Why Are Some Coffee Beans Oily? A Strange Phenomenon

Expert Consulted: Carl. Utilizing my 6 years of experience in the coffee industry, I’m able to provide accurate and concise information, helping you make informed choices about beans, brewing methods, and equipment, thereby elevating your coffee experience. In this article, I’ll share my knowledge and experience to help you answer the question: Why are some coffee beans oily?

If you’re here, you probably purchased some coffee beans and upon opening them you noticed a strange appearance.

They might have appeared oily or even wet. But don’t worry, they are perfectly fine and I’m going to guide you through all the information you need to understand what it is and why it happens.

So, why are some coffee beans oily?

When roasting a coffee bean, you are breaking down the cell structure of the bean, which in turn releases CO2. The oily appearance we sometimes see comes from a chemical reaction caused by the CO2 coming in contact with oxygen. This is more common in dark roasts due to the longer roast time and more fragile shell.

That’s just the quick answer, but there is so much more to this. Please keep reading to find out all about beans and why their appearances are the way they are.

Why Are Some Coffee Beans Oily? Does The Roast Level Matter?

Yep, the roast level definitely matters. A lightly roasted bean has a much stronger shell which means it will keep its flavors and matte finish for longer.

However, no matter the roast level, if you leave your beans out for too long they will start being oily from all the oxygen getting to them

Whereas a darker roast is more prone to oil seeping out and it may lose its flavor faster than a lighter roasted bean. I should mention that a properly roasted dark roast bean will not begin seeping oil until a few weeks have passed.

The darker the roast, the more likely it is to have oil on it at an earlier point in time.

Medium roasts tend to be somewhere in the middle, as they’re not roasted as long as dark roasts they’ll keep that natural matte-looking finish and those tasty flavors for longer.

It’s worth mentioning medium roasts still won’t stay fresh for the extensive time that light roasts do.

Why Are Some Coffee Beans Oily? The Science Behind Oily Beans

As I glossed over in my quick answer at the beginning of this article, for coffee beans to get oily, some chemical reactions need to take place inside the bean caused by extreme heat.

During the roasting process, the beans are exposed to extreme heat, which causes them to expand and, at some point, crack.

This cracking (known to roasters as the first crack) releases moisture and CO2 that has built up, allowing for the delicious flavor compounds to escape the cellular structure when brewing your coffee, and also allowing some naturally occurring oils to come out.

When dealing with dark and extra-dark roasts, they continue to tumble and roast in high temperatures, expanding further while experiencing more chemical and browning reactions.

This leads to what roasters call the “second crack”, where the cell walls of the bean start to break down, releasing gases and oils that were trapped inside. This also produces a much more intense popping sound, causing the bean to expand more than the first crack.

The second crack tends to be when the roaster decides to finish the roast, as they should have reached their desired bean profile.

As the bean reaches higher and higher temperatures, the acidity will be lower and the coffee will start to lose those bright and fruity flavors.

Light roasts are where this is most evident, as they are typically stopped before the second crack, meaning they maintain that acidic and bright flavor profile you know and love.

On the other hand, darker roasts are typically stopped during or after the second crack, giving them a much lower acidity and a more intense, full-bodied flavor.

Does The Oil Affect The Flavor?

One issue you’ll face with oily beans is oxidation. When exposed for too long, the oxygen will come in contact with the oils on the surface of the bean and oxidize the lipids which can not only cause an unpleasant smell but also ruin the taste of the beans. (Source)

Beans with oil on the surface are more prone to staling. This is why darker roasts should be consumed earlier than light roasts, to avoid stale beans.

Conclusion

Does the roast level matter?

Yes, a lighter roast is less likely to have oil on it than a dark roast. This is because of the longer roasting time of dark roasts.

However, leaving any bean out for too long will cause oil to accumulate due to the oxygen the bean was exposed to.

Coffee beans become oily because of chemical reactions taking place during and after the roast process.

This reaction is CO2 coming in contact with oxygen, which in turn may cause oil to rise to the bean’s surface.

The oil itself doesn’t necessarily affect the taste of the coffee, but, if oil is present that could mean the bean is getting or already is stale.

I hope this answers the question: why are some coffee beans oily?

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