Why Do We... Measure Everything: Extraction Yield vs. Strength in Coffee

Most people who visit specialty coffee houses will know.
Everything is measured. Although few baristas will ever call their coffee perfectly extracted, (being self-critical, overachieving coffee nerds), everything is thrown at trying to achieve a consistent, perfectly extracted and tasteful cup of coffee; scales, thermometers, sieves, refractometers, and so on.

So why can a specialty coffee bar not exist without all possible types of calculating, weighing, meter contraption thingies available? Because getting a perfect brew depends not only on taste but also science; the science of extraction.

After working as a barista and coffee trainer within specialty coffee for 8 years, I moved the majority of my work to the more mainstream market  a little more than a year ago, dipping my toe into Douwe Egberts Master Blenders 1753 (how long can a company name be?) and since last month, Gio Coffee (where I will also drop blog post on coffee on occasion). 

Why? (This is what a lot of specialty baristas asked me, sometimes out of genuine curiosity, but more often out of sheer disbelief.) Because everybody deserves at least a decent cup of coffee, even if its on a busy work floor at no charge.

In my job I run into a lot of flavour problems, sometimes due to bad coffee beans, but more often due to a lack of knowledge (which is exactly why mainstream needs us pesky specialty coffee baristas). One of the problems I run into, which is linked to all the measuring coffee nerds do, is the lack of understanding of extraction.

photo via Shutterstock

Often people will say they do not like coffee. They don't like such a strong cup. When following up on this, I found out it is not the strength they do not like. It's the fact that the coffee is too bitter for them. Which is something you hear a lot in the mainstream scene, explaining the use of milk, milk powder, coffee milk, sugar, or anything else to hide the bitter taste.

The problem is that most think the solution lies in dosing less coffee (if for a minute we forget the adding of milk and sugar as a solution). While in reality, dosing less will often make your coffee more bitter instead of less.

So how does that work?

Where things go wrong with the mainstream cup of Joe is the ground coffee to water ratio -aka the brew ratio- and the understanding of the difference between strength -aka total dissolved solids- and extraction ratio.

As a coffee brewer you can, amongst others, play with the grind size, the dose of ground coffee to water volume, and your extraction time. The extraction time is the amount of time the water is in contact with ground coffee. The longer the water is in contact, the more flavour is extracted and the more solids are dissolved.

Huh? Let me explain.

dividing an espresso shot

The first thing people need to understand is that all brewing coffee really is, is extracting flavour from the bean into water. The level to which this is done is very important for the flavour of your cup of coffee.

To explain this better I often compare brewing coffee to using a tea bag (I apologise in advance to specialty tea lovers, the analogy is over simplified). 

If you dip your tea bag into your cup of hot water for only a few seconds, your tea will taste of water and will be under extracted. You have not extracted all the flavours you want from the tea bag into the water. If you leave it in too long, your tea can turn very bitter and astringent. It will be over extracted. You have extracted too many flavours from the tea bag into the water.

see the changes in extraction; an espresso shot divided in three equal time slots

You can divide the extraction of coffee into three segments. The primary extract, the secondary extract and the tertiary extract. In the primary extract the more acidic and sweet aromas and flavours are extracted. The more bitter and astringent aromas and flavours come out in the secondary extract. At some point, really, nothing is left to extract and water is just simply passing through the grounds and landing in your cup. This is the tertiary extract. 

In other words, the longer the water stays in contact with ground coffee, the more flavour it extracts, and the more secondary (and tertiary) flavours will end up in your cup.

The strength, or weakness, of the coffee can be measured with the percentage of total dissolved solids (TDS). Which, to put it shortly, is what would remain in your cup if you let all the water evaporate. It's the actual coffee particles that have dissolved into the water while brewing. The higher the percentage of TDS, the stronger the coffee. 

two cups of instant; same extraction, different strengths

The strength of the coffee is different than the level of extraction of the coffee. You can have a very weak coffee with a high extraction level and a very strong coffee with a low level of extraction.

If you find it difficult to understand, thinking about instant coffee helps. If you have instant coffee the coffee has been brewed already. You cannot change the level of extraction, or the flavour, any more. You can however change the strength, by adding more or less instant to your water.

A lot of baristas use a refractometer to measure the level of TDS and the level of extraction. But, if you do not want to invest over 800 USD, it boils down to this:

If you find that your coffee is too bitter (meaning too much secondary extract; the water extracted unpleasant flavours from the coffee) you can:

- dose more coffee and grind more coarse

Unless you are using a french press, or another brewing method that allows you to submerge the ground coffee in the water, you need to adjust both. If your water is passing through the ground coffee, and you dose more without grinding more coarse, the water will stay in contact with the ground coffee too long and your cup will still taste too bitter.

If you only grind more coarse the water will not be in contact with the ground coffee long enough, which will result in under extraction.

If you are using a brewing method that submerges the ground coffee in the water you can also choose to adjust only one variable. However, if you only dose more, keeping your grind size and brewing time the same, you run the risk that your percentage of TDS will be too high (the coffee will actually be too strong). The other way round, if you dose the same, but grind coarser, your percentage of TDS will be too low and the coffee will taste too weak or too watery.

If you find your coffee is too weak or watery (which means that the TDS level is too low), you can:

- dose more coffee

- grind more fine

In this case you do not need to do both. One could be enough. If you dose more coffee to the same amount of water, there will be more coffee particles to dissolve. Resulting in a stronger cup. If you grind more fine, you will get more ground coffee particles for the same total weight of ground coffee, resulting in more coffee particles to dissolve, giving a stronger cup. 

a refractometer (photo by blog.dahtac.org)

As always, it's all about balance. In the end, when brewing coffee it's all about 'meten is weten' (or for the non Dutchies who do not understand this old rhyming wisdom; measuring is knowing). Know your coffee to water ratio, experiment with your extraction time (contact time between ground coffee and water) and get your grind size right.

For those who want to know, most agree that an extraction level between 18-22% and a TDS between 1.25-1.45% (for non pressurised brewing methods) or 10-13% (for espresso) is ideal. Although preference in strength can differ per country. And if you do not have a refractometer to measure this for you, use the guidelines above and taste, taste, taste...