Extraction is defined by the amount of a roasted coffee bean (actually several beans) that have been dissolved using the solvent, water. Water is the universal solvent and is extremely effective. Despite how effective it is, there are several factors that influence how effective it is at coaxing (or extracting) flavor solubles from coffee…
The Water Quality: Specifically, mineral composition. In loose terms the higher the alkalinity the less organic acids will be dissolved and therefor the less acidity the resulting cup will have. The higher the total hardness, the more overall solubles will be dissolved. These two major extraction influencers are also mildly dependent on each other. Solubles are what create flavor in coffee, without the right water we can’t dissolve said solubles!
Temperature: Certain compounds such as malic and citric acid dissolve easier at lower temperatures (below 200°F) than others such as sugar. Generally speaking though, 195°F – 205°F is the most ideal temperature for most coffees.
Grind: How accessible these solubles are is based on surface area. The finer the grind, the more surface area there is and therefor the more assessable the solubles are and the faster they will dissolve or extract in water.
Time: The amount of time grind particles are hanging out with the water. The longer the contact time the more solubles will be extracted from each particle, and therefor the more extracted the coffee will be.
Agitation or Turbulance: Aggressive pours or added pressure in methods such as espresso or aeropress brewing will increase the speed in which these solubles are being extracted.
Another thing to keep in mind is TDS or Total Dissolved Solids. This is interconnected with extraction, but is also it’s own variable. One way of looking at it is; Extraction is what was taken from the bean and TDS is the amount of the resulting brew is made up of what was taken from the bean. i.e. Coffee is 98-99% water, a brew that is 98.5% water would have a TDS of 1.5% and therefor would taste stronger than and a brew that is 99% water, which has a TDS of 1%. In this scenario, the balance of flavors in the cup might be equally balanced or the coffee could be equally extracted. You can control TDS in a few ways, one is your coffee to water ratio. The wider your coffee to water ratio is, the weaker your brew.
All of this is depending on You, the Brewer/Barista being able to define over and under extraction using tools. One wonderful tool is a refractometer, which will measure exactly how many of these solubles are dissolved in the resulting cup and therefor you will be able to calculate the amount of coffee that was extracted from the bean. You could also dry out the bed completely after brewing, weigh it and divide it by the starting weight of the coffee, but who has a day or two to wait to see what the extraction percentage of a brew is? The good news is if you do not necessarily have a refractometer or a day or two to spare, you can still define whether something is under-extracted, over-extracted, strong or weak. All you really need is your palate! At the end of the day, we are not just trying to match a bunch of theoretical numbers and science… We are looking for the coffee to taste wonderful!
Here are some key guidelines on defining under and over extraction with or without a refractometer, as well as some pointers for making an adjustments to compensate: