Extraction 

In this blog post, we will discuss extraction, specifically in relation to filter coffee and percolation. 

Extraction is a term often discussed in speciality coffee, but what does it really mean? Broadly speaking, extraction refers to the process of taking soluble flavour compounds from ground coffee particles and transferring them into water. This process is known as diffusion.

When we talk about extraction, we often refer to two key issues: under-extraction and over-extraction. Under-extraction occurs when not enough flavorful compounds are dissolved in the water, while over-extraction happens when too many undesirable compounds are extracted. Both situations can lead to undesirable flavour profiles in your cup of coffee. In coffee brewing (Percolation and Immersion), Diffusion is the main catalyst of extraction. For a tasty brew, it’s all about finding the balance of extraction that best suits a certain coffee. 

The extraction of coffee can be measured using a refractometer, which is a scientific instrument that measures how much light bends (or refracts) as it passes through a liquid—in this case, coffee. This measurement is known as TDS (Total Dissolved Solids), which indicates the strength of the coffee. This value can then be used in an equation to determine the extraction yield (EY), which reflects how much of a given dose of coffee has been extracted into the water. The ideal extraction yield for a tasty cup of coffee typically falls between 18% and 22%.

For reference, most filter coffees have a TDS strength between 1.2% and 1.5%, while espresso typically ranges from 8% to 12% TDS. You can observe this difference visually: espresso is thick, viscous, and has a rich mouthfeel, while filter coffee at lower TDS levels tends to be thin, light, and tea-like.

When brewing filter coffee, many factors can impact the coffee's extraction yield. Key factors include grind size, agitation, temperature, and brew ratio, among others. To keep things straightforward, we’ll focus on these main points.

Grind Size:

When water diffuses through coffee, it needs to penetrate the cell walls and pores of each coffee particle to extract flavorful compounds. For efficient extraction, the grind size must be small enough to allow the water to penetrate but not so fine that it hinders the process. Coarse grind sizes are preferable for percolation since using a lot of fresh water can lead to the extraction of harsh, undesirable flavours if the coffee is ground too finely.

Agitation:  

Agitation refers to the disturbance of the coffee bed. This can happen in several ways during brewing. The most significant factor is that each time you pour water into the brewer, you agitate the coffee bed. This disturbs the coffee grounds, exposing them to fresh water and dispersing the more concentrated liquid. Swirling the brewer also agitates the coffee, which can enhance extraction but may risk clogging the brewer with fine particles.

Water Temperature:  

Generally speaking, the hotter the water, the faster it can extract flavours. Hot water has more energy and can diffuse flavour compounds more quickly. However, very hot water may also extract undesirable compounds, negatively affecting the flavour. Finding a balance in temperature is crucial.

Brew Ratio:  

Brew ratio simply refers to the amount of water used in relation to the amount of coffee. Water acts as a solvent; the more water you use, the higher the potential for extraction. For example, if you have a 20g dose of coffee and use 250g of water versus 350g, the latter will have a lower concentration but a higher extraction yield due to the increased water volume aiding in diffusion. In percolation brewing, each time you introduce fresh water, you provide another opportunity to extract flavours, as the water concentration gradient plays a role in this process.

Understanding how much you want to extract is key to enjoying a tasty cup of coffee. Managing your variables and knowing how they influence your coffee is essential. 

If your cup tastes sour, thin, or hollow, it is likely under-extracted. You can improve this by adjusting the variables we discussed: grind finer, increase the water temperature, add more water, or increase agitation. Conversely, if your coffee tastes bitter, dry, or astringent, these are signs of over-extraction. In that case, consider making the opposite adjustments. 

It’s important to remember to change only one variable at a time. This way, you can clearly understand the impact each change has on the coffee. If you alter multiple variables simultaneously, you won’t be able to determine the effect of each one on your brew.

By Bradley Garrathy. Head Roaster