Sugar breaking down under heat

All vegetables contain some natural sugars. When you expose them to high dry heat, those sugars start to break down at around 160°C (320°F). The process is called caramelization, and it’s purely about sugar reacting with heat — no proteins involved, which is what makes it different from the Maillard reaction. As Harold McGee notes in On Food and Cooking, caramelization produces over 100 distinct volatile aroma compounds at temperatures between 160–200°C.

As sugars decompose, they form new compounds: diacetyl (buttery), furans (nutty), esters (fruity). The result is the complex, sweet-savory flavor you get from a well-roasted carrot or a golden-brown Brussels sprout. Boiling those same vegetables at 100°C never gets hot enough to trigger caramelization — that’s why boiled vegetables taste flat by comparison.

One nuance worth knowing: not all sugars caramelize at the same temperature. Fructose caramelizes at a lower temperature (~110°C / 230°F), which is why honey and high-fructose foods like onions brown noticeably faster than foods with predominantly sucrose.

Why dry heat matters

Caramelization requires temperatures well above 100°C. That rules out any moist cooking method — boiling, steaming, braising. Water caps the temperature at 100°C, preventing caramelization entirely.

Dry heat (oven, air fryer, grill) removes surface moisture and lets the vegetable’s surface temperature climb past 160°C. The drier the surface, the faster this happens — which is why crowding the pan ruins roasted vegetables. When vegetables are packed together, they steam each other and the surface stays wet, producing grey mushy vegetables instead of golden caramelized ones.

Brussels sprouts and the bitterness question

Brussels sprouts contain glucosinolates — sulfur compounds that give them their characteristic sharpness. High heat breaks down some of these compounds, which is why roasted sprouts taste milder and sweeter than raw or boiled ones.

The cut surface is key. When you halve Brussels sprouts and roast them cut-side down, the flat surface makes direct contact with the hot air and caramelizes quickly. The outer leaves also get crispy and charred at the edges. That combination — sweet caramelized interior, slightly bitter charred edges — is what makes properly roasted Brussels sprouts genuinely delicious.

The right temperature

For most vegetables, 200–220°C is the sweet spot. Below 180°C, vegetables tend to steam in their own moisture before caramelizing. Above 220°C, thin pieces burn before the inside cooks through. Sweet potatoes and dense root vegetables can handle the higher end; delicate zucchini works better at 200°C.

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