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Why Do Coffee Beans Get Oily? Is That Rancid Smell from Oxidation? What's the Difference Between Light and Dark Roast?

Published: 2026-01-27 Author: FrontStreet Coffee

Last Updated: 2026/01/27, A couple of days ago, a customer visiting FrontStreet Coffee noticed that the Mandheling beans in FrontStreet Coffee's jar had very obvious "oiliness," so the friend asked FrontStreet Coffee: "Has this Mandheling gone bad because it's been stored too long, causing it to become oily?" In fact, coffee beans becoming oily is a completely normal phenomenon, because of the surface on the coffee beans

A couple of days ago, a customer visiting FrontStreet Coffee noticed that the Mandheling beans in the FrontStreet Coffee jar had very obvious "oily sheen." This friend then asked FrontStreet Coffee: "Is this jar of Mandheling beans oily because it's been stored for too long?"

In fact, coffee beans developing oil is a very normal phenomenon. The "oil" on the surface of coffee beans is essentially water-soluble fat from the beans themselves. The reason customers ask this question is because not all roasted coffee beans develop oil - beans that show oil are quite rare nowadays. Why? Because this is related to the roasting degree of the beans.

What's the relationship between bean oil and roasting degree?

Before coffee beans are roasted, their structure is very tight, with water-soluble oils wrapped around the bean's fiber matrix. When green coffee beans are poured into the roaster for roasting, they continuously absorb heat energy from the roasting machine. The beans expand as they absorb heat. At FrontStreet Coffee, there's a corner with a display comparing green and roasted beans. Customers who have noticed this will find that green coffee beans expand 1.5 to 2 times in volume after roasting. The deeper the roast, the greater the expansion.

Along with volume expansion, the cell wall pores of the beans are simultaneously stretched further, accumulating a lot of carbon dioxide produced during roasting. The deeper the roast, the higher the expansion rate, and the more carbon dioxide accumulates. After roasting is complete, when the beans return to room temperature and normal pressure, carbon dioxide and aromatic substances begin to slowly release outward. Additionally, the water-soluble oils start to emerge on the bean surface, forming the "oil" we see with our naked eyes.

In addition to the amount of oil being determined by the roasting degree, the timing of oil appearance is also determined by the roasting degree. For example, beans that are removed after the first crack but before the second crack might need several days to form visible oil on the surface. However, if beans are removed when the second crack is intense, we can directly see some oil emerging on the bean surface right after they come out of the roaster.

Why are oily beans so rare?

Since the concept of specialty coffee became popular, people have adopted lighter roasts to better taste the beans' inherent flavors, preventing coffee's natural taste from being covered by flavors formed during deep roasting. Not only for single-origin coffees that express flavor, but some shops have even started reducing the roasting degree for espresso beans, which traditionally need deep roasting to combat milk flavor, in pursuit of a more extreme flavor experience. This has led to oily, dark-roasted beans becoming rare in today's era dominated by light roasts.

If we go back 40 years, you'll find that whether for espresso or pour-over, the vast majority of roasted coffee beans worldwide had a thick layer of oil, because the mainstream roasting degree at that time was dark roast. It's quite interesting that dark-roasted coffee beans have dominated for hundreds of years since their discovery, mainly for two unavoidable reasons. One was the necessity due to early roasting equipment being unable to roast coffee beans evenly, and the other was due to inadequate coffee bean quality! Because it wasn't possible to roast coffee beans evenly, extremely dark roasts were adopted, preventing large differences between beans from causing unstable coffee extraction quality.

Oily coffee beans emit more enticing aroma, and the extracted coffee has a richer body and more intense flavor. However, it's important to know that this layer of oil accelerates the oxidation of coffee beans, so we must ensure proper storage to prevent coffee beans from losing their aromatic substances early under the influence of this "oil."