When green coffee beans are roasted, they undergo a dramatic transformation. Complex chemical reactions unlock flavours, change their size, and turn them brown.
Another development is the appearance of oils on the surface of the beans. These are formed either during the roast when lipids within the cellulose structure come to the surface, or after a roast when the beans are in storage.
Oils on the surface of beans carry important flavour compounds and can have a significant impact on the mouthfeel of brewed coffee. However, opinions about oily coffee are divided, with some suggesting it indicates low quality, while others claiming it’s a sign of freshness.
To understand more about oily coffee beans, I spoke with 2018 Hellenic Barista Champion and head of quality control at Samba Coffee Roasters, Michalis Katsiavos.
Where does the oil on coffee beans come from?
Although green coffee beans may seem relatively inconspicuous, they’re packed full of carbohydrates, amino acids, water, caffeine, and lipids.
When heat is applied during a roast, each of these react differently, causing a range of changes in the beans. One of the most significant of these changes is the development of oils on the surface.
This happens as heat compromises the endosperm (the reproductive interior) and makes the outer layer more porous, causing oils deep inside the bean’s structure to migrate to the surface. The longer the roast goes on, the more oils appear as deeper structures in the bean break down.
It’s for that reason that dark roasted coffee tends to be more oily than light roasted coffee: the beans have had more time exposed to the high temperatures inside the drum.
However, light roasted coffee can also become oily when stored for a long period of time. This happens as the oils slowly come to the surface in the days and weeks that follow a roast.
Michalis Katsiavos is head of quality control at Samba Coffee Roasters in Athens. He tells me that while the lipid content of green arabica beans is around 15%, the amount that appears on the surface after a roast is ultimately determined by the roaster.
“Oil is trapped inside every single coffee seed until it is roasted,” he explains. “When the cells crack due to the heat of the roasting drum, the coffee oil starts to oxidise in the surrounding area. Usually, coffee beans roasted darker will weigh less and have more oil.”
How does oil impact coffee’s characteristics?
All specialty roasters accept that roasting coffee beans will cause the development of oils on the surface. But where they tend to be divided is on the impact of these oils on the final characteristics of the coffee.
One of the issues is around oxidation. The second a roast ends, coffee beans are under threat from oxygen in the surrounding air. When it comes into contact with oils on the surface of the beans, it turns the lipids into peroxide, causing the development of rancid, unpleasant-tasting flavours.
Although this will affect any roasted coffee, it happens significantly quicker in oily coffee as more of the oils are exposed on the surface of the beans. In most cases, that means that dark roasted coffee should be consumed sooner after roasting than light roasted coffee to limit the impact of oxidation.
Michalis explains that whether consumers buy oily coffee or not largely comes down to their personal preference.
“First of all, the strength of a cup of coffee depends on the bean used and how these have been treated,” he says. “If our goal is a very strong cup with bold flavour then oily, dark roasted coffee is the right choice.”
As well as bolder flavours, this type of coffee is also better for making espresso due to the crema. Crema is formed during the extraction process when hot water and coffee bean oils emulsify, resulting in lots of tiny bubbles on the surface of the brew. Naturally, the more oily the coffee, the better the crema will be.
That being said, oil isn’t always a good thing. With light to medium roasts, it can be a good indicator of stale, flavourless coffee. The reason is that the oils which haven’t migrated to the surface during the roast, start to do so when they’re in storage.
While coffee needs a certain amount of time after roasting to degas, the presence of oils on the surface may suggest that the beans have been left for too long and have started to lose their aromas and flavours.
Can oily beans also affect the packaging?
It’s clear that oil has a significant impact on the characteristics of coffee – but can it also affect the way roasters package their beans?
Michalis tells me that it’s important to think not necessarily about the oiliness of their beans, but about the rate at which CO2 escapes from them.
“I’ve noticed that oily coffee beans can affect the packaging in terms of how much carbon dioxide is produced,” he explains. “This means that with darker roasts and oilier beans, we can see the package being inflated due to the higher degassing rate.”
Indeed, roasting darker tends to produce more gases, greater internal bean pressure, and a more expanded cell structure. These factors lead to a higher rate of degassing and an accelerated stale process after roasting.
To prevent the coffee bag from exploding, it’s advised that specialty roasters fit all their packaging with degassing valves. Degassing valves are one-way vents that allow CO2 to escape without letting oxygen enter.
However, there’s also the roasting equipment to think about. Michalis explains that oily coffee often requires extra attention because it can easily damage parts if they’re not properly cleaned.
“Oily beans tend to cause problems with our equipment,” he says. “For instance, oil can stick easily to the grinder hopper surface, making it very difficult to clean”.
This is confirmed by BestCoffee in a blog post, in which they acknowledge that oils can clog up the gears and burrs on grinders, not to mention jam up the pipework inside espresso machines.
This, alongside specific packaging requirements, are worth considering before offering oily dark roasted coffee.
Whether selling light, medium, or dark roasted coffee, it’s important for specialty roasters to choose the best packaging for their beans. At MTPak Coffee, we offer a range of sustainable coffee bag options, including kraft paper, rice paper, LDPE, and PLA, all of which are completely customisable.
Our BPA-free degassing valves are 100% recyclable and can be fitted to all our bag types, from stand-up pouches to flat bottom pouches. Roasters can also choose to include resealable zippers or aluminium zip ties to help preserve freshness even once the bag has been opened.
For information about our sustainable coffee bags, contact our team.