November 9, 2011

Coffee Chemistry

Welcome back. In Part I of this series, we discussed some of coffee’s chemical makeup and its role in developing coffee’s flavour. This time, we’ll briefly discuss a few key components and take a brief journey to explore their role in flavour.


Although roasted coffee contains over 1,000 compounds, the vast majority of these aromatic by-products were created in just one process: roasting. Through a series of complex reactions involving sugars and proteins, we as roasters convert this otherwise boring, grassy smelling green bean to one with rich complexity and great aromatic appeal. But what are these compounds?

Over the years, studies on coffee’s flavour has determined that of the 1,000 or so compounds found in coffee, only about 50 percent of them are volatile or can be detected by smell. But even then, not all of them serve as equally important flavour constituents. According to chemists, we can synthetically ‘recreate’ the smell of coffee by simply using a handful of compounds in the lab.

Ask any coffee lover what is the best part of coffee, and many would say it’s the aroma of freshly roasted coffee. Interestingly, many of these wonderful aromatic compounds in coffee are actually some of the most foul-smelling compounds known to man. One class in particular – the mercaptans – are compounds typically associated with the smell of rotten eggs or otherwise decomposing organic material. But one compound in particular – furfurylmercaptan (FFT) – has by far been associated with the most ‘coffee-like’ aroma at low concentrations and is extremely repulsive at higher levels. However, like all potential odorants found in food, the development of FFT and several others are highly dependent on roasting profiles and bean composition.

During roasting, the rate and application of heat plays a critical role in the development of important compounds. As roasting progresses, thermal degradation of sugars and proteins allow them to react through the Maillard Reaction and ultimately produce the flavour of coffee. These reactions, which occur at various stages in the roast, begin with the development of peanut-like aromas about 170 °C, then become more coffee-like at about 180 – 190 °C. Once we’ve reached 200°C, we begin to see the formation of phenols and furans – or compounds typically associated with caramely/burnt like attributes.

But as roasting progresses and the beans get progressively darker, we also see a corresponding decrease in the level of acidity. Since the overall flavour of coffee is based on both smell and taste, this decrease in acidity affects the ultimate profile of the cup. For most coffee buyers, coffee acidity is a prized attribute and one indicative of the plants’ growing region and unique terroir. For example, coffees cultivated in Brazil generally are lower in acidity than say a Colombian – and markedly lower than a Kenyan.The exact reasons for this are quite complex, but overall we can attribute these changes to over thirty organic acids found in coffee, each playing a distinct role in how we perceive acidity. Of them, chlorogenic acid makes up the vast concentration of acids in coffee; as roasting progresses, it is decomposed to qunic and caffeic acid – both of which have slightly bitter attributes. Other acids such as citric and malic, which contribute to the citrus-like notes in high quality coffee, are also decomposed during latter stages of roasting and are partly responsible for the decrease in overall acidity. But acetic acid, which is produced during the fermentation process in wet processed coffees, is a key acid that can make or break a great coffee. Ferment for too long, and levels of acetic acid increase, producing a defective fermented note; while too little, generally produces a coffee with a flat/muted flavour.


As you may have guessed, the combination of aromatics and taste is extremely complex, and over the years there has been an industry effort to standardise tasting methodologies and descriptive terminology.

As seen from the Flavour Wheels, these terms can be categorised, then drilled down to describe each attribute even further.

Take for example, sour – this base modality can be broken down into soury and winey according to the Flavour Wheel for more accurate classification. Thus far, the system works quite well, but perhaps the most striking aspect of the wheel is the clear separation between taste and aroma. When compared to the taste side, we see that things on the aroma side have additional layers of complexity.

Over the years, sensory scientists have been able to establish a good understanding of taste, but we are still light years away in truly understanding the issue of smell. It’s been estimated that coffee is by far one of the most complex food matrixes in existence, so it will take several more years to fully understand how we truly perceive this wonderful beverage.

Fortunately, science has made great strides in areas of chemistry and sensory science, which provide coffee professionals with the knowledge and tools to develop a better product.


Joseph A. Rivera holds a degree in food chemistry and is the founder/creator He has conducted numerous technical training seminars throughout Australia on exploring and understanding coffee science. For more information, contact him at


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