Art verses Science in Coffee Roasting

"As an open space where our craft is always on display, we often find ourselves in conversations about the process. So we thought it’s time to share more about the inner workings of running a coffee roastery—from the perspective of the roaster."

Science

Coffee roasting is both a science and an art, transforming the humble seed of a tropical plant into a complex, aromatic, and flavorful beverage enjoyed across the globe. At its core, roasting is the process of applying heat to green coffee beans to extract sugars, acids, and aromatic compounds. The journey from raw bean to roasted coffee is marked by a series of distinct stages, each contributing unique characteristics to the final cup. 

A skilled coffee roaster must deeply understand the thermal dynamics of coffee seeds, which differ from bean to bean in terms of density of cellular structure. 

A better way of understanding the process is more easily understood if you consider that you don't really roast the seeds. You evaporate the free moisture in the beans and then caramelise them. 

This process includes balancing three types of heat:

• Conductive (heat from touching the steel), 
• Convective (heat from the hot air in the oven mixed with the speed of the fan) 
• Radiant heat (heat generated from the steel and later during the roast heat from each bean)

All this happens while carefully managing variables such as drum speed, airflow, and charge temperature (oven temp to set before commencing the roast).

Environmental conditions—like ambient temperature, humidity, and atmospheric pressure—also impact combustion and heat transfer. Roasters must take these into account, making subtle adjustments to gas and airflow well in advance of key temperature milestones.

Mastery in this area ensures the ability to fine-tune development time, preserve acidity and sweetness, and consistently highlight the unique character of each coffee.

The whole process ultimately involves fives stages:

1. Drying Stage

Green coffee beans typically contain between 7% and 11% moisture. The first task of the roaster is to remove this moisture by gently heating the beans. During the drying phase, it is essential that the roaster has enough thermal mass to begin evaporating the water content without scorching the beans. This stage is foundational, preparing the beans for the more transformative reactions that follow. Though visually subtle, this phase is critical for setting up a smooth and even roast.

2. Yellowing Stage

As the beans continue to absorb heat, they transition into the yellowing stage. Here, they begin to emit a scent reminiscent of baking bread. Visually, they take on a pale yellow hue and start to expand slightly. It’s at this point that the outer layer, known as the silverskin, flakes off and becomes what is known as “chaff.” The development of colour and aroma indicates the start of significant chemical changes within the bean.

The roaster must during this stage, which lasts for six minutes to 11 minutes, the average rate of temperature rise must decline to maximise aromatic compounds. 

3. First Crack

The first crack is a pivotal moment in the roast. As heat continues to build, the moisture inside the beans turns to steam and carbon dioxide accumulates. Eventually, the pressure causes the beans to audibly crack open—similar to the sound of popcorn popping. The intensity of this crack can vary depending on the roasting approach; a rapid application of heat results in a loud, sharp crack, while a more gradual rise may produce a softer, more drawn-out sound.

It’s at this stage that the Maillard reaction begins—a complex series of chemical changes that caramelise sugars and develop the sweet, rich flavours associated with roasted coffee. This same reaction gives browned foods like seared steak their depth of flavour. Managing this phase is crucial, as it lays the foundation for balance between sweetness, acidity, and body.

4. Roast Development

Following the first crack, the beans enter the roast development stage. At temperatures above 200°C, the roast begins to deepen. Here, the roaster must carefully choose how long to continue the process. Too short, and the flavours may be underdeveloped (taste like grass); too long, and the brightness and acidity may be “baked out,” leading to a flat cup profile.

Roasters track this phase as a percentage of total roast time—commonly between 8% and 20%. The ideal duration depends on factors like the coffee's density, origin, desired flavor profile, and intended brewing method (espresso vs. filter). This is the stage where a roaster's skill and intuition truly shine, balancing time, temperature, and taste.

5. Second Crack

Beyond the development stage lies the second crack—a point where few specialty roasters dare to venture. At this stage, the beans become extremely dark, brittle, and bitter. The cell structure begins to break down significantly, releasing oils to the surface and resulting in a shiny appearance. If the roast is pushed too far, the beans can combust if the drum stops spinning, due to their flammable nature. While some commercial coffees are roasted this dark, most specialty roasters avoid it, preferring to preserve the nuanced flavours of the bean rather than mask them with char.

Art

Coffee roasting is a meticulous process that transforms green coffee beans into the rich, aromatic beverage that so many of us rely on daily. Each stage—from drying to roast development—requires attention, control, and a deep understanding of how heat and time influence flavour. 

Whether you're sipping a bright, floral filter coffee or a bold, chocolatey smooth espresso that resembles the texture of eating a fine chewing caramel dipped in chocolate, every cup is the result of a carefully crafted roasting journey of building a flavour profile of each coffee and blending coffees together to become one.

Roasting coffee and cooking might seem like two separate crafts, but they’re deeply connected. A good fundamental understanding of cooking makes a big difference in roasting because both are all about controlling heat, time, and the transformation of raw ingredients into something flavorful. Here's how that plays out:

Heat Control

Just like searing a steak or caramelising onions, roasting coffee is all about managing heat precisely:

You need to know when to go hot and fast vs. low and slow.

Too much heat, too soon? You burn the outside before the inside’s ready—just like a burnt pancake with raw batter inside.

Timing is Everything

In both cooking and roasting, there’s a perfect window when flavours are at their peak, with coffee:

• Undercooked = grassy or sour coffee.
• Overcooked = burnt, flat, or ashy flavours.
• In the kitchen or the roaster, timing is about knowing when to stop—not too early, not too late.
  

 Maillard Reaction & Caramelisation

If you’ve ever browned butter or toasted bread, you’ve seen these processes in action:

• The Maillard reaction (complex browning) creates deep flavours in both coffee and cooked food.
• The art is basically managing these reactions to develop chocolatey, nutty, fruity, or spicy notes.

Using Your Senses

Cooks constantly smell, listen, and look—and so do good roasters:

• Smell tells you if you’re progressing or burning.
• Sound (like first crack and second crack in coffee) signals key stages.
• Sight helps monitor colour development, like browning butter or roasting veggies.
  
  

Understanding Ingredient Potential

Good cooks know how to bring out the best in ingredients. Similarly:

• Roasters need to know the potential of a bean because once picked, the potential flavour can only diminish.
• It’s like knowing how a good tomato can shine with the right treatment.
  
  

 Balancing Art and Science

In both worlds, you need a base of science and chemistry (temps, reactions, times) but also a clear understanding of what you're trying to achieve in terms of flavour.