Coffee Roasting Flavour Chemistry

Coffee Roasting Flavour Chemistry

A we have noted in our article relating flavour and brewing (extraction) techniques there a four helpful flavour categories for coffee which help to rationalise the parts that make up the whole flavour experience:

Enzymatics (fruit acids, florals)
Maillard compounds (nuts, toasted grain, malt, wood, tannin)
Sugar browning (sweet, vanilla, caramel, chocolate)
Dry distillates (burnt sugars, tobacco, smoke, ash)

In this article, we'll touch on how the roasting process effects these which will help you to understand why you may like some coffees more than others so that you can seek out more of what you like and less of what you don't.

Enzymatics is the group referring to the acids within the bean at harvest and those which are modified during processing and roasting. There are many acids within a coffee bean but here I will mention four common organic acids (which are described in more detail in the original article here).

Quinic Acid exhibits bitter notes and is formed during roasting from the breakdown of Chlorogenic Acid which one of the more abundant acids in the green coffee bean. At its maximum during a medium roast it breaks down somewhat as the roast progresses. It is most notable in a sample at cooler temperatures. Consider this next time you are finishing a cup of light roast filter coffee and there is an astringency to the finish. 

Citric Acid is responsible for the bright lemon and lime notes you will sometimes experience in coffee. It is highest in the green bean and degrades with increasing roast degree. It is even higher in beans from unripe cherries and this can lead to excessive levels that are perceived as sour.

Malic Acid (think green apples...) forms in the seed just like Citric Acid and also degrades with increasing roast degree. It is more fragile and what remains in the bean after roasting is sometimes difficult for the coffee drinker to perceive.

Acetic Acid (think white vinegar...) is responsible for those fermented/winey/tangy notes in 'Natural' or 'Dry Processed' coffees. It is formed during the time that the fruit flesh ferments inside the skin and then, when roasting begins, it actually increases as sugars break down to form more acids. At higher roasts it will break down and is at its highest for a medium roast.

Maillard Compounds are derived from the browning reaction of protein and carbohydrate at temperature and are a function of the bean constituents and their interaction with the roasting process. This starts at approximately 135 Degrees Celsius at around 5-7 minutes into the roast. If under developed, these can lead to  woody, tannin and paper tastes. When properly developed, these will present as nut, toasted grain and malt.

Sugar Browning refers  to the caramelization of sugars inherent to the coffee bean. These sugars are in higher quantities in beans grown under good conditions, especially high altitude where the ripening is slow and steady. The sugar browning occurs in the roaster at temperatures above approximately 170 Degrees Celsius until the end of the roast. Critically, the complex polymerisation that occurs amongst the various sugars present is also rate and time dependent. So the rate of temperature rise and the length of time in this phase affects the final spectrum of sweet tastes.

Dry Distillates are created near second crack temperatures (where the coffee bean cellulose matrix fractures) and in their subtle form are favourable for espresso coffees with flavours of burnt sugar and tobacco. However, with well extracted coffee, the darker dry distillate flavours easily dominate the cup with smoke and ash which are unpleasant so these are something we avoid in our approach to roasting.

Tagged : Workshop, How-to, Tasting, Roasting

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