Vol. XI · Flavour Plate 8 of 12aroma, volatility, fat-soluble, log P, blooming spices, tadka, 爆香, oil, flavour release, low-fatMF

Aroma Molecules.

Fry cumin seeds in ghee and the whole kitchen smells of them; simmer them in water and much less happens. Most spice aromas are oils.

Two properties decide what an aroma molecule does in a dish: how readily it evaporates (XI.1), and whether it prefers fat or water. Chemists measure the second as log P, the logarithm of how much more of it dissolves in an oily solvent than in water: limonene from citrus peel, about 4.6, is ten-thousand-fold fat-loving; vanillin, about 1.2, much less so. Most spice and herb aromas are fat-loving oils — which is why they are called essential oils.

That has three consequences in the kitchen. Fat extracts aromas: heating whole spices in oil or ghee (tadka, 爆香, sofrito) pulls their aroma into the fat, which then carries it through the dish. Fat holds aromas: in a fatty dish they are released slowly as it is chewed, and linger; a low-fat version releases them all at once and fades. And fat protects them a little from boiling off. The workshop shares aroma molecules between the fat and the water of a dish.

Below: the share of each aroma molecule sitting in the fat of a dish against the dish's fat content (equilibrium partition from log P), and the shares at your fat content.

What you will learn
Workshop
Share in fat = K f / (K f + (1 − f)), where K = 10^logP and f is the fat fraction by volume (octanol–water log P from PubChem, rounded: limonene 4.6, linalool 3.0, cinnamaldehyde 1.9, vanillin 1.2). Octanol stands in for cooking fat; real fats differ somewhat.

Bloom in fat, finish with fat

With only 1 % fat in a dish, about 100 % of the citrus aroma limonene and 91 % of linalool sit in that little fat: fat-loving aromas gather wherever the fat is. Vanillin, much less fat-loving, is only 45 % in the fat at 5 % fat and 80 % at 20 %. That is why a few drops of oil carry so much spice flavour, and why vanilla works in low-fat custards while citrus and herb flavours in a fat-free dressing fade.

In practice: bloom whole spices — cumin, mustard seed, chilli, Sichuan pepper — in hot oil or ghee at the start, until they crackle and smell, then add the rest (the Indian tadka, the Chinese 爆香 of garlic, ginger and scallion); add fresh herbs and zest near the end, often with a final spoon of good oil or butter. When cutting the fat in a recipe, expect herb and spice aromas to arrive faster and vanish sooner, and compensate with a little more seasoning or a fattier garnish.

At the stove

Warm a teaspoon of cumin seeds in a tablespoon of oil until they crackle, and another teaspoon in a tablespoon of water. Stir each into a cup of plain rice. The oil-bloomed cumin flavours the whole cup.

Perfumers extract flower aromas into fat — enfleurage, pressing petals onto trays of purified lard and replacing them daily for weeks — for exactly the reason cooks bloom spices in oil.

Terms

Log P
The logarithm of how much more of a molecule dissolves in an oily solvent than in water.
Essential oil
The fat-loving aroma oil of a plant.
Blooming
Heating spices in fat to extract their aroma.
Tadka
Spices fried in ghee or oil and poured over an Indian dish.
Most spice and herb aromas are fat-loving: bloom them in fat, and expect low-fat dishes to release and lose aroma faster.

Sources

  1. NIST Chemistry WebBook (SRD 69): normal boiling points and enthalpies of vaporisation; octanol–water partition coefficients (log P) from PubChem. Vapour pressure from the Clausius–Clapeyron equation.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.