This is the first topic in the learning project series of posts. Please click through at the end to share a link for further reading on the topic.
Acidity is an important part of coffee, and weaves its way through the entire supply chain of the industry from variety to picking, from process to roasting and from brewing to tasting. I want to start with the basics, cover some of the tasting aspect and then look at acids specific to coffee.
We tend to define acids as having a pH lower than 7, or as having a sour taste, but these aren’t particularly scientific. In fact there are three different scientific definitions of an acid: The Arrhenius definition, the Brønsted-Lowry definition and the Lewis definition. I think the Arrhenius definition is going to serve us best here which is:
An Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H+) that is, an acid increases the concentration of H+ ions in an aqueous solution.
The important part to note here is the presence of H+ ions, as we’ll discuss this later. It should be noted that his definition specifies an aqueous solution, so you could argue that pure hydrochloric acid, or pure sulphuric acid wouldn’t count as acid but this would be nitpicking.
The measurement of the level of acidity is done through measuring the number of these H+ ions, and is usually communicated through the pH scale. While there is some disagreement on exactly what pH stands for but the Carlsberg Foundation (tied to the Carlsberg Labratory where Søren Peder Lauritz Sørensen did the work to first introduce the scale) claims it stands for “power of hydrogen”.
It is important to remember that the pH scale is logarithmic. This means that each whole value below 7 is ten times more acidic. Thus a pH of 4 is 10 times more acidic than a pH of 5, and 100 times more acidic than a pH of 6.
Acidity is often described as being one of the 5 tastes, which is deeply unfair to other tastes such as pungency, astringency, piquancy or metallic tastes. (Much like saying we only have five senses ignores senses like balance or the sensation radiant heat). It is a critical part of the foods we taste, though it doesn’t have the same functionality as sweetness (the suggestion that something is good to eat) or bitterness (the warning that a food may be poisonous).
We have a very good idea of how taste receptors, in our mouths, detect sourness. That doesn’t mean it is particularly simple though. What happens is that a hydrogen ion (H+) enters the cell and essentially triggers an electrical response that fires the nerve that sends the signal to the brain that we are tasting something acidic.
Types of Acid in Food
There are a variety of different acids in food that influence our perception and enjoyment of it. Adding acids to foods (either more traditionally by using an acidic liquid like a juice, or in the more modern way of using them in powdered forms as food additives) is likely as old as cooking and the ideas of recipes. They also play a crucial role in preservation as well as providing some anti-oxidative properties.
Here are some of the key acids in food:
Acetic acid – We are most familiar with this acid as vinegar (though only acetic acid produced through fermentation can be called vinegar). This is an acid that can be extremely unpleasant (it is awful when present in coffee through defective processing), but we do like to add it to fried potatoes a lot.
Citric acid – This is the acid found in citrus fruit, and if you give most people plain water with citric acid added to it they will describe as tasting of lemons. This is an incredibly popular food additive – it is the acid used in sour sweets/candy – and it is no longer efficient to extract it from citrus fruit. Instead it is now often produced by fermenting a carbohydrate (like molasses) using a mould called Aspergillus niger.
Fumaric acid – This one doesn’t really occur in common foodstuffs (unless you enjoy eating lichen and Iceland moss), but it is used extensively as a food additive and acidity regulator. It is often used in soft drinks and can also be added as a coagulent in things like stovetop pudding mixes.
Lactic acid – Most of us are as familiar with the pain of lactic acid build up in muscles, as we are with the taste of it in sour milk products like yoghurt or cottage cheese, or with the sourness it brings to sourdough bread. Outside of food it also is used in things like detergents and as an active ingredient in mosquito lures.
Malic acid – This is the acid found in fruit like apples and pears, though for a more explicit experience of malic acid then the delicious tartness of rhubard is an excellent example. It was first isolated from apples, and as such takes its name from the latin malum. Despite being a little more expensive than citric acid, it is used in many similar products such as sodas (particularly diet ones) and also in the sourest of sour candies.
Phosphoric acid – This isn’t really a naturally occurring acid, but we consume huge amounts of the stuff, mostly in one particular form: cola. This particular type of acidity is often described as being relatively harsh compared to other acids, but it seems to provide excellent balance in cola drinks specifically.
Tartaric acid – This occurs naturally in many fruits like bananas, grapes and tamarinds. I confess I am probably most familiar with it as the primary acidity found in wine (along with citric, malic, ascorbic and many others).
Most of these acids can be bought easily in powdered form, and simple (low concentration!) solutions can be made for tasting. The tasting, and blind identification, of acids has become a staple part of coffee tasting tests for a variety of certifications.
Acids in Coffee
There are a great deal of different acids in green coffee, byproducts of a cycle of chemical reactions called the Calvin Cycle. Some of these survive the roasting process intact, but many don’t. The longer and darker that a coffee is roasted the lower the perceived acidity tends to be when that coffee is brewed and tasted. This seems pretty simple – but when you dive into the chemistry a little bit you will see that it isn’t quite as simple as that.
A variety of the acids we’ve already mentioned (citric, acetic, lactic, malic and phosphoric) have been identified in brewed coffee, but two others have as well (at higher concentrations than those already mentioned). They are:
Quinic Acid – While it usually is in crystalline form, quinic acids melts at around 160C and coffee that is being roasted will comfortably exceed these temperatures. It is considered to add a positive acidity to the cup, and give coffees a “clean finish”.
Chlorogenic Acids (CGAs) - Chlorogenic acids (a group of acids, rather than just one) contain no chlorine. The name comes from the Greek χλωρός (light green) and -γένος (a suffix meaning “giving rise to”), because of the green colour produced when chlorogenic acids are oxidised. These acids play an important role in the generation of flavours during the roasting process.
CGAs degrade quite dramatically during the roasting process, with around 50% of them gone by the time a medium roast is reached. As CGAs break down the byproducts are both caffeic acid and quinic acid.
This is designed as only a brief introduction, but we all want to go deeper…
Now it is your turn! Submit a link to further reading on acidity, and vote for whichever topic you would to see focused on next. The link can be about acidity in coffee specifically, or about acidity in general. It can be examples of tastings to try, or more about how we taste and how acidity interacts with other tastes and flavours. Anything and everything!
Here are the links submitted by readers, for those interested in learning more about acidity:
Taste: The sour and bitter story
Brewing Sour Beers at Home
What is the difference between tannins and acidity?
Acidity and Coffee
What is acidity in regards to the taste of coffee?
Coffee Chemistry Acids
Low Acidity Coffee Reviews
“Coffee Gone Sour”
A pinch of salt in your coffee, sir?
Brew temperature – 180 vs 200
Sourness and Acidity
Coffee and acidity: the science and experience
Coffee Chemistry – Acidity
Teaching to Taste
What’s geology got to do with it?
Prufrock – Coffee Science
I found my “roots” to coffee in Africa
Effect of sugar on acid perception in wine (abstract)
A Structural Basis for the Biosynthesis of the Major Chlorogenic Acids Found in Coffee (full)
Alchemy in the roasting lab (pdf)
Improving the flavour of fruit products with acidulants (pdf)
Taste Receptors in the Gastrointestinal Tract III. Salty and sour taste: sensing of sodium and protons by the tongue (full)
Biosynthesis of Chlorogenic Acids in Growing and Ripening Fruits of Coffea arabica and Coffea canephora Plants (pdf full)