When heated, the bean undergoes a change whereby the acids and aromas are unlocked. If over roasted, the beans will lose its acidity and taste bland. The trick is to stop the reaction when the flavor is at its peek and hasn't started to break down.

Heating coffee beans results in a loss of water mass. The less the water mass the lighter and less dense the bean becomes. At the same time, the bean will increase in size. Durring the roasting process, gasses are forced out of the bean at such a rapid rate that the beans will pop. As temperature increases, small chips are blown off the bean. As internal temperature of the beans increases a change in flavor occurs.

As the temperature nears 435 degrees, oil will begin to seep to the surface of the bean in patches. The roasting may be stopped at any time depending on the desired roast. If the beans are under roasted, they will taste bitter because the caffeine has not been broken down. The darker the roast the less caffeine. If the coffee is over roasted it will aslo taste bitter because the acids have broken down.

Most roasters advise stopping the roast at some point during the second popping phase. Some prefer to stop the roast
near the beginning of second pop. This will accentuate the coffee's unique characteristics. Others advocate roasting until
the second pop has nearly stopped. This will produce a mellower, smoother taste. My approach is a middle-of-the-road
one. Your technique should be adjusted to suit your personal taste.

The whole point of the roasting process is to develop and distribute the coffee oil throughout the bean. The coffee oil is
initially contained in one small part of the bean. As heat is applied, the oil's flavor is developed, and as the structure of the
bean expands, the oils spreads throughout the bean.

As the bean is roasted darker, the oil spreads throughout the bean. Some of it will reach the surface in patches, and the
beans may appear to "gleam" under bright light. However, the surface of the bean should not be excessively oily.

Coffeeman