If you open a fresh bag of coffee and see completely shiny beans – oily coffee – then the odds are you’re looking at either a very dark roast or a dark roast that has been on a shelf for way too long!  

As a roast cycle progresses, and the sugars in the coffee become more and more caramelized, you lose much of the coffee’s unique attributes and trade them in for carbonization and pyrolysis. These coffees display shiny oils on the surface, and may already smell rancid.

That’s not the coffee you want to be taking home! 

Oily Coffee Beans Means Either a Dark Roast or Your Beans Have Become Rancid.

  • Arabica coffee is made up of less than 15% oil. When coffee is roasted, the bean’s cellular structure becomes more porous and the bean becomes less dense as moisture leaves the bean and chemical reactions occur in the roasting process. This more open and porous structure allows oils to travel to the surface that are naturally occurring in coffee.
  • As the roasting process continues (to darker and darker roasts) more oils come to the surface. Light to medium coffees can have little to no oil – or some small quantities of oil that appear over time – whereas very dark roasts can come right out of the roaster shiny with oil.
  • Carbonization (burning coffee) can also rapidly accelerate the rate of surface oil emergence and rate of staling due to oxidation of oils.
  • Major oils in coffee are linoleic and palmitic acid, followed by oleic and stearic acid. Each of these oils has a different chemical makeup and are actually all extremely beneficial for your health! They have anti-inflammatory and antioxidant properties, among other benefits. (Here’s a neat breakdown about coffee oils)

So how do you know if you’re getting a good roast? 

As delicious as caramelized sugar sounds (like creme brulee?) caramelization of sugars actually ends up bringing forward a very bitter flavor. 

As sugars are reduced, the aromatic compounds and sweetness (residual sucrose) in the coffee are also reduced, resulting in a higher degree of caramelization and flat, bitter notes. This degree of roast (dark and oily) will likely showcase flavors of smoke, char, and roasty coffee.

The Science Behind Oily Beans 

If you are seeing oily beans, there are a number of steps that have already taken place behind the scenes. In order for beans to get oily, chemical reactions must take place between the internals of the bean via heat application. As the beans are roasted and exposed to very high heat, the beans begin to expand and eventually crack, releasing built-up moisture and CO2. Once cracked (for roasters this is appropriately named “first crack”) the coffee’s structure is more porous. This allows for the delicious flavor compounds to escape the cellular structure when you’re brewing coffee, and it also allows naturally occurring oils to make their way out. 

Dark and ultra-dark roasts follow the same progression, they continue to tumble and roast at high temperatures, and continue to expand and undergo chemical and browning reactions. Eventually leading to “second crack”, where small micro-fractures occur in the coffee as it continues to expand and become more brittle.

Medium-dark roast coffee follows the same process. However, in this instance the beans are not roasted as long and while the same chemical reactions take place, you’ll get a slightly bolder flavor without as much surface oil. 

During the roasting process the beans change color, adapt flavors, and change size! Oils can surface during the roast phase or after the roast. These oils can help carry flavor for the beans, but can impact the taste of the coffee once brewed.

Phases of Coffee Development 

All coffee beans start as a green seed (bean). These beans are packed with amino acids, water, caffeine, and lipids. However, to get to the brown coffee bean we typically interact with, there are a number of steps that must happen behind the scenes to yield this finished result. 

When heat is applied to beans during roasting, all the beans react differently which can cause a number of changes in the chemical composition, size, shape, and color of the beans. One of the changes that can occur during this roasting process is the development of oils on the surface of the beans. 

When the heat is applied, it penetrates the beans all the way through, starting a chain of reactions making the coffee more porous. As the coffee becomes more porous, the oils that once lived deep within the bean make their way to the surface. The longer the beans are roasted, the more oils appear on the surface as the coffee continues to expand.

That is the simple science behind the shiny beans of dark roast and the matte appearance of light roast! Dark roast beans tend to have more surface oil due to a longer roast process.

Having Control Over Your Coffee

What if you like bold coffee? Does this mean you’ll be subject to drinking coffee that has gone bad? Not necessarily.

The truth of the matter is that you have control over your coffee! The boldness of the flavor comes down to how the coffee was roasted and brewed. Over-roasted beans will yield a bold flavor, reminiscent of a classic cup of joe. With a light roast, the flavors you will taste are those more unique to the specific terrior of the coffee. So, what does this mean for you and your morning cup?

This means that you get to decide how your cup of liquid gold will taste! Choose a coffee and a roast level that matches your preferences. Before you get to brewing, try to think how the roast level will affect the flavor. You get to control some of the character of your cup based on how you adjust the grind and which method you choose to brew with. By adding in more coffee to your water ratio or producing a finer grind, you can make your coffee work for you and get a stronger flavor out of a light roast! Just be careful, too fine of a grind size will yield sour flavors or clog your filter.

How does oil impact flavor? 

At the end of the day, coffee taste and brew comes down to personal preference. All specialty roasters know and accept that darker roasts can cause oil surfacing. However, when it comes to preference all artisans are different. 

One issue that arises with oily beans is oxidation, if exposed too long the oxygen will get in contact with the oils that have risen to the surface of the bean and oxidize the lipids which can not only cause an unpleasant smell, but ruin the taste of the beans. 

Chances of staling coffee become much higher with more oil. Therefore dark roast coffee (oily beans) should be consumed sooner than light roast to avoid spoiled beans. At the end of the day, it’s your cup of coffee. If you like it dark, find a local roaster to provide you with a fresh roast. Life is too short for stale coffee!

So, how do you like your coffee? 

You might like a dark coffee, there’s no judgment here! We love pairing our coffees with our mood and our food. 

But with dark coffee comes surface oils, and keep in mind these build up in grinders quickly, so maintaining a clean grinder is a must for dark-roast drinkers.

Be sure to clean out your grinder regularly and change out burrs when needed. Dark and oily coffee can coat and clog the grinder, and even leave coffee residue behind, altering the flavor of the next bit of coffee that passes through.

Limit coffees’ exposure to oxygen! There is a reason that coffee bags have one-way valves (no it’s not actually meant for sniff-testing) it’s meant to allow CO2 to escape while keeping everything else out. The more air your coffee is exposed to, the faster it will stale and lose its delicious, delicate flavor. This is why you’ll hear that whole bean coffee is superior, because it has less surface area open to the elements, and the flavor will stay fresher until it’s ground just before brewing.

To keep your coffee as fresh as possible, keep it in an airtight container away from light, and limit the time the bag or container is open.

Want to learn more about the science of coffee?

Study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6040754/

At Pinup Coffee Co we’re passionate about artisan coffee. 

We artfully roast our hand-picked beans to make the best coffee around, that tells a story of its origin and unique profile, without the addition of any flavored oils. 

We do roast some dark coffees, but even our dark roasts aren’t coated in oil. Some coffees are excellent dark (sumatra lovers!) When we roast our coffee beans we let their intrinsic flavors and qualities shine, finding the perfect roast level. 

Specialty coffee is “special”, grown in better conditions, carefully selected, and meant to be truly enjoyed. Our job is to unlock the true qualities of the coffee, as well as produce roasts that everyone loves. 

We roast our coffees with care and respect, bringing out the best parts, and sometimes those best parts really shine at a darker roast level. But we won’t, under any circumstance, char and carbonize to the point where our incredible coffees aren’t recognizable. 


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