Coffee is one of the most chemically complex items that we consume. This chemical complexity results in an equally complex flavour profile and though they share a lexicon, there are many more flavour molecules found in coffee than in wine.
Image: Dan Bollinger
The raw coffee bean is thought to contain around 300 aromatic molecules and this number increases to over 900 when the bean is roasted. When these unique aromatic compounds combine, they interact to produce even more flavours.
Coffee flavour is determined by the variety of bean, where it was grown, how it was grown, and by everything that happens to it after it leaves the tree, including processing, drying, roasting and brewing.
Varieties and Flavour
There are a huge number of coffee varieties and each of these has their own flavour profile. Most varieties are either complex tasting Arabicas or simpler tasting Robustas. Terroir, ie soil, altitude and climate also have an impact on flavour. Some coffees are farmed with chemicals which will usually have a levelling effect on flavour. For example, in the flat plantations of Vietnam and Brazil, plants are fed a chemical to ripen all the coffee cherries on a bush at exactly the same time, thus facilitating mechanical harvesting. This would have no logic in the mountainous terrain of, for example, Colombia where it will always be necessary to pick each bean by hand.
The production process, particularly cleaning and drying, has a profound effect on flavour. Most importantly, has the bean been processed with or without its sweet mucilage layer. This layer is high in sugar content, with the potential for fermentation. If this fruit layer is left to ferment on the bean during the processing phase it will have a profound effect on the flavour profile of the bean, increasing sweetness and leading to the development of flavours such as strawberry, blueberry and even creamy notes. Semi washed processing gives a honeyed sweetness, and the washed method, in which the mucilage is immediately stripped from the bean, produces a more acidic final drink.
The impact of roasting on flavour
A good coffee roaster has the skill to manipulate a bean to create the flavour profile of his choice. By adjusting the roast level, he can pull out or diminish aspects of the bean to achieve the required balance of sweetness, acidity, body, mouthfeel and aftertaste. Before roasting, green coffee beans have a fresh grassy smell and little or no taste. As the roast level increases, more oils are separated and pulled to the surface of the bean, thus impacting on the body of the brew - the oil is clearly visible on dark roasted beans as a glossy sheen.
The origin of the bean can still be detected in lighter roasts, which tend to have a toasted grain taste and pronounced acidity. Increased caramelisation of sugars in the medium roast disperses the grainy taste, leaving nutty caramel aromas. The nutty caramels continue to develop in medium-dark roasts while at the same time the flavours and aromas of the roasting process become increasingly noticeable, eventually giving a more spicy coffee. The origin of the coffee in dark roasts is completely eclipsed by bitter, smoky and even burnt roast tastes.
The impact of Extraction on Flavour
The same bean, subjected to different extraction methods, will produce cups with different flavour profiles. Grind size, coffee to water ratio and brew time all have an impact on flavour. Flavours extract from coffee at different rates but the order of flavour extraction, regardless of the method, is always salt, acid, sweet and finally bitter. This subject of flavour and extraction has been covered by us in Filtering Coffee - The Principles.
Coffee cupping. Image:UK Centre for International Development
The flavour experience is derived in part from a combination of taste, which is sensed on the tongue, and from aromas, which are sensed in the nose. It's worth noting that aroma is detected when breathing in and out through the nose, when the substance is held in the mouth.
The sensory experience of coffee can be considered first in terms of its sweetness, bitterness and acidity, alongside body, aftertaste and balance. Expanding from discussion at this level, a host of flavour descriptors can be applied.
Strictly speaking, the term sweetness when used in the context of coffee indicates that there are no harsh tastes or flavour defects. In practice however, it can describe an actual sweet flavour, ranging from brown sugar to vanilla.
The little sugar that is found in the coffee bean is not the white stuff in the sugar bowl but complex long chain polysaccharides. During roasting these sugars change to form sweet aromatics which give the perception of sweetness in the final cup. In fact, roast coffee contains only about 0.2% sugar. Add enough water to make a cup and this reduces to about 0.06%.
in coffee is thought to come from trigonelline and quinic acic - it's quinic acic that puts the bitterness in tonic water. Caffeine is also bitter so coffees with a higher caffeine level, notably robustas, are bitterer. Just a small amount of well-balanced bitterness is desirable as it contributes to fullness of flavour but bitterness tends to dominate more subtle flavours. One of the most unpleasant types of bitterness is the one caused by over-extraction . The longer the extraction time the more bitter the cup, so it follows that darker roasts, which are more soluble and therefore faster extracting tend to make a more bitter cup. Light roasted almost green coloured beans tend not to be bitter at all.
There are over thirty different acids in roasted coffee, including citric acid, malic acid, lactic acid and acetic acid. The presence of some acidity in a cup contributes to a more sophisticated flavour profile. Good acidity gives a round lively fruitiness ranging from citrus to berry, while a total lack of acidity results in a dull flat cup. The term "bright" or vibrant tends to be used for cups with a higher acidity. Extreme acidity, when all you can taste is sourness, is considered to be a coffee defect.
Coffees with higher acidities tend to come from higher altitudes and be fully washed during the production process. Some countries, including Colombia and Kenya specialise in bright acidic coffee. Acids are destroyed or denatured by very long and very high brew temperatures so these should be avoided to maintain the subtleties acidity adds to the cup.
or mouthfeel isn't actually a flavour but rather the palate's tactile experience of the drink in terms of its weight or viscosity. It's about how the coffee coats the tongue and feels between the tongue and the palate. The experience might range from light or thin to medium to full. A coffee with a full body will have a buttery, oily or even syrupy quality.
Body is largely determined by the amount of oil produced on the surface of the bean during roasting, and the amount of oil, proteins and fibres that are retained during the brewing process. Darker roasts have the highest body (more oil is produced during darker roasting) and a French press produces coffee with the most body. Drip methods, which don't allow the passing of oils through their fine filters, result in a lighter coffee.
or finish refers to how long the positive flavours linger on the palate after the coffee is swallowed. A good aftertaste is sweet rather than bitter or astringent and will be present long after the coffee has been consumed.
Good balance is when flavour, aftertaste, acidity, and body are perfectly nuanced to achieve an overall experience that is greater than the sum of the parts. No one flavour or aroma overwhelms the others and the flavours are sensed evenly across the tongue. The roaster may need to blend several different coffees to create a balanced coffee.
SCAA Coffee Flavour Taster's Wheel
Scientists are yet to identify every last flavour molecules present in coffee but in an attempt to standardise the tasting process, the World Coffee Research and The Specialty Coffee Association of America (SCAA) recently collaborated with hundreds of specialists to produce a Sensory Lexicon for coffee. The lexicon describes 110 flavour, aroma, and texture attributes which can all be quantified on to a scale.
This Sensory Lexicon provided the basis for a new SCAA Coffee Flavour Taster's Wheel which was released in January 2016. It was the first re-release of this standard reference tool in 20 years and the new design allows for the addition of new flavours, as and when the lexicon is updated.
At the heart of the wheel are nine basic flavour categories: spices, nutty/cocoa, sweet, floral, fruity, sour/fermented, green/vegetative, other and roasted. Once these categories have been identified, the taster can move out from the centre of the wheel to find more specific descriptors. For example, once the flavour 'roasted' has been identified, this can be further delineated as 'cereal', 'burnt', 'tobacco' or 'pipe tobacco'. 'Cereal ', can then be defined as 'malt' or 'grain'.
One can only imagine that the next release of the wheel will include the addition of another concentric outside ring as the lexicon continues in its attempt to get to grips with the flavour of this most complex of consumables.