The Maillard reaction and caramelization are separate chemical reactions that happen to show up in the same pan at the same time, which is the entire reason people confuse them. Caramelization is sugar breaking itself down under heat with nothing else involved. The Maillard reaction can't run at all without an amino acid — a fragment of protein — reacting with a sugar alongside it. A pot of caramel made from nothing but sugar and water is caramelization, full stop. A seared steak's crust, built mostly from protein with only a trace of sugar, is almost entirely Maillard.
Why sugar can brown alone and protein can't
Caramelization is one molecule falling apart. Heat sugar past its threshold and its own bonds start breaking, rearranging into new ring-shaped compounds with no other ingredient required. That's why a pan of nothing but granulated sugar turns from clear to amber to deep brown by itself.
The Maillard reaction is different in kind, not just in degree — it's a reaction between two different things, a sugar and an amino acid, and it structurally cannot happen with only one of them present. Melt sugar alone and you get caramelization every time. Heat egg white alone, which is almost pure protein with barely any sugar, and it barely browns even scorched, because there's no sugar partner around for the amino acids to react with.
The temperature gap most cooks have backwards
People assume both reactions kick off around the same heat. They don't, and the mismatch runs the opposite direction from what you'd guess. The Maillard reaction gets underway around 280°F (140°C). Caramelization's starting point depends entirely on which sugar is involved — fructose breaks down as low as 230°F (110°C), but table sugar, the one in virtually every caramel recipe, doesn't get going until roughly 320°F (160°C), a full 40 degrees hotter than where Maillard browning already began.
That gap is why the dark fond stuck to a pan after searing meat isn't caramelized, whatever the recipe calls it. There's almost no free sugar in meat to caramelize in the first place, and the pan never reaches the temperature that would matter even if there were. What's stuck to the bottom is Maillard product, built from proteins and a trace of glucose, formed at a heat most of the sugar in that pan will never see.
Why they don't taste alike even at matching color
A deeply caramelized pot of sugar and a well-seared steak can land on nearly the same shade of mahogany brown and taste like they came from different kitchens. Caramelization has no nitrogen to work with, so its output is limited to sugar-derived compounds — mostly furans — that read as sweet, buttery, sometimes faintly rummy, and occasionally bitter if pushed too far. None of that is savory.
The Maillard reaction has nitrogen on hand because amino acids supply it, and that nitrogen gets folded into the new molecules it builds — pyrazines, thiazoles, compounds with a roasted, nutty, or outright meaty character. Color tells you how far a reaction has gone. It doesn't tell you which reaction you're looking at.
Onions: both reactions, running at once
A pan of onions carries enough sugar to caramelize and enough protein to Maillard, which is why the finished dish tastes deeper than either reaction manages on its own. But they don't start at the same moment. Onions are mostly water, and a wet surface can't climb past 212°F (100°C) until that water has cooked off — well short of what caramelization needs. Early in the pan, while there's still moisture to boil away, most of the color and aroma building up is the Maillard reaction working on the onion's surface proteins. Caramelization only takes over once the pan has gone dry enough to get hot enough, which is most of why the process takes as long as it does.