Why are some beans shiny?
Shiny coffee beans are coated in natural oils that have migrated to the surface during the roasting process. This is most common in medium-dark to dark roasts, where the heat has broken down the bean's cellular structure. While some oil is normal, an excessively oily surface can also indicate that the beans are older and have begun to lose their freshness.
Sources show where the underlying information came from. They do not automatically make a claim correct, so disputed history, evolving science and uncertain claims should be labeled as such in the explanation itself.
The sheen on a coffee bean is a visual indicator of the and the physical changes that have occurred inside the bean. During roasting, the heat causes the bean to expand and its internal structure to break down. As the roast progresses into the medium-dark and dark stages, the cell walls become porous enough for the coffee's natural oils to migrate from the center of the bean to the surface. This is why dark roasts are almost always shiny, while light roasts remain matte.
While a light sheen of oil is perfectly normal for a well-developed roast, an excessively oily or 'greasy' appearance can be a sign of a few things. First, it is a clear indicator of a darker roast profile. If you prefer a lighter, more origin-focused cup, you should look for beans that are matte or have only a very slight sheen. Second, oil migration is a continuous process. As beans sit after roasting, more oil will naturally move to the surface. Therefore, if you see a very oily bean that is supposed to be a light or medium roast, it may be an indication that the coffee is older and has lost its peak freshness.
It is important to note that these oils contain many of the aromatic compounds that give coffee its flavor. When they are trapped inside the bean, they are protected. Once they reach the surface, they are exposed to oxygen, which causes them to oxidize and go rancid. This is why very dark, oily roasts tend to have a shorter shelf life than lighter, drier roasts. The oils are essentially 'on the clock' the moment they hit the surface.
In practice, use the appearance of the beans as a guide when shopping. If you are looking for a bright, complex cup, choose beans that are matte or have a very light sheen. If you prefer a bold, heavy-bodied cup, a darker, oilier bean might be exactly what you want. Just be aware that the oilier the bean, the more important it is to consume it quickly to enjoy it at its best. Always store your beans in an airtight container to slow down the oxidation of those precious oils.
- ·https://gevi.com/blogs/coffee-knowledge/how-to-understand-coffee-tasting-notesVerify source for reference 1 ↗