Why are some coffee beans more difficult to grind?
There are many factors that a brewer should consider when grinding coffee beans, such as the region of origin, the roast, the processing method and the brewing method, etc. Each of these factors affects the coarseness of the grind and determines whether or not the coffee will achieve the best extraction results.
For a number of reasons, each type of coffee bean is slightly different at the grinding stage. For example, some beans are difficult to grind by hand, while others take longer to grind in an electric grinder. So, what exactly is the reason for this? Why are some beans particularly difficult to grind?
Talking about the "brittleness" of coffee beans
First of all, before we look at how difficult it is to grind certain beans, we need to make a distinction between the "density" and the "brittleness" of a bean. These two concepts are distinct but often confused.
The weight of a single bean divided by its volume is the density of the bean. In practice, of course, we measure volume to calculate density. We fill a bucket with coffee beans and divide the weight of the beans in the bucket by the volume of the bucket to get the density. Higher density beans tend to have a higher sugar content, which can be achieved by prolonging the developmental stages of the beans. At higher altitudes and lower temperatures, the cellular activity of the coffee tree is reduced, thus providing more time for the seeds to absorb the sugar from the pulp. It is widely recognized that increasing the density of coffee beans is beneficial to quality.
However, "density" alone is not the decisive factor in the ease of grinding coffee beans. There is also the concept of "brittleness". As coffee beans go through the roasting stage, their brittleness changes. For example, it is almost impossible to grind raw coffee beans in a regular coffee grinder, but it is effortless to grind dark roasted coffee beans. Raw coffee beans have a very compact cell structure with very strong and tough cell walls. However, during the roasting process, the water inside the cells evaporates, causing the cells to expand in size and leaving holes in the cellular structure, making the beans looser and thus more brittle. Of course, in terms of physics, density and brittleness are not necessarily related. For example, glass is very dense but fragile, while a sponge is very low density but almost unbreakable.
What happens when you grind coffee beans?
To break the coffee beans into finer particle fragments, the grinder's blade has to first fracture the structure of the beans. During this grinding process, the blade applies two types of force to the coffee bean: compression to crush the bean, and tension, shear and torque to tear the bean apart. Typically a grinder's blade uses all four of these forces simultaneously to grind roasted, cooked coffee beans into various sized particles.
When a force is applied, the coffee bean stretches or contracts, deforming like a spring, until it can't take the stress and breaks. If the force is withdrawn before rupture occurs, the bean regains its original shape, which is called elasticity. Although it sounds paradoxical, materials can actually be both brittle and elastic at the same time.
A material that is brittle is less elastic and can easily break when deformed by a force. For example, glass can be slightly bent, but if it is pressed hard enough, it can easily break, and the deformation is still small enough to break.
How does roasting affect the brittleness of coffee beans?
The roasting process has the greatest impact on the brittleness of coffee beans, which is largely attributed to the fact that the cellular structure of the coffee bean changes dramatically during roasting.
Roasting makes coffee bean cell walls brittle, partly due to dehydration. Roasters utilize heat and airflow to reduce the water content of the bean, causing it to become dry. As water is lost, the beans become more brittle.
Additionally, at first burst, water vapor, carbon dioxide, and other volatiles inside the cells, build up pressure, causing the cell walls to compress against each other and eventually break. Breaking the cell walls is an essential step. Only by breaking the cell walls can water enter and dissolve the components, which are the main substances that make up the unique flavor of coffee.
In addition, different levels of roasting have different effects on the brittleness of coffee beans.
Under the same grinding scale, the median particle size of coffee beans that come out of the first blast (i.e., light-roasted beans) after grinding into powder is 305 microns, while coffee beans that have undergone the second blast (i.e., dark-roasted beans) after grinding into powder has a median particle size of 120 microns. This illustrates how deeper roasting increases the brittleness of the coffee bean.
Interestingly, although it was stated earlier that density and brittleness are not necessarily related, the roasting process affects both density and brittleness. Due to the loss of water and organic matter, coffee beans lose weight during the roasting process, while expanding in size, which means that the density of coffee beans gradually decreases and the brittleness gradually increases during the roasting process. If you look deeper, you may be able to find some kind of correlation between the brittleness and density of a certain batch of beans.
So! What's the point of all the above knowledge?
As the roast deepens, the coffee bean is losing weight and losing water, and the cell walls become weaker and weaker. This means that a darker roasted bean will be finer ground than a lighter roasted bean, under the same grinding conditions. Dark-roasted beans are more brittle than light-roasted beans, and therefore more prone to breakage.
This also affects our actual brewing. Because dark-roasted beans are more finely ground (under the same grinding conditions) compared to light-roasted beans, extraction will be faster. Combined with the fact that the cellular structure of dark-roasted beans is looser, and that flavor substances such as aroma molecules, sugars and acids are more soluble in water, the traditional view that dark-roasted beans are easier to extract and therefore need to be adjusted to a coarser grind is quite justified.
So how do we adjust the coarseness of grinding?
While coarseness is the most important factor to consider when grinding, paying attention to the degree of roast and crispness of the beans will help us brew the desired flavor.
Lightly roasted beans are less soluble and less brittle, so they need to be ground finer to increase the surface area of the ground coffee and extract more aromatic compounds.
Also, because light-roasted beans are typically more difficult to grind, they also require a longer grinding time, which is more evident in some timed espresso grinders. But don't worry too much, although the grinding time will be longer, the actual time taken will not be too long and you won't lose too much aromatic substances. Generally speaking, ground coffee retains the vast majority of its aromatic substances for up to five minutes. If the grinding time is longer than five minutes, it is possible to reduce the liveliness and richness of the coffee flavor.
From a grinder maintenance point of view, beans with a high degree of crispness are theoretically "friendlier" to the cutter, as they reduce the rate of wear on the cutter. Lightly roasted beans, on the other hand, are likely to wear out the cutter faster. Unfortunately, this is meaningless to the majority of people, as there is no discernible difference.
However, beans that are on the darker side of the roast usually produce more fines because they are more brittle, and then the fines tend to clog up the grinder, which in turn affects the particle size distribution of the grounds and makes the extraction of the coffee uneven. So, whether you are using light or dark roasted beans, please clean and check the blade regularly to avoid the above problems.
All in all, the crunchiness of the beans is not the most important concern of the brewer. A proper brewing program is the most reliable guide to consistent quality. Before deciding on any grind or brewing adjustments, it's more important to taste it for yourself with your tongue.
In the end, bean brittleness is not a big issue for most coffee lovers and brewers, but there is no doubt that the science behind brittleness is interesting, and that it is indeed one of the factors for a perfect extraction.
The next time you brew your own coffee, consider how well the beans are roasted and adjust the grind scale slightly. After all, the better the coffee, the more you'll be able to feel the difference in the slightest detail!