Before coffee is roasted, it is referred to as “green coffee”. The green coffee is primarily made up of caffeine, lipids, carbohydrates, proteins (amino acids), and organic acids (although inorganic acids exist in coffee as well). These groups are quite stable in the green phase, and it is the carbohydrates, proteins, and acids that will undergo significant reactions during roasting to produce coffee. The important groups of carbohydrates in beans are from the monosaccharides and the polysaccharides found in beans. The disaccharide Sucrose (C12H22O11) also plays a vital role later on in coffee. Generally speaking, Monosaccharides represent the simplest forms of sugars, such as glucose and fructose, and usually follow the formula Cx(H2O)y. These sugars serve as building blocks for polysaccharides like starches and cellulose, which are usually long repeating chains of a basic unit or monosaccharide.
All together, carbohydrates represent about 50% of green coffee’s base. Amino acids are present in green coffee at levels of about 10-13% of dry matter. Amino acids are molecules containing an amine, a carboxylic acid group (an organic acid that contains at least one carboxyl group of COOH) , and a side chain (a chemical grouping that is attached to the main group and is specific to each compound). Lipids represent around 11-17% of coffee. They do not play a huge role in the chemical processes of creating coffee- rather, they act as conductors of aroma and taste later on in the coffee process. Caffeine, an astringent stimulant, develops in coffee as a defense mechanism in the coffee plant. Its content remains stable throughout roasting and brewing. It is highly water soluble. Finally, green coffee beans contain numerous acids. Chlorogenic , Citric, Phosphoric, and Quinic Acids represent some of the most important acids in the green coffee bean. A small amount of acetic acid is also present before roasting. It forms when the coffee cherry is fermented to remove the outer pulp. Other than Phosphoric acid, the major acids at work in coffee are organic.
In the first phase, the addition of coffee beans drastically drops the temperature of the roaster. The process is endothermic as the beans absorb heat to give off moisture. Around 100 degrees Celsius the temperature stabilizes as water turns into steam. This leads to an increase of pressure. Once the moisture content has been brought down, temperature increases rapidly again, and the reaction goes exothermic as sucrose begins to decompose at around 190-205 degrees C- forming steam and CO2. The increase in pressure from this reaction bursts the cells of the bean as the bean “cracks”. This crack causes the bean to almost double in volume. The process goes endothermic again until about 225 degrees C and beans “crack” once again. This increases volume again. Subsequent roasting pushes lipids through the cells to the surface of the bean, and as the bean roasts farther pst the second crack, the volume of the bean decreases due to decomposition. Chemical Processes during Roasting:
Caramelization- Caramelization occurs when sucrose begins to decompose and causes the first mechanical crack of the coffee bean. The sugars produce water and carbon dioxide during the reaction, but also color and aromatics like furans (responsible for caramel- like aromas) and HMFs (hydroxymethylfurfural) (responsible for pure, sugar aromas). As the sugar caramelizes further, the aromas increase, but the original taste of sweetness decreases.
The Maillard Reaction- The Maillard reaction (discovered in 1912) is the reaction that takes place during the browning of any food. This reaction varies wildly depending on the specific reactants- It happens in seared meat and toasted bread as well as roasted coffee beans. Despite the fact that the reaction comes out with too many variants (and too many unknowns) to go into extreme detail, basic principles govern the reaction for all variants. At its most fundamental form, the Maillard reaction is the reaction that occurs between a reducing sugar and an amino acid. Proteins are all made up of amino acids, so all proteins have the potential to undergo the Maillard reaction.
The reducing sugar required for the reaction is any sugar with an aldehyde group. Sucrose is not a reducing sugar, so it’s numerous other sugars like fructose, lactose, and glucose that undergo the process. The sugars and amino acids react to form molecules called melanoidins that have the brown color characteristic of the reaction. The process is complicated because different sugars and different amino acids produce different compounds. Futher complicating the browning process, the new compounds (melanoidins) react even further creating new substances. The Maillard reaction is the cause for many of the volatile aromatic compounds characteristic to coffee, as well as other non-volatile compounds.
Strecker Degredation-Strecker Degradation falls under the scope of the Maillard reaction. It is an intermediate step in the overall process, and involves amino acids. Rather than reacting with reducing sugars like the basis of the Maillard reactions, it requires a carbonyl compound as a reactant. The reaction yields CO2, an aldehyde, as well as an amino-ketone. This process is a significant intermediate step because it yields two products that are hugely responsible for the different smells of coffee.
The Formation and Decomposition of Acids- From the four predominant acids found in green coffee beans- Chlorogenic, quinic, citric, and phosphoric – two decompose and two increase. Chlorogenic acid (C16H18O9) decomposes by 60% to form caffeic (not to be confused with caffeine- not the same thing!) and quinic acid. The decomposition of chlorogenic acid in the coffee bean is directly proportional to the length of roasting. It also occurs later on in solution once the coffee has been brewed. Citric acid decreases as a result of the roasting process, it is is unable to withstand the roasting process, however its content does not change after it has been roasted (it doesn’t change with brewing). The acid diminishes quickly as roasting levels pass the light roast stage. Phosphoric acid increases with the length of roasting, however scientists are still unclear as to why the phosphoric acid increases rather than simply remaining stable throughout the process. Other notable acids that are formed when coffee beans are roasted are lactic acid and acetic acid, these acids form due to the decomposition of polysaccharides during and following the first crack stage.
Coffee Brewing and Taste
Approximately 28% of the components in roasted coffee beans are water soluble. 72% are insoluble. Brewing a cup of good coffee depends on the balance of extraction of these components. Extraction below 16% is associated with weak, peanuty coffee, while extraction over 24% leads to a bitter brew. Taste of coffee depends on the degree of carmelization the coffee went through during roasting, the acidity of the cup, and the aromatic compounds such as aldehydes, ketones, and furans. As mentioned before, carmelization increases the aroma of the coffee, but lowers the overall sweetness of the cup. The overall “body” or “weight” of coffee simply has to do with the number of dissolved particles in the cup. Darker roasts generally have more body to them because the lipids have been brought to the surface of the bean, and are therefore more readily brewed into the coffee.
A more “acidic” brew (not a literal term- it depends on the acids types present in the brew, the pKa, anions species present, and the buffering capacity) lends a “brightness” to the mouth. Below are some common acids associated with the different flavor notes that accompany an “acidic” taste in the cup. -Phosphoric acid has been associated with enhancing the brightness of the coffee, although there is still dispute over which acid contributes the most to the overall “acidity”, especially because the majority of phosphoric acid is neutralized with the presence of potassium once it’s in solution. – Quinic acid provides a cleanness to the brew in low quantities, but in excess leads to bitterness and astringency -Caffeic acid is generally bitter and harsh
-Citric is bright and floral in moderation, but excess is overly sour -Acetic acid can taste fermenty and vinegary in excess but in small amounts give the coffee a “winey” flavor -Lactic acid is associated with a less prominent “bright” flavor, but like citric acid, excess tastes sour -Malic acid is a less prominent acid that is known for its apple flavor
As for the various volatile aromas in coffee a few of the predominant components include: – Furans which are sweet , nutty, fruity, and caramel smelling -HMFs give off a clean sweet smell
-Ketones are floral, buttery, caramel, vanilla-y, and milky
-Phenols in lower levels are spicy, vanilla-y, clove, and anise flavored, excess are associated with a woody, medicinal flavor.
Lipids in coffee play a significant role because many of the aromatics in the coffee bean are fat- soluble rather than water soluble. The higher the lipid content, the stronger the taste of the coffee is likely to be. This is why dark roasts generally have a stronger flavor than the light roasts (although it is also due to the increase in overall flavor compounds versus sugars and intact amino acids). Espresso brewed coffee also has a stronger flavor not only due to the increase of dissolved particles relative to the water content, but because it is brewed using emulsion rather than extraction. Emulsion involves pressure as well as water to extract the elements within the bean. The high pressure brewing gets more lipids into the shot than a traditional extraction of pouring water over the coffee grounds would.
Finally- caffeine has a mildly astringent flavor. It was not discussed much in this paper because caffeine is relatively stable and doesn’t change from green coffee to roasted coffee. Caffeine level is merely dependent on the caffeine present in the coffee cherry when it was picked and the amount of coffee brewed in ratio to the water. Contrary to popular belief, it is not actually the presence of caffeine that contributes most to the bitter flavor of coffee. Rather, it is a compound called trigonelline (C7 H9NO3), an alkaloid, and its products after degradedation during roasting, that are given the most scientific credit for coffee’s bitterness.