Oils on coffee beans

Open a bag of coffee beans and often they will have a distinctive sheen on the surface. This is coffee oil; to some it looks attractive and a sign of freshness. To others it could suggest a problem with the beans, like they’ve been sweating! What’s more, not all coffee will have oil on its surface. So what's the deal; is it good or bad thing?

To answer that we need to get something straight. Calling them coffee beans is misleading because really they start life as seeds from a fruit. As a seed, they contain everything needed to grow another plant and this includes some fats, or oil. So all coffee contains oil but only on some will it be visible. This just depends on how long it has been roasted.

Before roasting, coffee beans are green and very dense; not at all like the brittle brown things they become. The longer the roast, the darker the brown colour will become to eventually turning black when they burn. Of coarse you will never see this if you use Limini Coffee!

So the colour of the bean is an indication of roast level. Light roasts will have a paler colour, and a different taste, to medium and darker roasts. It is on dark roasts where you will see the most oil.

This is because the roasting process creates a high amount of pressure inside the bean. The water present inside evaporates, drying it out and making it more brittle. At the same time processes are happening inside the bean that create carbon dioxide, which essentially cracks it apart, expanding it up to twice the original size.

The heat used to roast coffee turns oil into liquid which are then pushed to the outside. As dark roasts have been subjected to higher temperatures and for a longer time they are the least dense and oil has more chance to migrate to the surface creating more of a sheen. A lighter roast typically has a surface more matt in appearance.

So oil on coffee is not a sign of bad or good quality but of roast level.

It could also indicate the freshness of roasted coffee. The oils in any coffee will naturally start to leak out of it porous structure given time, maybe weeks or months after the roast date, it just depends on roast level. It will be slower for light roasts and faster for dark. Eventually these oils will evaporate, meaning really old roasted coffee will look dull in appearance.

It’s why at Limini Coffee we always include a ‘roasted on’ date on the bags, so you can guarantee you will be making coffee with beans that are at optimum freshness.