Why is the surface of coffee beans shiny?

The shiny surface of coffee beans is because coffee beans contain a substance called caffeine. Caffeine is a naturally occurring chemical that is found in many plants, including cocoa, tea leaves, and certain herbs. In coffee beans, especially after the roasting process, caffeine seeps out from the inside and coats the surface of the beans.

First, during the roasting process, the high temperature causes the internal water to evaporate and causes chemical reactions. These reactions lead to a series of complex changes, one of which is to release the caffeine inside to the outer surface. When the water leaves the beans, it takes away the oils and other soluble components of the beans. These soluble components include volatile compounds with aroma and flavor characteristics, as well as pigments.

Secondly, the cooling stage after the roasting process also plays an important role in the formation of surface oiliness. When cooling after high temperature treatment (commonly known as "annealing"), the remaining moisture further migrates from the interior to the surface. This process causes a thin layer of oil to form on the surface of the beans, giving the coffee beans a shiny appearance.

In short, the shiny surface of coffee beans is due to the caffeine and other soluble components seeping out from the inside and covering the surface of the beans during the roasting process. This oil layer not only gives the coffee beans a glossy feel, but also, together with volatile compounds, is responsible for the aroma and flavor characteristics of coffee. When we appreciate a cup of brightly colored and fragrant coffee, we can also imagine that there are countless complex and beautiful chemical changes hidden in it.