In the roasting process the berries swell up by the liberation of gases within their substance. The aromatic oils contained in the cells are sufficiently developed or "cooked", and made ready for instantaneous solution with boiling water, when the cells are thoroughly opened by grinding.
The roasting principles of different green coffees vary. Trained study and a nice science in timing the roast and manipulating the fire is necessary to a perfect development of aroma and flavor.
The drinking quality is largely dependent upon the experienced knowledge of the coffee roaster and his scientific methods and modern machinery, by which the coffee is not only roasted, but cleaned, milled and completely manufactured to a high point of perfection.
In their National Association work, the wholesale roasters are giving the public new facts and valuable information, from scientific researches, investigations, etc.
Grinding. The roasted berry is constructed of fibrous tissues formed into tiny cells visible only under the microscope, which are the "packages" wherein are stored the whole value of coffee, the aromatic oils. Like cutting open an orange, the grinding of coffee is the opening of surrounding tissue and pulp, and the finer it is cut the more easily are the "juices" released.
The fibrous tissue itself is waste material, yielding, by boiling or too long percolations, a coffee colored liquid which is fibrous and twangy in taste, has no aromatic character, and contains undesirable elements.
The true strength and flavor of roasted coffee is ground out, not boiled out. The finer coffee is ground, the more thoroughly are the cells opened, the surfaces multiplied, and the aromatic oils made ready for separation from their husks. Hence it follows that:
Coarse ground coffee is unopened coffee—coffee thrown away.
The finer the grind, the better and greater the yield. With pulverized coffee (fine as corn meal) the fully released aromatic oils are instantaneously soluble with boiling water.
In ground coffee the oils are standing in "open packages," escaping into the air and absorbing moisture, etc., necessitating quick use or confinement in air proof and moisture proof protection.