Many recipes, especially sauce recipes, for will call for a liquid to be reduced by such and such an amount—one half, say, or three-quarters.
If you're lucky, the recipe might explain what this means, or you might be able to figure it out from the context. But let's take the guesswork out of it altogether and define exactly what reductions are, how they're used in cooking, and how to make them.
What Is a Reduction?
In cooking, to reduce a liquid means to simmer it until some of the water in it has evaporated, which intensifies the flavors, thickens the liquid and causes it to take up less volume. The concentrated liquid you end up with is called a reduction.
Reducing is thus a way of concentrating flavors, adding new flavors, and manipulating the texture and consistency of the liquid.
In the case of stocks that are made from bones, reducing also causes the gelatin in the stock to concentrate, giving the liquid a much more jellylike consistency.
The thing to remember about reductions is that the only thing that is reduced is the amount of water. Everything else—the flavorings, seasonings, all the solids—stay in their original amounts, but now they're concentrated. That's why stock recipes generally don't call for added salt. When that stock is reduced, any salt in it becomes concentrated, and this can cause the finished sauce made from it to be too salty.
For that reason, seasoning with salt is customarily one of the last steps in making a sauce. This also goes for dishes such as soups and stews that are likewise made from stock.
Making and Using Reductions
Any liquid that is simmered will reduce to some extent, as the heat evaporates some of the water in it, especially if the pan is uncovered. Most sauces are simmered for at least a brief time as a final step before seasoning, to fine-tune the consistency. But a reduction is something else. The goal of a reduction is to drastically reduce the amount of water in a liquid.
In other words, a reduction is made by reducing. But reducing is not always done for the purpose of making a reduction.
A typical reduction will call for reducing by half or three quarters. And in addition to reducing stocks and sauces, sometimes individual ingredients are themselves reduced before adding them to a sauce. Examples of this are wine or vinegar. Indeed, the procedure for making beurre blanc, a rich, butter-based emulsified sauce, involves reducing a given amount of vinegar until it is almost dry, which is referred to as au sec.
The procedure for making bordelaise sauce for example, starts with simmering red wine along with various aromatics, herbs and spices, until the wine has reduced by three-fourths, and then adding the reduction to a prepared demiglaze.
Reductions are also used in making dessert sauces. One of the simplest reductions is balsamic glaze, which is made by reducing balsamic vinegar until it is thick and syrupy and then drizzling it across fruits, vegetables or desserts.
Demiglaze: The Ultimate Reduction
Demiglaze, sometimes called demi-glace, is the basis for innumerable classical sauces, and is itself the product of several reductions. Demiglaze is comprised of half brown stock and half brown sauce, with the resulting mixture then reduced by half. Thus a quart of brown stock and a quart of brown sauce would be combined and then reduced to yield one quart of demiglaze.
But the brown sauce, also known as Espagnole sauce, is itself made from brown stock and tomato puree which has been thickened with roux and then reduced by at least one-third. Thus by the time the demiglaze is finished, the brown stock in the Espagnole sauce will have been reduced by at least two-thirds, while the other half of the brown stock has been reduced by half.
Many sauce recipes based on demiglaze are themselves further reduced through simmering, meaning the flavors of the the constituent demiglaze are even more intensified.
What are Glazes?
A special kind of reduction is produced by reducing stocks by three-fourths or more, until they are thick enough to coat the back of a spoon. These highly concentrated reductions are called glazes, and they are used in flavoring sauces as well as various meat and poultry dishes.
It's also possible to reconstitute a glaze by adding water, to produce a liquid with the same strength as the original stock. The flavors won't be quite the same, but it will work pretty well. Glazes will keep in the fridge for a few weeks, and will last for several months in the freezer.
The Best Pans for Reducing
When selecting a pan in which to make your reduction, you're always better off using a taller, narrower pan than a wide, shallow one. For one thing, a wide, shallow pan makes it more difficult to judge how reduced a liquid is. If the original liquid is only half an inch deep to start with, how can you tell when it's reduced by half? Or three quarters? A taller, narrower saucepan makes it easier to judge.
Also, reducing a sauce or stock in a wide, shallow shallow pan increases the likelihood of it burning if you don't give it your undivided attention.
Finally, when you go to pour your reduced liquid into another container or pan, a small saucepan is much easier to handle than a wide pan you might need to lift with two hands. Many small saucepans will feature pouring spouts for just this purpose.