Are Wrinkled Coffee Beans Defective? What Causes Oily Surfaces on Coffee Beans?

"Why are coffee beans wrinkled? Are they bad beans?" "The coffee beans are oily. How long have they been sitting?" These are common questions many customers have about coffee beans! FrontStreet Coffee is here to answer everyone's questions through this article.

Why Do Coffee Beans Develop Wrinkles?

Most people's impression of coffee beans is that they should have smooth surfaces, so seeing wrinkled textures can be deeply confusing. However, wrinkles on coffee beans are actually a normal phenomenon, primarily caused by two factors: the quality of the raw beans and the roasting process.

High-Altitude Coffee Beans Are Prone to Wrinkles

Generally speaking, high-altitude coffee beans have higher density. During roasting, heat transfers more slowly to the bean core, meaning they spend more time in the dehydration phase. As beans absorb heat and dehydrate, they contract, forming texture lines. Due to their high density and tight fiber structure, coffee beans don't expand significantly to smooth out these textures, thus appearing wrinkled.

Roasting Also Affects Wrinkle Development

Wrinkled textures on coffee beans typically appear in medium to light roasts, while medium-dark roasted coffee beans have smooth surfaces. Lighter roasts or longer roasting times are more likely to produce clear wrinkle patterns on the bean surface. Conversely, darker roasts or faster roasting with increased heat result in better texture expansion on the bean surface, sometimes appearing completely smooth.

However, whether coffee beans have wrinkles doesn't affect their flavor. Beans with more wrinkles are not defective beans at all. In fact, some coffee beans that appear ugly and heavily wrinkled can actually produce clean, fresh, and delicious flavors.

Why Do Coffee Beans Become Oily?

Many people believe that oily coffee beans indicate the coffee is not fresh, but this is actually a misconception. If you're fortunate enough to visit a Japanese-style coffee shop, you'll definitely notice the glistening, oily coffee beans stored in jars. The "oil" on the surface of coffee beans is actually the fat from the beans. During roasting, carbon dioxide is produced, which allows the fat in coffee beans to seep to the surface, creating the oily appearance.

Oily surfaces typically appear on dark roasted coffees. Dark roasted beans show a slight sheen immediately after roasting, and the surface becomes significantly oily one to two days after leaving the roaster, with aromas of caramel, cocoa, and cream. In contrast, stale coffee beans exhibit greasy, tar-like, and woody aromas.

Not only do oils seep out during dark roasting, but they also continue to appear after resting for some time, due to residual carbon dioxide still being released from the beans. This phenomenon is more common in medium or medium-light roasts. These coffees appear dry immediately after roasting, but about a week later, as large amounts of carbon dioxide are released, spot-like oils begin to seep out. These oil spots actually represent the optimal flavor period of coffee, debunking the myth that oily coffee beans are not fresh.

The true indicator of stale coffee is when gas emission from the beans is depleted, accompanied by the loss of flavor compounds. In practical terms, when brewing, the coffee grounds don't expand at all during blooming, the taste is empty, and there are woody and fibrous flavors. There is no necessary connection between whether coffee beans are oily and their freshness.

Important Notice :

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