Acidity: it’s a divisive word in coffee brewing. For some, it signals sharp, sour flavours. For others, it’s a revered attribute of high-quality coffee.
Make no mistake: sour coffee is unpleasant. But vibrant, bright, and complex acidity, the kind that reminds you of stone fruits or a sweet tangerine, is highly appreciated by coffee professionals and specialty coffee consumers.
Whether your aim when brewing is to highlight a coffee’s natural, juicy acidity or avoid nasty sour notes, the good news is that you can.
Brewing coffee with the Behmor Connected. Credit: Behmor
You can only accentuate the characteristics that are already in your coffee beans. For that reason, it’s important to know what you have on your hands.
Is it a hard- or soft-bean coffee? Hard beans are grown at cooler temperatures (which normally, but not always, correlate with higher elevation). Generally, the harder the bean, the more fruitiness and acidity it is likely to have.
How was it processed: wet/washed, natural/dry, or honey/pulped natural? This is how the coffee beans (which are actually seeds) were removed from the coffee fruit, or cherry. It has a big impact on the flavour of the coffee. Naturals and honeys, if processed well, tend to have enhanced sweetness and body, while washed processing lends itself to a clean coffee profile – the kind of profile that allows complex acidity to shine.
Then there’s the roast: is it dark, medium or light? The darker the roast, the more you taste the roasting process than the coffee itself. A very dark roast will often be bitter, while lighter roasts will display more of the coffee’s natural acidity.
Natural, honey, washed, and roasted coffee beans. Credit: Ana Valencia for Behmor
That cup of coffee you’re drinking? It’s 94–98% water. And this means that the flavour of your coffee is often affected by the water you’re using to brew it.
Water quality is a huge topic, deserving of its own article, but let’s take a look at the basics – starting with hard and soft water. Hard water has a high mineral content, especially in terms of magnesium and calcium. Soft water, on the other hand, has a low mineral content.
Steve Cuevas is the 2017 US Cup Tasters Champion, meaning he won in the US-wide competition to correctly identify different coffees and their qualities by taste and smell alone. “At higher concentrations,” he tells me, “some minerals act as ‘acid buffers’ and will lower the perception of acid… It will still be there, but it won’t taste like it. It will also extract the coffee faster, most likely pulling out bitterness if you try and brew with a time past two minutes,” he explains.
Thomas Chandler is a coffee roaster, molecular biologist, and chemist, so if anyone can tell me more about this, it’s him. He explains that carbonate is a key “acid buffer”: the more carbonate in the water, the less acidic the brew. But he adds that this is not the only compound that can act as a buffer and, what’s more, carbonate’s ability to do this depends on the presence or absence of other compounds in the water.
Soft water, on the other hand, tends to be rich in sodium and Steve says that this will allow the acidity to shine.
However, that doesn’t mean soft water is always better. Christopher Hendon, a chemist from the University of Bath, and Maxwell Colonna-Dashwood discovered that water with high levels of calcium and magnesium – i.e., hard water – will extract more flavours, including more acids. At Barista Camp 2015, Colonna-Dashwood explained that this type of water would also benefit from the presence of buffers.
So, what’s the solution?
This is getting pretty technical and can even sound contradictory at times; however, whether we’re making coffee at home or in a café, we can use this information to help us improve our brew. If your coffee is coming out dull and lifeless, lacking that sparkling acidity you know it possesses, or is coming out sour, but you know it isn’t due to the coffee beans or how you’re brewing it, try changing the water. Use a filter, try bottled water, and just see how the taste of your coffee changes.
Coffee being served. Credit: Behmor
Water may be the biggest ingredient in your coffee, but it’s the combination of coffee and water that creates its flavour – meaning that how you combine them will affect the brew’s taste.
Mixing water and coffee leads to extraction: the slow diffusion of coffee flavour and aroma compounds from the beans into the water. And the amount of coffee and water, brew time, coffee grind size, water temperature, and more all affect how many of those compounds are extracted.
This doesn’t just create a “stronger” or “weaker” cup of coffee, because certain flavour and aroma compounds are extracted at different times.
Early in the extraction process, you’ll get the fruity acids. Next comes sweetness and balance, and then, finally, bitterness. This means that under-extracted coffee will taste sour while over-extracted coffee will be bitter. You want the Goldilocks of coffee extraction: sweetness, balance, body, and the perfect amount of acidity for your preferences.
Roasted beans and the brew they become. Credit: Behmor
There really is no one-size-fits-all grind profile. However, there are some basic rules that will help you brew for greater, or less, acidity.
A coarser grind size will slow down the rate of extraction because there is less surface area. (Note that extraction rate, the speed at which flavours and aromas are extracted, is different to brew time, the amount of time that the ground coffee spends in the water.) This means that a coarser grind should lead to greater sparkling acidity – or, if taken too far, sourness.
Want more acidity? Grind coarser. Less acidity? Grind finer.
Of course, grind size is just one of many factors. The ideal grind size will also vary according to the coffee. For example, darker roasted coffees tend to be more soluble and will, therefore, extract quicker. Often, a coarser grind will suit them better.
Coarse and fine coffee grinds. Credit: Nicholas Yamada
Another big factor in coffee extraction is brew or contact time. The longer the brew time, the more is being extracted.
You can use beans you like, good-quality water, and the ideal grind size, and still end up with coffee that’s too sour or too dull if you have the wrong brew time.
As the team at Barista Hustle emphasise, grind size doesn’t change what is being extracted. It just changes when it’s being extracted. So if you have a coarse grind size but a long brew time, you still won’t get much acidity in your cup. And if you have a fine grind size but a very short extraction time, the cup might still taste sour.
So, shorten or lengthen your brew time to taste more or less acidity, respectively.
Fruity coffee with a fruity scone. Credit: Ana Valencia for Behmor
Another important factor that can affect the rate of extraction is water temperature. Joe Behm is the Founder and CEO of Behmor, and he designs and manufactures SCA-certified smart coffee brewers and roasters. The Behmor Brewer offers 1°F temperature control so that users can control exactly how much acidity, sweetness, and more is in their coffee.
The hotter the water, the quicker the compounds will extract. The cooler the temperature, the slower the flavours and aromas will extract. However, there are also some compounds that won’t extract at certain temperatures (this is why cold brew is known for its sweet, smooth taste and limited acidity). What’s more, remember that all of this interacts with grind size, brew time, and more.
Joe actually favours a higher temperature combined with a shorter brew time. “To bring out the acidity in a brew, I do it at higher temperatures to bring the acidity out, say in the 204ºF/95ºC to 205ºF/96ºC range,” he tells me.
Steve agrees – providing the water quality is good. He says that some people may be tempted to use lower temperatures to avoid bitterness in the cup. However, with good water (and all other variables in the recipe controlled for), he finds he gets a more pronounced acidity at 94ºC/202ºF than 91ºC/197ºF.
Coffee being brewed in a Behmor Brazen Plus. Credit: Behmor
The wonderful thing about brewing our own coffee is that we get to drink it exactly the way we like it. And although it gets a little technical at times, mastering these concepts will enable us to brew a delicious cup of coffee every single time.
So, go ahead and tweak your coffee recipes. Try different types of water. And experiment with water temperature. Because even just a 1ºF difference may be the key to turning a dull or sour brew into a coffee full of bright, fruity acidity.