For the average coffee drinker, the idea that an expensive grinder would produce better tasting coffee sounds ridiculous. Even I had my doubts about this advice when I was first getting started. This is probably because coffee looks like it's "dead." We think ground coffee is just a bunch of coffee dirt and that all the important parts of the production happen just before and just after grinding, not during.

In fact, the difference is extremely noticeable and it has to do with ../core-concepts/extraction and ../core-concepts/oxidation. A cheap propeller-style grinder indiscriminately breaks the coffee beans into random shapes and sizes. This will include "fines," which are very small particles of the coffee bean.

The smaller the particles, the more extraction will happen. Overextraction leads to immensely bitter, flavorless coffee.

Burr grinders, especially conical burr grinders like the Baratza line will move the beans around breaking them into smaller pieces only allowing the piece to fall through until it has reached the target size. Think of it like a coin slot in a vending machine. Each coin is sorted by type into little siloes based on its size and shape.

A good grinder will also give you the option to define that shape. This will vary mostly based on the brewing method but can also vary slightly bean to bean. I have to use a slightly different grind setting for Ethiopian beans versus other origins for example.

The best way to find the optimal size is to start with the grinder manufacturer's recommendations for your chosen brewing method then to follow this up with a little trial and error to dial it in.

Lastly, never grind any you are not planning on brewing immediately afterwards. The curve for grinding is arguable, but happens over a matter of hours. Resist the temptation to save time by grinding the whole bag or even just grinding a little more for the next day. Once beans are ground, the ../core-concepts/oxidation process accelerates. If you do this, freezing arguably helps but this is by slowing down the oxidation a small amount at best, if not negatively impacting the flavor by freezing.

By extension, the only way to make a good cup of coffee from preground beans is to purchase them immediately after grinding and brewing the entire bag immediately when you get home.

As mentioned in ../sourcing-beans, buying preground beans at the grocery store or in most cafes is doubling down on how terrible your coffee will be. Not only will your coffee be stale but the roast will be long expired as well.