Extraction Fundamentals

What factors affect coffee extraction?

It's a more complicated answer than most expect, and while it may be daunting, breaking down coffee to its smallest discrete units [its quanta, if you will] can help us develop a deeper understanding of what's happening during the brewing process and truly control the outcome of each cup.

The following, in no particular order, are the factors that most deeply impact soluble extraction:

Water Temperature

Higher temperature water extracts solubles quicker, lower temperature extracts slower. For a particularly dense coffee (ultra-light roast, washed, high altitude, etc.), higher temp may be needed, but with less dense coffees (medium/dark roast, intense fermentations, etc.) high temp may be too extreme.

Agitation

The more you make the coffee particles move within the bed, via kettle flow rate, rotation around the coffee bed, kettle pour height, swirling the brewer, stirring, etc., the easier it is for solubles to make their way out of the particles, thus increasing extraction. You can use agitation to control the even-ness of an overall extraction, by focusing on some areas of the bed while avoiding others

Contact Time

In general, the longer coffee is in contact with water, the more solubles get extracted. The rate at which solubles are extracted diminishes over time, however, so other factors of increasing extraction may be needed to achieve your desired outcome

• Filter - aside from the effect filters have on the shape of the coffee bed (discussed below), different coffee filters allow for different flowrates of brewed coffee through them. This can affect your overall contact time, as well as, in some cases, the amount of particles allowed into the final brew. Examples of faster flowing filters are Cafec Abaca cone filters, and Sibarist filters across the board.

• Agitation - how you choose to utilize agitation can affect how the smallest coffee particles are pushed into certain filters (in particular with conical burrs, or flat burrs with a less consistent grind size distribution), thus affecting contact time. Generally a heavier agitation brew will push more "fines" into the filter, increasing overall contact time, while brews with more gentle agitation tend to decrease contact time

Particle Size

The size of each coffee particle after grinding will affect overall extraction by changing the available surface area of coffee that water can contact and extract from. A finer grind means smaller particle sizes, thus more overall surface area, and a higher extraction. A coarser grind means larger particle sizes, thus less overall surface area, and a lower extraction.

• Distribution - another important factor to consider is the differential of size in coffee particles, and the overall distribution of differing particles sizes. A tightly packed distribution will lean toward an overall more even extraction, as a result of the particles extracting at a similar rate to one another; this can lead to a more clean brew on the positive end, and a boring one-tone brew on the negative end. A more widely spread distribution will lean toward a more mixed extraction, as the largest particles will extract at a slower rate then the smallest particles; this can lead to a more complex brew on the positive end, and a muddy brew on the negative end. Many factors contribute to the spread of particle distribution:

• • Grinder burr geometry/style have a large affect; conical burrs tend toward a wide spread, while flat burrs tend toward a tight spread. Different brands/styles of burrs will also have characteristically different particle distributions, eg. SSP unimodal vs. SSP lab sweet

• • The grinder's coarseness setting you choose also has interesting effects on the particle distribution; overall, grinding finer allows fewer "boulders" (abnormally large coffee particles) through the burrs, leading to a more even distribution. However, each individual set of burrs has its own range of particle distribution, graphs of this range can generally be found on the burr manufacturer's website

• • The feed rate of beans into the grinder can also have an effect; for pour over style grinders a slower feed rate can result in a tighter distribution, however in espresso-style grinders (with hoppers) a higher feed rate (or, more accurately, a more even and high downward pressure on the burr from a full hopper of beans) can lend toward a more even and consistent distribution.

• • The RPM the burrs spin at can also have an effect; generally, lower RPMs lend to a tighter distribution, and higher RPMs lend to a wider distribution.

Coffee Bed Shape

The type of brewer and shape of filter will affect how fast water flows through the exit of the brewer, and how coffee extracts, in two different ways

• Brewer shape - while there are so many different styles and shapes of brewers, most fall into two categories: conical and flat bottom

• • Conical brewers (V60, Origami, Flower Dripper) will tend toward a more mixed extraction, because of the taller tapered bed, which leads to a higher surface agitation, and a lower extraction toward the bottom of the cone. This generally results in a more complex brew that can have a nicer depth of flavor on the ideal end, and may turn muddled and intense of the worse end

• • Flat bottom brewers (Kalita, Stagg X, Orea) will tend toward a more even extraction, as the depth of the coffee bed is more even from inside to outside. The lends to a brew that can potentially have more clarity on the ideal end, and a flat/one-note brew on the worse end

• Filter shape - Different filters have different flowrates they allow through themselves. Their contact points with the brewer they're in will also affect the overall maximum flowrate (eg. an Origami brewer with a cone vs. flat bottom filter). The more points of contact a filter has with a brewer, the lower the maximum flowrate will be, as the brewed coffee will have fewer points to flow through other than the bottom.

  • Faster flowing filters: Sibarist, Cafec, Fellow, 

  • Slower flowing filters: Kalita, V60, Chemex

Roast

Many factors of a roast profile will affect the density of the coffee, mainly roast level. Lighter roasts are inherently more dense than darker roasts, leading to lower extractions from the former with similar techniques.

Origin

Coffees grow differently in different growing regions, based off of a myriad of different factors, such as varying altitude, soil content, and climate. While any number of factors can affect specific harvests of coffee, it's generally safe to say you'll need to put more energy into brewing a Panama coffee grown at 1800masl vs. a Brazil coffee grown at 1000masl

Process

Exotic processes are becoming more and more prevalent in specialty coffee, and we must give each more consideration before brewing to maintain consistency. Washed and Honey processes are easier on the exterior cell walls of coffees, thus are generally more dense. Whereas Naturals, and the wide spectrum of fermentation increase the porosity of those cell walls, thus leading to lower density coffees. 

A closing note:

Extraction is a spectrum. Some people prefer an even extraction for a clean, sharp flavor profile, while other enjoy a mixed extraction for a complex, and dynamic flavor profile. Taste can be pretty subjective, none of this is to say there is a right and wrong way to brew coffee, but there are trends we can follow and tools we can use to affect extraction.

Our hope is that with all this information, you can brew more intentionally and consistently, and achieve the exact cup that you desire, every single time.