Why do vegetables lose color when overcooked?

Answer: Heat breaks down chlorophyll

Heat breaks down chlorophyll ✓ — Correct! Heat breaks down chlorophyll pigments in green vegetables. Chlorophyll contains magnesium at its center. Prolonged heating causes acids in vegetables to replace magnesium with hydrogen, forming pheophytin - an olive-brown compound. This is why overcooked broccoli turns brownish-green. Quick cooking preserves the bright green! Other pigments like carotenoids and anthocyanins also degrade with excessive heat.

Nutrients evaporate away — Wrong. Nutrients don't evaporate - some may leach into water, but this isn't what causes color change. The color loss is specifically from heat breaking down pigment molecules like chlorophyll, not nutrient loss.

Steam pressure forces colors out — Wrong. Steam pressure doesn't squeeze color out of vegetables! The real cause is heat degrading chlorophyll and other pigment molecules at the molecular level. The color change is chemical, not mechanical.