Do you know what the source of the aroma you perceive when you eat food is?
This article explains the chemical nature of aromatic compounds and presents methods to extract them for use in cooking.
A food contains many volatile compounds, which can be perceived in two ways: by direct nasal route (which corresponds to the odor) or by retronasal route, when the food is placed in the mouth, which gives rise to the aroma (see the article The perception of aromas). The set of volatile compounds responsible for the perception of an aroma is also designated under the same term of aroma.
In this document, we will see what is an aroma, and the most used extraction methods.
2. What is an aroma made of?
Flavor molecules are low molecular weight organic compounds whose vapor pressure at atmospheric pressure and room temperature is sufficient to cause volatilization in the gaseous atmosphere and produce an olfactory stimulus upon contact with the olfactory mucosa.
These volatile compounds belong to the different classes of organic chemistry: hydrocarbons, aldehydes, acids, alcohols, esters, etc. They do not contribute any energy to the food in which they are found, but they play a fundamental role in the recognition of food and the pleasure we derive from tasting culinary preparations.
They are present in food in very small quantities, or even traces, except in the case of spices and herbs where their concentration can sometimes be very high (see the table below and also the article Spices and herbs). For example, cloves contain 15 to 20%.
|Type of food||Quantity of flavouring|
|Meat, fruit and vegetables||50 ppm|
|Aromatic herbs||0,1 à 1 %|
|Spices||1 à 20 %|
Their number in a food product is variable, but generally several hundred. In coffee, for example, over 800 flavour compounds have been identified to date. Not all these compounds play the same role. Some are more essential to the characteristic note of the product: they are called key compounds of the aroma. Such is the case of the mushroom note communicated by oct-1-en-3-ol (see figure 2).
3. Extraction of aroma compounds
It is possible to concentrate or extract the aroma compounds using various techniques: dehydration, distillation, steam distillation, etc. Depending on the technique used, various products are obtained: essential oil, oleoresin (retinoid or concrete), infusion, alcoholate, etc.
3.1 Concentration by dehydration
This technique consists in eliminating most of the water from the plant material. We thus obtain either a dry product which will be used as aromatic raw material (ex: dehydrated garlic), or an aqueous concentrate (ex: orange concentrate).
3.2. The expression
This technique consists in recovering the volatile compounds by mechanical means (abrasion, compression, incision, perforation). We thus obtain an essential oil. This technology is used industrially to obtain essential oils from citrus fruits. In general, we couple the recovery of essential oil with the extraction of juice.
3.3. Dry distillation
It consists in entraining the volatile compounds by the steam produced by heating the vegetable matter. The product obtained is an empyreumatic oil. This technique is used in particular for the manufacture of smoke aromas from wood (cade, birch), but its importance remains limited.
3.4. Steam training
This is the oldest and still the most widely used technique for extracting volatile compounds. It consists in percolating the raw material with steam. The essential oil is recovered from the condensates by decantation (see figure 3). The residual water may still contain a small proportion of certain volatile compounds and can be used under the term floral water.
Figure 2 - Principle of steam drive
1 = Boiler, 2 = Extractor, 3 = Refrigerator, 4 = Separator or "Florentine vase", 5 = Colombage
The boiler (1) produces steam, which is sent to the extractor (2), in which is the plant product from which we want to extract the aroma compounds.
After condensation in a cooler (3), the water phase is separated from the lipid phase in a florentine vessel (4).
The lipid phase used in this method is less dense than the aqueous phase. The lipidic phase, containing the majority of the aroma compounds, is recovered and is called essential oil.
The aqueous phase is reused by being re-injected into the boiler, this is called the colombage (5). It is also possible to recover this residual water, containing a small proportion of aroma compounds, which is called floral water.