Post Reply: The Reactions Taking Place During Coffee Roasting

<blockquote><div class="quotetitle">Quote from <a class="profile-link highlight-moderator" href="#">Omari Kitula</a> on July 26, 2025, 3:33 pm</div>Coffee roasting is more than just heating green coffee beans; it’s a complex chemical transformation that gives coffee its signature aroma, flavor, and color. When green beans are roasted, they undergo a series of physical and chemical reactions that convert a grassy, raw seed into the rich, aromatic beans we brew. Here are the major reactions that occur during coffee roasting.

1. The Drying Phase: Evaporation of Water

Before chemical reactions starts, green coffee beans contain about 8–12% moisture. During the initial stage of roasting (up to 160°C / 320°F), most of this moisture evaporates. This phase is crucial because:

》It prepares the bean for chemical changes.

》Helps in preventing scorching or uneven roasting.

At this point, no major flavor development occurs, but the stage is set for the reactions to come.

2. Maillard Reaction: Flavor and Color Development

When the temperature reaches around 140–165°C (285–330°F), the Maillard reaction begins. This is a reaction between amino acids and reducing sugars, and it is one of the most important processes in coffee roasting.

What Happens?

》Complex flavor compounds called melanoidins are formed, giving coffee its characteristic brown color and rich aroma.

》Hundreds of intermediate compounds are produced, contributing to nutty, caramel, and roasted notes.

The Maillard reaction is the foundation of coffee flavor, similar to what happens when you toast bread or sear meat.

3. Caramelization: Sweetness and Body

Around 170–200°C (338–392°F), sugars in the coffee bean start caramelizing. Unlike the Maillard reaction (which involves proteins), caramelization only involves sugars breaking down under heat.
Effects of Caramelization
》Adds sweetness and body to the coffee.

》Produces caramel-like aromas.

》Balances acidity with smoothness.

However, if roasting continues too long, sugars burn, leading to bitter flavors.

4. Pyrolysis: Complex Aroma Formation

At higher temperatures, especially after the first crack (around 196°C / 385°F), pyrolysis occurs. This is the thermal decomposition of organic compounds without oxygen, leading to the release of volatile compounds that give coffee its unique aroma.

Key Compounds Formed

》Aldehydes, ketones, and esters→ fruity, floral notes

》Phenolic compounds→ smoky, spicy flavors

》Carbon dioxide (CO₂)→ contributes to crema in espresso

Pyrolysis continues into the second crack (around 224°C / 435°F) in darker roasts, producing more intense, smoky, and sometimes bitter flavors.

5. Chlorogenic Acid Breakdown: Acidity Control

Green coffee beans contain chlorogenic acids (CGAs), which contribute to coffee’s acidity and antioxidant properties. During roasting:

CGAs decompose into caffeic and quinic acids.

Light roasts retain more acidity; darker roasts become less acidic but more bitter due to quinic acid.

6. Formation of Aromatic Volatiles

Roasting produces over 800 volatile compounds, responsible for coffee’s complex aroma. Some major classes include:

》Furans – sweet, caramel-like notes

》Pyrazines – nutty, earthy aromas

》Phenols – smoky characteristics

The balance of these compounds depends on roast level, bean variety, and roasting method.

Coffee roasting is both an art and a science. From the Maillard reaction to caramelization and pyrolysis, each stage creates flavors and aromas that define your cup of coffee. Whether you prefer a light, fruity roast or a bold, dark roast, understanding these reactions helps appreciate the complexity behind every sip.</blockquote>

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