When making sauces and gravies, one of the standards for measuring the thickness of the liquid is whether it “coats the back of a spoon.” The same standard is used when making the custard base for ice cream.
What exactly does it mean? When does the liquid coat the back of the spoon? Surely, any liquid will coat a spoon if the spoon is dipped in it.
A liquid coats the back of a spoon when, after dipping the spoon it in, you run your finger along the back of the spoon and the indentation remains. That indentation will not stay if the liquid is too thin. It only stays if the liquid is thick enough.
It’s really not every complicated, is it? But, as with most things, a visual guide is helpful.
This technique for measuring the thickness of a liquid is useful when the liquid is thickened by reduction, meaning, the liquid is simmered to allow evaporation. What if the sauce or custard is thickened with flour or starch? In such cases, the thickness is determined by the amount of flour or starch that goes into the liquid rather than the amount of reduction that the liquid has undergone. You can still measure the thickness by checking if the liquid coats the back of a spoon. If the liquid is still too thin, you have two choices — let it simmer some more or add more flour or starch.
Once you’ve mastered this very easy technique, you’ll have sauces and custards that are just the right thickness every time.