Why grind size matters when brewing cold brew!
The basics of coffee extraction
To understand why grind size matters, it’s important to first understand how coffee is extracted.
About 28% (by weight) of a roasted coffee bean is soluble! So a brewed coffee, hot or cold, is simply parts of a coffee bean dissolved into the brew water.
Simple enough? Well, kind of.
Every dissolvable component of coffee has a slightly different solubility. So, salts, sugars, acids, phenols, fats and lipids etc. all take different amounts of time to dissolve. Some are dissolved straight away, others take longer.
“The first and most soluble parts of coffee are fruit acids and organic salts (light, bright, fruity flavours), closely followed by light aromatics created from the Maillard reactions and sugar browning during roasting (nuts, caramel, vanilla, chocolate, butter etc.) and lastly heavier organic matter (wood, ash, malt, tobacco etc.).”
The light aromatics are particularly tasty. These add sweetness to our cup.
Coffee guru Matt Perger describes the impact of grind size using a great analogy.
"When talking about grind size you could also say ‘how much coffee flavour am I going to hide inside the grinds, away from the water?’ or ‘how much should I delay the extraction of a portion of the total flavour?’. A finer grind will hide less flavour, and reduce that delay. A coarser grind will hide more flavour and increase the delay.”
In a nutshell, the coarser you grind, the longer it takes to extract the coffee flavours. Why? Because you are hiding away some of the flavour in a larger particle - there is less available surface area.
Complicating our recipe is the fact that grinders are not perfect. Every particle that comes out of your grinder will be a slightly different size. There will be lots of particles that almost match your designated grind setting, and a bunch that are smaller and larger. The number that vary from your chosen setting will depend on the grinder.
Why does this matter? If our ground coffee is made up of particles of different sizes, then it means that we will be extracting flavours from those particles at different rates!
This is one reason why it is a skill to prepare a good cup of coffee. We are trying to achieve balance - making sure our brew time is long enough to extract as much sweetness as possible, but not too long, so we avoid the bitter, ashy flavours of an over extracted cup.
And all the while, each individual coffee particle is being ‘extracted’ at a slightly different rate. See, baristas have a tough job.
How does the above science apply to cold brew?
The solubility of coffee increases with temperature. So using cold water means we are slowing down the flavour extraction process. This increases our brew time.
And we learnt above that the longer we brew, the more chance we have of extracting some of the undesirable flavours in our coffee.
So what’s the answer?
The longer brew time means we need to hide away a bit more of the coffee flavour to make sure our cold brew doesn’t taste over extracted. We need to grind coarser!
It follows that your cold brewing time will determine your grind size. A short and sharp cold brew, around 8 to 10 hours, would require a finer grind size (on the cold brew spectrum). For a longer brew we need to go a bit coarser.
There is no gold standard. The recipe you choose will come down to taste. So get experimenting!
Note also that the roast profile of the coffee you are using may impact on the solubility of the coffee flavour. For example, lighter roasts tend to be less soluble and require a finer grind setting.
Please share your thoughts and experiences in the comments below.
,  Matt Perger, Barista Hustle - https://baristahustle.com/blog/surface-area-and-time/