Complexity of coffee (part 1)

Coffee has long become an integral part of our lives. For some people, it’s a daily pleasant habit, a way to cheer up and, if necessary, clarify one’s mind. For others, coffee preparation is a real ceremony with its own peculiarities and rituals. And finally, some people dedicate their entire lives to studying this unusual product.

To get a cup of quality drink every single time you prepare it, it is necessary to know and understand how capricious coffee beans are at the stages of growing, processing, roasting and cooking, with hundreds of chemical compounds responsible for the taste, aroma and essence of the drink susceptible to change.

Experts believe that the essence of coffee is most revealed in espresso. Espresso is a small heavy porcelain cup, half filled with the dark drink, covered with a velvety thick reddish-brown foam, called the crema. The crema consists of tiny and thin bubbles of gas, spreading the distinctive coffee aroma. The word “espresso” refers to a portion of coffee made to order and cooked with hot water rapidly running through a layer of crushed compressed and roasted coffee under pressure. The result is not just a concentrated mixture of water-soluble dry granules. The drink contains a variety of aromatic compounds in the form of tiny emulsified oil droplets, enriching espresso with its characteristic flavor and aroma.

According to coffee experts, espresso is the ideal method of coffee preparation, due to the specially created technical conditions that fully disclose, enhance and demonstrate the properties of coffee beans.

The high quality of beans is not just an accidental phenomenon, but the result of maintaining strict control over many factors on the plantation, at the factory – by coffee manufacturers – and, of course, in the cup.

A single serving of espresso requires 50 to 55 fried coffee beans. Even one imperfect bean can spoil it all. That is why human sense of smell and taste in the process of evolution arose and developed as protective mechanisms that warned our ancestors against spoiled, unhealthy products. Thanks to modern technology, it is possible to identify 50 almost perfect beans economically and consistently.

Growing coffee

Growing coffee

Coffee beans are the seeds of plants belonging to the Rubiaceae family, which contains at least 66 species of Coffea genus. The two commercially exploited species are Arabica (accounting for two-thirds of the world coffee production) and C. Canephora, more commonly known as Robusta (accounting for one third of the world coffee production).

Robusta has 22 chromosomes, while Arabica – 44. Thus, Arabica cannot be hybridized with other coffee cultures. Robusta is a heavy-yielding and disease-resistant tree, up to 12 meters in height, best grown in a warm, humid climate. It is characterized by a sharp earthy aroma and a high caffeine content, ranging from 2.4 to 2.8%.

Arabica, which originated in the Ethiopian Highlands, is a medium- and low-yielding thin tree, three to six meters high, requiring a moderate climate and considerable care. The coffee, cooked from Arabica beans, has an intricate complex aroma, reminding one of flowers, fruit, honey, chocolate, caramel or toasted bread. The caffeine content does not exceed 1.5% by weight. Because of its high quality and taste, Arabica is sold at a higher price than the hardy Robusta.

Coffee cherries

Providing a good level of precipitation, after 210 days, red or yellow fruits (commonly called “cherries”) appear on Arabica trees. Each cherry contains two oblong seeds – coffee beans. Since fruits of two different flowers cannot grow on the same branch, picking of ripe fruits is the most effective method, as automated harvesters fail to tell the difference between ripe and unripe cherries.

Undoubtedly, the quality of beans also depends on the plant genetics, the soil where it grows and the microclimate, including such factors as altitude, daily temperature fluctuations, precipitation and sunlight. Along with the roasting process, these agricultural and geographical factors are responsible for taste differences among numerous varieties of coffee.

Coffee processing

Cherries must be processed immediately after harvesting. Manufacturers resort to two processing techniques: drying in the sun (natural method) and washing (wet method).

To dry the collected berries in the sun, they are placed in the courtyard and often mixed, so that they can evenly warm up and be ventilated. Then the dried cherries are automatically processed. At this stage, the dried flesh and parchment coat are removed. This is the simplest and the oldest method of bean processing.

There exists an alternative approach: first, the cherries are automatically cleaned, washed and dried, and then the parchment coat is removed. This technique requires the manufacturer to have special equipment and a large amount of water. As a rule, such coffee is more expensive.

The goal of these techniques is the same – to reduce the moisture content in green beans from 65% to 10–12%. Producers of high-quality beans use a variety of sophisticated technologies to minimize the percentage of defective coffee beans, including ultraviolet analysis for moldy beans detection.

The beans, processed in different ways, are visibly different themselves. For instance, the seeds processed by wet method have a more shiny, clean and smooth surface, while the beans, processed by dry method have remnants of a silvery coat, as well as a peculiar yellowish shade.

Mature green coffee beans

A mature green coffee bean consists of thousands of cells with a diameter of 30 – 40 micrometers and thick walls varying from 5 to 7 micrometers in size. During roasting, all key chemical reactions occur in the cells, resulting in a tempting coffee taste and aroma. Cells of immature beans have thinner walls. Also, such beans lack important aromatic precursors formed at the last stages of maturation.


One of the possible further stages of processing prior to coffee roasting is decaffeination. Many people want to enjoy the taste and aroma of coffee without experiencing its stimulating effect on the body. That is why decaffeination became so popular. For the majority of coffee varieties, the qualitative characteristics of the drink after decaffeination remain the same. The process itself takes place at industrial enterprises. There are four ways to remove caffeine from green beans, depending on the substance used to extract the alkaloid. It can be water, ethyl alcohol, supercritical carbon dioxide or dichloromethane. All four methods can be subdivided into the main stages:

  • soaking green beans;
  • caffeine removal;
  • solvent removal (if applicable);
  • drying the beans up to the normal moisture;
  • careful monitoring of decaffeination conditions, such as temperature, pressure and time, as caffeine extraction occurs by passive transport. Due to the difference in caffeine concentration inside the bean and outside it (in the solvent), caffeine is extracted from the cells, until these concentrations become equal.
Decaffeinated and ordinary green beans

Decaffeinated green beans

Ordinary green beans

Decaffeination methods

  1. Water. Green coffee is immersed in water, where caffeine dissolves after a certain time (by concentration gradient), then water is drained off. However, along with caffeine, beans lose the majority of aromatic compounds. To prevent this significant drawback, water-soluble coffee components are added to the solution. Caffeine is then removed from the solution by means of coal or other absorbents, and the solution saturated with aromatic compounds may be reused.
  2. Ethyl alcohol. It is found in many fruits contributing to this or that characteristic flavor. It is also found in various concentrations in foods, including green and roasted coffee. To remove caffeine from coffee beans, a mixture of water and ethyl alcohol is used. Thus, ethyl alcohol circulates around the water-soaked coffee beans in reservoirs. Caffeine is extracted from the beans. Then a mixture of water, ethyl alcohol and caffeine is drained. The procedure is repeated several times until the caffeine content is 0.1% or less.
  3. Supercritical or liquid carbon dioxide. Carbon dioxide is an easily accessible substance that is contained in large quantities in the air we breathe, and in the water we drink every day. Under certain conditions, it may be used for selective extraction of caffeine from green coffee beans, while most of other necessary components remain intact. To use carbon dioxide in the supercritical state (an intermediate state between liquid and gas), a very high pressure is necessary – up to 250 atmospheres. Therefore, this method requires special expensive equipment. Liquid CO2 may be used to remove caffeine at a lower pressure (65 – 70 bar) and a lower temperature, but it takes a much longer time to extract.
  4. Methylene chloride (or dichloromethane). This is the most common method of decaffeination. Dichloromethane acts selectively on the bean, removing only caffeine. Dichloromethane circulates around the water-soaked coffee beans in reservoirs. Then the mixture merges and the cycle repeats until the caffeine content is 0.1% or less. The solvent itself is almost completely removed by simple water vapor and leaves no traces.

Find out more in Part 2.