Mocopan Coffee’s Jared Chapman explores the four variables of grinder calibration that impact over- and under-extracted coffee and why coffee is a game of constant tasting.
Taste. It’s what we’re all about in the coffee industry. Striving for that sweeter, more delicious cup of coffee. This has led to a lot of work being put into understanding the many variables that affect the flavour we end up with in the cup. With more information out there on variables like particle size distribution and total dissolved solids (TDS), it can be overwhelming for a budding barista to start to understand the basics without getting confused by all the detail.
The purpose of this article is to stick to the basics of grinder calibration to help those who are still learning to better understand the process and basic variables in calibration and their effect on flavour.
To illustrate this, I’ll be referring to the diagram on the right, which provides a simple way to look at the impact of these variables on flavour.
As you can see, the “sweet spot” is where we want to aim in our extraction. This sweet spot will be different for every coffee based on a multitude of factors, but where the sweet spot lies for the particular coffee will determine our recipe. The two other flavour descriptors we are going to use are “sour” and “bitter” for under-extraction and over-extraction respectively. While there are many other traits in coffee that are over- or under-extracted, these are generally the easiest to identify when first tasting coffee.
The reason these flavours are dominant in under- or over-extracted coffee is that each part of an espresso extraction tastes different – more sour, sweeter, or more bitter. The goal is to have a balance of these flavours to achieve a well-rounded espresso. If we don’t extract the coffee correctly, we’ll have too much of a particular flavour, so it may not be unpleasant, but it won’t be balanced. The main two ways this can happen is by the amount of time that coffee is exposed to water, and the amount of water it is exposed to.
These two things will be impacted by the four variables we are going to explore: grind (particle size), extraction time, dose, and yield.
This is essentially how coarse or fine our ground coffee particles are. The easiest way to think about it is to think of a coarse grind being like pebbles, and fine grind being like sand. Now, imagine you have a bucket of pebbles next to a bucket of sand. You pour water in the top. Which bucket does the water flow through fastest? The coarse pebbles.
Now, how does this translate to flavour? Looking at our diagram, if we have grind that is too coarse, our coffee is likely to taste sour. On the other hand, if our grind is too fine, we will over-extract our coffee and it will be too bitter.
Extraction time is the amount of time the water is in contact with the coffee, and has a large impact on the flavour of the coffee. Remember what we said before. Coarse grind makes the coffee run faster. So, if we take another look at our diagram, unsurprisingly, an extraction that is too fast also results in an under-extracted and sour coffee. Likewise, fine grind makes the coffee run slower, thus an extraction that is too slow results in an over-extracted and bitter coffee. You’ll notice that all of our other variables can impact time, resulting in us taking too much or not enough from our coffee.
We’re going to look at dose in isolation for a moment. Dose is the amount of ground coffee we’re putting in our basket, in grams. To understand the impact of dose on extraction time, and thus flavour, imagine that we’ve got our buckets again. This time, the grind size is the same in both buckets, but one is half full and the other is full. In this case, which one will the water flow through faster? The half full one.
Again, how does this translate to flavour? According to our diagram, if we have too low a dose, our coffee is likely to taste sour (as it will run too fast). On the other hand, if our dose is too large, we will over-extract our coffee and it will be too bitter.
Yield is a measure of the amount of liquid we extract from the coffee, in grams. It can be a little confusing that we measure liquid in grams instead of millilitres but the reason is simple: consistency. Essentially, as coffee ages, it will extract with less gas, and thus a smaller crema (crema is largely made up of gas). This means if we measure the same coffee extracted at different ages, the volume will be inconsistent due to the crema, but because the crema weighs almost nothing, the weight will still be consistent.
As we can see, a yield increase results in over-extraction and a yield decrease results in under-extraction.
Now, it’s important to understand that this diagram simply looks at what happens if we change each of the variables in isolation and keep the others the same. For example, if we make the grind finer or coarser without changing any other variables.
If we start changing multiple variables at once, things get a little more complicated. For example, if we decrease our dose (thus making our coffee more bitter), but increase our yield (making our extraction more sour) this won’t balance the flavour. Also, adjusting some variables can potentially affect other variables, for example adjusting grind can affect our dose also. We’ll cover these concepts in detail another time.
Most importantly, don’t be afraid to experiment with your coffee and taste, taste, taste. There is no right or wrong and you may just come across something that unexplainably tastes great too.
This article appears in the April 2019 edition of BeanScene Magazine. Subscribe HERE.