Coffee Extraction
The way coffee is extracted has an enormous effect on the end result. After such a long journey from farm to cup, it’s a massive shame when the brew produces a terrible cup. It is therefore important to know how coffee is extracted and what you should do to bring the best out of every coffee.
How is coffee extracted?
Water passes over the grounds and performs both a surface and deep extraction. Around 30% of coffee is soluble in water but that high an extraction yield will produce a bitter, astringent and choking cup. A deeper extraction of the grounds will also bring out more coffee oils, which are perceived as a fuller body in the resulting cup. Paper filters are sometimes used to capture these body-giving oils.
Water quality also plays a part; water with a Total Dissolved Solids (TDS) count from 80 to 150ppm is ideal for brewing coffee. If the TDS count is too high before brewing, less flavours are dissolved into the end brew. Too low a TDS count and too many unwanted compounds are dissolved into the cup. The higher the TDS or soluble concentration (taken from the coffee only) after brew completion, the stronger the coffee will be. Add water to a coffee that is too strong to reduce the TDS ppm count.
Strength is not the amount of caffeine, the bitterness or perceived coffee taste. You can brew both weak bitter and strong bitter cups depending on the amount of coffee used, contact time and grind size. This graph from the SCA shows all possible outcomes and at what point extraction yield and soluble concentration create the optimal cup. This optimum is when coffee is at its sweetest and most balanced. Explore with different brew ratios and recipes to find it.
Brew Ratio and Brew Recipe
A brew ratio is the amount of coffee to water. For example, 20 grams of coffee to 260 millilitres of water. This may be displayed as 1:13. The brew recipe takes into account the ratio, grind size, timing of the brew and methodology.
Pour over and Full Immersion brewing
There are two regularly-used, simplistic terms for brewing coffee, pour over and full immersion. Pour over is as it sounds, the water is poured over the ground coffee, picks up compounds and passes through. It generally produces a cup with less body and more pronounced flavours, unless high pressure has been used (espresso). Other examples of this brew method are the Chemex and V60.
Full immersion brewing is when the coffee is steeped in water and left in contact until pouring. The body is generally fuller with richer flavours. Brew examples are the Ibrik and French Press.
Sometimes both methods are used for a single brew, creating a new set of results. Examples are the Clever Cup and the inverted method when using the Aeropress. Check out our brew guides for more information.
So what should I be looking for?
Coffee tasting is subjective; we all want to find a cup that best suits our taste buds. That’s why if you like the coffee you’ve brewed, you’re not wrong. However, research taken in both the 1950’s and repeated in the last decade both show that certain extraction levels are generally preferred by all people, a level which lies between the ranges of 18-22% of the aforementioned 30% solubility.
This is because it is in this range that sugars, sours and bitters are most harmonious, all extracted to a point that offers greater balance. When tasting your coffee to check if it is well extracted, it is this balance that will give you the answer, from your first sip to the aftertaste. Have a look at this brilliant article on baristahustle for tasting extraction.
Some basic rules to keep in mind
The longer water is in contact with coffee, the greater the extraction.
Finer ground coffee is extracted more quickly than courser grounds. Think of these two pour-over methods - espresso and Chemex. An espresso (albeit with higher pressure) takes around 30 seconds to extract, a Chemex takes up to 4 minutes.
Water will pass through coarser ground coffee more quickly than a finer ground. Think about the speed water passing through two tubes, one filled with sand and the other with gravel. However, don’t confuse this with level of extraction.
Agitation – Agitation is the movement of the coffee grounds during brewing e.g. pouring water, stirring. This increases the grounds contact with water and speeds up the extraction process.
Water temperature – For hot water brewing, the ideal brewing temperature is between 90-96C Celsius. Darker roasts can be brewed at lower temperatures than lighter roasts because of their looser structure; they are more brittle and more easily extracted.
Over Extraction
This is when the extraction is over 22% (over 20% for some). The coffee will be bitter, unbalanced and unlikely to have much perceived sweetness and acidity. To avoid this, try one of the following (only one, don’t change too many variables at once):
Reduce contact time, the time water is in contact with the coffee.
Coarsen the grind.
Use more coffee.
Use less water.
Under Extraction
Extraction is below 18%. The coffee will be underdeveloped, watery, sour in lighter roasts and ultimately disappointing. To avoid this, try one of the following (only one, don’t change too many variables at once):
Increase contact time.
Fine up the grind.
Use less coffee.
Use more water.
What are the other factors?
Sometimes extraction is not the problem and it is just a bad coffee. Here are some reasons that might be:
A Defective bean. One bad bean can destroy an entire cup.
A poorly processed coffee.
A low grade coffee.
A badly roasted batch.
Freshness. Coffee is going ‘stale’ after 2 weeks.
Unripe coffee.
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