The chemical composition of coffee beans determines the flavor expression in the final cup. From moisture and minerals to carbohydrates, organic acids, proteins, chlorogenic acids, fats, and volatile compounds, each component plays a specific role in both the green bean state and during roasting. Moisture content varies with processing stages, potassium dominates among minerals, sugars degrade significantly during roasting and contribute to aroma, while acids directly influence the brightness of the taste. This article will systematically outline the chemical composition of coffee beans and their effects on quality and flavor, helping coffee enthusiasts gain a deeper understanding of the material basis of specialty coffee. Front Street Coffee always pays attention to the chemical changes of coffee beans from green bean to cup, providing professional reference for enthusiasts.
Moisture:
The moisture content of coffee beans fluctuates significantly depending on the processing stage and product form. Wet beans with mucilage layer can have moisture as high as around 50%, dried green beans typically maintain between 10% and 13%, while roasted coffee beans have moisture content reduced to below 5%. The reason water can exist in coffee beans, similar to other foods, is that the beans contain large amounts of hydrated colloidal macromolecules, such as proteins and polysaccharides, which allow water to be retained within the bean in various physical and chemical binding forms.
Minerals:
Although minerals account for a low proportion in coffee beans—about 4% of the dry weight of green beans—they hold undeniable importance. They consist of various elements, among which potassium is the most abundant, accounting for about 40% of the total mineral content, followed by calcium, magnesium, phosphorus, sodium, and sulfur. Additionally, they include over thirty trace elements at PPM levels such as zinc, manganese, copper, and rubidium. It is important to note that the mineral content in coffee beans cannot be directly equated with the mineral levels in coffee or coffee powder products, because the latter introduces water during preparation or processing, and differences in water quality can lead to changes in mineral composition. The most critical factors affecting the mineral content and types in green beans are soil conditions and fertilization management during cultivation. During coffee preparation, at least 90% of the minerals in roasted coffee beans can be extracted, so the relationship between potassium or other mineral content and the soluble yield of coffee can be used to assess extraction rate.
Carbohydrates:
Carbohydrates in coffee beans can be divided into two major categories: polysaccharides and low molecular weight sugars, the latter including monosaccharides, disaccharides, and trisaccharides. They can also be distinguished by reducing and non-reducing sugars, and also contain derivatives such as pectin. The contribution of carbohydrates to coffee is reflected in three aspects: taste, aroma, and color. In terms of flavor, carbohydrates not only release coffee aroma after roasting but also adsorb volatile aroma components, giving coffee its unique flavor characteristics.
Low molecular weight sugars:
Sucrose is the most predominant free sugar in green coffee beans, and its content varies depending on variety, origin, and maturity. Overall, Arabica coffee beans have higher sucrose content than Robusta. Other simple sugars, including reducing sugars, can also be detected in green bean extracts. Green coffee beans also contain glucose and fructose; Arabica has lower levels of these two sugars than Robusta, and Arabica's total reducing sugars are also lower than Robusta's.
After roasting, changes in low molecular weight sugars vary depending on the roast level. Among them, sucrose is lost most rapidly, with a loss rate of 97% at light roast, 99% at medium roast, and 100% at dark roast. Other sugars such as glucose, fructose, and arabinose also undergo considerable loss.
Polysaccharides:
Polysaccharides are extremely important components in green coffee beans, accounting for about 40% to 50% of the dry matter. By type, they include galactan, mannan, arabinan, and cellulose; these substances constitute the matrix of the coffee bean and are closely related to the bean's hardness.
After roasting, a considerable amount of polysaccharides is still retained. One study showed that the differences between different roast levels are not significant, with retention rates between 70% and 75%, among which cellulose has the highest retention rate and arabinan the lowest. In terms of extraction, some polysaccharides are dissolved out, depending on the solvent type. When water is used as the solvent, more arabinose, galactose, and some mannose polysaccharides are dissolved.
Organic acids:
In brewing coffee, the expression of acidity is crucial. Under good conditions and techniques, a special taste with refreshing acidity can be developed, which is an essential condition for high-grade coffee. Generally speaking, the deeper the roast level, the less the acidity develops, and it may even disappear completely, giving way to another characteristic of pure coffee.
The non-volatile acids contained in raw Arabica coffee beans include citric acid, malic acid, oxalic acid, and tartaric acid. Testing has not found the presence of large amounts of volatile acids. After green beans are roasted, the acid content changes greatly, with volatile acids showing the most significant changes. The acid loss rate in green beans after roasting is about 15%, and more noteworthy are the chemical changes of acids. Formic acid and acetic acid tend to increase in the early stages of roasting and continue to increase with deeper roast levels, only showing a decreasing trend at the end of roasting; citric acid and malic acid gradually decrease as roasting progresses, forming their decomposition products.
Proteins and amino acids:
Calculated as crude protein, green coffee beans contain about 13% to 16%. After deducting nitrogen-containing compounds such as caffeine and trigonelline, the true protein content is about 8.8% to 9.7%. Green coffee beans also contain various enzymes, such as lipase, protease, carbohydrase, galactosidase, and peroxidase. Green beans contain about 0.15% to 0.25% free amino acids, with Robusta having higher content than Arabica. These free amino acids have a relatively large impact on coffee flavor, but a relatively small impact on taste.
Chlorogenic Acids:
At least three isomers of chlorogenic acid exist, and their function mainly lies in the physiological regulation of the coffee plant, including promoting growth, root formation, antibacterial action, pest and disease resistance, and serving as a precursor to lignin. In green coffee beans, Robusta has higher chlorogenic acid content than Arabica. Wet-processed green beans have about 40% lower chlorogenic acid content than dry-processed ones. After roasting, this substance disappears through different reactions, generating extremely complex products that are closely related to coffee quality.
Fats:
The fats in green coffee beans consist of coffee oil present in the endosperm and wax present in the outer layer of the coffee bean. Coffee oil contains not only triglycerides but also a considerable amount of other fat components; it shapes the characteristics of coffee and is extremely important for coffee.
The coffee oil content and composition in green coffee beans vary depending on factors such as variety. Based on multiple studies and statistics, Arabica coffee beans have an average content of 15% of dry matter with a standard deviation of 0.78%; Robusta coffee beans have a content of 10% with a standard deviation of 1.41%.
Volatile compounds:
Volatile compounds are the main source of coffee flavor and are particularly crucial to coffee quality. There are many types of coffee volatile compounds, and their presence directly affects the aroma quality of coffee. These substances mainly derive from non-volatile substances in green beans being broken down or reacting during roasting. Thermal decomposition, other reactions, or interactions between components—such as the results of the actions of carbohydrates, amino acids, organic acids, and phenolic compounds—form coffee's unique aroma and flavor. Factors affecting the composition of coffee volatile compounds include: coffee bean variety, cultivation climate, soil conditions, green bean storage, roasting temperature and time, roasting equipment, etc.
Green coffee beans do not possess the special aroma of coffee, so they are not directly consumed; they must be roasted to generate large amounts of volatile aroma substances. After green beans are roasted, analysis has confirmed at least 660 major volatile aroma components, making it the food with the most types of volatile aroma components among all foods and beverages. Aromas generated during roasting, such as hazelnut, buttery, and caramel notes, or grassy, smoky, burnt, spicy, and bitter notes, mostly come from volatile substances. In addition, differences in roast level also affect the flavor characteristics of coffee.
Pectin and lignin:
Pectin is a substance formed by the combination of various polysaccharides, with its main components being polymers of galacturonic acid, glucuronic acid, and rhamnose, etc., with content reaching over 3%. Lignin is the insoluble residue remaining after plant material is treated with sulfuric acid and caustic alkali, known as coffee fiber, with a content of about 2.4%.
Nitrogen-containing compounds:
The nitrogen-containing compounds in green coffee beans can be divided into alkaloids, trigonelline, nicotinic acid, proteins, and free amino acids, described as follows:
1. Alkaloids:
Mainly caffeine. Its content in green coffee beans varies greatly depending on variety. Robusta has higher content, averaging about 2.2% of dry matter; Arabica has less, averaging about 1.2%. In recent years, low-caffeine varieties have been cultivated in Java and Côte d'Ivoire, with caffeine content of only 0.2%. Caffeine can be removed by various methods to produce various decaffeinated products. Decaffeinated instant coffee powder has a caffeine content below 0.3%, and general commercial products are controlled between 0.1% and 0.2%. Caffeine can be said to be the spirit of coffee and is also the most controversial component.
Although caffeine has no off-flavor or odor, it has a pronounced bitterness. Some have attempted to use the degree of coffee bitterness to judge the amount of caffeine content, but this has not been fully successful, because caffeine's bitterness accounts for only a small part of coffee's bitterness, thus having little impact on the bitterness of decaffeinated coffee.
After being digested by the human body, caffeine can be rapidly absorbed and metabolized, and excreted through urine. The increase in caffeine content in human blood depends on the amount in the stomach. The most significant physiological effect of caffeine on the human body is stimulation of the central nervous system; the dosage required to produce changes in brain activity is extremely high, much higher than normal intake. Other physiological effects of greater concern are those on blood pressure and the cardiovascular system.
2. Trigonelline:
Trigonelline is a hygroscopic colorless crystal with excellent solubility in water. It also has mild physiological effects, mainly acting on the central nervous system, bile secretion, and intestinal peristalsis. Trigonelline's direct impact on coffee quality is very slight; its bitterness is only one-quarter that of caffeine, and due to its very low presence, its impact on taste is not significant. Its content in green coffee beans varies by variety, with Arabica having higher content than Robusta. Trigonelline decomposes rapidly during roasting, with a loss rate of about 50% to 80%, depending on roasting temperature and time. In addition, trigonelline decomposes to generate various compounds, including non-volatile nicotinic acid, as well as 29 volatile substances, among which 9 have been identified as substances containing coffee aroma.
3. Nicotinic acid:
Nicotinic acid content in green coffee beans is very low, but it increases after roasting, mainly because it comes from the decomposition of trigonelline. However, research findings show that at high roasting temperatures, nicotinic acid continues to decompose into volatile chemical substances, so the actual increase is not substantial.
The above content is compiled by CoffeeHunters, a coffee news website.
