The Chemistry Behind Coffee's Flavor And Aroma

Coffee's flavour and aroma are the result of chemical and biological reactions. The aroma is the release of flavorful compounds from coffee through the air. There are over 800 known aromatics in coffee, with new ones being discovered regularly. The aroma is perceived nasally and retronasally. The former is through smelling the coffee through the nose, and the latter occurs when the coffee is either in the mouth or has been swallowed, and the aromatic volatile compounds drift upward into the nasal passage. The aroma is dependent on the concentration of the compound and its odour threshold. The roasting process plays a key role in creating the various volatile compounds that make the unique coffee aroma.

The roasting process

The level of roast can also impact the aroma and flavour of the coffee. As the beans are roasted for longer, they undergo more chemical reactions, which can change the compounds in the beans and make them more detectable. This is why darker roasts tend to have a stronger aroma. However, it's important to note that roasting to darker levels can destroy some of the unique characteristics of high-end coffee.

Volatile compounds

The aroma of coffee is perceived by two different mechanisms: nasally, by smelling the coffee through the nose, or retronasally, when the coffee is in the mouth or has been swallowed and the aromatic volatile compounds drift into the nasal passage.

There are over 800 known aromatics in coffee, with new ones being discovered regularly thanks to advances in testing equipment. However, only a small number of these contribute to the perceived aroma. Researchers suggest that there are around 20-30 individual volatile compounds at work in a cup of coffee.

The volatile compounds in coffee include hydrocarbons, alcohols, aldehydes, ketones, carboxylic acids, esters, pyrazines, pyrroles, pyridines, furans, furanones, and phenols.

Pyrazines, for example, are the second most abundant class of aromatic compounds in coffee and contribute to the roasted, walnut, cereal, cracker, or toast-like flavours in coffee. Furan and furanones are what usually cause us to perceive caramel and roasted coffee notes. Pyrazines can be related to notes of nuts and roasted coffee. Ketones usually provide fruity or musty notes, and buttery flavours are created by smaller ketones such as 2,3-butanedione.

The Maillard reaction

The reaction results in the formation of several important flavour and colour compounds, which contribute to the overall sensory character of the coffee. The specific compounds produced depend on the types of sugars and the amount of perceived acidity. The Maillard reaction also affects the structure of the viscosity, with longer times during the reaction correlating with increased viscosity.

Small changes in heat application during the Maillard reaction can significantly impact the flavour profile of the coffee. The reaction usually begins around 300°F and continues through the first crack and post-crack sugar browning. Shorter Maillard reaction durations during roasting are associated with increased perceived sweetness and acidity, while longer reaction times lead to a more balanced flavour and increased viscosity.

Coffee bean freshness

The freshness of coffee beans is a key factor in preserving and enhancing the desirable aroma and flavour of coffee. Coffee aroma is responsible for all flavour attributes other than mouthfeel and sweet, salty, bitter, and sour tastes perceived by the tongue. It is the release of flavorful compounds from coffee through the air, and fresh coffee will have a much stronger aroma than older, stale coffee.

The roasting process makes coffee rapidly lose flavour, which can be sensed by smell. Less smell equals less flavour. The process of roasting, especially to darker levels, destroys the compounds that give high-end coffee their unique characteristics. The best-smelling coffee will be one that is freshly roasted, and buying coffee from a roaster directly will ensure maximum freshness.

The aroma of coffee is created by volatile compounds, which are vapours and gases released from the brewed coffee and then inhaled through the nose, contacting the olfactory (nasal) membranes. The number of aromatic compounds found in coffee is well over 800, and more are being discovered regularly thanks to advances in testing equipment.

The aroma compounds are created during the roasting process, particularly during the Maillard reaction, which is a reaction between proteins and sugars in the coffee beans. The more advanced phases of the roast lead to the caramelisation of sugars, which produces furans and furanones, which have caramel notes. The breakdown of amino acids in aldehydes during the Strecker Degradation also creates aroma-producing volatile compounds.

The freshness of coffee beans is paramount in preserving and enhancing the desirable aroma and flavour of coffee.

Chemical compounds

Coffee's flavour and aroma are the result of chemical compounds. There are over 800 aromatic compounds found in coffee, with new ones being discovered regularly. These compounds are created during the roasting process, particularly during the Maillard reaction, which is a reaction between proteins and sugars in the coffee beans. The roasting process also destroys the compounds that give high-end coffee their unique characteristics.

The aroma of coffee is created by volatile compounds—organic compounds that easily evaporate at room temperature and pressure. These compounds are released from the brewed coffee and then inhaled through the nose, where they come into contact with the olfactory (nasal) membranes. The aroma of coffee can also be perceived retronasally, which occurs when the coffee is either in the mouth or has been swallowed and the aromatic volatile compounds drift upward into the nasal passage.

The aroma precursors are various forms of carbohydrates, proteins, and acids. During the roasting process, these are transformed into volatile compounds. Different compounds create different sensory characteristics during roasting, and the level of roast can further impact how we perceive the aroma. For example, 2-methylpyridine is responsible for roasted notes, while pyrazines can create nuttiness or a burnt smell. The more advanced phases of the roast lead to the caramelisation of sugars, which produces furans and furanones with caramel notes. The breakdown of amino acids in aldehydes during Strecker Degradation also creates aroma-producing volatile compounds, such as 3-methylbutanal, which creates fruity and sweet aromas.

Ketones are also very aromatic and are the result of the auto-oxidation of fatty acids, such as propanone. They usually provide fruity or musty notes and buttery flavours. Unpleasant aromas can also be produced during roasting, such as phenols, which are produced by the breakdown of free phenolic acids during extended roasts. They have ashy and smoky aromas.

The perception of coffee aroma depends on both the concentration of the compound and its odour threshold, which is the minimum concentration at which we can detect its smell. The ratio of a compound's concentration to its odour threshold gives the compound's 'odour activity value' (OAV), which gauges its importance to the overall aroma. It is likely that a relatively small group of compounds that share both a high concentration and a low odour threshold make up the fragrance we know as coffee aroma.

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