Coffee scum is the oily film that is seen in coffee or that coats the inside of the cup after the coffee has been drunk. This film is caused by natural coffee oils contained in coffee beans. Coffee oil consists of 71 percent fatty acids, oils that are similar to those in margarine or soaps. The oil has a hydrophilic area that is somewhat soluble in water. The appearance of coffee scum is influenced by factors such as water quality, the grade of coffee bean, how the bean is roasted and water filters.


Hard Water

Coffee scum is more prevalent in areas where hard water is used. This is because minerals like calcium in hard water bond with the fatty acids in coffee, making the coffee scum more visible. Hot water from the coffee maker bonds the ingredients more easily, helping to form the insoluble scum. Soft water does not contain enough calcium to form coffee scum.

Water Filters

Water filters can be used to lessen the incidence of coffee scum. Most water filters contain activated carbon, which absorbs organic contaminants when water is passed through the filter. The activated carbon filters can be used along with a water softener to reduce heavy metals and scale in the kettle or coffee pot. The water filters can be connected to a dedicated water tap on the sink for drinking water only. The filters need to be changed every six months to a year to remain effective.

Grades of Coffee Bean

The type of coffee bean used to make coffee can influence whether coffee scum occurs. Higher-grade coffee beans are less likely to produce scum due to how the beans are produced. Higher-grade coffee beans are also less likely to be acidic and have a smoother taste. The freshness of the bean and whether the coffee is certified organic will also affect the taste and acidity of coffee.

Types of Roasting

The way a coffee bean is roasted can affect whether coffee scum occurs. Flame-roasted coffee beans have a burnt taste and are more likely to produce scum. A coffee bean that is slow roasted produces a smoother taste with no burnt flavor or bitterness. This method of roasting also produces a more stable caffeine molecule, which means less of a jolt to the nervous system.

References and Resources

Dear Dr. Brew: Oily Film on Coffee