Why Does My Coffee Have a Film on Top?

A freshly brewed cup of coffee gets you up and ready to face what the day brings. But have you ever brewed your cup of joe only to notice an oily film on top? This oily film must have raised your curiosity. So, why does your coffee have an oily film on top, and where does it come from? Read on for answers.

 

hot coffee cup in a person hand

 

Why Does My Coffee Have a Film on Top?

The oily film that gathers on top of your coffee is referred to as coffee scum. This film also coats the inside of your coffee cup after drinking the coffee. Coffee scum is caused by the natural oils that are present in coffee beans. 71% of these natural oils are fatty acids similar to those in margarine and soaps. The beans become oilier as they undergo roasting. So, the darker the roast, the oilier the coffee beans are. Light and medium roasts are less oily and will less likely produce an oily film.

 

Is the Oily Film in Coffee a Bad or Good Thing?

The presence of an oily film in your coffee isn’t an indication of bad coffee. Actually, most coffee makers will tell you that oily coffee is an indication of freshness. Additionally, oily coffee beans aren’t an indication of bad or substandard beans. As mentioned earlier, coffee beans contain natural oils, and the longer the roasting process, the darker the resulting bean and the oilier the bean becomes.

According to research, the presence of coffee scum may be an indication of bad or good coffee depending on several factors. For example, light roast and medium roast coffee beans are less likely to form the oily film since they haven’t been roasted long enough to produce the oils. So, if light and medium roast coffee beans produce an oily residue in your coffee, that’s a bad sign. This is an indicator of expired or incorrectly stored coffee beans.

Alternatively, the presence of a coffee scum from dark roasted coffee beans is an indicator that your coffee is fresh and healthy to drink. Other factors that influence the appearance of coffee scum include the type of water, water temperature, brewing method, type of roast, water filters, and coffee bean quality.

Type of Water

There are two types of water – soft and hard water. You’ll notice more coffee scum if you use hard water to make your coffee. This is because hard water contains more minerals such as calcium that easily bond with the fatty acids in your coffee making the coffee scum more visible. On the other hand, soft water has low calcium content and therefore no bonding occurs which results in no coffee scum.

Water Temperature

Using hot water such as boiling water from a kettle to brew your coffee increases the visibility of the coffee scum. The high temperatures bond easily with the molecules in your coffee, which increases coffee scum visibility. The oils present in your coffee are insoluble in water so they don’t mix with other molecules. Instead, they rise to the top hence the coffee scum.

Brewing Method

The brewing method you use impacts the visibility of coffee scum. For example, using a drip method will less likely produce a film since the method uses a filter. On the other hand, methods such as the French press and Turkish coffee will likely produce coffee scum since there are no filters used and the brewing temperatures are very high. Filter-free brewing methods are more likely to produce a coffee scum.

Type of Roast

The type of coffee roast used can affect the appearance of a coffee scum. Flame roasted coffee beans have a higher concentration of unsaturated fats so you may notice more coffee scum in brews from these beans. Additionally, dark roasts tend to bring more oils to the brew surface unlike light and medium roasts which means you may notice more film.

Filter Type

Coffee filters can help eliminate the oily film on the surface of your coffee. However, not all filters are effective. The most effective coffee filters are those that contain activated charcoal. The activated charcoal binds to the organic impurities present in your coffee, which includes the fats. This reduces the likelihood of oily film in your coffee. Additionally, water filters that contain activated carbon can be used to reduce the coffee scum appearance. When used with a water softener, the chances of a coffee scum are less likely since soft water doesn’t react with the compounds in coffee. Additionally, activated carbon filters and water softeners also reduce heavy metals, which makes your water safe for consumption and for brewing coffee. Water filters should be changed every six months for effective results.

Coffee Beans Quality

Coffee beans are divided into different grades depending on freshness, production process, and the coffee bean variety. Higher-grade coffee beans are less oily due to how they are produced. They are often slowly roasted which results in lower amounts of oil. Therefore, if you use high-grade coffee beans, you’ll be less likely to notice a film on your coffee. Alternatively, lower-grade coffee beans will likely produce more film since they are more acidic and their production process is more rigorous. However, coffee scum isn’t an indicator of substandard coffee beans. It’s pretty normal to have an oily film on your coffee.

 

How to Brew Coffee Without Coffee Scum

While some people enjoy coffee with the oily film, others cannot stand it. Therefore, if you’re not a fan of coffee scum, there are ways you can avoid it. These include using coffee filters and water softeners, using light and medium roasts, using high-grade coffee beans, and brewing your coffee at low temperatures.

 

Conclusion

The oily film in your coffee results from the natural oils present in your coffee beans. Hence, there is no need for concern if you see this film in your morning cup of coffee. However, keep in mind the factors that contribute to the appearance of coffee scum. This will help you know whether the appearance of an oily film in your coffee is a good or bad thing. Moreover, if you are not a fan of oily films, there are ways to avoid them. These include using light to medium roasts, using water filters with activated carbon, and using soft water instead of hard.

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