Complexity of coffee (part 2)

Coffee roasting

Coffee roasting

Coffee roasting is a pyrolytic process (ie the process of thermal decomposition of organic and many inorganic compounds), which significantly increases the chemical complexity of coffee. The aroma of green coffee contains about 250 different volatile compounds, while in the fragrance of roasted coffee this figure exceeds 800.

Under the influence of temperature, the residual water inside each cell turns into steam, which contributes to various complex chemical reactions between a huge number of sugars, proteins, fats and minerals. At high temperatures, from 185 to 240°C, sugar in combination with amino acids, peptides and proteins enter the process of caramelization, which is called the Mayar reaction. The final products of this process are brown sweet-bitter glucosamines and melanoidins along with carbon dioxide, form a characteristic, dominant taste of coffee. At the same time, a wide range of aromatic compounds with a lower mass (these are volatile substances) give coffee a familiar aroma. During roasting, the pressure within each cell increases to 25 atmospheres, steam and carbon dioxide try to escape from the grain, but meet thick, low porosity cell walls coated with oil. Some cells eventually burst, creating a characteristic jerky sound. During roasting, the coffee bean volume increases in half or more; and the mass of grains decreases by one-fifth.

Coffee roasting

Depending on the temperature, roasting can last from 90 seconds to 40 minutes. 12 minutes are considered the best option. The thermodynamics of intracellular reactions differs depending on the roasting. A short roasting time that requires a lot of heat energy, minimizes weight loss, but gives a cup of coffee a metallic bitterness associated with the presence of polyphenols, which do not have enough time to react in the right way. Long-term roasting is often used in poor countries, where many consumers can afford only inexpensive, defective grains. Under prolonged exposure of the temperature the grains lose unnecessary flavors and odors. Unfortunately, the desired taste and aroma also go away, as a result coffee becomes rather bitter.

The higher the final roasting temperature, the worse desirable flavor and the stronger bitterness are. And vice-versa, low roasting temperature can’t fully open the aroma and acidity usually comes to the front when drinking.

Aroma of coffee

Aroma of coffee

Researchers usually analyze the flavors emitted during the roasting of coffee beans by gas chromatography in combination with olfactometry (this is a measurement of the severity of the sense of smell with the help of special devices - olfactometers). Mass spectrometry is often used to determine the chemical composition of each odor. By inhaling the aromas of roasted coffee, you can recognize the aromas of roses, Darjeeling tea, chocolate, vanilla and violets, as well as truffles, soup, cheese, etc. The presence of molecules of chemicals such as ethyl butanoate and ethyl glycolate, which are responsible for the unpleasant smell of immature grains, spoils the cup. In addition, the molecules of methylisoborneol and trichloroanisole (TCA) add characteristic earthy, chemical smell to Robusta. TCA is also called "Rio Taste," because it was first discovered in coffee grown around Rio de Janeiro, it can also be felt in the corked wine. Perception of this smell is a kind of smelling ability for a person.

Making coffee

Making coffee

The next important step in turning coffee beans into an espresso cup   - is the extraction of active substances from ground coffee under the influence of hot water. The interaction of hot water and coffee in the preparation of espresso is different from the preparation of coffee using a filter. So, with the drip method of cooking, water freely passes through large particles of ground coffee. After 4-6 minutes of contact with hot water, most of the soluble substances in the roasted coffee pass into the solution. Thus, a cup of coffee will contain a large amount of water-soluble acids and caffeine. Preparing the same espresso allows less acid and caffeine (50-71% caffeine) to dissolve in the drink.

The preparation of espresso coffee requires special equipment that can heat water to a temperature of 90 to 94°C and create a pressure of up to nine atmospheres. Ground coffee of tiny grind is put into a perforated basket and compacted firmly. Compressed particles of ground coffee adhere to each other thanks to a thin layer of oil. This oil binds the particles together, forming a labyrinth from small air paths. The hydraulic resistance of such a layer of compacted ground coffee should be slightly less than the steam pressure, which will allow water to pass through coffee at a rate of about a milliliter per second.

Using the recommended 25-30 seconds for the extraction results in about 25-30 ml of coffee with a dense foam (crema).

Crema is a dense, red-brown foam that is on top of the espresso

Crema is a dense, red-brown foam that is on top of the espresso (Fig., Shown in an enlarged view). It consists mainly of tiny bubbles of carbon dioxide and water vapor (large circles), surrounded by a shell. Crema also includes emulsified oils that contain key aromatic compounds (particles with a red border) and dark fragments of the cellular structure of coffee. If the color of the foam is light, this means that there has been insufficient extraction of substances for one or more of the following reasons:

  • grind too coarse;
  • the water temperature is low;
  • small time.

If the foam has a dark color or there is a gap in the form of a hole in the middle:

  • too fine grinding;
  • too much coffee was used during the laying.

Excessive extraction is characterized by the presence of white foam with large bubbles. This happens when the water in the coffee machine is too hot. Just a white spot in the center of the cup means that the time to prepare the espresso was too long.

When preparing espresso, the components present on the surface of the ground coffee, including aromatic oils and pieces of the cellular structure, are also washed away. The high pressure created by the espresso machine emulsifies a small amount of oil, about 0.1 gram per cup. Undamaged cells of ground coffee produce gases (in particular carbon dioxide) passing through tiny pores in the cell wall. Some very fine particles of ground coffee can also get into the drink along with cell wall fragments that make the crema a look of tiger skin.

The final result is a multiphase colloid system in which water molecules are associated with a dispersion of gas bubbles, oil droplets and solid fragments, each of which is less than 5 microns in size. The colloidal nature of the dispersion gives the drink a high body, a viscosity and a low surface tension.

One of the most common problems that can often be found in small coffee shops with insufficiently trained staff   is an increase of extraction time. Such excessive extraction (beyond the recommended 30 seconds) leads to the inclusion of undesirable and less soluble aromatic compounds in the beverage (on the graph they are marked in red). As a result, the espresso received with a bitter and burnt taste with a smoky aroma suppresses all other flavors and aromatic characteristics and pecularities of coffee.

diagram of extraction

Coffee beans go a long way from the plantation on which they were grown, until they fall into the hands of a barista. Sometimes this time interval can be many months, years for some Arabica varieties. And at each stage it is necessary to follow their parameters and conditions. Only with their constant control and regulation the necessary chemical reactions will take place in the grains which will ultimately lead to the appearance of such an intense, rich flavor and unique taste in coffee.