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The Academics of Acid in Coffee

Five Senses CoffeeGuest Writer 2 February 2016

Bright, tangy, sour. Juicy, tart, winey. Some of the best coffees out there have exciting elements of acidity but objective understanding of this critical facet of coffee quality is often low so let’s break it down a little.

Acidity in coffee plays a large part in defining the overall cup profile and can affect many aspects of our tasting experience.

In the right quantities, acidity adds further layers of complexity and depth in the cup, but what exactly is acidity? Where does it come from and how do we describe it?

People often confuse the scientific description of acidity with the coffee tasting description of acidity. In science, acidity is rated on a pH scale. On this scale, a pH of 7.0 is considered neutral which means that the positive and negative ions in the water are in equal levels and essentially cancel each other out. If a substance is acidic, it has a low pH. For example, lemon juice is acidic and has a pH of 2.0. When it’s added to neutral water, it will release hydroxide ions, thus lowering the pH.

At the opposite end of the scale, soapy water has a pH of 12 and is ‘basic’ (read: the opposite of acidic) – soap, when added to water, releases hydrogen ions making it ‘basic’. The presence of acids in coffee will lower its pH, therefore the pH of coffee tends to vary but it’s generally around 4.5. However in coffee (and other food and drink generally), we think of acidity in terms of our sensory experience – is this snappy or bright? Tart or maybe sour? We don’t often think about where it falls on the pH scale. In fact, experience has shown that even when the pH indicates a certain acidity level, the sensory experience can be perceived quite differently.

So what contributes to our perception of acidity in coffee? Well, it’s actually a mix of many factors – a combination of different acids, the sweetness and how coffee is extracted and served.

There are a huge number of naturally occurring acids in coffee. However, the acidic profile of a cup is defined and shaped all the way along the production chain from the fruit to the processed seed through to roasting and, finally, the brewing process. All of these factors affect both the type and quantity of acidity in the finished brew. For the sake of simplicity, we will consider four main factors which greatly influence the acidity of coffee.

1) It starts with the plant.

Cellular respiration (plant growth and fruit development) predominantly results in the formation of chlorogenic, citric, malic and phosphoric acids. These can all be affected by whether a plant is grown in shade or at altitude. Shade-grown and high altitude coffee tends to have higher levels of organic acids, chlorogenic acids, caffeine and sugars (sucrose).The slow formation of the fruit under these conditions essentially allows for more nutrients to be packed into the fruit, resulting in higher levels of sweetness and organic acids in the cherry.

We also know that plant species and variety play a huge part in the formation of acidity. For example, the Robusta coffee species exhibits much higher levels of chlorogenic acid (around double the quantity) than it’s other main counterpart, Arabica. Chlorogenic acid imparts a bitter, vegetal taste, so particular coffee varietals play an important part in the resulting taste in the cup. Different cultivars of the Arabica species generate different levels of organic acids (and both sucrose and fructose) rendering them fundamentally different in regard to their acidity profiles.

2) Processing method.

When we compare wet (washed) vs. dry (naturals) vs. semi-washed (pulped naturals), we can see the impact of processing on acidity too. Washed coffees are first pulped and then soaked. This process leeches content from coffee, including quantities of sucrose and fructose, leaving high levels of acidity.

Washed coffees typically have the highest overall level of acidity in their cup profile. Naturally processed coffees leave all of the fruit intact, which tends to increase the sweetness of the cup, in turn mitigating our perceived acidity. Pulped naturals, with the mucilage (like a sticky pith around the seed) still intact, sit somewhere in the middle with a mixture of defined acidity and an increased level of sweetness.

3) Roasting methods.

Roasting coffee both converts and breaks down acids and results in the formation of acetic, quinic and caffeic acids, while changing the level of citric and malic acid in the process. Let’s start with looking at how chlorogenic acid behaves during the roasting process. Chlorogenic acid is a family of acids consisting of over six individual acidic compounds. They are grouped into two categories -: mono-caffeoyl and di-caffeoyl. Di-caffeoyl does not break down during roasting, and when present in a brew, it imparts a metallic taste. However, when mono-caffeoyl breaks down, it increases the amount of quinic and caffeic acids in the cup. This will significantly increase during later stages of roast development – a little and you get a sense of deeper body, too much and you’ll get a harsh, unpleasant phenolic character.

Next up, let’s consider citric and malic acids. These are formed naturally during the development of the cherry on the tree – citric is fairly self explanatory tasting somewhat like lemon or grapefruit while malic acidity can be likened to the tart characteristic of a green apple.

Both citric and malic acid deplete during the roasting process. The darker the coffee is roasted, the more these acids are broken down, leading to a flatter, more body-orientated cup profile.

However, in higher quantities they can carry a harsh, astringent sourness. With the right balance, they add a great snappy, fruit-like characteristic which can mesh beautifully with cup sweetness. Finally, when sucrose degrades during the roasting process, it forms acetic acid which you would recognise as vinegar. Vinegar in your coffee certainly doesn’t sound too delicious but in small quantities, paired with some sweetness, it can add some delicious complexity and winey characteristics to the cup.

4) Brewing brings it all together.

While the type of acidity in coffee is largely determined by the aforementioned factors, the amount extracted during brewing will also affect three things; the balance of acidity, the perceived strength/dilution of acidity and the balance of acidity vs sweetness (which for most tasters impacts our evaluation of the quality of the acidity.) You’ll notice that as coffee cools, it exhibits increasing levels of sourness. While coffee is cooling, quinide (a tonic water-like phenolic compound) converts to quinic acid which affects the taste in the cup.

Here’s a quick recap on the most common acids you’ll find in coffee.

Citric acid – think lemons. Certainly the most obvious and detectable acid, citric acid is formed largely in the plant. It exists in high levels in green coffee and reduces in level as a roast progresses.

Malic acid – think pears and green apples. Formed from cellular respiration, malic acid is there from the beginning and imparts a tart and more lingering acidity. Tasty! Most fruity, punchy coffees will have malic acid to thank for that.

Acetic acid – the main ingredient in vinegar. Acetic acid is created during coffee processing and the roasting process, and although not as punchy or noticeable as citric or malic acid, it contributes to a rounded, clean tasting cup. Obviously vinegar in your coffee doesn’t sound too appealing, but a dash of acetic acid can create some great complexity.

Lactic acid – while not as noticeable or punchy as citric or malic acids, lactic acid tends to change the textural aspect of coffee, making it slightly creamy. It also deepens the body of coffee.

Phosphoric acid – is an inorganic (mineral) acid that’s often introduced to the coffee plant via fertilisation. Known to add a beautiful blackcurrant note to Kenyan coffees, it’s lighter and has a more sparkling acidity.

To calibrate your palate to some of these acids, try diluting some of each lemon juice (citric acid), unsweetened green apple juice (malic acid) and brown vinegar (acetic acid) into their own glasses of water. Have a taste of each in this format and then add a little sugar to check out the dramatic transformation when acidity is balanced out with a little sweetness.

When you next taste a coffee, take note of where the experience is popping on your palate. Is it in the same place or giving you same sensation as the lemon juice? You’re probably tasting citric acid in your coffee!

You should now have a much more solid foundation to talk about your experience with coffee’s acidity. Use your newfound acid knowledge to help diagnose and describe your coffees and then parse this info into tasting notes that accurately paint a picture of the experience for your customers – knowledge win!

 

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