Why Does Oil Appear in Coffee After It Cools? What Are the Oils in Pour-Over Coffee, Cold Brew Coffee, and Siphon Coffee?

The Mysterious Oil Layer on Coffee

Have you ever noticed this situation: When a cup of pour-over coffee sits for a long time without being consumed, a thin layer of "oil" always appears on its surface. What exactly is this layer of "oil"? Could it be some kind of deterioration phenomenon in coffee?

What is the "oil" that appears when coffee cools down?

Actually, it's not just pour-over coffee - various single-origin black coffees like cold brew and siphon coffee also have "oil" floating on the surface! When it comes to "oil," I'm sure the first thing that comes to mind is espresso's golden foam - "Crema." What is Crema made of?

Crema is formed by carbon dioxide wrapped in oils from coffee beans! That's right, this layer of "oil" is not a substance that appears because coffee has deteriorated, but rather something that already exists in coffee! It's just that the visibility has changed, allowing people to see it when they couldn't see it before. The culprit behind this phenomenon is "condensation." FrontStreet Coffee's little science tidbit: "Condensation" refers to the phenomenon where gas or liquid condenses when encountering low temperatures. For example, water vapor turns into water when cooled, and water turns into ice cubes when cooled further.

The visibility of oils in coffee is mainly related to the performance of the filtration equipment used in different coffee preparation methods. The better the filtration performance of the equipment, the less oil can be filtered out. Therefore, extraction methods using highly efficient filtration materials like paper filters naturally allow very little oil to seep through. Right after brewing, you can't see any oil floating on the coffee surface at all.

However, when the coffee cools down, this extremely thin oil will form a visible solid due to condensation! Even very thin oils can reveal their form through accumulation. FrontStreet Coffee has previously shared that if you use equipment with larger filter pores and weaker filtration performance, such as flannel or metal mesh filters, you can clearly see a noticeable layer of oil floating on the coffee surface after brewing is complete!

Roasting Degree and Oil Content

The amount of oil is not only determined by the filtration performance of the equipment, after all, the equipment filters "already produced oils." Therefore, the key factor that determines the amount of oil is - "the degree of coffee bean roasting"!

How does the roasting degree affect the oil content in coffee beans?

During the roasting process of green coffee beans, the internal tissues continuously expand due to heating, which then produces carbon dioxide and oils. When roasting is complete and coffee beans return from the high-temperature environment to normal temperature and pressure conditions, they continuously release carbon dioxide from within. At the same time, oils also flow outward. After carbon dioxide is released, it immediately dissipates into the air, while oils remain on the bean surface until extraction, when they join the coffee liquid with hot water!

Getting back to the main topic, you should know that the oils in coffee beans are closely connected to the roasting degree! When the roasting degree continuously deepens, the content of carbon dioxide and oils also continuously increases! Light-roasted beans have shorter roasting times and less oil content. Therefore, they are usually in a very dry state when they come out of the roaster. After degassing for some time, their surface will develop a thin sheen due to oils. Dark-roasted beans are different. When beans are roasted for longer periods, the oil content becomes very abundant. Beans will immediately have a visible sheen after coming out of the roaster. After 3-5 days of degassing, oils directly overflow to the bean surface, forming a very "eye-catching" liquid!

Does this oil layer affect coffee flavor?

So, will this layer of oil affect the coffee's flavor? Because oils always exist in coffee, the condensed oil will not have much impact on the coffee's flavor. However, it forms a solid state, making it perceptible to us. Therefore, condensed oil will change the coffee's mouthfeel from the original smoothness to a certain roughness! But since the oil has appeared due to cooling, we don't need to worry about whether it tastes good or not. After all, everyone can imagine the taste of coffee that has gone from hot to cold.

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