Kokesh Hour 3
The Chemistry of Coffee
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...
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