There are three factors that impact how much oil appears on the surface of the coffee beans.
Darker green beans tend to see more oils migrate to the surface of the coffee bean during the roasting process.
In general, coffee beans are roasted at 3 different levels, from light, medium to dark. Light and Medium roasted coffee beans would only reach the “First Crack” stage during the roasting process, whereas Dark roasted coffee beans would need to reach the "Second Crack" stage. The temperature would need to reach around 224C in order for the Second Crack to occur, as that’s when the oils within the coffee beans would migrate to the surface.
Oiliness is also an indicator of how fresh or stale the coffee beans are. So, irregardless of the beans’ varietal and roast levels, the oils would leak out of the beans over time, and oils from aged coffee beans would eventually be completely evaporated.
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