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Coffee Grinding Physics - Not As Complicated As It Sounds

You're probably thinking the physics or mechanics of grinding coffee sounds like a horrible, boring subject, right? I know, just point out a decent coffee grinder, and don't make it so complicated.

But bare with me, it's really not difficult to understand, and armed with a little more knowledge, you'll be able to make a better choice when choosing the right coffee grinder. Bottom line, understanding the mechanics behind coffee grinding will actually help you brew a better, more flavorful cup.

If you ask most people why they decide to purchase a coffee grinder and grind their own beans, they will usually tell you "it's the best way to preserve coffee bean freshness and deliver more flavor in the cup". Very true, freshness is key. Absolutely, for more coffee flavor, buy freshly roasted coffee beans, and don't grind your coffee until just before you are ready to brew. Ground coffee will grow stale and lose its freshness much more quickly than whole beans. Even if the ground coffee is packaged carefully in a sealed container.

However, freshness is not the only reason to grind your own coffee. Equally important is the ability to control how coarse or fine the coffee is ground and insure a consistent level of grind, that is, that each ground coffee particle is approximately the same size. Understand these critical coffee grinding mechanics and you'll be well on your way to producing a consistently better cup of coffee.

Probably the most widely misunderstood and overlooked step in preparing a great cup of coffee is the grinding process. Most people don't realize how much difference adjusting and tuning the proper grind will make. In fact, if you're attempting to master the technique of espresso preparation, it's virtually impossible to get it right without a suitable coffee grinder that enables the necessary control to adjust the grind level.

New home baristas typically struggle through a lot of trial and error trying to figure out why their espresso shots just don't measure up until they eventually discover the revelation that "it's all in the grind". OK, mastering the grind isn't the only consideration, but easily the one single area you can focus on to get dramatically improved espresso and coffee results.

When it comes to brewing better coffee, what are we after here?

Brewing the best cup of coffee generally means you want to achieve what is called the "optimum level of extraction". Sounds complicated, but it's really pretty straightforward. The coffee beans possess many desirable flavor elements, compounds and components including coffee oils that we want to extract into the water and enjoy as a cup of coffee. The optimum extraction means we want to extract as high a concentration of these flavorful coffee elements from the ground coffee beans to the water as possible without going too far.

An under-extracted brew will result in a weak or dull cup of coffee that doesn't pull the maximum flavor from the coffee beans. And an over-extracted brew will taste bitter, overshadowing many of the subtle taste characteristics, as less desirable compounds and elements are introduced into the coffee when the extraction is allowed to progress too long.

So how do you achieve an optimum coffee extraction?

Let's start with the four main variables that you have some influence over during the brewing process. And this does vary depending on which brewing method you use (automatic drip, coffee press, vacuum coffee brewer, espresso machine, etc.)

  1. Water temperature
  2. Brewing or steep time
  3. Ratio of ground coffee to water
  4. Level of coffee grind (how coarse or fine)

Not all brewing methods provide direct control over the water temperature. Automatic drip machines do not (and generally do not brew at a high enough temperature). French press and vacuum pot brewing does allow the control since you are typically boiling the water yourself.

Not all brewing methods provide direct control over the brewing or steep time. Automatic drip machines and vacuum pots don't, while a French press does.

All brewing methods do allow you direct control over the ground coffee to water ratio (how much ground coffee and water you choose to use), and the level of coarse/fine coffee grind (if you have the appropriate coffee grinder).

Also consider, as we'll talk about in a moment, that in all brewing methods, the coarseness/fineness of the coffee grind will indirectly influence the length of time that the water stays in contact with the coffee grinds. The water will pass through coarse grinds much faster than fine grinds. With a finer grind, the water will actually stay in contact longer with the ground coffee, thereby indirectly influencing a longer and fuller extraction. This is true, even to some extent, with an automatic drip coffee maker.

Of course, with a French press as well as other brewing methods (even when a paper or permanent filter is used), if the grind is too fine, you will wind up with sediment in the cup. Generally, most people find this gritty sediment undesirable.

In pursuit of the optimal extraction, the four variables (water temperature, coffee/water ratio, coffee grind and brew time) tend to be dependent on one another. With a French press, when you use a coarser level of grind, you may need to increase the steep time. Or, when you lower the water temperature, you will slow down the rate of extraction for certain compounds, which you could offset by using a finer grind and/or increasing the brew time. As an extreme example, the method of "cold brewing" coffee uses cold water to extract a concentrated coffee liquid, but requires as much as 12 hours to brew or steep.

Do you start to see a pattern here? In all methods of brewing, the coarse/fine grind level has direct influence over the rate and degree of coffee extraction and is always under your control if you have a capable coffee grinder.

OK, let's explore the coffee grinding details a little further.

There are two types of coffee grinders. At the lower price range, the blade grinder is the simpler device for grinding coffee beans. This is the type of grinder that most people have in their kitchens.

In the more expensive price range, the burr grinder, either flat or conical burrs, is the coffee grinder to choose if you want the complete and proper control over how fine or coarse to grind the coffee beans.

The other important benefit using a burr grinder is the ability to produce a consistent particle or granule size at any level of grind. In other words, whether you choose a very fine level of grind, or a coarser grind, all the granules, on average, will be very consistent and similar in size. A blade grinder simply doesn't have this capability.

Let's clarify what we mean by "consistent" particle size.

This doesn't mean that each and every particle is precisely the same size at each grind level. That simply would be physically impossible to achieve. The significant improvement in consistency that a quality burr grinder enables is a tighter focussed range of particle size. In other words, with a proper burr grinder, you will have a much larger percentage or ratio of particles within a narrower size range. And this promotes a more consistent and controllable extraction.

But, even with the most expensive burr grinder, it defies the laws of physics to grind coffee that results in all particles of precisely the same size.

What we're after is producing a narrower range of "average" particle size.

We'll see why uniform particle size is important in a moment ....

As we talked about, the size of the ground coffee granules affects the extraction rate and how fast the water flows through the grounds. This gives us the control to produce the optimum extraction by adjusting the level of grind.

With a good quality burr grinder, you have a dial or wheel-like control that adjusts the distance between the burrs (metal grinding disks). As the burrs are brought closer together, the resulting average ground coffee particle size will get smaller. Burr grinders are engineered by design to produce a very consistent average particle size at any level of grind, from the coarsest to the finest setting.

With a blade grinder, the average particle size is determined by how long you spin the blades. As the spinning blades continue to strike the particles over and over again, the coffee granules are broken into smaller pieces. However, it's hit or miss whether all of the particles come into contact with the blades an equal number of times.

Furthermore, there's no predictable control in a blade grinder that assures that all particles struck by the spinning blades will break apart into similar sized granules. So, as you spin the blades longer for a finer grind, your result will consist of a much wider range of particle sizes - some very fine powder or coffee dust, some bigger particle chunks and a range of particles in between.

Let's look at the best case first ... all the granules are about the same size

Each coffee particle, being of reasonable uniform size, will release flavor components to the surrounding water at about the same rate throughout the brewing process. Remember, for the optimum brewing result, we want to extract many of these components, but not all of them. With a uniform granule size, enabling a uniform rate of extraction, we can adjust the brewing time to allow extraction to continue for the proper amount of time.

Or, if we choose to keep the brewing time more constant, we could make the grind more coarse for less extraction, or more fine for more extraction. Again, this predictable control is only possible when the average particle size is uniform and consistent.

What happens when the coffee particles vary in size across a broader range?

The smaller granules extract faster, and therefore, release the flavor components faster to the surrounding water. The larger particles will extract slower, and for a given amount of brewing time, will release fewer flavor components to the surrounding water, causing under-extraction (weak coffee with less flavor). You don't have much choice but to tune the brewing time for the average granule size, somewhere in the middle. And you'll have problems at both ends of the particle size range.

The larger particles will under-extract, and the smallest particles will very likely become over-extracted. The wider the range of granule size in the ground coffee, the more the desirable components of the extracted brew are overcome by the undesirable, bitter tasting components, or diluted by the under-extracted weaker tasting portion.

If you are using a blade grinder today, you're likely familiar with the fine powder dust that's unavoidable with every grind. This coffee dust is bad. It produces the over-extracted bitter taste in coffee, clogs up your coffee filter, and will almost always result in some grit or sediment in the cup. OK, those are the negatives.

The positives? Blade grinders are much less expensive than the quality burr grinders. Bottom line, blade grinders are certainly a reasonable compromise between price and grinding coffee suitable for automatic drip machines if you use a paper filter.

Burr grinders produce very little dust. And only burr grinders provide the important control to produce a consistent and uniform particle size across a full range of levels, from coarse to very fine.

And, as mentioned, if you're an aspiring home barista, only a quality burr grinder will produce a fine enough grind and give you the precise control to adjust the grind in very small incremental steps. Without this level of more precise coffee grinding control, pulling great shots of espresso is almost impossible.

See our choices and recommendations for burr and blade grinders.

Mistobox delivers premium fresh-roasted coffee right to your doorstep, hand selected from the finest artisanal coffee roasters.
Mistobox delivers premium fresh-roasted coffee right to your doorstep, hand selected from the finest artisanal coffee roasters!



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