The raw coffee bean is the only ingredient you need to roast coffee. 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. During 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 also taste bitter because the acids have broken down.