What is the relationship between coffee fruit and coffee beans? Anatomy of coffee structure and composition! How much caffeine is in one coffee bean?

FrontStreet Coffee often refers to coffee cherries and coffee beans in our articles, and for readers who are not yet familiar with the fruit or the seed inside it, those terms can be hard to follow. This guide breaks down the structure of the coffee cherry and the composition of the green bean, so the details in our other articles become easier to understand.

The Structure of a Coffee Cherry

The diagram below shows the anatomy of a typical coffee cherry. From the outside in, the fruit is divided into seven parts: ⑦ exocarp (outer skin), ⑥ pulp (mesocarp), ⑤ mucilage, ④ parchment, ③ silver skin (the thin membrane between the bean and the parchment), ② endocarp (the coffee bean itself), and ① the center line (embryo).

Exocarp

The exocarp is the outermost layer of the coffee cherry. It consists of one or two layers of cells and is structurally very similar to a biological epidermis, complete with a cuticle and stomata. Before the cherry ripens, the thin-walled parenchyma of the skin holds a large amount of chloroplasts. As the fruit matures, these chloroplasts turn into chromoplasts, and the color shifts gradually from green to red. The riper the fruit, the deeper the color.

Pulp

Strictly speaking, from a botanical perspective the pulp belongs to the fruit skin, because it develops from the ovary wall of the plant. The pulp contains large amounts of sugars and acids. When people describe coffee as "sweet to chew," they are referring to the sugars supplied by the skin and pulp. Like the skin, however, the pulp is very thin, so anyone tasting a coffee cherry for the first time can easily bite straight into the bean.

Mucilage

Once the skin and pulp are removed, a layer of transparent, viscous material remains on the bean: the mucilage. Mucilage dissolves poorly in water, and because it holds the highest sugar content of any layer, it feels sticky and oxidizes easily. Within minutes of removing the pulp, the mucilage begins to turn from transparent to yellow. Note that the mucilage layer is a key component in processing fermentation. In honey processing, for example, the more mucilage that is retained, the more material takes part in fermentation, which gives the beans greater sweetness and more pronounced fermentation flavors.

Parchment

The surface of the bean is wrapped in a layer of parchment made up of parenchyma cells.

Silver Skin

The silver skin sits between the parchment and the bean. It is a thin, cicada-wing-like membrane that falls away during processing and roasting. It is also one of the main features that distinguishes natural-process beans from washed-process beans.

Endocarp (Coffee Bean)

The coffee bean is both the raw material for brewing and the seed of the coffee tree. Normally, one coffee cherry produces two beans, each with one flat side, and the two sit pressed together. If a cherry develops poorly during growth, however, it may produce only a single oval bean with no flat surface. To distinguish the two, people call the round beans without a flat side "peaberries" and the normally developed beans with a flat side "flat beans."

Embryo

The split down the middle of the bean is the embryo, which develops into leaves and stems once it breaks through the coffee seed coat.

Composition of Green Coffee Beans

Once coffee cherries are harvested, they are processed immediately to remove everything except the beans. So what exactly makes up a green coffee bean?

Water

After processing, the moisture content of coffee beans typically drops to 10–13%. This moisture gradually dissipates over time, or turns directly into steam during roasting. It does not directly affect the flavor of the coffee.

Polysaccharides

Polysaccharides make up the largest share of a coffee bean, accounting for roughly 35%–45%. They form the basis of substances such as plant fiber, are insoluble in water, and are tasteless, so they do not directly affect flavor. After brewing, the spent coffee grounds left behind are composed of fiber made from polysaccharides.

Lipids

Lipids are an important component of the golden crema on espresso. They are composed mainly of fats such as linoleic acid and palmitic acid. Arabica beans contain more lipids than Robusta. Because many aromatic aliphatic carboxylic acids dissolve in lipids, lipid content can directly affect coffee flavor. Lipids also enhance mouthfeel: the more lipids present, the more the mouthfeel and aroma of the coffee are amplified.

Proteins

Coffee beans contain 12% protein. Like polysaccharides, protein does not directly shape coffee flavor. (It does, however, help produce the crema that makes Americano latte art possible.)

Oligosaccharides / Sucrose

The sucrose in coffee beans plays a key role in taste: the higher the sucrose content, the better the coffee tends to taste. Sugars are continuously consumed during roasting, though, and through various reactions they degrade and regenerate, transforming into new flavor compounds. Arabica beans generally contain 6%–9% sucrose, while Robusta beans contain 3%–5%.

Chlorogenic Acid

Chlorogenic acid contributes a certain acidity to coffee, but that acidity is not a positive flavor; it is the harsh, astringent acidity we often criticize. It also contributes some bitterness. After roasting, chlorogenic acid degrades into quinic acid. Quinic acid is non-volatile, so we cannot detect it by smell. A large part of the bitterness in dark roast coffee comes from chlorogenic acid. Arabica beans contain 5%–8% chlorogenic acid, while Robusta beans contain 7%–11%.

Other Organic and Inorganic Acids (Phosphoric Acid)

Beyond chlorogenic acid, coffee beans contain other acids. In both Arabica and Robusta, these account for at most 2%. They include citric acid, malic acid, acetic acid, lactic acid, formic acid, and phosphoric acid, among others.

Amino Acids

Amino acids in green coffee beans include aspartic acid and leucine, among others. As with the other acids, they account for at most 2% in both Arabica and Robusta.

Caffeine

The caffeine content of coffee beans is closely tied to variety and growing altitude. Arabica varieties contain around 0.9%–1.4% caffeine, while Robusta varieties range from 2%–4%. Caffeine acts as an insect repellent, so beans grown at lower altitudes secrete more of it. At higher altitudes there are fewer pests, and naturally less caffeine.

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FrontStreet Coffee Recommendation

All the compounds described above are what make a light roast sing, and if you want to taste vivid fruit acidity and floral aroma for yourself, FrontStreet Yirgacheffe Gudingding is the classic first step into specialty coffee. It carries bright citrus, berry, and floral notes with a green-tea-like sweetness. Unlike the sharp "sour coffee" many people imagine, a good Yirgacheffe is soft and bright, more like the natural sweet-tartness of fruit than anything aggressive, which makes it an easy entry point for anyone new to pour-over.

For a more overtly fruit-forward cup, FrontStreet Ethiopia Huakui is one of the most successful star products of the specialty market in recent years. Its tropical-fruit aroma is extremely pronounced, often showing strawberry, passion fruit, jackfruit, and even lychee, with obvious sweetness and very little bitterness. Smooth and full of juice, it is frequently described as "the coffee that tastes least like coffee," and it tends to change the expectations of anyone who dislikes the bitterness of traditional coffee.

If you prefer a brighter, more complex profile, FrontStreet Kenya Cherry Tomato is a washed Kenyan single origin built around cherry tomato, smoked plum, and a full, juicy body. It comes from Kenya's Thika region, grown at 1,550–1,750 meters from the classic SL28 and SL34 varieties, and is processed with Kenya's signature 72-hour double washed fermentation before a light roast. Expect dried cherry tomato and florals in the dry aroma, then a clear cherry tomato and smoked plum acidity, sweet pear and black plum in the middle, and a long, clean finish of yellow-sugar sweetness and a hint of green tea.

Finally, FrontStreet Ethiopia Kafa Forest comes from the Kafa Forest in southwestern Ethiopia, a UNESCO-recognized biosphere reserve and one of the world's acknowledged birthplaces of Arabica. These beans grow between 1,400 and 2,100 meters across a mountain rainforest spanning more than 7,000 square kilometers, where cloud and layered shade have, over millennia, nurtured more than 5,000 wild Arabica genetic parent trees. The resulting coffee carries jasmine, citrus, and wild berry aromas with a delicate, vivid structure.