Why does coffee appear grease when it is cold? What is the oil of hand-brewed coffee, cold-extracted coffee and siphon pot coffee?

I don't know if you've noticed this: when a cup of hand-brewed coffee is not drunk for a long time, its liquid surface will always show a thin layer of "oil." What is the origin of this layer of "oil"? Could it be that coffee had deteriorated?

What is the "oil" that appears after coffee cools? Reasoning, in fact, not only hand-brewed coffee, such as cold extraction, siphon pot coffee and other single black coffee will have "oil" on the surface of the situation! When it comes to "oil," I think everyone will think of the golden foam of espresso coffee-"Crema". What is Crema made of?

Crema is made from coffee bean fat wrapped in carbon dioxide! That's right, this layer of "oil" is not a substance that emerges because coffee has deteriorated, but it already exists in coffee! It's just that the visibility has changed, so people can see it from the beginning. And the cause of this phenomenon is "condensation." Front Street Science: "Condensation" means that gas or liquid condenses because of low temperature. For example, water vapor turns into water when it is cold, and water turns into ice when it is cold. The degree of appearance of oil in coffee is mainly related to the performance of filter equipment used in different coffee making methods. The better the filtering performance of the appliance, the less grease can be filtered out. Therefore, using the extraction method with extremely high filtration performance such as filter paper, the oil that can seep out is naturally very small. When the coffee is freshly brewed, there is no visible layer of grease floating on the surface of the coffee.

However, when the coffee cools down, this extremely thin layer of oil will form a solid visible to the naked eye due to condensation! Even a very thin oil can be stacked to reveal its own form. Front Street has shared before, if you use flannel, metal filter and other large filter holes, filter performance is weak, then after brewing you can intuitively see a layer of obvious grease floating on the coffee surface!

The amount of grease depends not only on the filtering performance of the appliance, after all, the appliance filters the "generated grease." Therefore, the key factor that determines the amount of fat must be-"the roasting degree of coffee beans"!

How does roasting affect the amount of fat in coffee beans? During the roasting process, the internal tissues of coffee beans will expand continuously due to heating, and then carbon dioxide and oil will be produced. When the roasting is completed, the coffee beans return from the high temperature environment to the normal temperature and pressure environment, and the carbon dioxide in the body will be continuously discharged. At the same time, the oil will also overflow outward. The carbon dioxide dissipates into the air immediately, while the oil stays on the surface of the beans until it is extracted and added to the coffee with hot water!

Back to the topic, you know, coffee bean fat and roasting degree is closely related! As the roasting deepens, the carbon dioxide and fat content will also increase! Light baked beans have a shorter baking time and less fat content. Therefore, they are generally in a very dry state when they are out of the oven, until after a period of time, their surface will appear a thin layer of luster due to grease. Deep baked beans are different. When beans are baked for a long time, they are very rich in oil. After being discharged, the beans will immediately have a visible luster. After 3~5 days of exhaust, the oil will overflow directly from the bean surface to form a very "bright" liquid!

So, does this grease affect the flavor of the coffee? Because the oil is always present in the coffee, the condensed oil will not have much effect on the coffee flavor. However, it will form a solid state, resulting in our perception. Therefore, the condensed grease will change the taste of coffee from smooth to rough! But since it's already oily because of the cold, then we don't need to worry about whether it tastes good or not ~ After all, from the taste of hot and cold coffee, you can imagine ~~

- END -

Front Street Cafe

No.10 Baoanqian Street, Yandun Road, Dongshankou, Yuexiu District, Guangzhou City, Guangdong Province