4 Jun 2014

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...

10 comments:

  1. Cerianne interesting, I havn't finished reading but a question on this `The longer the water is in contact, the more flavour is extracted and the more solids are dissolved.` :

    In the Xmas issue of the Economist of 2003, I had read that Espresso extracts the flavours better (and faster) with less caffeine than the filter (or similar) method: slower with more caffeine and less (or is it different?) flavours!?

    Can you comment?

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  2. Different methods do have different ways of extraction. Expresso, due to the pressure used, can extract quite quickly. However the fact that an espresso has such a complex flavour profile is not only due to the pressure, but also the roast and the grind. Also when measured, espresso often has quite a low extraction yield. Meaning an espresso machine (literally) does not extract better. Filtermethods often extract much more. The ideal extraction of espresso and filter do not differ that much. What is different is the flavour and the strength, not the extraction. The difference in flavour is due to water pressure, roast profile and grind size.
    On the caffeine level, it depends. Caffeine is extracted more towards the secondary extraction. So if your water is only in contact with the ground coffee, you would expect less caffeine. In that sense espresso will have relatively less caffeine than most other methods. However, some believe this is compensated to the the fact that the dosing for espresso is relatively much higher. Also, it really depends on the beans you use, and again, on the roast profile. But I have not worked with any type of measuring device for caffeine. So I would not know the details of it all.

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  3. Thank you for this article! I've been wondering about this and it explains much of my experience (and frustration) in experimenting with brews.

    Can I ask for your recommendations for cold-brewing? I have experimented for months with this and while I've got a good enough result so far, I simply have not been able to achieve what I've been aiming for. A nearby coffee shop had the best cold brew I've ever had and is consistently extremely flavorful with an incredible profile. I've tried to get secrets from them from time to time and have garnered a little info but still can't get it.

    What I'm currently working with is 15% coffee to water (1:6.666...) for 24 hours at room temperature. Coarse grind just doesn't seem to extract enough, and I've played with finer grinds but so far to no avail. Perhaps I iust haven't found the right grind? Any recommendations would be greatly appreciated - I'm hoping it really is down to the right grind at this point because I've tried countless other variables.

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    Replies
    1. Hi Luke. Good to hear you liked it. Glad you found it be of use.

      I always use a course grind myself for cold brew. However I do find that a much longer extraction time is the way to go, leaving the coffee to extract for about 3 whole days. The cold brew then loses that weird dusty flavour and becomes very clear and clean. It is quite strange to experience. I also leave it in the fridge, not at room temperature. Maybe something to try?

      Also, what water do you use? Something like Spa, with a very low TDS, often allows for more extraction.

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    2. Thank you, Cerianne - much appreciated!

      I've wondered about longer steeps as well, but figured it must be totally overkill since places like Intelligentsia think anything past 14 hours is too much. I thought 24 hours was already pushing it... But I'll definitely give this a go, in the fridge.

      I just use purified bottled water - not spring - as I try to keep it as clean as possible.

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  4. This comment has been removed by the author.

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  5. Very good article. Thank you!

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  6. Thanks this is a very well set out article with good information.
    Do you have any top tips for the refractometer? I've just picked one up.
    Also going to read more of your posts so I apologist if you've covered it

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    Replies
    1. Hi Joseph! Glad you liked it. I do not have a lot of tips for the refractometer. Anything specifically you find difficult? For the most part it is very important to calibrate your refractometer with distilled water before use. The device can be very useful, but most important is still taste. If it tastes good, you are fine. If you think the taste is a bit off, the refractometer can tell you in which direction you can find a solution. If you have any specific questions, don't hesitate to ask :) Otherwise, I hope you enjoy the rest of the blog as well

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  7. Thanks for sharing, but actually I'm still a bit confused, a basic question I'm still struggling with, I know all the formula for extraction rate, but still think it doesn't make sense to me.

    so a higher tds means more dissolved solubles in the brew, then isn't the extraction rate proportionate to it, as you should have extracted more in order to get more dissolved solubles? for expresso, I got 7-10% tds, but somehow just get under extraction value for it (<18%), why is that? and does the extraction measure, the whole coffee bed, or an average approximate for single grind/ bean? and one more thing, what situation can we get - 1) high tds and low extraction rate and 2) low tds and high extraction? and why is that?

    Thanks, it maybe a bit too much, but it'll be greatly appreciated if you can help with it, my head is in chaos at the moment.

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