Coffee roasting is a complex process that transforms the physical and chemical properties of green coffee beans into the aromatic tawny beans that we know and love. As an experienced barista and coffee roaster, I’ll walk you through the complete journey of coffee beans from their raw state to the final roasted product. From changes in color, size, and weight, to chemical reactions like pyrolysis and the Maillard Reaction, we’ll cover everything that happens during this transformative grill.

This article explains what happens during coffee roasting from physical and chemical changes to flavor and caffeine impacts. It covers the complete transformation from green to tawny bean.


  • Coffee beans undergo physical and chemical changes during roasting like pyrolysis, Maillard reactions, and caramelization.
  • The grilling process typically takes 8-20 minutes depending on the degree of grill.
  • Coffee beans double in size, decrease in weight, and change from green to yellow to tan during roasting.
  • Controlling temperature, heat transfer, and air flow are key to achieving an even, consistent grill.
  • The flavor profile and caffeine content of the beans are significantly impacted by the grilling process.

During the coffee grilling process, there are a few key takeaways to keep in mind. Firstly, volatile compounds are released, giving coffee its unique flavors and aromas. The first crack, which occurs about halfway through the grill, signifies the beans reaching a crucial stage of development. The end of the roast is determined by the desired level of grill and individual preferences.

To ensure a smooth transition, the beans should be brought to room temperature before grinding and brewing. Home roasters have the flexibility to customize the final profile, and factors like exothermic reactions, lipids, and other chemical changes can have a significant impact on the final taste of the coffee. 

Overview of the Stages of Roasting

The complete coffee grilling process consists of three main stages:

  1. Drying
  2. Change of color
  3. Development

It typically takes between 8-20 minutes from start to finish depending on the type of beans, the desired roast profile, and the equipment used. Let’s look at what happens in each stage. Roasting Your Own Coffee Beans is a rewarding experience that allows you to have control over the flavor and aroma of your coffee.

Overview of the Stages of Roasting

Coffee grilling is both an art and a science. As a passionate coffee enthusiast, I’ve learned that how coffee beans are roasted can profoundly impact the flavor and aroma of your morning cup of joe. In this comprehensive guide “how many types of roasting”, I’ll walk you through the major types of coffee grilling and how they differ.

Let’s take a look at the stages of grilling coffee seeds. In the first stage, the raw coffee seeds start to tawny, and physical changes take place. As the temperature rises, the beans absorb heat and undergo chemical transformations. It is during this process that the Strecker degradation occurs, releasing various volatile compounds that contribute to the flavor of the coffee. These stages of roasting are crucial in developing the desired taste and aroma of the final product.

The Drying Stage

The first stage of roasting is the drying phase. The raw green coffee seeds contain approximately 10-12% moisture. When they are first loaded into the roaster, the temperature is about 93°C (200°F). The beans begin releasing steam and start to dry out. These beans are carefully selected for their quality and flavor. So, if you’re ready to take your coffee experience to the next level, here’s what you need: green coffee beans for roasting

During this stage, the beans will double in size as they heat up, turning from green to yellow and tan colors. The roaster has to control the temperature carefully to ensure even drying without cooking the beans. This stage lasts 2-4 minutes.

 The drying stage is an important step in the grilling process as it is where the beans lose most of their moisture content. During this stage, heat causes a reaction in the beans, causing them to release water vapor. This process helps intensify the flavour and aroma of the beans. Additionally, the heat also causes the breakdown of larger chemicals, such as quinic acid, into smaller, more volatile compounds with high vapor pressure at room temperature.

Change of Color Stage

At about 180°C (356°F), the beans begin to take on more color changing to yellow, then tan, and light brown. This is due to chemical changes taking place inside the bean.

One important reaction is pyrolysis – the decomposition of organic material by heat in the absence of oxygen. This process produces oils and carbon dioxide that escape from the bean.

Change of Color Stage

The beans will emit a grassy, earthy aroma during this phase as organic acids break down. Controlling the rate of temperature increase is critical for an even color change. This stage lasts 2-5 minutes.

During the change of color stage in coffee grilling, the beans tawny as they are grilled. This is because chemical compounds within the beans break down into smaller chemicals that have a high affinity for tawny pigments. This stage is crucial in achieving the desired darkness and flavor profile of dark grilled coffees, which are commonly used for filter brewing methods.

Development Stage

The final stage of roasting involves rapidly increasing the temperature to 204–224°C (400–435°F). This brings out the rich, dark brown colors associated with high-quality coffee.

Two important chemical reactions occur during this stage:

  • The Maillard Reaction: This browns the beans and produces the flavors associated with grilled  coffee. It is a reaction between amino acids and reducing sugars.
  • Caramelization: The heat breaks down the natural sugars in the bean, producing the caramel and nutty notes.

Controlling heat transfer and increasing airflow ensures even development. Cracks will appear on the surface of the beans releasing oils and carbon dioxide. Once cooled, the beans are ready for grinding and brewing!

During the development stage, fruits undergo various changes in their chemical composition. Citric and tartaric acids are among the compounds that contribute to the taste profile of fruits. As they ripen, the fruit becomes sweeter and the bitterness and astringency decrease. The cell walls in the plant are at their strongest during this stage, but they gradually soften as the fruit matures. When the fruit is processed and dried, these cell walls are dissolved in water, resulting in the formation of hundreds of different flavors.

What happens during coffee Roasting

Several noticeable physical changes happen to the beans during the full grill process:

  • Size: Beans double in size as they expand from the internal heat and release of gases.
  • Color: From green to yellow, tan, light brown, and finally dark brown.
  • Shape: Beans lose their oval shape and become misshapen and wrinkled.
  • Weight: Beans lose 12-20% of their weight as moisture evaporates. An average 140g green bean will grill down to about 112-120g.
  • Density: The beans lose density as the cell structure breaks down and becomes porous.
  • Oil: Oils appear on the surface as cell walls break and oils diffuse out.

Monitoring and Controlling the Grilled

To achieve an even, consistent grill, coffee roasters have to monitor and control several variables:

  • Temperature – Increasing heat steadily between 93-224°C depending on grill stage.
  • Heat Transfer – Convection and conduction ensure even heating inside the beans. This is affected by the roaster drum speed. Faster transfers more heat.
  • Air Flow – Moving hot air through the grilling chamber provides oxygen and removes smoke.
  • Time – Typical grill time is 8-20 minutes depending on desired profile.

Roasters often track temperature, time, and color changes to determine grill degree from light to dark. Factors like humidity, altitude, and bean density impact the process. It takes skill and experience to master coffee grilling!

Monitoring and controlling the grill is a crucial process in coffee production. It involves various chemical reactions that take place during grilling. One important reaction is the breakdown of chlorogenic acid, a compound responsible for the acidity in coffee, into quinic and caffeic acids.

Additionally, amino acids react to form aldehydes and ketones, which contribute to the coffee’s aroma. The presence of polysaccharides, such as cellulose, determines the strength of the coffee’s cell walls. This two-part series of reactions also involves the caramelization of sugar molecules, resulting in the rich flavors and dark color of the grilled beans. 

How Roasting Impacts Coffee Flavor & Caffeine

The grilling process profoundly affects the flavor, aroma, and caffeine content of the final coffee seeds:

  • Flavor – The Maillard reaction and caramelization produce the nutty, chocolatey, and other flavor notes.
  • Acidity – Longer grilling reduces acidity as organic acids break down. Light roasts are more acidic.
  • Aroma – Pyrolysis creates aromatic oils and compounds that give each coffee its unique smell.
  • Sweetness – Caramelization of natural sugars adds sweetness especially to darker roasts.
  • Caffeine – Light roasts have slightly more caffeine. Longer grilling destroys a small percentage of the caffeine.
  • Bitterness – Grilling produces bitter flavors that balance the sweetness in dark roasts. Overly dark roasts can become too bitter and burnt tasting.

The skill of the roaster comes from optimizing this balance of flavors based on the inherent qualities of each coffee origin and variety.

How Roasting Impacts Coffee Flavor & Caffeine

Roasting is a critical process that significantly impacts the flavor and caffeine content of coffee. Each type of roasting, whether it be light or dark, for filter or espresso, includes a number of physical changes that take place within the coffee seeds. During roasting, compounds are created that contribute to the distinct flavors and aromas of coffee. In addition, the roasting process also affects the caffeine content, with darker roasts typically having slightly less caffeine than lighter roasts. Therefore, finding the perfect daily grind requires understanding how roasting influences both flavor and caffeine.


What equipment is used for grilling coffee seeds?

Coffee can be roasted in drum roasters, hot air roasters, and tangential roasters. Drum roasters are the most common. They tumble the beans while heating them with gas or electricity. Hot air roasters use a cyclonic column of hot air to circulate the beans. Tangential roasters use a rotating glass chamber heated by gas burners.

How are light, medium, and dark roasts different?

Light roasts are roasted for a shorter time at lower temperatures to preserve more of the bean’s natural flavors. Medium roasts strike a balance between flavor and body. Dark roasts are roasted longer at higher temperatures resulting in a fuller body and robust flavor.

Does the coffee roast affect caffeine levels?

Yes, but only slightly. Light roasts retain more caffeine since less is burned off during the shorter roasting time. But we’re only talking about a difference of 2-5% caffeine loss between light and dark roasts.

Can you slow cool coffee beans after roasting?

Yes, some specialty roasters will slow cool beans over several hours before packing. This can enhance flavor development compared to fast cooling. But it requires precise control and is not practical for large commercial roasting.

What happens if you over grill coffee beans?

Over roasted beans will taste bitter, acidic, and burnt. If taken to extremes, the beans can burn and turn rancid. Proper temperature monitoring is critical to end the roast before overheating begins.

How should you store grilled coffee beans?

Store beans in an airtight container away from air, heat, light, and moisture. Roast date should be marked. Consume within 1-4 weeks for peak freshness. Freezing extends shelf life but can diminish flavor.


That covers the complete journey of the coffee bean during the roasting process! As you can see, roasting transforms and enhances the bean at a chemical and physical level. Mastering this craft takes years of training to expertly manipulate temperature, heat transfer, airflow, drum speed, and time to create the perfect roast. 

With origins like smooth Colombian, bright Ethiopian, and bold Sumatran taking distinct paths from green to brown, the roaster brings out the best qualities in every bean. So enjoy your freshly grilled coffee, brewed to perfection!

As a long-time caffeine enthusiast and industry veteran, I’m always on the lookout for brands that are truly innovating the caffeine experience. That’s why I was so excited when I first discovered The Roasting Plant Coffee – a New York-based caffeine company that has perfected the art and science of caffeine toasting.

Want to read more about coffee beans? Please visit the website:

Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *