Why Does My Coffee Look Oily On The Surface? – Barista HQ

There's nothing better than a fresh cup of coffee, but have you ever noticed a shiny layer on the top? You might be wondering why does my coffee look oily, but it's actually very common with certain types of coffee. This article will explain why it happens and what you should do about it.

The technical name for this oily layer on your coffee is 'coffee scum.' This term is pretty grim and probably makes you think it's dirty and unsafe to drink. However, it's not as bad as it sounds, and coffee scum is naturally occurring within your drink.

Coffee oil can rise to the top of your cup for a number of reasons, and it's actually very common. Even Starbucks coffee looks oily sometimes (though you may not notice the film on top of the coffee in the cup), so it's not a sign you've done anything wrong. Here are the main reasons why it happens:

1. Brewing Method Used

Certain brewing methods will naturally produce more oily coffee than others. The oily film on top of the coffee will usually be removed by filters, so drip brewing is less likely to produce a film. Similarly, if you're using a coffee maker with paper filters, it's less likely you'll have a shiny layer on your coffee.

Turkish coffee and french press can often result in a greater oil residue. If you want to avoid the oily residue in your cup, it's worth using a brewing process that includes filters. [1]

2. Roasting Type

The type of roast coffee that you use makes a huge difference. Whole coffee beans contain a high amount of unsaturated fat, and the oil level in your roasted coffee beans will impact how much coffee scum is created. This will vary depending on which type of roasting process is used.

Light roast coffee tends to have the lowest amount of oil; however, medium roast and darker roasts have greater levels of natural oils. The roast type makes a big difference too. Flame roasted coffee beans are popular because of the slightly burnt flavor, but they tend to be some of the most oily. To avoid getting that oily film on your cup of coffee, you should choose lighter roasts.

3. Bean Quality

The quality of your bean will impact how much oil is produced. High-grade coffee beans tend to be roasted for longer, which helps remove the fat from oily beans. Lower quality coffee beans tend to be roasted for a shorter time, exuding more oil into your cup.

The differences in coffee scum tend to be down to the production process, and just because there's an oily layer doesn't mean they aren't fresh coffee beans. Oily coffee beans can still be good quality; it just means they're probably dark roast beans or medium roast coffee beans.

4. Filter Type Used

Using a filter is the easiest way to remove that shiny oil level from your coffee cup. Your coffee machine and espresso machine will usually have some kind of filter built-in, but not every filter is the same.

Activated charcoal filters are the best for stopping a layer of coffee scum from forming because the fatty acids will bond with it. This makes it less likely to rise to the top and can actually give you a smoother taste.

5. Water Temperature

Coffee enthusiasts know that the temperature of the water will impact the flavor of your espresso, but it can also change the level of oil on the top. Hotter water causes the molecules in the coffee to bond more effectively with the water, but the fatty acids within the coffee are insoluble.

This means they’ll sit separately and rise to the top if you use water at very high temperatures.

6. Hard Water

Hard water has a higher mineral content than regular water. Calcium is the most common impurity, and it can impact the taste of your water and coffee. The hard water bonds with the fatty acids and helps them rise to the top more easily. This means there's more oil and the coffee scum is much more noticeable.

If you have soft water in your area, this won't be a problem, but if you do have hard water, you will need to use water filters or a water softener to stop the scum from forming.

Why Does My Coffee Look Oily On The Surface? – Barista HQ

Is Oil In Coffee A Good Or Bad?

It's natural to assume that oil in your coffee is a sign that it's gone bad or that you aren't using fresh beans. That's not the case, and even if it's oily, it can still be great coffee. However, it may be a bad sign if the oils are present in light or medium roasted coffee.

Dark roasted coffee beans are naturally more oily, and when combined with hot water, they'll produce a thicker layer of coffee scum. This is actually more prevalent in certain brands of coffee, and it's why dark roast Keurig coffee looks oily. Coffee scum, therefore, isn't a real concern with dark roasts, and some people even argue that the fatty acids give the coffee more body.

If you are using a medium or light roasted coffee, there could be several factors influencing how much coffee scum there is, but your best bet is to check the expiration date. The coffee is very unlikely to be unsafe, but it may not have the best taste, so you might choose to use different beans.

While coffee oil contains some antioxidants which are good for you, some people believe that the oil can increase your cholesterol levels. This hasn't been conclusively proven, but some studies have found a relationship between coffee and cardiovascular disease. [2]


Why Should You Avoid Oily Coffee When Making Your Own?

What exactly is Coffee Scum?

Coffee scum is the layer of oil that can gather on the surface like a film on top of coffee. This is caused by the oil in coffee beans and is more prevalent with certain types of beans or specific roasting processes. These oils are mainly made from insoluble unsaturated fats, which don't dissolve during the brewing process and float to the top.

If you are using your own coffee maker, then you should avoid making oily coffee at home. The accumulated oil from the roasted beans can start to clog up your machine and prevent it from being able to brew properly. It can also damage your water filters and impact the mechanism so that you can't grind your beans.

There are two ways to avoid damaging your home coffee maker:

  • 1
    Use Light Roasts
    There are more oils in dark roasted coffee beans, and light roasts produce less oily coffee. That means fewer fatty acids are flowing through your machine, so the oils are less likely to cause damage.
  • 2
    Use Fresh Beans
    It's worth using beans that are as fresh as possible as there will be fewer oils. Check the sell by date and look at how your coffee beans look before you grind them so you can keep the oily beans out of the system.

Common Myths Around Oily Coffee/Coffee Beans

  • Oily Coffee Is Unsafe
    Coffee scum doesn't look good, but some people believe it's unsafe and that it can even have a negative impact on your nervous system. This isn't the case, and there are no real health risks associated with drinking it. In fact, the oily layer contains some antioxidants from the coffee that can be good for you.
  • Coffee Scum Means You’ve Made It Wrong
    Coffee making is a bit of an art, and many people assume you've done it wrong if there's oil on top. How you brew the coffee can make a difference, but it's generally down to the type of bean you choose and how the coffee bean is roasted. Don't question your technique; just check which beans you're using.
  • Oily Coffee Means You’re Using Cheap Ingredients
    Many people believe that an oil film means you're using cheap beans and that your water quality is poor. Cheap coffee is often thought to be old and lower grade, but this isn't the case. In fact, every coffee can become oily, usually due to the roasting process.
coffee beans and pods

Common Oily Looking Coffee Questions

Does coffee contain oil?

Every coffee bean will contain some natural oils. Arabica beans tend to be 15% oil, while Robusta beans are around 10% oil. This oil is mostly saturated fat which floats to the surface of your coffee.

Can you remove oil from coffee after brewing?

Yes, you can filter your coffee after brewing to remove the oily layer from the top of it.

What is the purpose of coating coffee beans in oil?

Coffee bean oils remain inside until they are roasted. If you have an oily coated bean, then it's because of a chemical reaction between the inside of the bean and the oxygen.

How can you tell if a coffee bean is oily?

Oily coffee beans tend to be darker and weigh more. These are thought to have a bolder flavor, though many coffee enthusiasts insist that the oil content doesn't impact the taste.

What kind of coffee beans are not oily?

Light or medium roasted coffee beans are less oily because they've been exposed to higher temperatures for a shorter period. This means there's less breakdown of the beans, and the oils remain within the coffee beans.

What is the oil in coffee called?

Coffee oil is chemically known as caffeol. The oily layer in your coffee cup is called coffee scum.

Do coffee filters filter out oil?

Some filters are good at absorbing the oil and preventing it from forming a layer on the top of your drink.


Conclusion

The oily looking layer on the surface of your coffee is completely natural and forms as a result of the saturated fats within the coffee beans. It's very common with dark roasts, but even with medium or light roasts, it can occur because of the brewing method you use, the water quality, or other factors. 

It's usually perfectly safe to drink, but it can mean that your beans are out of date, so check the expiry date before making your morning cup.

References:

  1. https://www.health.harvard.edu/staying-healthy/whats-the-healthiest-way-to-brew-coffee
  2.  https://academic.oup.com/aje/article/153/4/353/129046