Around 95% of a cup of espresso coffee is water. For filter coffee, it is as much as 98%. How we brew coffee is simple. Most methods involve either pouring hot water over coffee grounds or immersing the coffee in the water for a set amount of time.
By doing this, we are extracting flavour from the coffee. Some flavours in coffee are undesirable, so it is important to ensure we control what we extract from the coffee, and how much.
While there is nothing better than visiting your local cafe and drinking a flat white made on an espresso machine, it can be easy to forget that a lot of thought goes into making sure that coffee tastes great. While brewing coffee is simple, making it taste fantastic requires some thought, and the right equipment, including using a coffee grinder to get the perfect grind size.
In this guide, we will look into the science behind espresso coffee and how we can improve our coffee by changing the grind size.
The basics
There are a number of variables that affect the extraction, and therefore the taste, of coffee. The main ones include brew time, temperature, roast level, and grind size. (We are going to make volume a constant here and assume that we are making a standard double espresso of around 36g.)
If a coffee doesn’t quite taste right, a trained barista will know how to change these variables to improve the taste. When problem-solving, the first thing they would look to change is the grind size, set on your coffee grinder. This is, in part, because it is easy for the barista to change, and it needs to be recalibrated often.
If we take a look at the other variables, we can see why. The roaster will have performed their own check, so it is unlikely to be the roast level. Within the specialty coffee industry, it is generally accepted that espresso coffee tastes best when brewed at 91 to 95°c for 25 to 30 seconds.
These parameters have become standard through years of trial and error. Most coffee machines can regulate temperature and the brew time depends on the grind size (we will address this point in a moment.) If the coffee is brewed within these parameters, then it is unlikely that temperature or brew time is the problem. So, that brings us back to grind size.
Why does grind size matter?
As we have seen, the wrong grind size is often the reason why coffee doesn’t taste right. So why is that?
Well, grind size will affect how quickly the water passes through the coffee and how much it extracts.
When a grind size is small, the water passes slowly through the coffee, if too fine, it can even get stuck, this will lead to a longer extraction time. Alternatively, too large a grind size leads to the water passing through quickly, so we would expect to see a much quicker extraction time. A helpful analogy is to compare a bed of sand to stones. Water moves much quicker through stones than sand, the same applies to the size of coffee grounds.
Generally, when the water passes through the coffee grounds quickly, we get a very viscous espresso that has intense flavours and high acidity. Contrarily, when it passes through slowly, the espresso is watery, with little flavour, and it can even taste bitter.
These are the two extremes, and what we are trying to achieve is something in the middle. A good-tasting espresso has a balance of acidity and bitterness, with a reasonable level of viscosity.
Photo: coffeefriend.co.uk
Understanding extraction
It is not only the speed at which the water passes through that affects the taste. The amount of solubles extracted from the coffee depends on the grind size. A smaller grind size means a greater surface area, while a larger grind size means a lesser surface area. The smaller the grounds, the greater the surface area, and therefore the more solubles we extract from the coffee.
There are lots of flavours that we don’t want to draw out from the coffee, so it is important that we don’t make the grind size too small. According to the Specialty Coffee Association (SCA), the best tasting coffee has a total dissolved solids (TDS) reading of 18 to 22%. By this we mean, we are looking to extract around 20% of the solubles from the coffee grounds. If, for example, we extract too much on our first attempt, we can simply make the grind coarser.
Espresso grind size
Generally, espresso coffee is finely ground because it has a very short brewing time. Because the brewing time is quick, we need to have a larger surface area. To help with the extraction and to ensure that the water doesn’t get stuck, the coffee machine pushes the water through at a high pressure.
This pressure gives espresso coffee its distinctive viscosity and flavour and is impossible to replicate with other methods. Adjusting the coffee grinder to get that sweet spot is where the barista’s skills come in handy. The grind size controls not only how much of the coffee we extract, but how long it takes.
Summary
We hope that this deep dive into grind size has helped you understand how coffee extraction works. Though it can be confusing, knowing how and when to adjust your grind size is an important step towards improving your own espresso making.
