How do you mix corn starch slurry and how do you use it?
When making the slurry, stir cornstarch into cold water until it has the consistency of cream. This can be set aside until it's needed, but be sure to stir it briefly before you pour it into the sauce to redistribute the starch granules in the water. You should pour it into your sauce toward the end of its preparation.
According to McGee you should use roughly 2/3 as much starch as you would flour. If you aren't using a recipe, add your slurry a bit at time until it's a thick as you like it. Also remember that the sauce will thicken a bit as it cools, so it should be a little bit thinner on the stove than you intend to serve it.
You can add cornstarch to any cold liquid, like orange juice or milk. When it's properly mixed, you can add it to the warm (hot) liquid you want to thicken.
I usually start small, with maybe a teaspoon of starch and a tablespoon of water. I guess that's roughly 2/1 water to starch by volume.
You do need more starch slurry for more liquid, but I advocate for going cautiously and starting with a single "dose" and then seeing if it's enough.
When making a slurry, I find it easier to avoid lumps by adding the liquid to the starch a little at a time. Then to use it, add a little slurry at a time to your sauce and bring it up to a bare simmer. Then add more slurry as necessary to reach the desired thickness.
Another idea if you don't care about the added fat is to make a roux with the cornstarch. Since the roux is cooked you avoid the raw starch taste. For roux, I never had any lump problems adding the starch directly to melted butter.
Corn starch (UK=cornflour) granules 'explode' like popcorn when heated, but on a miniature scale - this is what makes it thicken stuff (Starch gelatinization). This happens just below water boiling point (as PoloHoleSet has posted).
If you add conrnstarch slurry to liquid that is too hot, the outside grains 'pop' and form a thick sticky layer around the rest, which is one reason why you get lumps. Let the liquid cool a little before you add the cornstarch, stir it constantly, reheat it gently, don't boil it hard.
Save back a small (6-8 oz) jam or relish jar. Add your water and corn starch, tighten the matching lid and shake vigorously, voila! A perfect lump free slurry ready to pour or drizzle into your sauce.
most starches will separate from water if you let them sit. this is why a roux is used; it changes the gravity of the flour and prevents it from sinking to the bottom of the liquid where it burns
