Structure of coffee beans

A coffee bean is really 2 seeds that are side by side. Peaberry accounts for about 5% of the world’s coffee, has only a single bean inside the cherry, and is a natural mutation.

The coffee cherry's outer skin is called the exocarp. The exocarp, also referred to as the peel, skin, or epicarp, is the outermost layer of the coffee fruit. Beneath it is the mesocarp, a thin layer of pulp, followed by a slimy layer called the parenchyma.

Mesocarp is the “flesh” of the coffee fruit- during cherry maturation. This layer is rich in sugars and has a huge influence on the taste of the coffee bean.

Pectin is responsible in protecting the coffee beans. It’s made up of a cellulose layer.

The beans themselves are covered in a paper-like envelope named the endocarp, more commonly referred to as the parchment. This is a thick shell-like layer that surrounds the bean and is formed of three to seven layers of sclerenchyma cells (fibrous cells that serve as the principal support cells in plants). The cells of the endocarp harden during coffee fruit maturation, thus limiting the final size of the coffee seed, or bean.

Inside the parchment, side-by-side, lie two beans, each covered separately by yet another thin membrane. The biological name for this seed skin is the spermoderm, but it is generally referred to in the coffee trade as the silver skin. The silverskin is a very thin and fine layer of skin which envelops and adheres tightly to the seed of the fruit which is the coffee bean.

The endosperm is the primary reserve tissue of the seed and is composed of only one tissue though the cells in the outside and indoor part of the endosperm differ in oil material and cell wall density. Most of the coffee bean itself is comprised of endosperm, which is the tissue that is created near the time of fertilization in most flowering plants. The endosperm surrounds the embryo and supplies nutrition (starch) but may also contain proteins and oils.
Structure of coffee beans

Physical properties of coffee beans

Coffee is the roasted kernel of the coffee cherry.  Each coffee bean is actually the seed of a coffee cherry, the fruit of the coffee tree.

In the coffee process, first the fruit flesh (pulp and parchmentlike hull) is removed, and then the kernel is dried, resulting in green coffee beans.

A typical coffee bean has the shape of a half ellipsoid with well rounded edges.  It contains about 12% of water and most of this is lost in the early stage of roasting, followed by a loss of up to 8% of the dry bean material.

The green coffee bean has a very tight structure where less than 7% of its pore volume has a diameter of 10 nm or more.

The two coffee beans are covered by a thin parchmentlike hull, which is further surrounded by pulp.
Physical properties of coffee beans