Why Do Coffee Beans Get Oily?

The Makeup of the Coffee Bean

Coffee beans are made up of three basic materials: a fibrous hull that surrounds the actual berries, a mixture of protein and fat, and moisture, sealed into the bean by the hull. According to coffeefaq.com, the outer hull is composed of "silverskin (a vestigial remainder of the fruit's development, also called the spermoderm). The silverskin is covered by a parchment skin (the endocarp), which is covered by a slimy layer (the parenchyma), surrounded by a thin layer of pulp (the mesocarp), all covered by an outer skin (the exocarp)." All of these work to keep the bean safe and dry.

Where the "Oil" Comes From

When the coffee bean is roasted, the outer hull cracks open under the heat and the berries are exposed. The fats and proteins break down and mix with the moisture found in the hull, which under heat turns to steam and liquefies the fats and proteins creating an oily substance. Heat causes oil to seep out of the berries. The longer they are exposed to the heat of roasting, the more oil is expressed and the darker the coffee bean becomes. "Dark roasts" are those in which the beans have been roasted long enough for significant amounts of oil to be expressed. "Light roasts" are those in which the bean is roasted for only a short time, not long enough for much oil to seep out or become very dark in color.

Oily Beans and Taste

Oily coffee beans, then, generally just indicate a darker roast than other beans which do not appear oily. They can be used to prepare coffee the same way as lighter roast beans, and will have the flavor of dark roast coffee.

References

• The Science of Oily Beans
• Coffee Faq
• Supermarket Guru: The Appearance of Coffee Beans