In Focus: Acids in Coffee

Posted by Stella Cochrane on

Our CMCR KL Academy Trainer, Pablo Bialaszynski, took some time out of his busy schedule recently to investigate the complex effect of acids in coffee.

Here's what he found:

'When we think of acids, we often think of a pH level chart and reminisce about school science experiments with lemon juice and vinegar. In this case, we’re going to focus deeper on the source of coffee’s acids and the sensorial experiences that come with them.

This post aims to provide a perspective that simplifies acidity, rather than complicate a topic that is already tricky. You might find a greater appreciation for your cup of specialty coffee with a little more understanding.

There are a few stages of a specialty coffee’s journey that could alter acidity in the final cup and inevitably how we sense and perceive it. It could be the type of Arabica, the environment of where it’s grown, the method of processing, as well the effects of roasting.

yellow bourbons   coffee cherries    drying coffee

There are two main groups of acids, known as 'organic acids' and 'inorganic acids'. 'Organic acids' - as the name would suggest – can be easily considered as naturally occurring in the plant and fruit's development via cellular respiration. While 'inorganic acids' are acids present that could be a byproduct of, for example, a fertilizer or soil rich in certain minerals.

Organic acids are more prominent in gradually higher altitude Arabica; and there are over two dozen of these acids! Different cultivars can be composed of different levels of these same acids. An effective analogy is the one of apples. Have a look at these different apples below, could you try to imagine how they taste individually?

apples

Now what’s remarkable is that you didn’t need to be an aficionado of apples to know which of these would be the juiciest and sweetest, or has a tart sourness and crisp feel.

These apples – like specialty Arabica – are rich in acids and sugars. In this case, over 90% of apples acids are malic acid. And while an orange would also have malic acid, we would never compare 'an apple-like flavour' to that of an orange, simply because of its undeniable character, rich in citric acidity, or citrus. In fact, stone fruits like peaches have both malic and citric acids in almost equal amounts!

Of course, we couldn't talk about acids without considering the environment that the coffee grows in, as this is a big factor effecting its end quality. Higher elevations (thus lower levels of oxygen) promote the maturation of harder and denser seeds within the fruit and the cool nights slow down the ripening of the fruit. Bright citric acid slowly matures into sweeter malic acid and ultimately changes how acidity can differ in presentable flavour.

Altitude map

Different processing methods will also effect the perception of acids in the coffee. Washed processed coffees are higher in perceived acidity and this is attributable to the removal of mucilage which contain most of the fruit’s sugars. A 'natural' or 'dry' processed coffee retains the mucilage and therefore has a greater presence of sweetness which can mitigate perception of acidity. An easy analogy is that of making lemonade - We know drinking lemon juice concentrate isn’t a delight, and diluting it with water might make the sourness less painful, but all it takes is some sugar to turn it into something more palatable.

Now that we have some good understanding of how certain acids form, let’s look at how roasting coffee can alter that. The concept of roasting can be viewed pretty simply; a light roasted coffee generally preserves acidity. while a darker roasted coffee degrades specific acids whilst increasing a very select handful of others. There are many chemical reactions in roasting; as well there are many ways to go about it.  

roasting stages of coffee

The fruity acids like citric and malic degrade at higher temperatures and longer roasts, while certain groups like chlorogenic acids have components that break down and increase during the roast. Quinic and caffeic acids are a result of this degradation and are commonly associated with bitterness, astringency and the body found in darker roasted coffee. Naturally, we wouldn’t want acidity to be the only character, it would overwhelm the drink, nor do we want a cup full of bitterness - Balance is key and this is where skill and experience in roasting comes into play.

Here are the few acids that we can easily identify in taste!

Citric acid – as you would find in oranges and lemons, it is easily detectable. While it’s known for bright and juicy sensations, this acid has one of the greatest presences in green coffee. It reduces in darker roasts.

Malic acid – quite like the crispy joy of biting into a green apple or sweet pear, this is a tart and lingering acidity. A prized quality in specialty-grade Arabica coffee.

Acetic acid – the taste of vinegar, how delicious. A common byproduct of washed coffees that contribute to a rounded, clean tasting cup. Poor fermentation can accentuate this acidity but the right amount of it makes for nice complexity.

Tartaric acid – the grapey and winey notes in coffee, which is understandable considering grapes, are rich in this acid. Too much however will be sour and unpleasant, this acidity is one of the main acids in wine.

Quinic acid – the byproduct of cholorogenic acid decomposing during roast. It increases in concentration in darker roasted coffee, stale coffee and coffee that was brewed and cooled or sat in a coffee pot too long over a hot plate. It does give coffee a clean finish but is better known for the sourness of coffee that turned cold.

Phosphoric acid – introduced during fertilization, this inorganic acid is commonly found in Kenyan coffees. With a hint of sparkling in texture, it’s the reason for the delicious currant notes. It’s believed that the larger presence of this inorganic mineral makes Kenyans so iconic.

As you can see, there’s much room for flavour expression in specialty coffee!

apples              citrus fruits

There is a plentiful of things for us at Common Man Coffee Roasters to consider. Tasting different coffees from one harvest to another, new and old cultivars from varying regions and countries, analyzing and improving our understanding of how different coffees react in the roast; and, inevitably how we brew our coffee and serve our consumers. Put simply, making delicious coffee common.

Next time you find yourself in a specialty café, look out for tasting notes in the coffee you ordered, even if they’re as funky as raspberry, cola, liquorice, banana or coriander.

Thanks for reading, this calls for a great brew!

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