Coffee Chemistry 101: The Art of Acidity in Coffee

A term that was typically limited to the quality labs of large commodity trading houses and roasteries across the world prior to the dawn of the Third Millennium CE – ACIDITY –  is the vital element that brightens or spites the cup of coffee in your hand. It has traversed a long path from being a complicated scientific term used by quality experts to an absolute metric used by conscious coffee consumers to make a purchase decision. But how do we understand acidity? Does it really influence the experience and quality of your cup? And is it necessary to allow this particular aspect of coffee to influence how you choose which beans to brew? Join me on this short but hopefully productive journey to find the answers to these questions and to possibly unravel new questions that might expand our understanding of the world’s favourite beverage. We will, as you will soon notice, discover how acidity is not limited to a science but is quite an art in itself given the learning curve, intuition and pure skill that is necessary to inject/induce/introduce (or whatever other perfect term you can deduce) the ideal style and degree of acidity into your coffee.

What is Acidity?

Let’s try and be absolutely unsnobbish with the definition of this term – acidity is that juicy, sparkly bright sensation that lights up your palate when you sip on your coffee. It’s similar to the sensation you get when you bite into an apple or orange (if you’re thinking pleasant) or into a raw lemon (if you’re thinking otherwise). The terms one would generally use to describe acidity would include lively, tangy, sharp, bright, juicy, sparkly and the list goes on. But if you grasp the general sense of what the term means, we could conclude that it describes to us how “alive” or “awake” a coffee is.

But does that mean the quality of a cup is proportional to its degree of acidity? Absolutely not! While acidity can enliven and make delicious a cup of coffee, it can absolutely destroy it if it is present in unsavoury amounts. Too much acidity can make your cup overly sour which will mask some of the sweet and subtler notes dissolving them into oblivion. Too little acidity and your coffee will taste flat and lifeless drowning your brewing efforts in vanity.

 

As we will learn later in this article, the degree of acidity in your coffee is influenced by a variety of factors – some of them that can and others that cannot be influenced by human intervention. But this is where the art of acidity becomes important as the human handler at various levels of the coffee value chain treats his or her coffee in a manner that ensures it has the perfect acidity or the exact acidity that is preferred by the client.

Our Q-Grader here at Naivo – Jessie Ovian says, “We need to keep in mind how the coffee is going to be brewed as that gives us an idea of how much acidity is the right amount in the coffee. For example, if the end consumer prefers coffee with milk, this changes the way we look at things as a higher portion of milk itself brings a large amount of acidity to the cup.”

This gives us an idea of how product developers or roasters think. While one aspect of focus is directed toward flavor development, another aspect is on how the acidity in the cup affects the physical system of the consumer. For example, a coffee that is already quite acidic mixed with milk can yield all sorts of unpleasant sensations in the belly of the consumer.

Jessie goes on to note, “I often observe that lots from East Africa that we tend to roast lighter to enhance their floral and bright characters end up curdling milk due to their sparkling acidity. So, I always recommend keeping these coffees away from milk.”

 

Here’s an interesting point to note. In India, most blends used for traditional South Indian style filter coffee and also instant coffee (which is broadly consumed with milk across the country) contain a large proportion of Robusta. This is due to the fact that Robusta exhibits lesser acidity compared to Arabica/ In addition to inducing desirable bitterness as well as body into the cup, the Robusta helps to control the higher degree of acidity that is being drawn from the portion of Arabica present in the blend. The fuller-bodied characteristic of Robusta in combination with its acidity-controlling power makes the filter coffee blend suitable to consume with milk.

A Peek Into the Science

Maybe we can spare a few minutes to be coffee snobs? Or maybe not, lets try and keep the science simple.

A deep enough dive into the chemistry of coffee reveals to us that acidity can be credited to two categories of acids – organic acids and chlorogenic acid. Organic acids include citric, malic, quinic, succinic, acetic, and tartaric acids. Each of these compounds bring their own aspect of flavor to the cup, a few of which are listed below.

• Malic Acidity – remember biting into an apple? That’s the kind of lively flavor this kind of acidity brings to your cup. Imagine biting into a juicy green apple. A coffee with good malic acidity will devolve a subtly similar experience to you when you take your first sip.
• Citric Acidity – remember biting into an orange, a mausmbi or a grapefruit? Coffees with citric acid are in general, well, more citrusy in their flavors.
• Tartaric Acidity – this is the kind of acidity that one would generally identify in wine – particularly red wine. It is a grape-like acidity and is easily distinguishable from other categories of acidity. It is rarer, however, than its counterparts.
• Acetic Acidity – this is a vinegarish type of acidity that is unpleasant and something that you typically don’t look for in your cup.

 

Now the second category – chlorogenic acid is something that’s present as a whole molecule but then gets broken down during the roasting process into quinic and caffeic acids. These acids are generally responsible for the bitter, smoky and astringent flavors in coffee. Logic dictates that the darker a coffee is roasted, the larger the amounts of these acids that are generated hence resulting in smokier and bitterer notes.

 

Let’s now understand how different phases of the coffee journey relate to acidity and how it is influenced by the various activities that take place in each of these phases.

The Journey of Acidity

Origin

Soil plays a major role in developing the chemistry of coffee beans and hence it is obvious that it should play a large role in defining the acidity inherent in raw coffee beans before they are shipped off to a trading house or roastery. We won’t dive deep into what composition of soil and what nutrients present in them construct acidity in coffee beans as we need to save some content for masterclasses and advanced, structured courses at the university level. Keeping it simple, we’ve noticed that coffees that we’ve sourced from Kenya, Ethiopia and Tanzania had more malic features compared to Latin American coffees from Colombia or Panama which exhibited more citric features. Indian coffees exhibited citric acidity when they were high-grown arabicas while special processed coffees fermented in different ways also exhibited malic acidity. We’ll leave the deductions of exact correlations to the hardworking PHDs. I hope you’ve got the basic gist which is that soil (and also post-harvest processing) influences acidity!

Varietal of plant can also influence acidity. For example, over the years we’ve noticed that Chandragiri is much brighter in acidity compared to Kents, Cauvery and Selection 795 all from the same estate. Now one factor we can attribute this observation to is genetics. However, certain varietals are grown only in certain conditions and it is inevitable that the conditions also impact flavor.

Finally, elevation plays a huge role. It’s a well-known fact that higher grown coffees are more acidic than lower grown ones. This might not be due to altitude alone but also due to temperature. Cooler temperatures at higher altitudes allow for slower maturing/ripening of coffee cherries – the extra time allowing for development of more complex flavours in the fruit which will eventually be imbibed by the two little beans being nurtured inside.

Post-Harvest Processing

Post-harvest processing primarily deals with how coffee beans (actually seeds) are freed from their bondage to the surrounding fruit and mucilage. The sweetness and acidity of a cup can be greatly influenced by choosing how the coffee is processed and playing with a multitude of factors.

The wet/washed process involves pulping the cherries and rinsing them which gets rid of all the fruit around the beans. The sugars are given very little opportunity to ferment around the beans and influence their end flavor. Due to this reason, washed coffees are generally bright in acidity as there is no overpowering sweetness present to mask the acidity.

In case of sundried coffees (often referred to as naturals), the entire fruit is left to dry around the beans. In this case, all that sugar material (sucrose and fructose) is available for microbes to act upon and ferment and this leads to a great deal of sweetness getting imbibed into the beans. In this case, the sweetness tends to stand out and overpowers the perceived acidity.

 

Several alternative methods such as honey processing, anaerobic digestion, carbonic maceration etc. also involve leaving some or all of the fruit around the beans and allowing for drying in different environments (sun, shade, oxygen-free environments) which all create opportunities for different types of fermentation to take place. It is common in the current age to also use barrels previously used to ferment alcohol to age coffee in or ferment coffee along with fruit such as pineapple, papaya and watermelon in aerobic or anaerobic environments. Controlling a variety of all these relevant factors allows for growers to play chess between acidity and sweetness and give birth to various unique lots of coffees with unique flavor profiles.

So, there we have it! Processing influences acidity; as simple as that.

Roasting

A roaster is always focused on bringing out the best in a lot of coffee. The focus is always on what’s already present in the coffee and how to unravel it. This is undeniably why sourcing is such an important task for any roaster. Acidity is never added to a coffee, it is brought out of a coffee. There are several factors that a roaster can control to dictate the final outcome of acidity in the end product. They are much simpler than you might be starting to imagine.

The simplest factor is roast level. Lighter roasted coffees are generally brighter in acidity than darker roasted ones. This is because with the extended application of heat, all those molecules of organic acids responsible for fruity, floral and other such subtle flavors start breaking down and losing their influence on the cup. The natural flavours of the coffee get replaced by smoky, toasty, bitter flavors associated with the process of roasting itself. You could say the acidity gets hidden under these flavours.

 

Another factor is the intensity of heat application that can draw out different levels of acidity from the coffee. Generally, higher intensity of heat draws out more acidity. However, roasters will control the overall heat transfer to ensure their beans don’t get completely scorched. In a traditional drum roasting setup, roasters manipulate heat application, air flow and drum speed over the entire period of the roast to achieve a particular heat transfer curve which gives them a replicable mathematical visualization to execute and achieve a specific flavour profile for any given coffee.

There is no golden curve that a roaster can follow for all coffees. Roast profiles are determined depending on the nature of the coffee, characteristics of its origin, its processing and its transformation with time depending on interaction with environmental factors such as temperature and humidity. Bringing all these factors into a replicable symphony allows a roaster to understand what needs to be manipulated to achieve desirable levels and styles of acidity.

Brewing

The final but albeit most impactful, delightful and powerful operation executed at the end of the life of a coffee bean is brewing. The reputation of a coffee cannot be completed without the right approach to brewing that reveals all of the value that was molded into potential in all the prior phases of growing, processing and roasting. You could source an exquisite Heirloom Arabica from the high heavens of Ethiopia, polished to perfection at the hands of an experienced Roastmaster and end up brewing a flat, unimpressive cup simply because you chose the wrong grind size or set the wrong temperature on your kettle. Three simple factors influence acidity in the brewing phase – grind size, water temperature and brewing time. Acidity is influenced by extraction level and these are the three control variables that define extraction.

In the process of extraction, natural flavours including the floral, fruity and subtler flavours (all closely associated with acidity) are extracted first followed by the sweet flavours and finally the smoky, bitter flavours. Under-extraction will yield an overly acidic, sour cup while over-extraction will yield a bitter and harsh cup. While brewing, you are typically looking to find the perfect balance between natural, sweet and bitter notes. For the three control variables, below are some important points you might want to keep in mind while brewing,

• Finer grind size leads to faster extraction due to more surface area of coffee being exposed to hot water.
• The higher the water temperature, faster will be the extraction due to more chemical activity brought about by the larger amount of energy available.
• A longer brewing time will result in more extraction as the contact time between hot water and coffee grounds is increased, which again results in more chemical activity. Coffee brewed with colder water generally yields a cup with lower acidity. This is the reason why cold brews (as seen in the title image of this article) are very low in acidity. However, the time required to brew coffee with cold water is longer than with hot water as the energy available for extraction is very low; the logical explanation why cold-brewing takes 24 to 48 hours to complete.

Increasing or reducing these three variables in harmony with each other will help you find the perfect balance and unleash not only the most delicious acidity, but also the perfect body and sweetness in your cup.

 

 

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I believe or at least hope that I’ve covered the most essential topics relevant to acidity. It is quite a complex science on its own and merits its own graduate level course. But as an enthusiast with even the simplest interest in coffee, it would help to understand the fundamental variables that can make or break your cup.

Let’s finish with a very important insight here – every level of the coffee value chain defines the deliciousness in your cup. The commitment of the grower, the adventurousness of the processor, the genius of the roaster and the experimentation of the brewer – all of these attributes come together to conjure the magic in your coffee cup!

The next time you’re drinking your coffee, don’t forget to slurp and cherish all that teamwork. Cheers!