Coffee contains many aromatic oils, which we refer to as coffee oils. These oils give coffee the specific flavor and aroma, and they are extracted into the beverage during brewing. Coffee oils are relatively volatile, and they get lost as coffee beans get older. Coffee oils also evaporate from the hot coffee. Over-extraction also causes the evaporation of the oils.
Dark roasted coffee beans are often oily on the outside because the longer roasting time has drawn the oil out from the coffee bean. This then means that the coffee bean will have less oil to be extracted during brewing.
Oil in coffee can be either advantageous or disadvantageous, depending on individual perspective. Coffee oils provide a lot of the signature flavors that most coffee drinkers associate with coffee. However, some coffee drinkers prefer to avoid coffee oils, because the oil contains the molecules cafestol and kahweol and some studies have linked high consumption of coffee oils with higher cholesterol.
Although this is the case for some people with a predisposition to higher cholesterol, there are also many health benefits to consuming coffee oils, such as decreased risk of type 2 diabetes. This is due to the fact that the compounds in coffee oil increase the uptake of glucose in muscle cells. The compounds in the coffee oils also have anti-inflammatory and anti-tumorigenic properites.
Some types of brewing allow for more oil to be present in the beverage. French press and espresso coffee contain more coffee oil than filter coffee because their filters are courser than filter coffee filters. These courser filters allow more coffee oils into the final cup.