Particular brew methods are designed to extract desirable soluble and insoluble flavor compounds without extracting undesirable bitter flavors that leach out of the bean later in the brew cycle. If coffee particle size varies significantly, then your brew will be extracting at substantially different (and unpredictable) rates, resulting in a coffee that is partly under-extracted, partly brewed right, and partly over-extracted. Flavors that popped in the cafe are fogged, and your brew may taste a somewhat flat or bitter due to over-extraction of particles that are too small for your brew method.
An ideal (non-existent) grinder would grind 100% of the particles to exactly the size. Professional quality grinders are developed to grind as many particles as possible to the same size. Ordinary home grinders are designed to break up the beans into small pieces, but not necessarily accurately. Here's what the difference might look like:
Wealthy home aficionados can tighten up that bell curve with a shop-quality grinder at home ($1000+). Depending on the state of your palate and your wallet, a slightly less accurate home grinder might also do the trick (about $200). The rest of us must make do with the grinders we have. Here are a few tricks:
- If you have a blade grinder, and the grinding chamber floor is flat, tilt the grinder as you grind to avoid re-chopping already ground particles. Here's a blade grinder pictured in a great grinder review over at Gimme! Coffee.
- If you have a hand burr grinder, keep an eye on the wobble in your handle over time. As you crank on that handle morning after morning, you'll start to put pressure on the metal axle itself and on the fittings that align that axle. Over time, your burrs will wobble more and more, generating an increasingly erratic grind size. Home-barista.com has a great article on maintaining the axle of the popular Hario hand grinder.
- Alter your brew methods to suit your grinder. It sounds odd, but if your grinder can produce a decently consistent turkish coffee grind, but only pebbles and dust in the French press range, learn to brew turkish!
- Finally, cheap grinders wear out. Replace your grinder when you can't stand it any longer!