Caffeine is a white crystalline xanthine alkaloid that works as a psychoactive stimulant. In 1820, caffeine was first isolated from coffee by a German chemist and in 1821 by French chemists who later coined the word caffeine. The substance can be found in fruit, leaves and seeds.In certain plants it acts as a natural pesticide that kills insects who trying feeding on the vegetation.Research shows that most caffeine consumed by humans is extracted from the bean of the coffee plant and the leaves of the teach bush.he substance can be found in fruit, leaves and seeds. In certain plants it acts as a natural pesticide that kills insects who trying feeding on the vegetation. Research shows that most caffeine consumed by humans is extracted from the bean of the coffee plant and the leaves of the teach bush.