Many people love coffee because it has a very distinctive flavor from the roasting process. roasting. Of course, to experience the benefits of caffeine to stay focused on work. This coffee flavor arises due to high temperature heating during the roasting process. With heat energy, the coffee beans undergo water evaporation. During the coffee roasting process, the green beans undergo physical and chemical changes. Physically, the changes can be seen in their color, size, and volume.

It is the chemical changes that actually affect the final flavor of the coffee. It happens because of the interaction of chemical compounds in the coffee bean.

Physical Stages of Coffee Beans

There are several stages of physical changes in coffee beans during the roasting process. These stages occur simultaneously and are interrelated.

The form of change can also be seen directly by looking at it. However, a quantitative test can also be conducted with the help of a weight density meter. Observing physical changes is usually done before roasting, and after.

Discoloration of Coffee Beans

In the roasting process, coffee beans are roasted in a drum at a temperature of about 195°C. In the first one minute of roasting, the beans will experience a temperature drop to 100-105°C. Then, it increases again according to the supply of heat energy.

This decrease is because it absorbs and raises the temperature of the green bean. In this process there is also evaporation of water from the coffee beans.

The water content in the green bean will shrink so that the coffee beans turn brownish in color. This color change can be measured using a Lovibond meter based on the brightness level.

In the Lovibond tool, the brightness value is abbreviated to (L), this measure is based on the amount of light at a particular wavelength. The surface of the coffee bean will reflect back these waves. If the L value is high, it means that the coffee beans have a bright color.

Green Bean Weight Change

During the roasting process, weight shrinkage occurs due to the evaporation of water in the coffee beans. For example, in a roast of about 14 minutes, the water content in the beans shrinks to 4% from 12.50% at the beginning.

Within this 4% moisture content, several types of organic compounds begin to synthesize. In addition to producing chemical compounds, carbon dioxide (CO2) gas evaporation also occurs.

The reduction in bean weight is also influenced by the shedding of the epidermis. The darker the roasting level, the more shrinkage there will be.

The weight loss in the roasting process is known as the yield. It serves as a measure of the functional level of a roasting machine.

Change in Bulk Density of Green Bean

The density of green beans can be seen from the ratio between the weight and volume of coffee beans.

For example, green beans initially have a density of 615 kg/m3 and a moisture content of 12.50%. Then, after roasting for seven minutes, the moisture content drops to 8%.

Of course, the bulk density of coffee beans is reduced to 506 kg/m3. It drops again to 400 kg/m3 after roasting for 14 minutes.

In addition, there is an expansion of the green bean volume, which is an effect of the pressure of water vapor andCO2 gas. This expansion occurs within the cell walls of the coffee bean. For example, dark roasting causes the beans to expand by 30-40% of their original size.

Green Bean Stages Chemically

As mentioned earlier, coffee contains chemical compounds. This compound will produce the distinctive flavor of coffee. Chemical compounds in coffee beans include:

  • Carbohydrates,
  • Compound,
  • Nitrogen,
  • Fats,
  • Acidic compounds.

When the beans have not entered the roasting process, these compounds are not active and do not interact with each other. However, due to the heating process the compounds will combine to produce new compounds.

In this stage, several chemical reactions occur, ranging from the Maillard reaction, caramelization reaction, pyrolysis and acidity.

Stages of Maillard Reaction

The reaction stage starts intensely when the moisture content is low at 140-170°C. This is where the compounds that produce the coffee flavor start to emerge. There are 3 phases in the Maillard reaction.

First Phase

When protein compounds break down, they turn into amino acids. At the same time, carbohydrate compounds break down into the monosaccharides glucose and fructose.

Then, the synthesis of amino acids and monosaccharides produces amadori compounds. This compound is unstable, so it undergoes rapid shrinkage of water content.

Rapid shrinkage of moisture content produces carbonyl compounds. The change follows the Strecker reaction mechanism.

Second Phase

The Strecker reaction includes the incorporation of alpha amino acids and dicarbonyl compounds. In this process the seeds turn a brownish-yellow color. This reaction produces a volatile compound called pyrazine and a non-volatile compound called pyridine.

The formation of aroma occurs due to the presence of pyrazine compounds. The threshold value level of pyrazine is the lowest, this affects the vapor of this compound is easily detected through smell. Meanwhile, pyridine compounds cause the bitter taste.

Third Phase

This phase produces the compound melanoidin, which results from the condensation reaction of several compounds from the second phase. In the third phase, there is a contribution to the formation of flavors and a change in color to dark brown.

Stages of Caramelization Reaction

When the amino acid content of the green bean gets lower, it means that the caramelization reaction has started. This low level of amino acids occurs due to the Maillard reaction. Sucrose or sugar compounds dehydrate and condense into caramel. The reaction occurs at a temperature of 170-200°C.

The outcome of this reaction depends on the degree of dehydration, when the heating reaches 170°C. Then, the sugar in the seeds releases 4 molecules of water orH2O, and turns into caramellic compounds.

At this point the color of the coffee beans turns brown and by this compound there is a tendency for sweetness to appear.

Sucrose can turn into caramelene compounds by releasing 8 water molecules. This happens if the heating temperature is increased again.

This caramelization process changes the color of roasting green beans to dark brown. There are also products of this reaction called furan compounds. Furan compounds create a caramelized sweetness and a nutty flavor.

Pyrolysis Stages

When the temperature rises again at the roasting level of 200°C, the reaction stage rises to the pyrolysis phase. At this stage, the reaction changes the content of the complex organic compounds that the coffee beans have.

Due to the high temperature and lack of oxygen, gaseous and solid fractions of carbon compounds are formed. Pyrolysis product gas resides in the cell walls of coffee beans. The layer is strong and has properties that are difficult to penetrate.

The increase in temperature during the coffee roasting process causes the production gas pressure to increase. This causes the cell walls to break, resulting in a cracking sound.

Some compounds eventually form carbon atoms, known as charcoal. It has a darker color, but there is a layer of oil on the outside.

In this stage, the coffee beans have a morebitter flavor with decreased acidity.

Acidity Stages

Coffee beans have a complex flavor ofacidity that combines with other flavors. This creates an interesting sensation on the tongue of the connoisseur.

However, there is alsosourness, which is a taste that is toosour to cause an uncomfortable sensation on the tongue of coffee lovers.

There are 3 types of chemical compounds that contribute to the sour taste in green beans, including :

  • Aliphatic Acids,
  • Chlorogenate,
  • Phenolics.

The concentration of the three compounds can affect the pH value of the roasted coffee beans. Therefore, the acidity level is measured based on the pH value.

Before roasting, the pH of the green bean reaches 5.7. Then, after roasting to the first crack (light) level, it decreased to 5.20.

This decrease is after the decomposition of sucrose, glucose and fructose compounds. Then, it turns into a maximum of aliphatic acid compounds, namely:

  • Citric acid
  • Malat
  • Lactate
  • Pyruvate
  • Acetic Acid

The higher the temperature when roasting, the faster acidic compounds form. This is why the pH value of roasted coffee is lower.

The appearance of acidic compounds in coffee beans persists up to the medium-temperature roasting level. Approximately 48 seconds after the first crack roasting level.

Then, the pH rises again at the second crack roasting level, with a value of around 5.30. This happens because the aliphatic acid undergoes a change to CO2 gas.

The process continues until about 48 seconds after the second crack roast level. Faster decomposition occurs if the temperature rises to 210 C. Thus, the pH of the coffee beans rises to 5.40.

The acidic compounds are broken down through the high-temperature heating process. Up to very dark roasting, chlorogenic acid turns into quinic acid.

Quinic acid is what causes sourness when brewing roasted coffee.

It is interesting to know the physical and chemical stages that occur in green beans. These stages can occur due to heating orroasting.

Source:

https://www.cctcid.com/2019/07/22/perubahan-fisis-dan-kimiawi-biji-kopi-selama-penyangraian

https://coffeeland.co.id/roasting-coffee-proses-penting-dalam-menentukan-karakteristik-kopi

https://repository.pertanian.go.id/bitstreams/28fe8cba-05fe-4938-8a23-7cbf5cf50136/download