My favourite part of doing a roasting course was watching green coffee beans turn into a caramel, chocolate-brown colour through a little viewing window while their powerful aroma filled the room.
Over a couple of short minutes, thousands of chemical reactions take place together with irreversible physical changes that allow us to extract the aroma and flavour locked inside beans.
Let’s look deeper inside a bean to see exactly how this transformation happens on a microscopic level.
Microscopes normally use the power of light beams to magnify an object of interest. In the case of coffee, optical microscopes allow us to appreciate grind shape and size, or have a closer look at the surface of a coffee bean. Image 1 was taken using an USB microscope at 400x magnification. Look closely. In this image you can see the centre of a roasted Arabica beans with a bit of the silverskin visible.
To dive deeper, and really zoom into the plant cell level, we need a more powerful tool. Electron microscopy is designed to give a real close-up of the material. Instead of light beams, it uses electron beams to create an image, which can reveal details down to the nanometre (nm) range.
Just to appreciate how small a nanometre is, think of these few examples: a nanometre is how much your nails grow in just one second. It’s a million times smaller than a millimetre, getting pretty close to the size of atoms. An atom is about a tenth of a nanometre across. This level of magnification gives us a sensational view of the bean’s internal structure.
Let’s take a step back and take a quick look at what a plant cell looks like in general. In many ways, they are similar to an animal cell, with a couple of important differences.
Both plant and animal cells have a cell membrane, where most of the lipids (oils) are found within the cell, including most of the oils that give great mouthfeel to a well-extracted espresso. On top of this, plant cells have a rigid cell wall, made of cellulose and a couple of other carbohydrates. The cell walls serve as a ‘plant skeleton’, forming a very solid structure that can hold up even a tall tree. If we extract around 20 per cent of bean solids into an espresso, most of the 80 per cent left behind is precisely this cellulose framework.
We can also notice that the plant cell has a big empty ‘bag’ in the middle called a permanent vacuole. There is usually one very large vacuole, which can take up more than half of the cell’s volume. The vacuole holds water or nutrients such as proteins, fats and carbohydrates, which from our point of view, are key flavour compounds.
In the case of a green coffee bean, the cell wall is about 10 micrometres thick, the cell itself is approximately 20 to 30 micrometres in cross section, and the vacuole takes up about half the cell volume. The presence of vacuoles gives the bean a structure a bit like Swiss cheese, and indicates that we are dealing with a porous material.
What happens during roasting?
The change in plant cell structure during roasting has been a subject of a number of scientific studies, so we have some knowledge of what happens at a structural level. For example, a series of scanning electron microscopy (SEM) images taken by Niya Wang at University of Guelph in Canada shows changes in the coffee bean’s cell structure during the roasting process.
Here, we can quite clearly see how the pores expand during first crack, then open up even more at second crack. The pressure increases inside the pores (due to the build-up of gases such as carbon dioxide, steam, and volatile organics) and causes the cells to expand, a bit like blowing up a balloon. The cell walls then compress against each other, and eventually rupture. This breaking down of barriers between cells has obvious importance for how water infiltrates the bean while brewing.
According to a paper by Redgwell et al, roasting breaks down about 12 to 40 per cent of the structural carbohydrates. While cellulose is remarkably stable and shows almost no degradation, other polysaccharides become soluble as they degrade, contributing to the taste and viscosity of the coffee beverage.
Besides the micron-scale porosity, plants also contain an extensive network of nano-scale pores called plamodesmata between cells (just like there’s usually a series of narrow alleyways connecting two city blocks). Neighbouring cells use these tiny channels to communicate with each other and share nutrients. As author Schenker et al shows in the paper “Pore Structure of Coffee Beans Affected by Roasting Conditions” this network of tiny pores also affects the ageing of the roasted bean because it allows oils to emigrate to the surface. As these oils come into contact with air, they start to oxidise, developing rancidity. It is one of the reasons it is important to keep roasted beans away from oxygen as much as possible.
How does grinding affect cell structure?
Image 4 below shows an SEM image of ground coffee. This sample was not frozen or cross-sectioned as in some of the above images, so reveals grinds in their natural state.
The cell structure is mostly shattered to sharp shreds in the grinder, but in this case, the cell wall structure is still visible. Breaking up these walls exposes the inside of the cells, where many of the soluble materials are stored.
As we have learned from looking at the plant cell structure, many flavour compounds such as amino acids, sugars, and fats are stored in the vacuoles, or in the cell membrane (such as oils). With the cell wall broken, we can extract these materials more easily and enjoy their contribution to the overall taste experience.
Dr. Monika Fekete is the Founder of Australian Coffee Science Lab.