tags: [] - coffee/green-beans - coffee/plant-science aliases: - Coffee bean anatomy - Green bean structure - Coffee seed anatomy

Coffee Bean Structure¶

Tags: #coffee/green-beans #coffee/plant-science Aliases: Coffee bean anatomy, Green bean structure, Coffee seed anatomy Related: Green Coffee MOC | Coffea | Chlorogenic Acids | Density and Coffee Quality | Peaberry Status: ✅ Complete

Overview¶

The coffee bean is the seed of the coffee cherry (Coffea spp.), and the raw material for roasting and brewing. A standard coffee cherry contains two seeds positioned with their flat faces towards each other; when only one seed develops, the result is the round mutation known as a peaberry. The bean consists of a thin papery outer membrane (the silverskin or spermoderm) enclosing a dense endosperm that holds all of the chemical compounds — sugars, proteins, lipids, chlorogenic acids, caffeine, and volatile precursors — that are transformed by heat during roasting into the flavour complexity of brewed coffee.

Bean Anatomy¶

Silverskin (spermoderm): The innermost protective membrane of the coffee seed; a thin, translucent-to-silver papery layer that adheres loosely to the endosperm surface. The silverskin is shed during roasting (it becomes chaff) but often remains lodged in the centre cut.

Endosperm: The main body of the bean; a dense, honeycomb cell structure that stores all the chemical compounds for germination. The endosperm contains the full range of proteins, carbohydrates, lipids, acids, caffeine, and volatile precursors, and is the site of all roasting chemistry. High-altitude beans have a more densely packed endosperm (hard bean).

Centre cut (crease): A groove running along the flat side of the bean where the two seeds faced each other inside the cherry. The silverskin often remains in this groove after roasting.

Chemical Composition of Green Coffee¶

Component	Proportion (dry weight)	Role
Carbohydrates (polysaccharides, sucrose)	40–55%	Structure; sucrose (6–9% in Arabica) is a major substrate for Maillard reactions and caramelisation
Proteins and amino acids	10–13%	Maillard reaction precursors; create brown colour and flavour during roasting
Lipids (triglycerides, diterpenes)	12–18% (Arabica); 10–12% (Robusta)	Body, mouthfeel; cafestol and kahweol (diterpenes) in unfiltered coffee
Chlorogenic acids (CGAs)	6–10%	Antioxidants; acidity contribution; bitterness on breakdown to quinic acid
Caffeine	0.8–1.4% (Arabica); 1.7–4.0% (Robusta)	Alkaloid; bitter taste; heat-stable through roasting
Organic acids	1–2%	Citric, malic, quinic acids; acidity and brightness
Minerals	3–4%	Potassium, magnesium, calcium, phosphorus; reflect terroir
Moisture	8–12%	Critical for storage stability; 10–12% is the standard green bean moisture target

Physical Properties¶

Density: High-altitude beans grown in cool conditions develop a denser endosperm cell structure (hard bean). Density is measured in grams per litre or by sink/float testing and is a recognised quality indicator — denser beans generally carry more chemical complexity and withstand roasting heat better. Low-altitude or fast-grown beans have looser cell structure (soft bean) and roast faster.

Size: Bean size is measured by screen size (holes measured in 1/64-inch increments). Screen 18 and above (AA/AB grade equivalents) indicates a larger bean. Size does not inherently indicate quality but uniform sizing is important for consistent roasting.

Peaberry: A natural mutation (occurring in approximately 5–10% of beans) in which only one seed develops inside the cherry, resulting in a round rather than flat-sided bean. Peaberries are often separated and sold at a premium, though the quality benefit over flat beans is debated.

Colour: Arabica green beans are typically green to blue-green; Robusta tends towards yellow-green to brown-green. Washed coffees are brighter green; natural processed coffees are more muted. Fading colour indicates oxidation and age-related quality loss.

Changes During Roasting¶

The green bean is transformed profoundly by heat:

Volume expansion: Beans expand 50–100% as CO₂ pressure builds and cell walls rupture at first crack
Colour progression: Green → yellow → tan → brown → dark brown, driven by Maillard products (melanoidins) and caramelisation
Density decrease: Moisture loss and structural breakdown cause density to fall progressively; very dark-roasted beans float in water
Oil migration: At dark roast levels, lipids migrate to the bean surface through broken cell walls, producing a visible sheen and accelerating staling
Chemical transformation: Maillard reactions, caramelisation, acid development and degradation, and CGA breakdown create hundreds of new volatile aroma compounds; overall complexity increases to medium roast then declines in very dark roasts

Common Defect Beans¶

Defect	Description	Off-flavour
Quaker (underdeveloped)	Bean remained pale/blonde through roasting	Peanutty, astringent, raw
Full black	Severely over-fermented or dead bean	Fermented, phenolic
Full sour	Advanced fermentation damage	Vinegary, acetic
Insect-damaged	Coffee berry borer holes	Variable off-notes

Key Facts¶

Coffee bean = seed inside the coffee cherry; two flat beans per cherry (peaberry = single round mutation)
Main structure: silverskin (spermoderm) enclosing a dense endosperm
Chemical composition: dominated by carbohydrates (40–55%), followed by lipids (12–18%), proteins (10–13%), and chlorogenic acids (6–10%)
Arabica has higher lipid content and lower caffeine than Robusta; these differences drive quality and flavour distinctions
Moisture target for green bean storage: 10–12%; above 12% risks mould; below 8% risks brittleness and uneven roasting
High density (hard bean) = high-altitude, slow growth = quality indicator

Green Coffee MOC
Coffea
Arabica
Canephora
Chlorogenic Acids
Density and Coffee Quality
Peaberry
Chaff

References¶

Changelog¶

Date	Change
2026-05-02	Compliance review: full rewrite — original had no frontmatter, no metadata block, bold pseudo-headers throughout, no proper `##` section structure, no copyright; content preserved and restructured as encyclopedia article

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