Grinding Fundamentals.

Before we can appreciate a finely brewed cup of coffee, we must take the laborious task of extracting important flavoring materials locked within the bean itself. One easy way to do this would be to place whole roasted beans in hot water and agitate - eventually, after much time, the flavoring material would extract into solution. Obviously such a brewing method would prove too impractical. However if you cut the bean in half - thereby increasing surface area - extraction efficiency significantly increases. Break the bean even further, and the surface area increases further until it becomes practical enough to brew. By now your gotten the idea - breaking the bean in half decreases particle size yet significantly increases surface area - see Table 1 below. 

Its no suprise then, that creating smaller coffee particles increases surface area such that a there is greater liberation of carbon dioxide gas (CO₂) and absorbtion area for hot water to act upon. Smaller particles also shorten the distance from the center of each particle thereby increasing extraction of oils, solubles and other flavoring materials.

The whole process of reducing whole particles into smaller ones is called "grinding" and can be done using number of different methods. Some include crushing, rubbing, grating, cutting, tearing and compressing using a different type of grinderss. Well discuss these methods in greater detail in later, but for now, lets discuss some of the factors affecting grinding.

Factors Influencing Grind.

Several factors act on the bean which ultimately affect the way the coffee's behave during grinding. The most common include: moisture content, degree of roast and brittleness of the bean.

Moisture content: Typically roasted beans that have been subjected to water quenching tend to be softer than air cooled ones. As a result water quenched beans tend to distort unevenly during grinding and tend to produce inconsistent grind particles. 

Degree of roast: Typically beans of a lighter roasts will be more pliable and tenacious than darker roasted coffee. It is the greater loss in moisture in dark roasts that makes beans more brittle and as such, produces a greater number of fine particles than a lighter roasts.

Bean brittleness: the origin of the coffee also affects how the bean behaves during grinding. Natural coffee's will grind differently than wet processed coffees - given the same roast level. Also new crop coffees will always produce fewer fine particles than past crop coffees - due to differences in cell structure. Altitude also affects the way the bean will grind since those beans grown at higher altitude tend to be denser and harder than those grown at lower altitude. Its also been found that differences in cell structure between robusta and arabica as opposed to arabica produce difference in the number of particles.

Grinding Standards

In the 1940's the US Department of Commerce established a testing procedure for measuring the distribution of particles. The equipment - called a RoTap - consists of four wire screens stacked on top of each other. When attached to a vibrating machine, particles inside the sieve fall through the various size screens to create a particle distribution. The procedure involves measuring 100g of roasted ground coffee and placing it on the top sieve and allowing the machine to shake for 5 minutes. After the cycle, all the sieves are removed and the coffee remaining in each sieve is weighed. Using this method a particle distribution curve can be calculated then classified. Luckily the US Department of Commerce has taken defined some of the most common grinding specifications. See Table 2 below. 

Since grinding plays such a critical role in beverage quality, its important to match the grind level the brewing method. Obviously espresso requires a much smaller particle size than say French Press. Also, always grind the coffee as needed as increasing surface area rapidly causes coffee to go stale. Happy grinding!

References:
Lingle, T. The Brewing Handbook. SCAA.