A raw carrot tastes mildly sweet and a little grassy. Roast it, and the same carrot can taste almost like candy, because heat is breaking the vegetable's own sugar molecules apart into smaller compounds that read as sweeter, nuttier, and faintly bitter all at once. That's caramelization — sugar decomposing under its own heat, with no protein required and nothing added. Every carrot, onion, and ripe piece of fruit already carries enough sugar to run the reaction on its own skin.
Why fruit and honey scorch before table sugar even starts
Not all sugars fall apart at the same temperature. Fructose — the sugar that dominates ripe fruit and honey — starts breaking down around 230°F (110°C). Sucrose, the sugar that makes up most of a raw carrot or onion, and the same molecule as ordinary granulated sugar, holds together until closer to 320°F (160°C), a gap of almost a hundred degrees.
That gap is why a spoonful of honey drizzled over roasting carrots can turn dark and acrid well before the vegetables themselves have properly browned, and why a seared pineapple's cut face chars at the edges faster than a home cook expects. The fruit's fructose is already caramelizing at a temperature sucrose hasn't come close to touching yet.
Why nothing caramelizes while it's still wet
Water holds a surface at 212°F (100°C) and won't let it climb any higher until every drop has boiled away. Both sucrose and fructose need heat well above that ceiling, so a food has to shed its surface moisture before caramelization can start at all.
That's most of what a pan of sliced onions is doing for the first half hour on the stove. The onion is releasing water, not browning. The color and sweetness people wait for don't show up until the pan has gone dry enough to climb past 320°F (160°C), the point where the onion's own sucrose actually starts to caramelize.
Why it turns from deep and toasty to bitter so fast
Caramelization doesn't stop cleanly once the color looks right. The same heat that breaks sucrose into pleasant, nutty-smelling furans keeps running past that point, fragmenting those compounds into smaller ones that smell sharper and taste more acrid the longer they cook. There's no separate "burnt" reaction taking over. It's the identical chemistry continuing past where the flavor peaked.
That's why the last stretch before a dish goes from richly caramelized to bitter happens fast, and faster than the color alone suggests. By the time a pan of onions or a tray of vegetables looks noticeably darker, some of what's caramelizing has often already gone too far to taste good.