Now that your coffee shop offers at least four different boutique ways to serve coffee, you and your baristas find yourself baffled at the multitude of grind sizes that can be purchased or created. Grind size alone can dramatically change the flavor of your cup of coffee, making it bitter or smooth, and the size is different for the majority of the different types of coffee brewing techniques. Making sure you have the correct size, according to experts, comes down to three major factors: contact time, extraction rate, and flow rate which I will break down for you in this article to ensure your cup of coffee is just right. If you aren’t feeling like knowing each individual brewing time, and dislike doing the world’s smallest amount of math, get your shop a grind and brew chart to ensure your baristas are using the right grind without too much forethought. These charts can help your less experienced baristas get up to speed with your many brew methods and can be found, for free, online. In the pursuit of knowledge and the perfect cup, here is a breakdown of why (grind) size matters.
First, let’s talk about what the heck contact time is and why it matters in determining the grind of your coffee. Contact time is the amount of time water spends with the roast itself. Think about the difference between French press, with high contact time, and espresso coffee, with a relatively low contact time. The longer that the water sits to soak in your brew, the coarser you want your grind to be. If you mix up the sizes, and your contact time is too high or the grind too fine, you can over-extract your brew and end up with a not so savory and very bitter coffee. The same can be said in the reverse situation, in which you could end up with a weak brew. The worse complaint a barista can hear after a long day is that the brew they have crafted is too weak, and as a coffee consumer there is nothing worse than a first sip that reveals a very weak brew. Finding a balance between grind size and contact time can help create the best cup of coffee out of your beans.
Contact time in this sense is related to your extraction rate, which is a very complicated concept to apply, but easy to understand. Extraction rate covers everything that the water extracts from the coffee, and it can be tasted and therefore manipulated in a cup of coffee. Think about the difference between throwing a handful of beans into warm water, waiting a bit and then drinking that, versus a fine grind where water can easily pass through. When manipulating your coffee extraction, you want to note that under-extracted coffee, when you haven’t gotten enough flavor out of the brew, can taste sour or lack sweetness, a common way to really hinder your coffee and espresso from selling. This is because your under-extracted coffee grounds have absorbed all of the balancing flavors that make a good cup of coffee. In a quick trip down science way, you can best tell if your coffee is under extracted because acids and salts are more soluble than sugars, and your coffee will lack its sweetness because these sugars haven’t had enough time to dissolve completely. The same distaste towards your coffee can also come from burnt or hollow flavor in your espresso that can come from a high extraction rate. As you might have guessed, manipulating extraction rate is one-part contact time and one-part surface area, or rather the size of your grind. By having the right grind size, you can ensure your beans are picking up the right flavors and have a smooth finish.
Finally, we will cover flow rate as a way to get your grind measurement most accurate. Simply put, the finer the grind the slower the water. If you are using a coarse grind with a slower water rate brewer, think drip coffee, you are going to have a backflow in your coffee machine and could actually break the machine itself. A brewer that has a slower rate, which is good for a finer grind, provides you more time for a longer extraction and therefore adds a nice bold taste to your coffee. When water flows through your machine, it will take the path of least resistance. When you are grinding your coffee, a uniform grind is therefore naturally most ideal, otherwise, the water path will exploit some grounds more than others. Flow rate can also help you determine if you have the right grind size for your brewing method. If there is a rapid flow through that is picking up no sediment in your final cup, you probably have a pretty weak brew, which is due to the coffee being ground too coarsely. However, if your brewing method is causing leakage from a bad flow and has heaps of sediment in it, you probably have way too strong of a coffee, and your grind is too fine.
Remember that when trying to figure out what size grind you need for your coffee, you should include contact time, extraction rate, and flow rate into your thinking. You have a higher extraction rate if you have a larger surface area, and to get that larger area, grind the coffee finer. The higher the extraction, the less contact time is needed for your brew. Finally, a finer grind can reduce the flow rate of your water and therefore increase the contact time. When assessing why your coffee has turned into a dark bitter mess, look at your grind size and determine if it is right for the type of coffee you wish to produce. If the math is making you feel a bit of a head rush, go ahead and find a grind chart online and see if it matches your grind. Playing around with universal sizing charts and the three elements of a good cup of coffee can turn around your brew in both your shop and your home. A skilled barista in your café should be able to identify these different grind sizes and be confident enough to adjust the size, flow, contact time, and extraction rate until they produce a signature cup of coffee.