| Barista Proposes, Roaster Disposes |

The chemical reactions that occur during coffee roasting play a crucial role in creating the flavor of the coffee. Some specific reactions can lead to bitterness if the roasting process is not carefully controlled:

1. Maillard Reaction: This is a chemical reaction between amino acids and reducing sugars. It creates flavor and color compounds in coffee. If this process goes on for too long or occurs at too high a temperature, it can lead to the formation of bitter compounds.
   

2. Caramelization: This is the process where sugars in the coffee beans break down and form caramel. When controlled properly, this process creates sweet and delicious flavors. However, if caramelization goes too far, it can produce bitter and burnt tastes.
   

3. Pyrolysis: This is the thermal decomposition of organic compounds in the coffee beans. Pyrolysis can lead to the formation of bitter and burnt compounds if the roasting temperature is too high or the roasting time is too long.
   

4. Formation of Phenolic Compounds: Phenolic compounds in coffee beans can be altered during roasting, leading to bitterness. If over-roasted, these compounds can become excessively bitter.
   

5. Oxidation of Fats: During roasting, the fats in coffee beans can oxidize, creating compounds with unpleasant smells and bitter tastes.
   

6. Decomposition of Chlorogenic Acids: Chlorogenic acids are compounds in coffee beans that can break down during roasting, forming bitter compounds.

To avoid bitterness, roasters need to precisely control the temperature and duration of the roast to optimize the chemical reactions in the coffee beans. This requires experience and skill to achieve the perfect roast level.