What Happens During Coffee Roasting: The Physical Changes

Roasting transforms coffee from a green seed to the aromatic, flavorful bean we love. But what happens during the process?

When we roast coffee beans, they undergo physical and chemical transformations. In this first part of a two-part series, learn more about the physical changes that take place inside the roaster.

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Roaster checking freshly roasted coffee beans. Credit: Devon Barker

The Importance of The Physical Structure

The anatomy of a coffee bean is essential to creating the delicious roasted end product we want. Without the specific physical structure, the chemical reactions essential for flavor and aroma wouldn’t take place.

In The Craft and Science of Coffee, Britta Folmer states that “ground green coffee powder exposed to similar temperature histories as in bean roasting does not produce the desired flavor compounds. The intact bean acts as an essential ‘minireactor’ for the chemical reactions. It controls the reaction environment in a way that the right precursors can react with each other in the right sequence.”

Green coffee beans being prepared for roasting. Credit: Neil Soque

Dramatic Changes

Green coffee beans are dense and compact seed structures. But when you introduce them to the roaster, their native form is completely changed. Let’s take a look at what physical changes take place.

Color

Perhaps the most obvious change that happens during roasting is in color. Before roasting, coffee beans are blue-green. They change to brown because of the production of melanoidins. These are polymers that form when sugars and amino acids combine under heat. Chaff, or silverskin, will also come off during roasting. This is the papery outer layer of the coffee bean.

Roasters and consumers use the level of color as a parameter of quality and profile.

Learn more in A Home Roaster’s Dictionary, From First Crack to Silverskin

A tray of green coffee beans. Credit: Julio Guevara

Moisture & Mass

Water makes up around 10–12% of processed and dried green beans, but roasting reduces this to around 2.5%. As well as water that is already present in the green beans, additional water is created by chemical reactions. However, this is vaporized during roasting.

The loss of moisture and the transformation of some dry matter into gases is why beans have a reduced overall mass after roasting. On average, beans lose 12–20% of their weight. Roasters often keep track of percent weight loss to help identify which batches might deserve extra scrutiny in quality assurance.

Different roasting profiles will influence when dehydration takes place. Changes in water activity at different points of the roast can mean a difference in chemical reactions and this may have an impact on final profile.

Learn more in Effective Data Collection for Coffee Roasters

Freshly roasted coffee cool down.

Volume & Porosity

Coffee beans have some of the strongest cell walls in the plant kingdom. They have external rings that reinforce the cell, increasing its stiffness and strength.

When coffee is roasted, the increased temperature and transformation of water into gas create high levels of pressure inside the beans. These conditions change the structure of the cell walls from rigid to rubbery. This happens because of the presence of polysaccharides (bonded sugar molecules).

The internal matter pushes out towards the cell walls, leaving a gas-filled void in the center. This means that the beans expand in volume as they decline in mass. Much of the gas build-up is carbon dioxide that will be released after the roast.

Roasting also increases porosity, making the beans less dense and much more soluble. This, of course, is vital to making them into a delicious drink.

Roasted coffee bean.

Oils

Coffee beans contain lipids, or oils. During roasting, the high internal pressure causes these compounds to migrate from the center of the cell towards the surface of the beans.

The lipids help to keep volatile compounds inside the cell. Volatile compounds are chemicals that have a high vapor pressure at room temperature, some of which are essential to creating the flavor and aroma of coffee. Without the oils, they might disperse.

Dark roasted coffee beans.

The longer the roast, the more pronounced the structural transformations. Bean density decreases continuously, more gases are developed as time passes, and in a very dark roast, you may see oil migrate to the surface of the beans.

These developments go some way to explaining why a dark-roasted coffee tastes different to a light-roasted one, but there are also significant chemical transformations that impact profile. We’ll take a look at those in part two.

Find out more in Coffee Science: Breaking Down Where Flavor Comes From

Inside a coffee roastery.

What Happens When: The Stages of Roasting

Different roast profiles have an impact on the final taste, aroma, and mouthfeel of your coffee. This is because there are different opportunities for chemical reactions to take place.

But whatever roast profile you choose, there are three main phases: drying, browning or the Maillard Reaction, and development. These popular terms actually describe different stages of chemical and physical changes.

Drying

The drying phase starts immediately after the turning point. (When you add cold beans to a roaster, the heat inside the machine falls before rising again. The point at which it begins to rise is the turning point.) During drying, water content begins to vaporize and pressure starts to build inside the beans.

A sample of roasted beans.

The Maillard Reaction

When you see the coffee beans start to turn brown, the Maillard Reaction has started. This happens at around 150oC/302oF. During the Maillard Reaction, gases including carbon dioxide, water vapor, and some volatile compounds are created. The internal pressure increases enough to break the cell walls of the beans, making a pop. This event is known as first crack.

Melanoids begin to develop during this stage. As well as changing the color of the beans, they contribute to the final mouthfeel of the coffee.

Development

After first crack, the roast changes from an endothermic reaction (the beans absorb heat from the drum) to an exothermic one (the beans release heat). During this stage, the physical transformations continue – beans increase in porosity, oils migrate to the walls of the cell, and the color darkens. A lot of chemical reactions also take place during this stage, which we’ll discuss in part two.

A roaster inspects a fresh batch. Credit: Devon Barker

Although it may seem straightforward, coffee roasting is a complicated process that includes a number of physical and chemical transformations. All of them are made possible because of the unique structure of the coffee seed.

So next time you empty the roaster or buy a bag of coffee, stop to consider the fascinating processes that each bean has been through to get there.

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Written by Verônica Belchior.

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