When coffee is roasted the heat starts a reaction as the coffee is roasting and the structure of the coffee begins to form gases and oil from the coffee bean itself. This is a natural process and part of the roasting process. This is the main reason why coffee is usually left to sit for up to a week as it settles. The settling time allows a majority of these gases to form and settle and allows the oil to seep through the bean.
There are about 2000 different chemicals in one single coffee bean. These chemicals usually evaporate during the roasting process or turn in to several hundred volatile aromatic compounds. There are many types of coffee, each with their own specific sugars and oily substances. Lipids for instance, are fatty substances that are insoluble in water. When coffee is roasted the heat starts a reaction as the coffee is roasting and the structure of the coffee begins to form gases and oil from the coffee bean itself, making the beans appear shiny or “oily”. This is a natural process and part of the roasting process.
With the invention of one way valve bags, coffee can be packed almost immediately after roasting as the one way valve allows the gases to escape but no oxygen to enter the bag. Exposure to oxygen is what makes the coffee begin to stale.
Coffee that’s oily after roasting is also a sign of freshness. However not all coffee will excrete the same amount of oil. Some may be very noticeable and look like they have been soaked in oil, whereas others may be very discrete with only faint remnants of oil.
Usually you will find that the darker the roast of the coffee the more oil you will find as the more roasting tends to bring out more oil.
The taste of a good coffee is also determined by the maintenance of the machine. If the machine is poorly or not maintained at all, it will have a negative impact on the flavours in the cup. That is why daily cleaning is indispensable. Also make sure that the coffee bean reservoirs are cleaned at least once a week, as oils may stick to the jar and become rancid. This will eventually cause the coffee to taste stale.