Why are there wrinkles on the surface of the roasted beans?

If you look closely at the surface of the roasted beans, you will find that some beans have a completely stretched-out grain on the surface, while others do not. How is this difference caused?

One reason lies in the raw coffee beans themselves, which are denser and more likely to leave a grain.

Dense, representative coffee beans, from Kenya and Guatemala.

Less dense representative coffee beans are produced in Caribbean countries such as Brazil, Cuba, and Jamaica.

However, even coffee beans from the same origin can have different densities.

Coffee beans from higher altitudes of origin have a higher filling density, so the stretching of the grain is not very good.

Another reason has to do with the roasting. When roasting there is water vapor and carbon dioxide released from the beans, so the surface grain will stretch out.

Especially with deep roasting, a lot of carbon dioxide is released, so the coffee beans will have a very good spreading effect.

If the roast is lighter or if it is roasted for a longer period at a lower heat, the grain of the coffee beans will be more defined.

Coffee beans roasted with the same raw beans will have a different degree of grain spreading because the roasting method or degree of roasting is different, and the taste will naturally be different.

Why is there oil on the surface of roasted beans?

The oily substance on the surface of coffee beans is the fat in the beans.

The carbon dioxide produced during roasting allows the fat in the coffee beans to penetrate the surface of the beans.

As the roasting deepens, more carbon dioxide is produced.

The fact that fat will seep out during the deep roasting process and will also seep out after the roast has been left for a while is due to the carbon dioxide still being released from the residual coffee beans.

Carbon dioxide is released from the coffee beans one by one for about a month after the roasting is finished, with the greatest amount being released in the first few days after the roasting is finished.

The oils, on the other hand, can leach out pretty much in the first few days of a good roast.

Some people like coffee beans with a lot of oil on the surface, and some people don't like it very much.

Will the oil leaking out from the top layer of the coffee bean cause premature deterioration of the coffee bean? Don't worry, the oils in the coffee beans will not change over a long period.

Freshly roasted light roasted beans should be dry and oil-free on the outside, and freshly roasted dark roasted beans should be oily on the surface.

When choosing coffee beans, the presence or absence of oil on the surface is a reference indicator for judging freshness, but not absolute.