Coffee contains many complex acids, such as citric, malic, and lactic, all of which contribute to its flavour and aroma.
One acid that plays a particularly vital role is chlorogenic acid (CGA). In essence, chlorogenic acid is an important biologically active dietary compound produced by certain plant species and is a major component of coffee.
During a roast, chlorogenic acids may convert into aroma compounds, contribute to the colour of the final bean, and add bitter-tasting notes to the cup.
Therefore, the more roasters understand where CGAs come from and how to manipulate them during a roast, the better tasting coffees they may be able to create.
That said, converting CGAs into these flavourful compounds is a complex process, as one mistake may cause the entire batch to taste sour or astringent.
To understand more about chlorogenic acids in coffee, I spoke with the founder of Caffeina Consulting, Emma Haines.
What is chlorogenic acid (CGA)?
Chlorogenic acids are also known as esters, which are formed when an alcohol and a carboxylic acid combine.
Once these esters are formed, they can easily be broken down into their individual components via hydrolysis.
CGAs only exist in organic form, which means they contain carbon, and all chlorogenic acids are composed of caffeic and quinic acids. Notably, the number of side chains will determine whether these are -mono, -di, or feruloyl type acids.
Often, chlorogenic acids are referred to as a singular acid, despite them being a family.
“Within this ‘family’ of compounds, there are essentially two sides,” explains Emma, who is also an authorised Specialty Coffee Association Trainer. “These are mono-caffeoyl and di-caffeoyl acids.”
“Mono-caffeoyls are simpler and break down quite easily during a roast, whereas di-caffeoyls have a larger structure and are much more resistant to roasting,” Emma explains.
Typically, di-caffeoyl compounds are known for bitter tastes and are more prevalent in robusta coffee. It is believed this bitter note contributes to the plant’s defence mechanism, alongside caffeine.
Chlorogenic acids are also used in building lignin – an essential building material for plants. “Due to its natural occurrence in plants, it is naturally present in their seeds, hence its appearance in green coffee,” Emma says.
In coffee, CGAs account for around 6% to 8% of their dried weight in arabica coffee, and up to 10% in robusta.
“While 8% may not seem overly high, it is around 6 to 8 times the concentration of caffeine,” Emma explains. “This puts the importance of this group of acids into perspective.”
What role does chlorogenic acid play in coffee?
Essentially, chlorogenic acids help immobilise caffeine molecules, as caffeine is toxic to plants despite it being a valuable defence mechanism.
In nature, caffeine and CGAs exist as a complex, which means they are usually found together. One of CGAs most significant features is their chemical instability and degradation during heat application.
For example, during a roast, some CGAs will convert into aroma compounds, contributing to the overall aroma and flavour of the beverage.
That said, this will differ as the type and concentration of CGAs will vary between plants, depending on its genetic structure, variety, the age of the plant, and the environment in which it is grown.
CGAs also contribute to the colour of the coffee. Most CGAs are broken down during a roast and may recombine with numerous other compounds to form melanoidins – which give coffee its infamous brown colour.
When coffee is roasted dark, the quinic and caffeic acids break down into their individual components, each of which is innately bitter. This helps explain why dark roasted coffee is often more bitter.
While there are many other bitter-tasting compounds, CGAs are a larger contributor.
It is important to note aroma compounds predominately come from the breakdown of sugars during roasting, which form furans during the Maillard reaction.
Typically, these compounds have caramel, malty, and sweet aromas, which play a critical role in determining the quality of coffee.
“Many acids are produced during roasting,” Emma says. “However, citric acid is present throughout, as it is an important contributor to the perceived acidity in coffee.”
Malic acid is also present in the green seed and is attributed to the crisp green apple acidity many specialty drinkers love. That said, both of these acids will break down during roasting.
Formic, acetic, lactic, and glycolic acids are formed during roasting, with sucrose being a precursor for these formations. This is why different green coffee sucrose levels will result in different amounts of acid post roast.
Can we taste chlorogenic acid in a cup of coffee?
Given the complex nature of CGAs in coffee, it can be challenging to pinpoint exactly what taste or aroma contributions they have.
“Typically, we know that the longer the roast, the more decomposition occurs and the less we are able to perceive these acids,” Emma explains.
“When CGAs decompose and essentially ‘split’ to create quinic and caffeic acids, we can perceive these as sour and astringent. Both impact the final cup in different ways,” she adds.
Essentially, acids are one of the most essential components of coffee for their ability to affect the taste and create these flavour “precursors”.
The perception of acidity itself is an important contributor to coffee quality. It helps distinguish a higher score when cupped by professionals, and is also deemed preferable by consumers during hedonic testing.
Alongside their sour taste, some acids in coffee will also have aroma qualities. These include acetic aromas, such as vinegar, and formic acids that are perceived as pungent and fermented.
Quinic acid also contributes to the mouthfeel and astringent sensation, while caffeic acid tastes bitter.
“Studies on the organoleptic properties of CGAs are still limited, but 5-caffeoylquinic acid has been attributed to being minimally acidic, while the di-caffeolyl mixtures have been noted as having more bitterness and a metallic mouthfeel,” Emma explains.
Due to this lack of understanding of pure CGAs, it is difficult to understand whether the pure CGAs pre-roast and the post-roast compounds are similar.
“The breakdown of CGAs starts around, or just before, first crack,” Emma says. “Progressive roasting will decrease the concentration of all CGAs.”
According to a recent report, around 45% and 54% of CGA were lost in a light roast compared to their green form, whereas up to 99% were lost in darker roasts.
The more roasters understand about CGAs, the better they can manipulate them during a roast to create a coffee offering that will delight consumers and keep them coming back.
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A special thank you to Dr. Joseph Rivera of coffeechemistry.com for contributing to the resources for this article.