Searing doesn’t retain water—it eliminates it.
Exposing the surface of meat to high heat through searing is a key step to achieving a mouth-watering steak. Typically, the steak is placed in a very hot pan and left until the surface turns brown and forms a crust. You may have heard an old adage that the purpose of searing meat is to “seal in the juices”, and this brown coat of armor appears to be the “seal” that satisfies the adage. When you look closer, though, this idea doesn’t hold water.
Here’s a closer look.
Water can easily pass through proteins whether they're folded, as in the raw steak on the left, or cooked and relaxed, as in the seared steak on the right.
You can see that individual proteins are much larger than individual water molecules, often over ten times larger. The connections between the molecules in muscle tissue have gaps big enough for water to pass through. In fact, when searing a steak, you know that water passes through the tissue because you can hear it. Cooks are taught to listen for a loud sizzle when the steak hits the pan, indicating that the pan is hot enough. This sound is water on the surface of the steak rapidly boiling into steam. As the surface of the meat cooks, proteins in the meat tighten up and squeeze out more water, which continues the sizzling. If there were an impermeable water barrier on the outside of the steak, the sound would stop.
So, if searing meat actually makes meat release water, why would anybody do it? The brown crust may not help retain moisture, but it does add incredible flavor. The brown compounds are products of a set of chemical reactions known as Maillard reactions, which occur between amino acids and sugars. These reactions happen at temperatures above 140°C (280°F) and require a high-heat method of cooking, such as roasting or frying. Liquid water can only reach a temperature of 100°C (212°F)—it doesn’t even get close to hot enough to cause Maillard reactions. That’s why meat doesn’t brown when it’s boiled and why you might sear meat prior to stewing it.
Maillard reactions don’t just occur in meat. Amino acids are the building blocks of proteins, so as long as proteins, sugars, and high-enough temperatures are present, tasty brown food can result. Maillard reactions are responsible for the brown crust on bread and dark roast on coffee. No Maillard reaction creates a barrier to water, but all Maillard reactions create compounds that we perceive as delicious.
Why is the idea of searing to seal in juices so pervasive? It’s appeared in many publications dating back to 1850, including some from Justus von Liebig, a pioneering organic chemist, as well as Auguste Escoffier, possibly the most Western famous chef and cookbook author at the turn of the 20th century. You can read more about how this idea spread through history in Harold McGee’s excellent book On Food and Cooking. Want to convince yourself that two experts at the top of their fields in the lab and the kitchen were wrong? Simply cook two steaks to your favorite temperature, searing one and not the other. Weigh them both before and after cooking to see which one lost more moisture during the cooking process. The seared one likely weighs less after cooking because it’s lost more water, but don’t worry. You’ll make up for it by salivating over the tasty compounds the high heat produced.