You may have heard your favourite baristas using the word ‘extraction’ when talking about brewing the coffee you’re about to drink, but what do they mean? In short, they’re referring to how much ‘stuff’ their brew water has managed to take out of the dry dose of coffee.
Not every part of the coffee bean can be dissolved in water; in fact, only around 30% of the coffee is soluble, with the other 70% made up primarily of roasted plant matter. However, achieving a cup of coffee that uses all that 30% is very difficult; doing it in a way that also tastes nice is even harder. Most baristas will aim a bit lower than that maximum potential Extraction Yield (EY).
For example, a well-made espresso typically uses only 18-22% of the 18-gram dose put into it — a combination of plant acids, plant sugars, as well as caramelised sugars created during the roasting process. This means that in a 36g espresso shot with an EY of 20%, around 3.6g is extracted coffee, and the rest is water.
If this were a pour-over brew, using much more water, we’d still be aiming to get around that same EY%, but the resulting drink will be far more diluted. With an 18g dose and 300ml of brew water, we get a 260ml yield of beverage that still only contains 3.6g of extracted coffee, but dissolved in much more water. The relative quantities of extracted compounds will also differ from that of an espresso, which is why a filter coffee tastes different from an americano coffee.
The amount of ‘stuff’ or Total Dissolved Solids (TDS) in a liquid can be measured with a refractometer — a handheld device that shoots rays of light through a small sample of liquid and measures the speed at which they pass through and back again; the more ‘stuff’, the slower the rays move. The device uses this information to calculate for us the TDS content of the liquid sample, from which we can infer the TDS of our brewed coffee. Through a simple calculation formula, we can use our yield weight, TDS, and starting dose to work out the EY% of our coffee.
To summarise, Total Dissolved Solids (TDS) is how much actual coffee is in what we drink, while Extraction Yield (EY%) is how much of our coffee dose ended up in our cup.
Formula for calculating extraction yield:
EXTRACTION YIELD = TDS x BREW YIELD
DOSE
Why is this important to know? The relationships between TDS, EY%, and flavour is a can of worms that we’ll open up in another post — the suggestion that more extraction = better flavour is unfortunately neither true nor false, but instead rather complicated. For now, it’s worth acknowledging that these data are useful for getting an idea of how effective or efficient we’ve been in brewing our coffee. For instance, everybody wants to get the most out of the coffee they work hard for to be able to buy.
If my espresso only has an EY of 15%, that means I’ll be throwing away 0.27g of coffee that I could be drinking! This may not sound like much, but for businesses going through dozens or even hundreds of kilos of coffee a week this can quickly add up to a waste of money and a loss of profits. It also means needlessly throwing away the hard work put in by peoples across the globe who grow, process, and roast our delicious coffees, when their incredible efforts deserve to be appreciated.
Further, these data can offer some insight into how consistent one brew is to another — if you’re using the same amount of coffee and water and producing similar TDS & EY% results, you’re probably doing a great job of brewing consistently. If the results vary greatly from brew to brew, you may need to adjust some part of your method or technique to remedy this.
However, TDS and EY% aren’t the be-all and end-all to making great coffee. This piece only scratches the surface on the complexities of coffee extraction, but there are more to come that will explain all — extraction & flavour, how to boost your extraction, and how to improve the evenness of your extraction.
Author’s note — much of the information presented here is greatly simplified, and the more you learn the more you’ll realise how few ‘simple’ statements can be made when it comes to coffee. Similarly, any facts or numerical data are generalised and not to be taken as absolute features of the glorious cup o’ joe.
Blog written by Jonas from the Wogan Brew Bar.
