When coffee is roasted, it changes both physically and chemically. Intrigued? Read on to find out more.
You may not realise it, but the coffee that you are drinking while reading this blog has gone through a lot of changes to move on from its raw green state to its roasted state. In this post, we’ve taken a closer look at how coffee changes when it’s roasted.
How Coffee Changes When It’s Roasted – The Physical Changes
The most obvious physical change that occurs to coffee as it is being roasted is its colour. The longer coffee is roasted, the darker it will become.
When roasting coffee, the beans will start in their raw state.
Colour-wise, they are a sort of dull green colour. As the temperature in the roaster increases, the beans will turn more toward yellow. From here the beans will start to turn light brown and then, as they continue to roast, start to turn dark brown.
Once a coffee has entered the ‘development phase,’ there are names given to the beans as the roast progresses. ‘Cinnamon roast’ is the term given to the lightest of roasts. Here, the beans are dropped very early into the first crack (when the beans enter an exothermic reaction).
From here, the beans can be classed as one of five alternative terms, depending on the desired roast profile. These are ‘City’, ‘Full City’, ‘Viennese’, ‘French’ and ‘Italian’.
The darker the roast, the further toward ‘Italian’ the term for that particular coffee heads. However, most roasteries describe their coffees as ‘Light Roast’, ‘Medium Roast’ or ‘Dark Roast’ to help avoid any confusing terms.
Check out ‘The Different Types of Roasting’ for more.
Moisture & Mass
Water makes up 10-12% of raw green beans. However, during the roasting process, the moisture content in the beans is reduced to around 2.5%. Additional moisture is created through chemical reactions; however, this is turned to vapour during roasting.
As a result of the beans losing some of their moisture content, their overall mass is reduced. Beans lose around 12-20% of their weight during roasting.
In lighter roasts, water makes up about 90% of the lost weight in a bean. The other 10% of the percentage of weight loss is made up of around 2% of organic matter. This includes small amounts of chaff, carbon monoxide and nitrogen.
When beans are roasted darker, they lose a higher percentage of organic matter with medium roasts losing 5-8% of their organic matter, and dark roasts losing up to 12%.
While beans lose their weight during roasting, they expand to 150-190% of their original volume. This means that as the beans lose their weight but expand in volume, their density halves.
Coffee beans contain oils. During roasting, these oils rise to the surface of the bean from the centre of each cell that makes up the coffee bean. These oils help to keep volatile compounds inside the cell.
A quick chemistry lesson for you – volatile compounds are chemicals that have high vapour pressure at room temperature. They are responsible for creating the flavours and aromas of coffee.
If the oils weren’t there to keep the volatile compounds in, those flavours and aromas would be gone way before the beans were roasted.
When coffee beans are roasted very dark, they will have an oily appearance. This is due to the oils reaching the surface of the bean.
How Coffee Changes When It’s Roasted – The Chemical Changes
The Maillard Reaction
When coffee reaches a temperature of around 150°C, the Maillard Reaction begins. This process begins while the beans are absorbing heat (endothermic reaction) and continues throughout the beans releasing heat (exothermic reaction at first crack).
Heat causes the carbohydrates and amino acids within the beans to react. The control of this allows us to control how fast the beans change their colour and how they start to develop new flavours.
As the beans begin to roast, the natural sugars within them begin to caramelise. Coffee Roasters will plan how long they desire for the beans to stay in the roaster.
The roasting process allows the roaster to control how long the natural sugars within the beans are caramelised for, and thus, control what particular flavours are produced.
For instance, if the beans were dropped from the roaster just as this reaction took place, they would more often than not hold a subtle, caramel sweetness.
If the reaction were allowed to continue, the beans would develop a toffee sweetness leading to a bonfire toffee sweetness until eventually, the sugars would be completely burnt, and the natural sweetness and flavours would be gone. Think of it like making toffee sauce in a pan at home.
This reaction is dependent on the Maillard Reaction. This is the process where amino acids interact with molecules created during the Maillard Reaction. This creates compounds such as aldehydes and ketones.
These compounds also contribute to the creation of flavours and aromas.
Acids in coffee beans help to create flavour. During roasting, some acids will disappear, and some will be created.
An acid is a substance with a pH level that is lower than 7. The ions that the acid gives off activate neurons on the tongue. These neurons send signals to the brain, which we associate with the various flavours that are found in coffee. Pretty cool right?
As the green beans are being roasted and are undergoing their own chemical reactions, the concentration of acid within those beans is constantly changing. As the temperature inside the roaster increases, most acids degrade, however some increase.
The acids found in coffee are found in lots of other things too — mainly fruit.
There are a load of acids that are responsible for the flavours that we associate with coffee. The three that we are most interested in are citric, phosphoric and chlorogenic acid.
Citric acid is found in Arabica beans that have been grown at a high altitude. From this acidity, you can expect to find notes of citrus fruit such as lemon, lime or orange.
Phosphoric acid is much sweeter than most acids. It can turn what would normally be a sour-tasting citrus flavour into a sweeter, more pleasant flavour such as grapefruit or mango.
Coffee contains a high level of chlorogenic acid. During roasting, this is broken down into caffeic and quinic acid. However, quinic acids are responsible for bitterness, astringency and sourness. This is why the darker the roast, the more bitter a coffee will be and lighter the roast, the fruitier the acidity.
And there we have it, that’s how coffee changes when it’s roasted; these are some of the many ways coffee changes both physically and chemically.
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