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

Since FrontStreet Coffee often mentions terms related to coffee cherries and beans in our articles, which can be difficult for friends unfamiliar with fruits and beans to digest, today we'll share another breakdown about the structure of coffee cherries and beans to help everyone better understand the key points in our articles.

The Structure of Coffee Cherries

Below is a common coffee cherry anatomy diagram. A fruit is mainly divided into seven parts, looking from outside to inside: ⑦ Exocarp (outer skin), ⑥ Pulp (mesocarp), ⑤ Mucilage, ④ Parchment, ③ Silver skin (thin film between coffee bean and parchment), ② Endocarp (coffee bean), ① Center line (embryo).

Exocarp

The exocarp is the outermost layer of the entire coffee cherry! It consists of 1-2 layers of cells with a structure very similar to biological epidermis, having a cuticle and stomata. When the coffee cherry has not yet ripened, the thin-walled parenchyma of the skin contains a large amount of chloroplasts. When the fruit matures, these transform into chromoplasts, and the color gradually changes from green to red. The riper the fruit, the deeper the color!

Pulp

Technically speaking, from a biological perspective, coffee pulp is part of the fruit skin because it develops from the ovary wall of the plant. The coffee pulp contains large amounts of sugars and acids. When people say that chewing coffee is very sweet, they're referring to the sugars provided by the skin and pulp. However, like the skin, it's very thin, so friends trying coffee cherries for the first time can easily bite directly into the coffee bean.

Mucilage

When we remove the skin and pulp, we can see a layer of transparent viscous substance on the coffee bean - this is the mucilage layer. Mucilage is difficult to dissolve in water, and because it has the highest sugar content, it feels sticky and oxidizes easily. In the brief minutes after we remove the coffee pulp, the mucilage will gradually turn from transparent to yellow. It's important to note that the mucilage layer is an important component in processing fermentation. For example, in honey processing, the more mucilage retained, the more substances participate in fermentation, which gives the coffee beans higher sweetness and fermentation flavors.

Parchment

The surface of the coffee bean is covered with a layer of parchment, which is composed of parenchyma cells!

Silver Skin

The silver skin is sandwiched between the parchment layer and the coffee bean - it's a thin, cicada-wing-like cytoplasmic membrane. It falls off during coffee bean processing and roasting. It's also a major characteristic that distinguishes between natural and washed processed coffee beans.

Endocarp (Coffee Bean)

The coffee bean is both the raw material for coffee making and the seed of the coffee tree. Typically, one coffee cherry produces two coffee beans, which have one flat side and stick together. It's worth noting that if the coffee cherry experiences poor development during growth, it might only produce one oval coffee bean without a flat surface. To distinguish these two different situations, people call the round beans without flat surfaces "peaberries," while normally developed beans with flat surfaces are called "flat beans."

Embryo

The split in the middle of the coffee bean is the embryo part, which develops into leaves and stems after breaking through the coffee seed coat.

Composition of Green Coffee Beans

After we harvest coffee cherries, we immediately process them, removing all substances except the coffee beans. So next, let's look at what substances make up a green coffee bean!

Water

Typically, the moisture content of coffee beans drops to 10-13% after processing. This moisture gradually dissipates over time or turns directly into steam during roasting. It doesn't directly affect the flavor of coffee.

Polysaccharides

The highest proportion in coffee beans is undoubtedly polysaccharides! Accounting for about 35%-45%. They form the basis of substances like plant fibers, are insoluble in water and tasteless, so they don't directly affect the flavor of coffee. After we brew coffee, the remaining coffee grounds are composed of fiber made of polysaccharides.

Lipids

Lipids are an important component that forms the golden crema in espresso. They are mainly composed of fats such as linoleic acid and palmitic acid. Arabica coffee beans have a higher lipid content than Robusta. Because many aromatic aliphatic carboxylic acids can dissolve in lipids, the lipid content can directly affect the flavor of coffee. Additionally, it can enhance the mouthfeel of coffee. The more lipids, the more the mouthfeel and aroma of coffee are enhanced.

Proteins

Coffee beans contain 12% protein. Similarly, it doesn't directly affect the flavor formation of coffee. (Americano latte art relies on it~)

Oligosaccharides/Sucrose

The sucrose in coffee beans plays a key role in the taste of coffee. When the sucrose content in coffee beans is higher, the coffee tastes better. However, sugars are continuously consumed during the roasting process. With the support of various reactions, sugars continuously degrade and regenerate, transforming into new flavor substances. Generally, the sucrose content in Arabica coffee beans ranges from 6%-9%, while the sucrose content in Robusta coffee beans ranges from 3%-5%.

Chlorogenic Acid

Chlorogenic acid brings a certain acidity to coffee, but it's important to know that its acidity is not a positive flavor but rather the negative acidity we often criticize, with an astringent quality. Besides this, it also brings some bitterness! After roasting, chlorogenic acid degrades into quinic acid! Quinic acid is non-volatile, so we can't detect it through smell. A large part of the bitterness in dark roast coffee comes from chlorogenic acid. The chlorogenic acid content in Arabica coffee beans is 5%-8%, while in Robusta coffee beans it's 7%-11%.

Other Organic Acids & Inorganic Acids (Phosphoric Acid)

In addition to chlorogenic acid, coffee beans also contain other acids. Whether in Arabica or Robusta, these acids account for at most 2%! They include citric acid, malic acid, acetic acid, lactic acid, formic acid, phosphoric acid, and so on.

Amino Acids

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

Caffeine

The caffeine content in coffee beans is closely related to the coffee variety and growing altitude. The caffeine content in Arabica varieties is around 0.9-1.4%, while Robusta varieties range from 2-4%. Caffeine acts as an insect repellent, so when coffee beans are grown at lower altitudes, more caffeine is secreted. At higher altitudes, there are fewer pests, so naturally, there's less caffeine.

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