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How to Roast Coffee Beans

The main chemical reaction during roasting is the Maillard reaction, a non-enzymatic browning reaction between amino acids and reducing sugars that occurs when the beans are heated. This reaction is responsible for the formation of new flavor and aromatic compounds, as well as the browning of the beans. It typically begins at around 150°C (302°F) and continues throughout the roasting process, with the rate and extent of the reaction influencing the final flavor and aroma of the coffee.

Other important chemical reactions that occur during roasting include:

  • Caramelization: the breakdown of sucrose into caramelized compounds, which contributes to the flavor and aroma of the coffee.
  • Pyrolysis: the thermal decomposition of organic materials, which can lead to the formation of volatile compounds and the breakdown of cellular structures.
  • Strecker degradation: a reaction between amino acids and reducing sugars that can lead to the formation of new flavor compounds.
  • Decarboxylation: the loss of carbon dioxide from the beans, which can affect the flavor and aroma of the coffee.
  • Dehydration: the loss of water from the beans, which can affect the texture and flavor of the coffee.

These chemical reactions, along with physical changes such as the expansion and cracking of the beans, transform the green coffee beans into the flavorful and aromatic roasted coffee that we know.

The yellowing stage during coffee roasting is a crucial phase where the coffee beans undergo significant chemical and physical changes. This stage marks the transition from the drying phase to the Maillard reaction phase, which is characterized by the beans turning yellow and emitting a hay-like aroma. During the yellowing stage, the water content in the beans is evaporated, and the beans begin to change color, flavor, and aroma.
The Maillard reaction, a non-enzymatic browning process, starts to occur, which is responsible for the development of the characteristic flavors and brown color of roasted coffee. The yellowing stage is often divided into three aromatic inflection points: Grass, Hay, and Bread.
  • The Grass stage occurs around 90°C – 125°C (200°F – 250°F), where the water in the beans is converted to gas, and the coffee starts to exhibit a grassy aroma.
  • The Hay stage takes place around 150°C (300°F), where sugars and amino acids interact, leading to the caramelization of sugars and the emergence of a hay-like aroma.
  • The Bread stage occurs around 160°C – 180 °C (330°F – 350°F), where the Maillard reaction continues, and the coffee develops a rustic, brown color and a bread-like aroma. Roastmasters use various methods to determine when the yellowing stage has been reached, including visual cues, such as the beans turning yellow, and aromatic cues, such as the emergence of a hay-like or bread-like smell.

The exact temperature at which the yellowing stage occurs can vary depending on the type of coffee beans and the roasting equipment used. Overall, the yellowing stage is a critical phase in the coffee roasting process, as it lays the foundation for the development of the coffee’s flavor, aroma, and color.

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