Week 5: Spices, aromatics and oil

What galangal, shallot, onion and garlic contribute, and how chopping, crushing and slicing change the result at a cellular level — plus what determines when oil choice matters, ahead of this week's storage exercise.

22 March 2027

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An aromatic is chosen for what it changes about a dish before any heat is applied — a decision this week treats as a preparation question, not a seasoning afterthought. Galangal, shallot, onion and garlic form the aromatic base of an enormous range of cooking traditions, and the allium members of that group share a mechanism worth understanding before any of them are touched with a knife.

Aromatics: allium first

Intact allium tissue keeps a flavourless precursor compound and the enzyme that acts on it in separate cell compartments. Only when the cell wall is broken do the two meet, and the reaction that follows produces the pungent, characteristic smell and flavour associated with the raw vegetable. This is documented for garlic specifically: cutting or crushing ruptures cells and brings the enzyme alliinase into contact with its substrate, alliin, producing allicin — a mechanism reviewed in detail by Borlinghaus et al. (2021) in Molecules. Onion works by a related but distinct pathway, identified by Imai et al. (2002) in Nature.

Crushing ruptures far more cells per gram than slicing does, releasing more enzyme and substrate into contact and producing a stronger, faster reaction — which is why a crushed clove of garlic smells sharper, sooner, than a sliced one of the same size. Chopping sits between the two.

Bay leaf, lime leaf, oregano, basil: timing and heat

Leaf aromatics are mostly a matter of timing: their flavour comes from volatile compounds that are themselves heat-sensitive. A study of basil subjected to different heat treatments found the volatile compound profile changed measurably with heat exposure, and that milder treatment preserved more of the aroma associated with fresh basil. Bay leaf is added at the start because its flavour compounds need time and heat to diffuse into a dish and are robust enough to survive the exposure; basil is added at the end, or off the heat entirely.

Oils: smoke point and when it matters

Every oil has a smoke point, and reported values vary between sources because it is a property of a specific batch of oil, not a fixed constant of the oil type. It falls over the life of the oil: Crosby (2018), writing in the Institute of Food Technologists’ Food Technology magazine, cites research finding that the formation of as little as 1% free fatty acid in soybean oil causes a 25% decline in its smoke point — meaning an oil that has already been heated, or stored badly, or kept too long, smokes at a markedly lower temperature than the same oil fresh. Oil choice matters more as intended heat increases and matters comparatively little where heat is gentle or absent.

Why this sits before the tutorial

This week’s tutorial exercise is a storage-duration calculation, not a cutting exercise: an aromatic once cut behaves like any other cut produce for the purpose of a cold-chain clock, and an oil’s useful life is itself a storage question — how long it keeps its smoke point before that 1% free fatty acid threshold is crossed. The lecture supplies the reason those clocks run the way they do; the tutorial runs the clock itself.

Outline