How to measure extraction of coffee?
You might have heard a barista saying “this coffee is over-extracted”. Term over-extracted is just an easy way to say that it has too much bitter notes and the brewing went wrong some way. Let’s dig deeper into the world of extraction!
So what means extraction? It is how much (by weight) or how many percent of our dry coffee has been dissolved in the water that you are brewing your coffee with. So if we put it simple, you could say e.g. that from your dose of 20 grams, 4 grams found its way to the cup. Usually extraction is communicated by Extraction Yield % which would be in this case 20 % (4 g/20 g).
Extraction Yield % can be calculated by this formula:
Extraction Yield % = Brewed Coffee (g) x TDS (%) / Dose (g)
e.g. Extraction Yield % = 36 g x 10 % / 18 g = 20 %
Brewed coffee means how much liquid you actually have in your cup, not the amount of water that you used to brew the coffee. For example to make one cup with Hario v60 you might use 250 grams of water to brew the cup but you will end up with something close to 210 grams of coffee. Exception to this are immersion brews (Aeropress, French press, cupping etc.) where you need to use the brewing water quantity.
Dose means the amount of dry coffee that you are using. Measure it before and after grinding so that you have an exact number. Remember that 0,1 grams makes a difference.
TDS comes from words Total Dissolved Solids. Fancy words but simply put it means what is the strength of your coffee. TDS is measured in percentages and usually the range for different type of coffees is 1-12 %. Higher the percentage, stronger the coffee. To measure TDS you will need a refractometer.
So what do we do with this number?
Correct or ideal extraction yield % is seen to be 18-22 % which originates from research by Coffee Brewing Institute (prof. E. E. Lockhart in MIT) in the 1950’s and the research has been verified by SCAA in the recent years. Percentages under 18 are widely seen as “under-extracted” which means that you haven’t taken enough from the dry coffee to the cup. Under-extraction leads sour or too acidic flavours and kind of empty experience when tasting. On the other hand, percentages over 22 are seen as “over-extracted” which means that too much from dry coffee has ended up into the cup. Over-extraction leads bitter and/or burnt notes in the cup and the experience can be “a bit too much”.
Extraction should not be associated with the strength of the coffee. Strong coffee DOES NOT mean high extraction and actually it more than often means low extraction. Same way, mild coffee does not mean low extraction. This is quite often problem when making espresso as pushing more water through coffee will make the drink milder but at the same time it will extract more. So you will end up with a mild and over-extracted espresso (not good).
18-22 % not always right
To make things more complicated, 18-22 % extraction does not always mean good extraction (balanced flavours). Different origins, roasts, grinders, brewing methods etc. extract coffee different way so the scope of good extraction for some coffee might be 18,5-19,5 % and to other coffee 20,2-21,1 %. Also recently some geeks (Matt Perger and Scott Rao) have debated that there's no such thing as over-extracted coffee but it is just your or your grinders fault. This means that some parts of your ground coffee is extracted more than other parts. It could be result of channelling in brewing or poor grind particle size distribution. You can read more from here.
How can you increase your extraction?
There are few basic ways to extract more from your coffee:
Grinding finer means that you have more surface area where the water can extract flavour. If you cut a coffee bean in half, you will double its surface area and it will be twice as easy to extract flavour from it.
As you grind finer it will easier to extract but also you will increase the brew time at the same time which will also increase extraction. The more water and coffee spend time together, the more coffee will give for itself to the water.
Increasing brew water quantity
When making espresso or drip filter coffee the purpose is to run water through the coffee bed. The more water you will have running through the coffee, the more you will extract as fresh water will have the full potential of extracting.
Higher temperature in brewing water
You will have to have some sort of energy to extract flavour from coffee. The most common and easiest energy source for extracting is water’s thermal energy. More temperature, more extraction.
Higher pressure when making espresso
When making espresso, pressure is another energy source for extracting. It works the same way as water temperature; more pressure, more extraction.
Turbulence when making filter coffee
Turbulence means some sort of movement in the coffee bed when making filter coffee. Moving the grounds and water will expose more surface area to the water so that it is possible to extract more. More turbulence, more extraction.
Appropriate brewing practices -> avoiding channelling
Channelling means that water is running through the coffee bed unevenly. There are numerous reasons for channelling such as poor dripping / water distribution in drip filter coffee or cracked coffee puck in espresso making.
If you want to lower your extraction, the ways work the other way around (coarser grind -> less extraction).