Oil production principle: the oil production of coffee beans is closely related to the release of carbon dioxide. Coffee beans produce carbon dioxide when roasted. Generally, with the deepening of roasting, more carbon dioxide will be produced. When coffee beans are roasted to a certain extent, they will permeate with the released carbon dioxide, which is what we call "oil production".
Extended data
There are usually two kinds of "oily" beans:
First, deep roasted fresh coffee beans.
With the deepening of roasting, more carbon dioxide will be produced. When coffee beans reach deep roasting, they will be highly dehydrated and produce oil.
Generally, from the first day to the second day after baking, a large amount of "oil" began to appear on the surface. At this time, the aroma and flavor of deep roasted coffee beans also reached the peak. However, if the deep baked beans are dry and oil-free, special attention needs to be paid. Maybe you'll make a cup of fake coffee!
Second, stale lightly roasted coffee beans.
Shallow roasted coffee beans have just been roasted, but about 5-7 days after roasting, punctate oil droplets will appear on the surface of coffee beans, which may be caused by uneven heating and local dehydration. Such "spotty oil" does not mean bad taste, but the best tasting period.
However, if the lightly roasted coffee beans are left for several weeks, the surface will be shiny, smell very light and have a greasy taste, indicating that the flavor of coffee beans has lost and should be avoided.