Reducing

Reducing in cooking is a method of thickening food by heating it so that some water evaporates away, leaving the body of the liquid thicker.

Reducing a liquid also concentrates the flavour.

“Reduction intensifies flavor. The method involves increasing the concentration of particles in a fluid by boiling off some of the liquid until the desired thickness is reached. This is thickening by increasing the packing density of the molecules or particles in the sauce.” ^[1]Brenner, Michael; Sörensen, Pia; Weitz, David. Science and Cooking: Physics Meets Food, From Homemade to Haute Cuisine. New York: W. W. Norton & Company. 2020. Kindle Edition. Page 175.

Scientifically, what happens is that you are reducing the space between molecules in the liquid:

“Now why does it work? It only works because there were already enough molecules in there to cause the thickening. They were just too far apart. There was too much water. So we increased their concentration by driving water off, through evaporation.” ^[2]Nathan Myhrvold. In: Science & Cooking: From Haute Cuisine to Soft Matter Science (physics). HarvardX SPU27.2x Module 2 –  Viscosity. Accessed November 2022.

Note that this implies that reduction will only work with liquids that had sufficient non-water, non-volatile molecules and particles in them to start with. You can’t reduce pure water.

Reduction caveats

Unless otherwise specified, reducing is usually done with a quick boil. This is usually only expedient, as this kind of futzing usually has to go on at the stove while everything else is just about ready and just waiting on the sauce that you are reducing. And, unless otherwise specified, you are usually aiming to reduce the volume of the liquid by half.

Be careful not to walk away though and forget about it while it is boiling like crazy, or you will reduce it to charred gunge at the bottom of the pan.

Sometimes, though, reducing can take a long time, as in reducing wine for a wine sauce, or crushed tomatoes for a pasta sauce. For this reason, with respect to time at least, and perhaps also cooking fuel, reducing can be one of the least efficient methods of thickening a food item.