How does cooking or processing create smell?

Key odorants of wheat bread crust and crumb (VERIFIED): A review of bread aroma chemistry reports 2-acetyl-1-pyrroline, (E)-2-nonenal, 3-methylbutanal, 2,3-butanedione, methional and (Z)-2-nonenal as primary odorants of wheat bread crust, with 2-acetyl-1-pyrroline suggested as the key crust odorant behind its cracker-like note; the crumb has a different set, and these odorants are generated mainly during baking. Scope: Wheat bread; crust and crumb differ, and odorants depend on ingredients, fermentation and baking.

Caramelisation forms aroma-active volatile compounds (VERIFIED): Heating sugars during caramelisation forms both brown-colored products and volatile compounds responsible for characteristic caramel aroma. Scope: Supports aroma formation during sugar caramelisation; exact products depend on sugar type and reaction conditions.

How curing forms vanilla aroma (VERIFIED): In traditionally cured vanilla beans, most glucovanillin was hydrolysed in the first stages of curing, and the aromatic compounds freed from their bound forms can be converted further, significantly changing the aroma profile of the cured bean. Scope: Vanilla planifolia beans cured by one traditional method; it describes how the aroma forms, not a ranked list of key odorants.

Aroma of toasted oak wood (VERIFIED): A sensory panel described toasted sessile oak with six aroma descriptors (fresh wood, fresh green, sweet, roasted, spicy and smoky), and thymoquinone (pencil and cedar odour) and verbenone (fresh, menthol and tea-leaf odour) were identified as contributing volatiles. Scope: Toasted cooperage oak (Quercus petraea) studied for wine and spirits; untoasted or other oak may differ.

Thiosulfinates in the smell of fresh garlic (VERIFIED): A review reports that the pungent odour of fresh garlic comes mainly from thiosulfinates and their breakdown products; the cloves' reactive sulfur substances are readily converted into many volatile compounds when garlic is processed. Scope: Fresh garlic; heated, black and aged garlic have different volatile profiles.

1. Smell bread before and after toasting it. Describe what changed in one sentence.

Smell and Science is free, like all of SmellBook Learn. It covers where smells come from: the volatile compounds behind them and how researchers find the ones that matter. Every finding is a VERIFIED claim shown with its scope. It is not medical or safety advice. Lessons on smell receptors, perception and adaptation will be added when registered sources support them.