Vol. XI · Flavour Plate 1 of 12taste, smell, aroma, retronasal, flavour, volatility, olfaction, temperature, noseF

Taste and Smell.

Pinch your nose and a strawberry and a raspberry taste the same: sweet and a little sour. The difference was never on the tongue.

The word flavour covers several senses working at once. The tongue and mouth taste five qualities — sweet, salty, sour, bitter and umami (XI.2). The nose smells: about four hundred kinds of receptor, working in combination, tell apart many thousands of aroma molecules. Nerves in the mouth feel chemical irritation — the burn of chilli, the cool of mint, the tingle of Sichuan pepper (XI.7) — and texture and temperature (XI.10).

Aroma reaches the nose two ways: sniffed from the plate (orthonasal), and breathed up from the mouth to the back of the nose as we chew and swallow (retronasal). The second is the larger part of what we call taste: block it and a raspberry is just sweet-sour. Aroma molecules must evaporate to be smelled, and evaporation rises steeply with temperature. The workshop warms four aroma molecules.

Below: the vapour pressure of four aroma molecules against temperature relative to 20 °C (Clausius–Clapeyron with NIST data), and how much more of each reaches the nose from food at the temperature you choose.

What you will learn

Separate taste, smell, irritation and texture.
Explain retronasal smell.
Explain why warm food smells stronger.
Use aroma deliberately.

Workshop

Vapour pressure p = 101.325 kPa × exp(−ΔH/R (1/T − 1/Tb)) for each molecule's normal boiling point Tb and enthalpy of vaporisation ΔH (rounded from the NIST WebBook): limonene (citrus) 176 °C, 39.5 kJ/mol; hexanal (cut grass) 131 °C, 43 kJ/mol; ethyl butyrate (fruity) 121 °C, 39 kJ/mol; vanillin 285 °C, 64 kJ/mol. Relative to 20 °C.

Most of flavour is in the nose

Warming food from 20 to 60 °C multiplies the vapour pressure of limonene, the citrus aroma, about 7.0 times, and of vanillin about 23 times: heavier aroma molecules hold back more when cold and gain more from warmth. At fridge temperature (4 °C) they fall to 0.39 and 0.22 of their 20 °C values. That is why cold food needs stronger seasoning, why a cheese or a melon straight from the fridge tastes of little, and why vanilla ice cream smells of more once it begins to melt in the mouth.

Lighter molecules are far more volatile to begin with: at 20 °C limonene's vapour pressure is about 933 times vanillin's. They reach the nose first and are lost first in cooking — the reason citrus zest, fresh herbs and toasted sesame oil go in at the end (XI.9), while vanilla and spices with heavy aroma molecules survive baking. Taste the dish at the temperature it will be eaten; hold a nose-pinch test now and then to remind yourself how much of flavour is smell.

At the stove

Pinch your nose, close your eyes and let someone give you a jelly bean or a spoon of fruit yoghurt. Chew, then let go of your nose: the flavour arrives all at once, from behind.

Humans have about 400 working olfactory receptor genes, mice about 1 000; but by breathing aroma back from the mouth, people make far more use of smell while eating than most animals do.

Terms

Taste: The five qualities sensed by the tongue: sweet, salty, sour, bitter, umami.
Aroma: Volatile molecules sensed by the nose.
Retronasal smell: Smelling aroma that rises from the mouth to the back of the nose while eating.
Volatility: How readily a molecule evaporates.

Flavour is taste plus smell, and most of it is aroma reaching the nose from the mouth; warmth releases more of it.

Sources

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.
H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.