March 28, 2019

What Happens During Coffee Roasting: The Chemical Changes


Do you know what happens when you roast coffee? What changes take place to make green beans into the flavorful, aromatic ones we love?

In part one of this two-part series, we looked at how the coffee bean’s anatomy plays an important role and outlined the physical changes that take place during roasting. Now let’s take a look at some of the chemical changes, including how flavour and aroma develop.

Read part one, What Happens During Coffee Roasting: The Physical Changes

some bags of coffee with green coffee on them

Bags of green coffee at a roastery. Credit: Ana Valencia

The Main Chemical Reactions

Introducing coffee beans to the heat of the roaster sparks hundreds of different chemical reactions. Roasting degrades some compounds, alters others, and creates new ones.

You may hear people mention a chemical process called pyrolysis. This is when an organic material is heated above its decomposition temperature, producing volatile compounds and leaving behind a solid residue containing a lot of carbon, or char. In coffee roasting, we avoid getting the beans hot enough to cause charring, but they do undergo chemical changes associated with pyrolysis including the caramelization of sugars and production of volatile compounds.

Here are the main chemical reactions that impact your daily cup of coffee.

  • The Maillard Reaction

This process begins at around 150°C/302°F, when the beans are still absorbing heat endothermically, and continues in the exothermic part of the roast. Heat causes a reaction between the carbohydrates and amino acids in the beans. This causes changes in color, flavor, and nutritional content.

The change in color is due to the production of melanoidins. These are large molecules that not only turn the beans brown, but contribute to mouthfeel and body.

Small changes in the temperature and length of time spent in the Maillard reaction can have a big impact on the final profile of a coffee.

Coffee that has spent longer in the Maillard reaction has been reported to have an increase in viscosity. A shorter Maillard duration can create more perception of sweetness and acidity. This is in part because the acids that produce fruity and sweet notes are destroyed if the coffee remains in the Maillard reaction for too long.

When roasters experiment with roast profiles, it includes varying the length and intensity of Maillard reaction and recording its effects on profile.

Find out more in The S-Curve Roast Profile: Exploring Roasting Basics

some green coffee on a farmer's hand

Green coffee beans. Credit: Ana Valencia

  • Strecker Degradation

This is a process that depends on the Maillard reaction. Amino acids react with carbonyl-grouped molecules to create compounds such as aldehydes and ketones. As roasters, we don’t need to understand exactly what these compounds are – the important thing to recognize is that this reaction is essential for the creation of aroma- and flavor-producing compounds.

  • Caramelization of Sugars

At around 170°C/338°F, heat causes large, complex carbohydrates to break down into smaller sugar molecules that can be dissolved in water. This means that the perceived sweetness level of your finished brew increases. This reaction continues until the end of the roast and it also contributes to the sweet notes in the coffee’s aroma, such as caramel and almond ones.

a coffee roaster ready to work

A coffee roaster. Credit: Miguel Regalado

Volatile & Non-Volatile Compounds

You may hear mention of volatile and non-volatile compounds in roasted coffee. In general, volatile compounds are responsible for aroma and some non-volatile ones contribute to flavor. But what are they?

Volatile compounds are organic chemicals that have a high vapor pressure at room temperature. Many of them are formed in Strecker degradation, in the development stage of roasting. When aroma-producing volatile compounds disperse, we experience that signature coffee smell. These include:

  • Aldehydes, which add fruity, green aromas.
  • Furans, which contribute caramel odors
  • Pyrazines, which have an earthy scent.
  • Sulfur-containing compounds, including 2-furfurylthiol. Some of these have an aroma that is commonly described as “roasted coffee” but there are others that don’t smell as appealing in isolation. For example, methanethiol smells like rotten cabbage.
  • Guaiacol, which has smoky, spicy tones.

Carbon dioxide is a volatile compound that doesn’t contribute to aroma, but which does have an impact on body.

a roastery into a coffee shop

A roaster at work. Credit: Viktor Nikolaienko

Non-volatile compounds are simply substances that are stable at room temperature. That is, they don’t vaporize. Some of these compounds are changed during roasting, whereas others remain stable through the process. Many non-volatile compounds contribute to flavor and profile.

Examples include caffeine, which is responsible for some bitter flavors. Caffeine is naturally occurring in coffee and remains unchanged by the roasting process. Other non-volatile compounds include sucrose, which provides sweetness, and lipids, which provide body and mouthfeel. The melanoidins that create color and body are also non-volatile compounds.

freshly roasted coffee beans

Freshly roasted beans. Credit: Caleb Minear

The Role of Acids

Acids play an important role in creating flavor and are sensitive to heat. Roasting can degrade some acids and create others.

For example, the citric and tartaric acids that produce fruity and sweet notes are broken down during roasting so a long or overly hot roast can hugely reduce the sweetness of the final profile.

Coffee contains a high amount of chlorogenic acid, which roasting breaks down into caffeic and quinic acid. Both chlorogenic acid and the resulting quinic acid are thought to provide bitterness and astringency.

Learn more in Why Are Some Coffees More Acidic Than Others? A Brew & Roast Guide

coffee beans being roasted

Freshly roasted beans. Credit: Gregory Hayes

Coffee roasting includes a number of chemical transformations that contribute to the flavor, aroma, and body of your finished cup. Many of these reactions are sensitive to variations in temperature and length of exposure to heat. So a small change in roasting technique can have a profound impact on profile.

Understanding what happens during roasting and knowing why these changes occur can help you make more informed choices. If you have an overview of how chemical compounds are created and changed during the process, you can better understand what went wrong, or right, with your batch and use the information to make your next one better.

You might also like A Guide to Achieving Consistency in Coffee Roasting

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