What makes coffee smell? How do we smell coffee?

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We enjoy the unique flavor and rich aroma of coffee, chocolate, caramel, nuts, fruit and flowers, which may be felt when brewing or sipping coffee, but what is the chemical reaction?

Each sensory property consists of different chemical and biological reactions. This article shows you how the aroma of coffee comes from.

Where does the coffee aroma come from?

The raw beans of unroasted coffee have no obvious aroma, and a variety of aromatic compounds can be created only through roasting, thus forming the unique aroma of coffee, but the flavor recognized by most people is actually fragrance, because the only taste that the tongue can feel is sweet, sour, bitter and salty. When the receptors of the nose feel the aroma and combine with the sense of taste, it is the flavor we feel.

However, the complex aroma is actually related to the chemical composition of raw beans. Compounds that produce different aromas will be present in different concentrations in raw beans, including variety, climate of origin, fruit maturity, treatment, etc., will affect the chemical composition of raw beans.

Which compounds form aroma?

The substances that form the aroma are made up of different forms of carbohydrates, proteins and acids, which are converted into aromatic molecules through the baking process (especially the caramelization reaction). Different compounds produce different sensory properties during baking, and the depth of baking will further affect the aroma we feel.

2-methylpyridine shows a baking aroma.

Pyrazine produces nutty flavors, such as 2, 2, 3-dimethylpyrazine. Or a charred smell, such as 2-ethyl-3-methylpyrazine.

The deeper the baking will lead to the caramelization of sugar. This produces furans and furanones, such as 4-hydroxy-2pyrrine-5-dimethyl-3 (2H) furanones with caramel flavor.

In the process of degradation, the decomposition of amino acids in aldehydes will also produce volatile compounds that produce aroma. 3-methylbutyraldehyde produces fruity and sweetness.

Ketones are also very aromatic and are the result of automatic oxidation of fatty acids such as acetone. Ketones usually produce fruity or moldy flavors (β-damasone is a ketone with the smell of fruit tea), while butter is produced by smaller ketones such as 2mae 3-butanedione.

The baking process can also produce an unpleasant smell. Phenols such as guaiacol are produced by the decomposition of free phenolic acids during an overly long baking process. They produce a strong smoky smell.

How is the aroma felt by us?

We now know how coffee forms some volatile compounds during roasting. But it must be noted that the aroma will be felt in different ways.

It is estimated that more than 900 aromatic volatile compounds will be produced in the baking process, but only about 30 of them can be perceived as flavor and aroma, and we can't feel most of them at all. These compounds also need to be balanced on us in order to have a specific sensory experience.

When we smell coffee powder, aroma molecules interact freely with our olfactory receptors. When coffee powder is mixed with water to make a cup of coffee, other molecules are extracted, giving us a different sensory experience.

Polarity (differences in molecular electrodes) is the driving force for the extraction of volatile compounds from coffee. During the extraction process, the highly polar molecule of 2pyr3-butanedione (with creamy aroma) is extracted first, while β-damazone may take longer to be extracted in a perceptible amount because of its smaller polarity. To put it simply, different compounds are extracted at different rates, so we feel different aromas at different points in time.

The interaction of compounds may also alter sensory perception. For example, 2meme 3-butanedione, which is usually thought to have a caramel flavor, can increase the sweetness of coffee.

Fabiana Carvalho is a neuroscientist who specializes in the study of human memory, sensation and sensory nerves. She says there is an increase or decrease in flavor intensity between the equilibrium states of the compounds. Fabiana gave an example of an experiment that showed that subjects thought the cup of sugar water was sweeter when strawberry essence was added to a sucrose solution of the same sweetness.

By understanding how the aroma is formed and perceived, we can use information to judge how we flush out the coffee we like, and we can distinguish that different stages of extraction will produce very different flavors and aromas.

Try some different brewing techniques, or sample and taste the coffee at different extraction points to see what flavor you feel.

Translated from Perfect Daily Grind

Translated by Cheng Zhen Coffee

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