What temperature should milk be when adding to roux? Possible Duplicate:
  Right way to defrost roux  




I've always heard that it's best to add roux to liquids that are at or near a boil, however I ran across this video recently where the chef says specifically "add cold milk to the roux. Cold milk, hot roux, no lumps."

Is there a standard rule for adding roux to sauces? Do things change if you're using milk vs. stock or other liquids? And can anyone explain the chemistry as to why cold or hot liquid would be better?
I've heard a few different answers (theories) as to why you should use different temperature liquids to the roux, most of it's related to starch gelatinization.  I'm also not a fan of scalding milk when I don't have to, as it can bubble over if you don't pay attention and/or taken on a bit of a scorched taste.

I've always added cold milk when making a bechamel in part because that's the way I learned growing up, but if I'm thickening a gravy I'm generally using warmer liquid (drippings, let to sit so you can separate out the fat).  I rarely use a roux to thicken something near a boil ... for that, I'll use a slurry.

There are differences when dealing with adding cold liquid to a roux -- you need to add the liquid slowly, and stir it well before each addition, which also gives it a chance to come back up to temperature.  Of course, I recommend always adding the liquid slowly, as I find it easier than trying to work out lumps afterwards.