Why does brown butter taste nutty?
Browning butter isn't caramelizing the milk sugar, and it isn't just "toasting" in some vague sense -- it's a genuine Maillard reaction that had to wait its turn.
The kitchen answer
Butter can't brown until its water has cooked off, because water always boils away before anything gets hot enough to brown. Once that happens, the milk proteins and milk sugar left behind in the fat brown fast -- which is why brown butter seems to go from "melted" to "burnt" in what feels like seconds.
What's actually happening
Butter is roughly 80% fat, but the rest is water and milk solids (protein and lactose). As long as water is still present and boiling off, the butter's temperature is pinned near 100°C -- it can't climb higher until the water is gone, the same reason a pot of boiling water won't exceed 100°C no matter how high the burner is turned up.
Once the water has evaporated, the temperature is free to climb past that point, and the milk proteins and lactose suspended in the remaining fat undergo the exact same Maillard reaction responsible for seared meat and toasted bread crust -- reducing sugar (lactose) plus protein, under heat, producing the browned color and nutty, toasty flavor compounds.
What does that mean for your recipe
The long, seemingly uneventful simmering stage isn't wasted time -- it's water leaving. Once the audible bubbling and sputtering quiets down and the milk solids at the bottom of the pan start to turn golden, the reaction has effectively begun and moves fast from there, which is exactly why brown butter is usually finished off heat, using residual pan heat, rather than left on the burner until it visibly looks done.
What can go wrong
- Butter that goes from pale to black abruptly: the Maillard reaction, once it starts, proceeds quickly -- there's a much narrower window than the long water-evaporation stage suggests.
- Bitter, acrid brown butter: pushed past browning into genuinely burnt milk solids, the same overshoot problem covered in Heat & Transformation.
- Butter that never browns: pulled off heat too early, while water is still present and temperature is still pinned near 100°C.
- Uneven browning: milk solids settling and scorching locally on the pan bottom before the rest of the butter reaches the same point -- a light-colored pan makes this easier to catch in time.
Try it yourself
Watch the transition happen, not just the result:
- Melt butter in a light-colored pan over medium heat.
- Watch and listen: vigorous bubbling and sputtering (water evaporating), then quieting, then color change at the bottom.
- Pull it off heat the moment it turns golden-brown and smells nutty.
Watch for: the sound changing before the color does -- the sputtering genuinely quiets down as the water finishes leaving, giving you an audible warning that browning is about to start.
Go deeper
This is functionally the same principle as why a wet piece of meat won't sear properly until its surface moisture evaporates first -- water boiling at the surface pins that surface near 100°C and blocks the Maillard reaction from starting, regardless of how hot the pan itself is.