. Put your starch into a cold liquid and shake or stir until everything is evenly distributed. This works best with pure starches such as corn starch.
- Buerre manié. Knead your starch into butter first, then add to your liquid. This is not all that common, because with a pure starch you might as well use the slurrie, and with flour, you'll want to use option 4.
- Roux. The most popular method with flour Melt your butter, wait for the water in the butter to boil off, add the flour. (Bonus points if you can tell me why we boil off the water first.) Cook for about five minutes until the flour flavor is gone, or more for specialized applications (especially cajun applications). A regular, or blonde, roux is lightly toasted, and they go all the way to deep, dark colors.
Once you have your starch molecules properly separated, you can add them into your boiling liquid without fear of clumping.
As for the pure starches vs. flour thing that I mentioned above, the thing with a pure starch and its long, long molecule is that it is far too complicated to be tasted by our tongues. So it doesn't really matter how we get it into the sauce, as long as it goes in separated bits. With flour, though, there are a lot of things in that powder other than just untasteable starch. Raw flour tastes like uncooked sugar cookie dough without the butter, sugar, and salt. Is that not a good enough description? Well, pop into the pantry and taste a little flour. That flavor will pervade your sauce, and your sauce will suffer for it.
In order to combat that problem, we cook the flour, and the easiest and best way to do that is to cook it in butter. Well, I lie. Despite, or because of, its French origins, making a roux out of butter is one of the hardest way to make a roux, precisely because butter has water mixed in with the fat. Save yourself some trouble and use anything else, as long as the anything else will taste good with your sauce. Canola oil, olive oil, bacon grease, duck fat, schmaltz, suet, whatever. Mix the flour in, cook for 5 minutes or until things smell a little toasty, and add to your sauce.