Introduction
Coffee is a brewed drink made from roasted coffee beans and is nowadays unarguably of paramount importance in the lives of billions of people as emphasized by the fact that it is globally the second most consumed drink after water. Coffee was first introduced in Europe in the 16th century, although, according to legend, it was discovered in 850 AD by a goat herder in Ethiopia and subsequently spread to neighboring Arabic empires were the coffee brew rapidly gained in popularity. Coffee plants are commercially cultivated in plantations in about 70 countries, ranging from Ethiopia to Vietnam and Brazil. Coffee beans are obtained from berries from the coffee plant. Although 70 coffee plants species are known, the two most important ones are the Arabica and Robusta coffee plants. Coffee prepared from Arabica beans has a mild flavor and superior quality and aroma, while coffee from Robusta beans is characterized by a stronger and bitterer flavor. Typically, ripe coffee berries are picked, processed to remove the outer layer and dried. The beans are roasted, ground and infused with near boiling water to produce a cup of coffee.
(Bio)chemical composition
Chemically, coffee is remarkable complex and its exact composition depends on many of factors such as variety, roasting, grinding and brew conditions. The variability of coffee is underscored by a recent study which analyzed the chemical makeup of espresso coffees obtained from 20 different coffee shops in Glasgow, finding that the caffeine content ranged from 50 to 322 mg per cup (1). The most important chemicals in coffee are (2): chlorogenic acids, caffeine, diterpenes (cafestol and kahweol), trigonelline and melanoidins (figure 1).
Chlorogenic acids
Coffee beans contain different (poly)phenols that together makeup 6-10% of the dry weight. The majority of (poly)phenols are chlorogenic acids (figure 2, adapted from 2), including caffeoylquinic acids. Upon roasting, the amount of chlorogenic acids is reduced. Despite this reduction, coffee remains the richest source of dietary chlorogenic acids as a single espresso contains 24 to 423 mg of chlorogenic acids (2). Coffee is often said to possess superior antioxidant activities due to the presence of chlorogenic acids. However, the vast majority of these compounds are metabolized after coffee consumption and the plasma levels of unmetabolized chlorogenic acids are too low to make a significant contribution to the antioxidant system. Nevertheless, chlorogenic acids display bioactivity because: (i) these compounds have been implicated in modulating expression of genes required for drug detoxification, (ii) they possess anticarcinogenic activity through inhibition of DNA methyltransferase (iii) and may function as antithrombotic agent, protecting against the development of cardiovascular diseases. Moreover, it was reported that chlorogenic acids inhibit the adsorption of Streptococcus mutans (the major causative agent for human caries) onto saliva-coated surfaces, suggesting that coffee protects against S. mutans-induced tooth decay (3).
Caffeine
The amount of caffeine in green coffee ranges from about 1% of the dry weight in Arabica to 2% in Robusta, respectively. A typical cup of coffee contains between 50 – 100 mg caffeine, while the caffeine content in espresso varies from 50 to 322 mg (1). The structure of caffeine is shown in figure 3 (adapted from 2). Unlike the level of chlorogenic acids, the amount of caffeine is not affected by roasting and remains therefore relatively stable. Caffeine is rapidly absorbed by the stomach and intestines and is subsequently distributed to all tissues via the circulatory system. Caffeine is degraded in the liver through the action of the cytochrome P450 enzymes, such as CYP1A1. The first step in caffeine metabolism is its demethylation, resulting in three dimethylxanthines (paraxanthine, theobromine and theophylline). These metabolites (figure 4) are further degraded by enzymes in the liver, which are ultimately excreted via the urine. The physiological effects of caffeine are well documented and are mediated through the adenosine receptor located in the brain. Adenosine is a inhibitory neurotransmitter, which signals tiredness and association with its receptor slows down neuronal activity. However, caffeine prevents the binding of adenosine to its cognate receptor and as a consequence neuronal activity is not decreased but increases instead, resulting in enhanced perception, reduced fatigue, increased heart rate and blood pressure. Importantly, regular coffee drinkers develop a tolerance towards the negative effects of caffeine. The biochemistry of caffeine is explained in the following movie:
Diterpenes: kahweol and cafestol
Kahweol and cafestol (figure 3, adapted from reference 2) are present in coffee as fatty acyl esters and contribute to the bitter taste of a cup of coffee. These compounds are common in boiled and unfiltered coffee and can be removed by filtering. Kahweol and cafestol contribute to the adverse effects of coffee because they have been implicated in the cholesterol-raising effect of coffee, although it was also reported that these compounds exhibit anticarcinogenic activity (4).
Trigonelline
Trigonelline is a pyridine alkaloid of which the amount in green coffee is about 1% of the dry weight. This compound is partially degraded by roasting into nicotinic acid and several other pyridine derivatives (figure 4, adapted from 2). The levels of trigonelline in a cup of coffee range from 40 to 110 mg. Moreover, this compound has been shown to exhibit different biological activities, including hypoglycemic, neuroprotective and antibacterial activities.
Melanoidins
Coffee contains a substantial amount of complex non-digestible carbohydrates, such as galactomannans and arabinogalactans. The fiber content of a cup of coffee varies between 0.14 to 0.65 g. During roasting, these carbohydrates react with amino acids generating structurally diverse polymers known as melanoidins. Their exact composition is not known but these high molecular weight compounds are in general brown colored and contribute to the dark color of coffee. Several studies indicate that melanoidins exhibit different biological activities, including antimicrobial, antioxidant and the ability to influence the gut microbiome (2).
Conclusion
The chemistry of coffee is complex because it contains hundreds of different compounds. Moreover, the chemical composition depends on a variety of factors and the exact formulation of a cup of coffee will therefore be hard to establish. Although it is clear that coffee contains different bioactive compounds, its complex chemistry complicates full assessment of its physiological properties. However, the combined data from different epidemiological studies indicates that coffee lowers the risk of chronic diseases such as type 2 diabetes, cardio vascular disease, cancer (endometrial and hepatocellular cancer) and neurodegenerative diseases (i.e. Parkinson’s). More information about the beneficial effects of coffee can be found in the articles below.
https://www.labroots.com/trending/clinical-and-molecular-dx/6412/daily-coffee-habit-extending-life
References
1. T. W. M. Crozier, A. et al. 2012. Espresso coffees, caffeine and chlorogenic acid intake: potential health implications. Food Funct: 3, 30–33.
2. Iziar, A. et al. 2014. Coffee: biochemistry and potential impact on Health. Food Funct: 5, 1695-1717.
3. Daglia, M. et al. 2002. Anti-adhesive effect of green and roasted coffee on Streptococcus mutans' adhesive properties on saliva-coated hydroxyapatite beads. J Agric Food Chem: 50, 1225–1229.
4. Cavin, A. et al. 2002. Cafestol and kahweol, two coffee specific diterpenes with anticarcinogenic activity. Food Chem Toxicol: 40, 1155–1163.