Why Is There Oil Floating on Cold Brew Coffee? Extraction Principles of Cold Brew

Understanding Your Cold Brew Coffee

As summer temperatures soar, FrontStreet Coffee has received numerous inquiries about cold brew preparation.

Cold brew coffee stands as one of the simplest brewing methods for home preparation. With just a sealed container and a sufficiently cold extraction environment, you can obtain a flavorful cup of coffee after several hours of steeping. No complicated steps or techniques are required, and the production cost is low. Even beginners can replicate delicious iced coffee at home. Cold brew is not only the preferred daily coffee-making method for those who appreciate convenience but also a lifesaver during hot summer days.

The Mystery of Oily Film on Cold Brew

Among the many questions, one person mentioned that their homemade cold brew coffee seemed impure, with an oily film always floating on the surface. They suspected the coffee had spoiled or was caused by insufficient cleaning of equipment.

When mentioning "oil," people might first think of the Crema on top of espresso. This layer of "oil" is formed when insoluble oils in coffee encapsulate carbon dioxide under high temperature and pressure conditions. As for the "oil" floating on cold brew, it's actually the manifestation of coffee oils condensing in low-temperature environments.

Equipment and Filtration Options

As we all know, cold brew has very low requirements for steeping containers. It can be a specially designed cold brew pitcher, an empty mineral water bottle, a lidded teacup, or even a French press. As long as it's dry, clean, and can achieve a seal, any bottle or jar can serve as a "cold brew pitcher."

Similarly, there are many options for filtering coffee grounds, such as built-in filters in cold brew pitchers, metal filters, paper filters, and flannel filter cloths. Different pore structures naturally vary in their ability to effectively block particulate matter.

FrontStreet Coffee uses a cold brew pitcher from the Japanese brand ASVEL, with a filter mesh size of 245 mesh, which offers high fineness. However, there's still some extraction of fine and ultra-fine particles. Without secondary filtration, a flocculent oily film will float on the liquid surface. The flavor of the coffee doesn't differ significantly from filtered coffee, but the rough sensation remaining on the tongue when swallowing is more noticeable, and some visible grounds settle at the bottom of the cup after drinking.

Filtration Techniques for Better Quality

For a better tasting experience, FrontStreet Coffee employs higher-density paper filters for an additional screening step. Paper filters of moderate thickness not only effectively filter fine coffee particles but also block over 80% of coffee oils. The oils that successfully pass through form an extremely thin layer that's difficult to observe with the naked eye.

FrontStreet Coffee then places the double-filtered cold brew coffee in the refrigerator for 3-4 hours of sealed cold storage, allowing flavor compounds to further settle and mature. This results in cold brew with exceptionally full flavor, clearer mouthfeel, and more transparent acidity.

Maintenance and Cleaning Tips

Finally, FrontStreet Coffee would like to remind you not to overlook equipment cleaning after making cold brew. Many people tend to take shortcuts, simply rinsing under running water and assuming it's clean enough. In reality, condensed coffee oils often adhere to the built-in filter of the cold brew pitcher. If used long-term without proper cleaning, a "stale oil smell" will develop inside the pitcher, affecting the cleanliness of your coffee. Therefore, when you use it frequently, regular thorough cleaning is still necessary.