While there are many important pieces of equipment that go into making a great cup of coffee the most important is without a doubt the grinder.  While this may seem like a bold statement, don't fear!  It's not to say that a high end espresso machine or a hand-blown chemex aren't important, more so that without a good grinder beforehand, their inherent advantages can be over shadowed by a poorly extracted coffee as a result of uneven grind size.  Just how does the evenness of a grind affect extraction?  Let's turn to what I'll refer to as "Particle Size Distribution" or PSD for short.

What is PSD?

In a nut shell, PSD refers to the spread of different particle sizes produced by a given grinder, and what portion of the whole sample each size represents.  It's easy to think that by adjusting our grinder to a given grind size we end up with an even, and consistent PSD, but in reality we always end up with a full spread ranging from "fines" (that dust-like super fine powder) to "boulders" which happen when oddly shaped pieces slip through the burrs without first being broken down further. 

Have a look at the diagrams below for a little more info on how an even Particle Size Distribution can result in even extraction and a much more balanced cup.

In this first set of diagrams have a look at a slightly exaggerated example of a uniform particle size on the left, and a more typical spread of different particle sizes on the right.
Once water is introduced and extraction begins, we can see how the larger particles in the sample on the right won't fully extract (less surface area to interact with the water) while the smaller particles will over extract in the same given time period.  Only a portion of the coffee will be properly extracted.
This result is typical and might be familiar to anyone who has found themselves frustrated by a coffee that is seemingly under and over extracted simultaneously (sour/bitter).  This can even give false readings when using refractometers to measure % TDS as it will average out to an apparent middle, despite in reality being over and under the ideal level of extraction.

PSD and Espresso

When it comes to espresso and PSD, another problem presents itself.  Espresso grinders (and machines for that matter) have been designed around an intrinsic flaw to the system: uneven PSD, specifically "fines".  When there is a high percentage of "fines" present in a sample of ground coffee the flow rate of water through that coffee is slowed down considerably as all of the gaps between the larger particles are filled.  To overcome this problem espresso machine manufacturers have historically raised the pump pressure in order to maintain a reasonable flow rate based on the ideal duration for a shot.  Not only does this exaggerate the uneven effect of over extracting and under extracting at the same time, but also limits the overall extraction considerably; in order to achieve a reasonable flow rate with high levels of "fines" (even with higher pump pressure) you must grind coarser which lowers the potential for extraction considerably. 
Have a look at the diagrams to the right.  Both images show a theoretical espresso grind size.  As the grind size gets smaller, PSD actually becomes more uniform (as compared to say grinding for a french press or chemex).  The sample on the left will let more water flow through despite the particles on the right being larger as there are less "fines" present.
The extraction is thrown off twofold: the difference in relative surface area/particle causes simultaneous under and over extraction, as well as the difference in flow rate means that water will be in contact with the coffee for longer, increasing that gap.
So what can we take away from this in order to make better coffee? Make sure your grinder is your best piece of equipment, as it has the most profound effect on how you extract your coffee.  We love Baratza grinders for this fact, combined with the company's attitude towards innovation (not to mention realistic pricing!).  You can see all of their products here, and buy them directly from our website or cafes.  They even go as far as to publish the PSD graph of each grinder on their website - now there's a first!
Well that's all for now, but as always let us know if you have any questions about anything you might have stumbled across during this email!  It can sometimes be a little "heady" to get through, but we're always happy to talk shop and help you make better coffee in whatever way we can!

Yours in coffee,