When you need to introduce a cold substance — flour or eggs or milk or cream — to a very hot liquid, e.g. soup or sauce base, you need to use a technique called tempering.
There’s lots of little kitchen tricks when making good sauces and soups, and tempering is a foundation skill that if you don’t use it… well, let’s just say floating clumps of greasy flour in your sauce are never appealing
You can find thousands of how-to’s on this most basic of kitchen tricks, including videos galore, so I won’t belabour the point here. Just a quick once over of the technique.
You need to bring your cold ingredient up to the temperature of your hot liquid, which if not done, will end up “cooking”, e.g. lumping, splitting, curdling or seizing the cold ingredient before it can be combined in to the base.
How do you do that?
Easily enough. It is a basic kitchen technique after all.
Bring a small amount of your hot liquid to your cold ingredient, and I do mean a small amount, and mix it until all of the liquid is combined.
And repeat again.
And keep repeating until the cold ingredient’s temperature reaches about two-thirds of the temperature of the target liquid.
You can combine ever larger amounts of the target liquid with your cold ingredient, usually adding half as much again with each addition; so start at a half-tablespoon of very hot soup to 4 tablespoons of flour for thickening.
Thoroughly combine the small amount of liquid with the flour.
Then add 3/4 tbsp of hot liquid.
Then add 1 1/2 tbsp of hot liquid to the flour slurry.
Then add 3 tbsp of hot liquid to the flour slurry. By then you should be ready to combine the now much warmer ingredient in to the soup, sauce or other dish.
There’s just one caveat, tempering chocolate is not the same as tempering a cold ingredient before adding to a much hotter base sauce, soup or custard.