Why Are Coffee Beans Always Oily? Are Oily Coffee Beans Not Fresh? Are Oily Coffee Beans of Higher Quality? What's the Difference Between Dark Roast and Light Roast Coffee?

Understanding Coffee Bean Oiling: Separating Fact from Fiction

If you frequently buy coffee beans to brew at home, you might have noticed that some coffee beans start to develop an oily surface after being stored for a while. Meanwhile, other coffee beans remain dry on the surface regardless of how long they're stored. This has led to an interesting phenomenon: some people believe that oily coffee beans are proof of high quality, while others think the opposite—that oily beans indicate moisture absorption and declining quality.

In reality, both of these perspectives are essentially incorrect. Coffee bean oiling is a normal phenomenon that neither serves as proof of premium coffee quality nor indicates moisture absorption (when stored properly)! Why? Because coffee beans naturally contain certain water-soluble lipids that can migrate to the bean surface under specific conditions, creating the "oiling" phenomenon we observe. Before roasting, these lipids are contained within the bean's tightly structured cellular matrix, effectively "locked" inside the coffee beans. It's only after the raw beans undergo roasting that these lipids begin to migrate to the surface.

As we know, during the roasting process, the continuous heat energy supplied by the roaster causes coffee beans to expand continuously. This expansion opens up the cell wall pores in the coffee beans, allowing them to store large amounts of carbon dioxide.

After roasting is complete, when the coffee beans return to normal temperature and pressure conditions, the carbon dioxide stored within them begins to escape outward. At this point, the lipids inside the coffee beans can migrate to the surface along with the escaping carbon dioxide, creating the oiling phenomenon. However, as FrontStreet Coffee mentioned, this oiling phenomenon only occurs with certain coffee beans, and these beans typically share a noticeable characteristic—they have a relatively dark roast level.

Factors Influencing Coffee Bean Oiling

As FrontStreet Coffee mentioned above, coffee beans expand continuously due to heat energy, and this expansion makes their structure more porous. Therefore, when coffee beans are roasted to a darker level, they receive more heat energy and expand more significantly. Greater expansion leads to a more porous structure. This porous structure can store more carbon dioxide while making the attachment of lipids looser, naturally allowing oils to migrate more easily to the bean surface. Below are two beans from FrontStreet Coffee's selection: FrontStreet Coffee's Papua New Guinea Paradise Bird (medium-dark roast leaning toward medium), and FrontStreet Coffee's Sumatra Golden Mandheling (medium-dark roast leaning toward dark). Both share the same roast date—roasted five days ago—so you can compare their respective degrees of oiling.

In addition to the degree of oiling, the speed of oiling is also affected by roast level. The same bean will oil more quickly when roasted to a darker level. The two beans shown above only developed noticeable oiliness several days after roasting. If the roast level were even darker, the coffee beans would show significant oiliness on the same day they were roasted. As FrontStreet Coffee mentioned above, roast level is just one influencing factor. Besides roast degree, coffee bean oiling is also affected by processing method, roasting technique, and bean density. For example, some heavily anaerobic processed coffee beans will oil more easily than conventionally processed beans at the same roast level, because the bean structure has already undergone significant changes during processing—similar to having lower density. Therefore, the same roast profile will result in a darker roast level for these beans, making them more prone to oiling.

The explanation of coffee roasting methods can be quite complex. We can simply understand it as differences in heat application and timing during roasting that cause variations in coffee bean expansion rates, thus leading to different degrees of oiling. However, since many people rarely encounter roasting in their daily lives, FrontStreet Coffee won't elaborate further here.

Conclusion: Oiling and Quality

In summary, we can conclude that whether coffee bean oiling occurs has little correlation with quality. Oiling also doesn't directly affect coffee quality, though it does require more attention to storage methods. Once beans have developed surface oils, they're more susceptible to quality deterioration. If not stored in a suitable environment, coffee beans can easily develop negative flavors. So this is something everyone should pay attention to in their daily coffee routine~

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473