Why do coffee beans have different sizes?

The size of a coffee bean is determined by a combination of genetics and environment. Different varieties of Arabica, such as Bourbon or Pacamara, naturally produce beans of different sizes. Furthermore, the conditions under which the coffee is grown—altitude, soil nutrition, and rainfall—influence how much energy the plant can put into developing its seeds. A plant that is stressed or lacks nutrients may produce smaller, less developed beans.

After the coffee is processed and dried, it is sorted at the mill. This is often done using mechanical screens with holes of specific diameters. This process, known as grading by screen size, is a standard way to categorize coffee for the international market. Larger beans are often perceived as higher quality and can command a higher price, but this is a historical convention rather than a scientific rule. While larger beans can sometimes indicate a more mature, well-developed cherry, a smaller bean from a high-quality variety grown at high altitude can easily outperform a large bean from a lower-quality crop.

For the roaster, bean size is a critical factor in achieving a uniform roast. If a batch of coffee contains beans of widely varying sizes, the smaller beans will roast faster than the larger ones, leading to an uneven roast where some beans are under-developed and others are burnt. This is why high-quality specialty coffee is carefully sorted to ensure that the beans in a single bag are as uniform as possible in both size and density.

In practice, you don't need to worry too much about bean size as a consumer, as your roaster has already done the work of ensuring the batch is consistent. However, it is a good reminder that coffee is an agricultural product with natural variation. If you notice different sizes in your bag, it might be a characteristic of the specific variety or a sign of a more rustic, traditional processing method. It is all part of the diversity that makes coffee such a fascinating subject.