Why do some coffee beans have a white line in the middle?

If you look closely at a roasted coffee bean, you will notice a distinct groove running down the center. This is known as the center cut. Biologically, the coffee bean is actually a seed that develops inside the coffee cherry, and the center cut represents the seam where the two cotyledons—the embryonic leaves of the seed—are joined together. It is a completely natural feature of coffee bean anatomy and is present in every single bean, though its visibility and color can change significantly during the roasting process.

In green, unroasted coffee, the center cut is often the same color as the rest of the bean. However, as the bean roasts, the center cut often retains a lighter, sometimes white or tan color. This happens because the center cut is a deep, protected crevice that is less exposed to the direct heat of the roasting drum compared to the outer surface of the bean. Additionally, the silver skin—the thin, papery layer covering the bean—often gets trapped inside this groove. Even after the rest of the chaff has been removed, some remnants may remain in the center cut, contributing to its lighter appearance.

Many coffee enthusiasts wonder if the center cut is an indicator of quality. While it is not a primary metric for grading, the appearance of the center cut can provide clues about the bean's processing and roasting. For example, in some high-altitude, dense beans, the center cut may be very tight and narrow. In other varieties, it might be more open. If the center cut is extremely dark or charred, it might suggest that the bean was roasted too aggressively or at too high a temperature, which can lead to bitter, smoky flavors.

Ultimately, the center cut is simply a part of the bean's structure. It does not dictate the flavor profile on its own, but it serves as a visual reminder of the bean's journey from a seed inside a fruit to the roasted product in your grinder. When you are evaluating the quality of your beans, focus more on the uniformity of the roast color and the absence of defects rather than the specific appearance of the center cut.