Unlocking Coffee's Aroma: The Science Behind The Scent

Coffee is one of the most widely consumed beverages in the world, and its aroma is a key part of its appeal. But what gives coffee its distinctive smell? The answer lies in chemistry – specifically, in the Maillard reaction, which occurs when coffee beans are roasted. This reaction between amino acids and reducing sugars is what gives coffee its brown colour, as well as its distinctive flavour and aroma. During roasting, volatile compounds are produced, which are what we perceive as aroma. These compounds then interact with our olfactory receptors, creating the sensory experience of coffee's aroma.

The Maillard reaction

When a reducing sugar and an amino acid react, the nitrogen in the amino acid bonds to the carbon chain of the sugar, giving off a molecule of water. The molecule that forms, glycosylamine, is unstable and first changes its structure in a process called the Amadori rearrangement. It then reacts again in one of three ways: by losing more water molecules to become a caramel-type molecule, breaking down into short-chain molecules, or reacting again with more amino acids. All three products of these reactions can react again with amino acids to form melanoidins, dark brown compounds that provide a lot of the colour in coffee and can have roasted, malty, bready, bitter, and burnt flavours.

The range of flavours produced by the Maillard reaction is essential to the complexity of aromas in roasted coffee. It is also why the sugar content of green beans is so important to the final flavour, even though very little sugar survives roasting intact. Unripe coffee beans (or 'quakers') are a good example of this – the lack of sugar means Maillard and caramelisation reactions cannot take place, resulting in pale and flavourless beans.

Roasting and volatile compounds

The aroma of coffee is created by volatile compounds, which are organic compounds that evaporate at room temperature and pressure. These volatile compounds are produced when coffee beans are roasted. Raw coffee beans have a different smell from roasted beans.

The roasting process chemically transforms the aroma precursors in green coffee beans—various forms of carbohydrates, proteins, and acids—into volatile compounds. The Maillard reaction, a chemical reaction between amino acids and reducing sugars, is a significant contributor to the formation of volatile compounds. This reaction occurs at temperatures between 140°C and 165°C (284°F and 329°F), causing starches to break down into simple sugars, which then turn brown and develop a distinct scent and flavour.

The level of roasting impacts the aroma profile. For example, 2-methylpyridine, a compound responsible for roasted notes, is formed during the roasting process. Pyrazines, another group of compounds, can create nuttiness with a light roast or a burnt smell with a darker roast. The more a coffee bean is roasted, the more the compounds change and become detectable, strengthening the aroma.

In addition to the Maillard reaction, other processes contribute to the formation of volatile compounds during roasting. Strecker degradation, which involves the breakdown of amino acids, produces volatile compounds that contribute to the aroma. The degradation of individual amino acids, particularly sulfur-containing amino acids, also plays a role. The breakdown of free phenolic acids during extended roasts produces phenols, which contribute ashy and smoky aromas.

How we perceive coffee aromas

The aroma of coffee is what gives it its flavour. While our tongues perceive basic tastes, such as sweet, salty, bitter, and sour, it is our noses that detect the aroma of coffee, which is made up of volatile compounds. These compounds are organic compounds that easily evaporate at room temperature and pressure.

Coffee aroma is perceived in two ways: nasally and retronasally. The former is the smell of coffee through the nose, while the latter occurs when the coffee is in the mouth or has been swallowed, and the aromatic volatile compounds drift into the nasal passage.

The roasting process is what creates the volatile compounds that give coffee its unique aroma. The Maillard reaction, a chemical reaction between amino acids and reducing sugars, is responsible for the distinctive brown colour and flavour of roasted coffee beans. This reaction occurs at temperatures between 140 and 165 °C (284 to 329 °F). During roasting, the beans lose moisture, increase in volume, and decrease in weight. The intense heat breaks down starches into simple sugars, which then turn brown and change the bean's scent and flavour.

While there are over 800 aromatic volatile compounds produced during the roasting process, only around 30 of them significantly contribute to the aromas and flavours we perceive. The compounds need to be in balance for us to have a specific sensory experience. The interaction of compounds can also change our perception. For example, 2,3-butanedione, which usually has a caramel aroma, may increase the perception of sweetness in coffee.

The complexity of coffee aromas also depends on the composition of chemical compounds in the green beans, which can vary depending on factors such as variety, weather conditions, maturation level, and processing choices.

The role of caramelization

Caramelization is a slower process than the Maillard reaction, another important reaction in coffee roasting. It involves the application of heat to green coffee beans, causing a chemical reaction in the sugars present. This reaction typically begins around temperatures of 150°C to 180°C, with water being lost from the sugar molecule, initiating a chain of events. The sugar is converted into a furfuryl compound, a type of furan, which imparts sweet, nutty, fruity, or caramel-like aromas and flavours to the coffee. These furans are derived mainly from sucrose and polysaccharides during the roasting process.

The longer the sugar is caramelized, the less sweet it becomes, so finding the right balance is crucial. By controlling the degree of caramelization, roasters can strike a harmony between the benefits of uncaramelized sugar sweetness and avoiding the astringency and sourness associated with light roasts. Additionally, the level of roast can influence how we perceive the aroma, with darker roasts often enhancing the aroma by changing more compounds in the coffee and making them detectable.

The caramelization process not only contributes to the aroma but also enhances the overall flavour profile of the coffee. The resulting furans and furanones, such as 4-Hydroxy-2,5-dimethyl-3(2H)-furanone, impart caramel notes that add depth and complexity to the coffee's flavour. This interplay between aroma and flavour is what makes the role of caramelization so integral to the overall coffee experience.

The importance of freshness

The aroma of coffee is responsible for most of its flavour attributes, aside from the basic tastes perceived by the tongue, such as sweetness, bitterness, and sourness. This is because the aroma of coffee is made up of volatile compounds that are released from the brewed coffee and inhaled through the nose, where they come into contact with the olfactory (nasal) membranes.

Freshly roasted coffee beans will have a much stronger aroma than older, stale coffee beans. This is because the roasting process makes coffee lose its flavour rapidly, which can be sensed by smell. The darker the roast, the more compounds in the coffee are changed and become detectable, strengthening the aroma.

The roasting process also destroys the compounds that give high-end coffee their unique characteristics, eliminating some of the benefits of paying more for coffee. This is why it is important to buy freshly roasted coffee directly from a coffee roaster, rather than from a store shelf.

When coffee beans are roasted, aromatic oils and acids weaken, changing the flavour. At higher temperatures, new oils start to develop, including caffeol, which is created at about 200 °C (392 °F). Caffeol is not a unique oil but is responsible for the origin of the coffee aroma.

The freshness of coffee and its roast are important factors in preserving and enhancing the coffee's desirable floral aroma. A stale coffee will smell musty and cardboard-like.

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