Put a wet steak in a hot pan and you get a grey, steaming slab. Pat the same steak dry and you get a dark, savoury crust. The pan is just as hot in both cases. The difference is a thin film of water, and a rule of physics that water refuses to break.
The brown crust is not burnt meat. It comes from the Maillard reaction, first described by the French chemist Louis Camille Maillard in 1912. Sugars and amino acids on the surface react with each other and break down into a cascade of new molecules. Some are brown pigments. Many are the aromas we read as roasted, toasty or meaty. John Hodge worked out the main steps of the mechanism in 1953.
The reaction is slow when it is cold and fast when it is hot. It runs quickly from about 140 to 165 °C. Below the boiling point of water it crawls, so slowly that it takes years in a chilly cellar and minutes only well above 100 °C.
Here is the catch. Wet food cannot get hotter than the boiling point of water, 100 °C at sea level, for as long as liquid water is left on the surface. All the extra heat from the pan goes into turning water into steam instead of raising the temperature. That is not a small effect: boiling off one gram of water takes about 2,260 joules, which is more than five times the energy needed to heat that gram from freezing to boiling.
So a wet surface sits at about 100 °C, far below the Maillard zone, until the last of the water is gone. Only then does the temperature start to climb, and only then does the crust begin. A hotter pan dries the surface faster. A wetter steak, or one crowded next to others that release steam, spends much longer stuck at 100 °C.
Start with the default: a wet steak in a 200 °C pan. The line shoots up to 100 °C and goes flat, and the steak stays pale. Only when the water is gone does the line bend upward into the shaded Maillard zone, and the colour starts to deepen. Now slide the water to zero and restart. The plateau almost vanishes and the crust arrives early.
This one idea explains most of the usual kitchen advice. Patting meat dry removes the water you would otherwise have to boil off. A hot pan, rather than a lukewarm one, drives that water away faster. Leaving space between pieces lets steam escape instead of re-wetting everything. Mushrooms and onions release a lot of water as they cook, which is why they stew first and brown later.
Chemistry has its own dial too. Alkaline conditions speed the reaction, which is why pretzels are dipped in a lye solution before baking, and why a pinch of baking soda browns onions faster. A little goes a long way: too much turns onions mushy.
And more heat is not always better. Very high temperatures push past browning into bitter, burnt flavours, and the same reaction can make acrylamide, a probable carcinogen, in starchy foods fried or baked hot. Caramelisation is a related but separate process: it is sugar alone breaking down, with no amino acids involved, and it needs higher temperatures than the Maillard reaction.
So the next time a pan hisses and the food just sits there pale, you are watching physics at work. The steam is the sound of the pan being held at 100 °C. The crust starts when it stops.