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Factors effecting acidity in the cup

May 27, 2013

There is an often repeated statement that lighter roasted coffee is more acidic than darker roasts. This is a misleading generalisation that becomes conflated into: acidic coffee is lightly roasted and coffee with little acidity is darkly roasted. There are many factors impacting the degree of acidity in the cup, and here I want to identify them.

Brewing Parameters
The brewing parameters can have a major impact on the perceived acidity of the final cup. This is even more true in espresso drinks, as everything is magnified. I won’t go into this in detail – that’s a job for a barista trainer – but suffice to say at this point that water chemistry, water temperature, extraction strength and extraction percentage all commonly have huge impacts on acidity.

Freshness
Immediately after roasting, coffee beans begin to de-gas. The primary gas given off from the beans during this is CO2 – which forms a partially ionized solution in water – Carbonic Acid. Very fresh coffee will therefore tend to be more acidic than more rested coffee, which has lost this CO2.

Roasting
Lighter roasts (those taken to around mid/late first crack approximately) will not tend to be very acidic. In these roasts, the acidic flavours (as well as other flavours) will not have been developed.

Beyond this point, acidity will begin to be developed. The extent to which it is developed depends upon the roast profile used – how much energy, and when, it is supplied to the beans during the roasting process, determines what compounds are created and what are broken down. Two lots of coffee, of apparently identical degree of roast, will possess different degrees of acidity depending on the how the profile used has caused acidic compounds to be developed and broken down.

At darker roasts, acidic (and other bean specific) flavour compounds will have been broken down, reducing perceived acidity. Additionally, caramelised & roast flavour compounds will have been created which will tend to mask acidity in the cup.

Green Bean Freshness
As a fresh product, green beans will undergo changes as they age. The changes are not really predictable, but it’s generally true that the intensity of flavours (including acidity) start to noticeably decrease as beans age beyond 12 – 15 months after picking/processing.

Processing
How the picked coffee beans are processed impacts acidity significantly. In general, washed coffees are the most acidic, pulped natural coffees less so, and natural processed coffees the least. This is a very broad generalisation, but processing certainly impacts acidity and other flavours to a high degree.

Green bean & terroir
There is a vast range of factors impacting the flavour of coffee even before processing – coffee plant varietal, farming practices, altitude, soil type, weather – to name just a few. Some farms produce very consistent cup profiles from year to year – other vary widely. Whilst it is possible to spot trends within these factors, generalisations should be handled very carefully – these factors are too interlinked for easy analysis.

This list of factors is not intended to be exhaustive or complete – although please feel free to comment if there’s something major I’ve missed! Rather, it’s to point out the wide variety of factors which influence acidity. Most people – whether working in the coffee industry or as customers – will rarely get to see the impact of just one of these variables.

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