It’s a line that comes up in recipes frequently: Reduce sauce by half, reduce liquid by 1/3, etc. But what does that mean, and how do you do it?
In the kitchen, the term “reduction” refers to a technique that delivers intensely flavored, thickened liquid simply by boiling. Whether it is a soup or a sauce, by bringing the liquid to a rapid boil, it turns into steam and escapes from the pan, in turn reducing its original volume.
The key to this technique is never to cover your cooking vessel, so put away your pot lids. If you cover your pan, the steam will be trapped inside and the reduction will take a very long time.
How do you tell if you are reducing correctly? Well if your instructions dictate to reduce by half or 1/3, a tip is to take note of where your liquid currently lines the inside of your pot. Once the boiling begins, the liquid will go down (that’s the reduction part), usually leaving a line of residue that circles the interior of your pot (see image of reduced tomato sauce). This is a good marker for you to tell if you are at your goal or if you should continue boiling.
A reduction sauce can be made of just one ingredient. For example you can get a balsamic or a red wine reduction by reducing a cup to a half cup. The goal is to thicken the liquid to a sauce-like consistency. You’ll see it referred to as “nape” in some French recipes. Basically, you want the liquid to cling to the back of your spoon.
Be careful not to over-reduce your liquid. If you are starting with a large amount of soup and know it will take half an hour for it to reduce, feel free to leave this unattended and move on to other kitchen tasks. However, if you are simply reducing a bit of Grand Marnier for a dessert sauce, stay by your pan. If you over-reduce, you’ll know it. The only thing left in the pan will be a sticky burnt coating, and you’ll have to start over.