Cooking science

Losing the aroma

Almost everything you can smell in food is volatile — it evaporates readily at kitchen temperatures. That is why you can smell it at all, and it is also why a herb added at the start of a long braise contributes almost nothing by the end.

If you can smell it, it is leaving

Aroma reaches your nose because molecules have left the food and travelled through the air. A pan that smells wonderful is a pan actively losing the compounds that make it smell wonderful.

Heat accelerates this enormously, and so does time and surface area. A simmering uncovered pot loses volatiles continuously for as long as it simmers.

This is the single best argument for adding delicate things late, and the reason a dish tasted at the end of cooking often has less aroma than it did halfway through.

What loses most, and when to add it

Soft herbs

Basil, coriander, dill, chervil, mint, parsley. Almost entirely lost within a minute. Add off the heat.

Hard herbs

Rosemary, thyme, bay, sage. Their compounds are held in resinous oils and survive long cooking. Add early.

Ground spices

Lose aroma quickly once ground, and again in the pan. Buy whole, grind as needed, bloom briefly.

Citrus zest

Highly volatile. Adds most when stirred in late, or rubbed into sugar so the oil is captured.

Wine and spirits

The alcohol carries aroma away with it as it evaporates. Reduce early for depth; add late for the smell of the drink.

Garlic

Raw pungency is volatile and largely gone after long cooking, replaced by sweetness. Add early for sweet, late for sharp.

Vinegar

The sharp aroma cooks off. A final splash at the end is a different ingredient from vinegar added at the start.

Working with it

• Add in two stages. Half the herbs or spices early for depth, half at the end for aroma. This is what most good recipes are doing when they seem to repeat an ingredient.
• Lid on for aroma, lid off for reduction. A lid returns volatiles to the pot along with the steam.
• Grind spices fresh. Pre-ground spice loses most of what it had within months.
• Toast and bloom whole spices, which develops new compounds as well as releasing existing ones.
• Finish with fat. Many aroma compounds are fat-soluble, so a knob of butter or a drizzle of oil at the end carries them.
• Taste at the end, not the middle. What you smell mid-cook is partly what is leaving.
• Store dried herbs and spices sealed and dark. They lose aroma on the shelf as surely as in the pan.

A worked example

The same dish, treated two ways.

1. Put all the herbs and spices into a stew at the start and simmer for three hours.
2. The result is deep, rounded and slightly flat. The volatiles have gone and what remains is the heavier, less fugitive part of each ingredient.
3. Now do it again, holding back half the ground spice, all the soft herbs, the citrus zest and a splash of vinegar.
4. Stir the reserved spice in for the last five minutes, and the herbs, zest and vinegar off the heat.
5. The result has the same depth and a top note the first version lost.
6. That difference — depth from early, aroma from late — is most of what separates a good stew from a dull one.

Common questions