Coffee brewing is an entire science as you balance three extraction-defining factors to get the perfect espresso. Usually, you work based on a hunch and adjust various factors until you get the ideal cup. From there, it becomes habitual. But when you get a new coffee maker or switch to grinding your own coffee, the novelty forces you to ponder how different factors relate to each other.
The grind size increases brewing time, so bigger grounds require longer brewing to produce coffee as strong as smaller grounds. Large grind size results in coarse (big) grounds. The finer (smaller) the grounds, the stronger the coffee, which is why brew time is reduced to avoid over-extraction.
In this article, you will learn more about how grind size affects extraction and how brew time is related to coffee flavor. You will also discover the limits of the grind size and the brewing time. Finally, we will discuss over-extraction and how to prevent it.
Grind Size and Extraction
As a rule of thumb, the finer the grind size, the better extracted the espresso. The surface area principle dictates the relation of grind size to coffee extraction. A coffee bean’s surface interacts with its environment to release its essence. My article on whole bean coffee explains how it is possible to brew coffee without even grinding it.
The reason coffee beans are ground is to increase the number of surfaces in the batch. To best illustrate this example, we need to imagine a coffee bean cut into a cube. Let’s suppose you have two such coffee cubes. Each one of them has six surfaces interacting with the water. For every cube, four sides and a top and bottom surface all interact with water and release flavors and caffeine into the mix.
That’s a total of 12 sides (2 cubes) in the water. For any given minute, a specific amount of coffee flavor will be released into the water. Now imagine both the cubes were cut in half. This would not increase the total weight of the cubes, but it would turn them into four cubes.
The total number of sides would increase to 24 instead of 12. At this point, 24 sides would release essence into the water. For that given minute, the extraction would be twice as much. But if doubling the size, the extraction time was halved (half a minute), the extraction wouldn’t change.
Of course, coffee grounds are not cubic, but the surface area principle still applies. As long as coffee is ground further, it will continue to have more surfaces that can interact with water and release flavors, oils, and caffeine into the resulting beverage.
The Grind-Size Limits
Coffee grounds aren’t infinitely grindable. There is a point in coffee grinding where the reduction of the grind size has diminishing returns. Fine coffee grounds are no smaller than 0.27 mm, and grinding further is harder with not much in the way of extraction efficiency. If, with grounds that fine, the coffee is still weak, the problem is most likely in the brewing temperature. But sometimes, it can simply be in the brewing time.
Brew time and extraction
Coffee grounds have a time-contingent extraction rate. It can be measured in any unit of time. For illustration purposes, we shall use a minute, given that an espresso shot is brewed in 30 seconds. So the average extraction rate of coffee is two espresso shots per minute. If the same batch of coffee grounds is brewed for 15 seconds, it would have half an espresso shot’s worth of coffee flavors, oils, and caffeine.
Cutting the brewing time in half, again, would get even weaker coffee, almost a quarter as strong as an average espresso. But taking the same example to the other extreme, one can wonder if brewing espresso for 4 minutes would get eight espresso’s worth of coffee. After all, the extraction rate of coffee is two shots per minute.
The Brew Time Limits
There is an upper limit to coffee extraction because while the extraction action can be increased by increasing the brew time, one cannot extract flavor that isn’t there. The brew time is limited by the actual content of the coffee grounds. When the grounds’ extract is exhausted, the brew time does not yield better results.
The Relationship Between Brew Time and Grind Size
Both the grind size and the brew time are limited by the actual content of the coffee grounds. If the grounds don’t have any essence, it doesn’t matter how fine they’re ground because the surfaces can’t release anything into the water. Similarly, it doesn’t matter how long they stay in hot water. They have nothing to release.
When the grind size hits its limit, the brew time can be adjusted to improve the resulting coffee. Similarly, when the brew time is fixed, the grounds can be churned finer to make better coffee. Look at the table below for the exact adjustments.
| The Coffee Needs to Be: | The Constant: | The Variable: |
| Stronger | Grind size can’t be changed | Increase the brewing time |
| Stronger | Brew time is fixed | Get finer coffee grounds |
| Weaker | Grind size is fixed | Cut the brew time |
| Weaker | Brew time can’t be changed | Use coarser (thicker) coffee grounds. |
Over-Extraction Explained
Whenever we discuss brewing time and coffee grind size, we have to acknowledge over-extraction and why it isn’t always the best for coffee. Just because you can get more out of the coffee grounds by increasing brew time or grind fineness doesn’t mean you should.
Over-extraction is a phenomenon where the appealing flavors and essence of coffee get mixed with bitter oils and acidic contents of the grounds because of excessive brewing action. It happens when the brew time is increased, and the coffee grounds are finer.
Final Thoughts
Grind size is directly proportional to brew time. The bigger the coffee grounds, the longer it takes to extract strong coffee from them. The finer the grounds, the quicker the brewing has to be to prevent the over-extraction of acidic soils. And if all else fails to make coffee stronger (or weaker), you need to start thinking about the brewing temperature.
