Grinding Coffee

Issues

The process of grinding coffee deserves attention apart from other steps in the brewing process. Getting the grind right is an essential prerequisite for success with any brewing method. It is also important to consider when coffee is ground, the degree of uniformity in granule size, and the performance of the grinding equipment.

When grinding coffee, we are primarily concerned with achieving a certain average granule size. The size of the coffee granules will affect the extraction rate and the rate of water flow through the grounds. To achieve a certain average granule size in a burr grinder, we adjust the distance between the burrs. As the metal grinding disks are brought closer together, the resulting ground coffee will consist of smaller and smaller particles. In a blade grinder, the average granule size is determined by the duration of the grind. Granules are struck by the blades and broken into smaller pieces over and over again for the duration of the grind.

When the granules in a sample of coffee are all about the same size, they will each yield flavor components to the surrounding water at about the same rate during the brewing process. Each granule is expected to contain the same quantities of flavor components. We want to extract some, but not all of these components. A uniform granule size, and a corresponding uniform rate of extraction, allows us to adjust the brew time and other brew process factors to extract just the right amount of flavor components from each granule.

Now, consider what happens when a sample of ground coffee has granules of widely varying sizes. The smaller a granule is, the faster it will yield it's flavor components to the surrounding water. The largest granules will be underextracted, but may contribute some of their the most readily extracted flavors. The brew process will most likely be tuned to the average-sized granules, so these are likely to contribute an optimum balance of flavors. The smallest granules will be overextracted and will contribute to the brew's bitterness. What you have then is decent brew mixed in with some over and underextracted coffee. The more diverse the granule sizes in your ground coffee, the more the good portion of the brew is contaminated with the bad portions.

Equipment

Of the two types of grinders in common use, burr grinders and blade grinders, the burr grinder produces the most specific and uniform coffee granule size. Most burr grinders can be adjusted to produce the coffee granule size that is appropriate for a specific brewing method. The adjustment is usually made with a calibrated dial or lever that moves the grinding burrs closer or farther apart. The grinding burrs are grooved metal disks or concentric cones, one of which spins. Whole coffee beans are cracked into smaller pieces at the edge of the disks and then these pieces are broken down further between the burrs until they are small enough to pass through the smallest space between the burrs and out of the grinding chamber. Burr grinders produce very little coffee dust. The dust in ground coffee consists of very fine particles that will have a bitter influence on the brew. This dust will also clog filters and/or pass through filters to make mud in the bottom of your mug. Dust is bad, which brings us to blade grinders.

Blade grinders are simple, practical, and affordable, but they produce quite a bit more dust than burr grinders. The wide range of granule sizes in blade-ground coffee makes blade grinders appropriate only for coarser grinds. The dust in blade-ground coffee is likely to clog, and perhaps ruin espresso machines. Metal filters used for drip brewed coffee may also clog. Paper filters are more forgiving of dust, and will continue to let water flow through. So a blade grinder may be practically used to grind coffee for a drip brewer that uses a paper filter. The coarse grind used for an infusion method such as the press pot can also be done in a blade grinder. But regardless of the coarseness of the grind, the ever-present dust, or even just the smaller-than-average particles in blade-ground coffee, will influence the brew with overextracted flavors. With drip grinds and coarser grinds, this influence on flavor may be acceptable, or even preferred by some. But generally, this influence is a compromise for the convenience and low-cost of a blade grinder.

Grinders should not heat coffee excessively. Some heat will inevitably be produced by friction and the breaking down of the beans' cell walls. But worn or improperly adjusted burrs and other mechanical factors inside of grinders can cause excessive heat to build up. This heat can destroy flavors in the coffee. If the ground coffee coming out of your grinder is significantly warmer than the beans you put in, you may need to replace the burrs. When running large quantities of coffee through a grinder it may be necessary to let the grinder cool between grinds.

Practice

If at all possible, coffee should be ground just before brewing. Grinding coffee allows gasses and volatile flavor compounds to escape faster than they would from whole bean coffee. Whole bean coffee itself loses these flavors fairly quickly after roasting. Grinding accelerates this process. It's best to buy whole bean coffee in small quantities and then grind only enough for each brew.

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