Anatomy of a Coffee Bean: Parts, Layers and Grades

A coffee bean is the seed of the coffee cherry, a small red or yellow fruit that grows on the coffee plant. Each cherry usually holds two beans, which grow with their flat sides facing each other. Working from the outside in, the parts of a coffee cherry are the outer skin (exocarp), the pulp (mesocarp), a sticky layer of mucilage, a papery hull called the parchment (endocarp), a thin silver skin, and finally the bean itself, the endosperm, with a tiny embryo at one end. Processing removes the outer layers, and roasting turns the green bean’s sugars, acids and oils into the flavors in your cup.

The Parts of a Coffee Bean

• Outer skin (exocarp): the cherry’s skin starts green and turns red, or yellow or orange in some varieties, as it ripens. Pickers aim to harvest only ripe cherries, because unripe ones make sour, grassy coffee. The dried skin and pulp are sold as cascara, a fruity tea.
• Pulp (mesocarp): unlike most fruit, the coffee cherry has very little flesh, just a thin, sweet layer under the skin.
• Mucilage: a sticky, sugary, pectin-rich layer that clings to the parchment. How much of it stays on during drying is what separates washed, honey and natural coffees; see coffee processing methods and honey-processed coffee.
• Parchment (endocarp): a thin, papery hull that protects each bean. Coffee is often dried and stored “in parchment” and hulled just before export.
• Silver skin: a very thin, silvery film on the bean. Most of it comes off during roasting as coffee chaff, and a little stays in the crease of the bean.
• Bean (endosperm): the seed’s food store and the part that becomes coffee. It is hard, dense and green-gray before roasting.
• Embryo: a tiny plant at one end of the bean. In a fresh seed it can sprout into a new coffee plant, which is how coffee is grown from seed; see how to grow a coffee plant.

Why Coffee Beans Come in Pairs, and What a Peaberry Is

Most coffee cherries hold two seeds that grow face to face, which is why a coffee bean is flat on one side, rounded on the other, and has a crease, called the center cut, down its flat side. In about 5 to 10% of cherries only one seed develops. It fills the space alone and grows small, round and oval, like a pea: a peaberry. Peaberries are often sorted out and sold separately, especially from Tanzania and Kenya, and many roasters say their round shape roasts more evenly. Learn more in what is peaberry coffee.

What Is a Coffee Bean Made Of?

Green coffee beans are mostly carbohydrates, with oils, proteins, acids, minerals and caffeine. The figures below are typical ranges by dry weight; they vary with variety, growing conditions and processing.

Robusta’s higher caffeine and chlorogenic acids, and its lower sugar and oil, are a big part of why it tastes stronger and more bitter than Arabica. See Arabica vs Robusta.

What Happens to the Bean When It Is Roasted?

• It dries and browns: moisture is driven off, then sugars and amino acids react (the Maillard reaction) and sugars caramelize, creating hundreds of aroma compounds and the brown color.
• It grows and gets lighter: the bean swells, cracks open with an audible “first crack,” and loses roughly 12 to 20% of its weight, mostly water.
• Its acids change: chlorogenic acids break down as the roast goes on, which is one reason darker roasts taste less acidic and more bitter.
• It fills with gas: roasting creates carbon dioxide inside the bean, which slowly escapes over the following days and weeks. That is why fresh coffee “blooms” and why coffee tastes best about 2 to 4 weeks from roast.

Do light roasts have more caffeine? Only a little, and it depends how you measure. Caffeine survives roasting largely intact, but dark-roasted beans are larger and lighter. Measured by the scoop, light roasts have slightly more caffeine; measured by weight, dark roasts have slightly more. The difference is small either way. See coffee roast levels and how much caffeine is in coffee.

Coffee Bean Size and Screen Grades

Green coffee is sorted by size using screens, metal sieves with round holes measured in 64ths of an inch. A bean held by screen 18 is at least 18/64 inch (about 7 mm) across. Larger beans often, but not always, come from higher altitudes and slower-growing plants, and they roast more evenly when sorted to one size. Many countries use size in their grade names:

• Kenya: AA (about screen 17 to 18), AB (about screen 15 to 16), and PB for peaberry. See Kenyan coffee.
• Colombia: Supremo (screen 17 and up) and Excelso (about screen 14 to 16). See Colombian coffee.
• Central America: grades are often based on altitude rather than size, because high-grown beans are denser. Terms include HB (hard bean), SHB (strictly hard bean) and SHG (strictly high grown), with altitude cut-offs that vary by country but are usually above about 1,200 to 1,350 meters (4,000 to 4,500 feet) for the top grades.

Size and altitude grades describe the beans, not the flavor, so a big AA or Supremo is not guaranteed to taste better. You will often see these terms on the bag; see how to read coffee bag labels.

How Coffee Beans Are Graded for Quality

Beyond size, green coffee is graded by counting defects, such as black, sour, broken or insect-damaged beans, in a sample. The Specialty Coffee Association’s long-standing system uses a 300 g sample and five grades:

• Grade 1, specialty: no more than 5 full defects and no primary defects, no quakers (unripe beans that roast pale), and at least one distinctive attribute in the cup.
• Grade 2, premium: no more than 8 full defects and up to 3 quakers.
• Grade 3, exchange: 9 to 23 full defects.
• Grade 4, below standard: 24 to 86 defects.
• Grade 5, off grade: more than 86 defects.

Specialty coffee must also score 80 points or more out of 100 when it is cupped by trained tasters. See specialty vs regular coffee. The next time you buy a bag from a local roaster, you will know how much happened to each bean before it reached your grinder.