Why is the surface of some coffee beans shiny when roasted?

These "oils" distributed evenly on the surface of coffee beans are actually not "oils", but water-soluble organic substances that look like oils.

"Coffee oil" itself contains many aroma ingredients of coffee, which can be dissolved in water, so the surface of your brewed coffee will not be covered with greasy oil.

There are two reasons: oil beans.

A stale shallow baked beans b fresh deep baked beans.

A) stale moderately baked beans

The "medium-light baked beans" with light roasting heat and light brown appearance are dry and will not produce oil after baking. About six days after baking, the phenomenon of "dotted oil" begins to appear (dotted oil droplets appear on one side of the coffee bean). Slight "dotted oil" does not mean that it is not fresh, but sometimes the flavor of medium-shallow roasted coffee beans is at its peak. Continue to put, more than two weeks after the oven, the surface of medium-shallow baked beans gradually appeared a layer of bright oil, at this time, the flavor of "medium-shallow baked beans" has begun to decline, should avoid buying.

B) fresh deep baked beans

Deep-baked beans with dark brown appearance show a slight glossy appearance after baking, and a large amount of oil begins to appear on the surface from the second day to the fifth day after baking. The bright-looking "deep-baked beans" not only do not mean that they are not fresh, on the contrary, the deep-baked beans will gradually dry out three weeks after they come out of the oven, and finally become dry-flavored beans. Therefore, if you see coffee beans that are dry but dark brown in appearance, please pay special attention to their baking date, which is most likely to be spoiled beans.