What Components Make Coffee Taste Great? Unveiling the Secrets of Green Coffee Beans

Coffee trees bear cherry-sized fruits, and the seeds inside these fruits are what we call coffee beans. Typically, each fruit encases two flattened seeds that face each other. Once immersed in hot water, the green beans begin to release a subtle coffee flavor and aroma.

Roasting green coffee beans triggers extensive chemical changes—in the process, thousands of compounds are both generated and broken down. Roasters aim to develop beans that, after grinding and brewing, yield an exquisite flavor profile. Among the many effects of roasting, the beans undergo the following transformations:

• Size: They nearly double in size.
• Density: Their density is almost halved.
• Sweetness: Sweetness initially increases and then decreases.
• Acidity: Acidity gradually increases.
• Aroma: Up to 800 aromatic compounds can develop.
• Cracking Sound: A loud cracking sound is produced as pressure is released and water evaporates.

The purpose of roasting is to optimize the flavor of the soluble compounds in coffee. The soluble solids contribute to the taste experienced during brewing, while the dissolution of volatile aromatic compounds and oils releases the aroma detected by the nose. Together, the dissolved solids, oils, suspended particles, and the primary fiber fragments of the coffee bean create the body of the coffee.

Green coffee beans are very dense; approximately half of their dry weight consists of various sugars, while the other half is a mixture of water, proteins, lipids, acids, and alkaloids.

Structure

The structure of green coffee beans is a three-dimensional matrix of cellulose—a polysaccharide composed of nearly a million cells. This matrix, which encases the cellulose, contains hundreds of chemical constituents. The roasting process transforms these components into oils and soluble substances, ultimately determining the flavor of the brewed coffee.

The cellulose structure accounts for half of the bean’s dry weight. Although the fibers contribute only modestly to the overall flavor, they trap some volatile aromatic molecules, thereby enhancing the aroma, increasing the viscosity, and improving the overall mouthfeel of the coffee.

Carbohydrates

The primary carbohydrate in green coffee beans is sucrose, which makes up about 6–9% of the bean's dry weight and serves as a source of sweetness in a cup of coffee. Sucrose also influences acidity, as its caramelization during roasting produces acetic acid.

Lipids

The main lipids are triglycerides, constituting roughly 16% of the green bean's dry weight. Although lipids are not water-soluble, they remain in the brewed coffee—especially when using unfiltered or porous filtration methods. The oils help to retain the aroma and contribute to the coffee’s mouthfeel. Beans with a higher lipid content are often regarded as higher quality. However, lipids also pose a quality challenge, as they are highly susceptible to oxidation or rancidity during the storage of roasted beans.

Proteins

Proteins and amino acids account for about 10–13% of the green bean's dry weight. During roasting, the amino acids react with reducing sugars in a non-enzymatic browning process known as the Maillard reaction. This reaction produces glycosylamines and melanoidins, which impart a bittersweet flavor, a brown color, and aromas reminiscent of charred notes, meat, and toasted bread.

Caffeine and Trigonelline

Caffeine and trigonelline are two alkaloids, each making up approximately 1% of the green bean's dry weight, and they contribute to both the bitterness and the stimulant properties of coffee. In a cup of coffee, caffeine accounts for roughly 10% of the bitterness as well as most of the stimulating effect. Coffee trees produce caffeine as a defense against insect predation; therefore, trees grown at higher altitudes—where insect pressure is reduced—may yield beans with lower caffeine content.

Trigonelline is believed to be the major contributor to coffee’s bitterness and also plays a role in generating numerous aromatic compounds. Additionally, it helps reduce the formation of pyridine and niacin during roasting. Niacin, also known as vitamin B3, is thought to be responsible for coffee’s documented anti-cavity benefits. In a cup of approximately 200 grams of coffee, the niacin content can range from about 20 to 80 milligrams, depending on the roast level.

Moisture Content

Ideally, moisture should account for 10.5–11.5% of the weight of green beans. When the moisture content is too low, the beans tend to be lighter in color, and the brewed coffee may exhibit flavors reminiscent of dry hay or straw. Roasters must heat low-moisture beans with caution, as they can roast too quickly. Conversely, if the moisture content significantly exceeds 12%, the green beans are highly prone to mold, and the brew may acquire a grassy flavor. Excess moisture slows down heat transfer within the beans, requiring extra energy for evaporation. Thus, roasting overly moist green beans necessitates additional heat—typically achieved by extending the roasting time and increasing the heat intensity.

Organic Acids

Organic acids in coffee primarily refer to chlorogenic acids, which account for about 7–10% of the green bean's dry weight. Chlorogenic acids contribute to the coffee’s acidity, imparting acetic notes, astringency, and bitterness. Robusta beans tend to have higher chlorogenic acid levels, which likely results in a noticeably stronger bitterness. On the other hand, chlorogenic acids also offer antioxidant benefits for both the coffee beans and the drinker. Other organic acids found in coffee include citric, quinic, caffeic, malic, acetic, and formic acids.

Gases and Aromas

Volatile aromatic compounds provide coffee with its enticing fragrance. Although green beans contain over 200 volatile substances, their aroma is quite subtle. The roasting process, however, creates a vast array of aromatic compounds. To date, researchers have identified over 800 volatile compounds in roasted coffee.