What do coffee bean grades like AA and AB mean?

In the world of coffee grading, especially when dealing with East African origins like Kenya, you will often see labels like AA, AB, or PB. These designations are primarily based on screen size—the physical dimensions of the bean. The grading process involves passing the green coffee through a series of metal sieves with holes of specific diameters. Beans that are caught by or pass through these screens are sorted accordingly.

An 'AA' grade indicates that the beans are the largest, typically measuring 7.22 millimeters (screen 18) or larger. 'AB' beans are a mix of screen 15 and 16, making them slightly smaller. 'PB' stands for 'Peaberry,' which is a natural mutation where only one seed develops inside the coffee cherry instead of the usual two. Because peaberries are round and dense, they are often sorted separately and prized for their unique roasting characteristics.

Historically, the logic behind this system was that larger beans contained more essential oils and nutrients, which would theoretically lead to a better-tasting cup. While there is some truth to this—larger, more uniform beans often roast more evenly—modern specialty coffee has moved toward a more holistic view of quality. A smaller bean from a high-altitude, well-managed farm can easily outperform a large 'AA' bean from a lower-quality lot.

It is important to note that these grades are not universal. A 'Grade 1' in Ethiopia, for example, refers to the number of defects found in a sample, not the size of the bean. Therefore, when you see 'AA' on a bag, you should view it as a descriptor of bean size rather than a definitive score of the coffee's flavor profile. The best way to judge the quality of the coffee is to look for the cupping score or the roaster's tasting notes.

In practice, understanding these grades helps you navigate the market, but it shouldn't be the only factor in your decision. If you are brewing at home, bean size can affect your grind settings—larger beans might require a slightly different adjustment to achieve the same extraction as smaller ones. Ultimately, these classifications are a tool for trade and logistics, helping to ensure consistency in the supply chain, but the true quality of the coffee is found in the cup.