What is the Maillard reaction (and why is it important to a coffee roaster)?


First of all, pronunciation: it’s pronounced my-YAR (named after Louis-Camille Maillard, who ‘discovered’ it in 1912). When it’s done right, it leads to brilliant sweetness, when done wrong, you get the bitterness we want to avoid in roasting.

Not Louis-Camille Maillard.

One of the key aspects of roasting coffee is manipulating the stage of the roast where the Maillard reaction (MR) happens. To do so, it helps to understand some of the chemistry behind the process.

The MR isn’t exclusive to the coffee world. It is a key aspect of cooking. You’ll have definitely experienced it at some point, even if you don’t know it. The stage of cooking where things ‘brown’ is the MR, and you’ll see it with all sorts of things – onions, aubergines, steaks and potatoes! If you’ve ever burnt yourself in the kitchen – that’s the MR on your skin too…

The science of it:

The Maillard reaction takes place when sugars (yes coffee and steaks contain sugars) and protein are subjected to heat. To some extent, this is always happening, but you really start to see, smell and taste the effects above around 120℃.

When subjected to high heat, protein breaks down into its building blocks (amino acids), which then react with a group of sugars known as simple sugars. Coffee has some of these amino acids already present without needing to break protein down. The resulting reaction produces an abundance of flavour and aromatic compounds, and it’s what makes coffee so complex and interesting to taste.

While this reaction sounds complex already, it’s also what’s known as an autocatalytic reaction, meaning that the resulting compounds from the MR go on to react again with more proteins and carbohydrates, and make the reaction even stronger.

How it affects coffee specifically:

The molecular weight of the coffee is affected by how long we make the MR in the roast. As a roaster, it’s not just important to get this right and consistent between batches, but it’s one of the most important decisions we make before the roast begins – how can we consider the characteristics of the coffee before it goes through the MR?

The longer we extend the reaction, the more molecular weight we create in the coffee.

If we’re aiming for a more developed coffee the MR is our friend. Stretched out we’ll get a heavier mouthfeel and rich caramel notes. Indeed, the process of making caramel is a masterclass in the MR! Not enough, and there’s no oomph, too much, and you’ve got burnt caramel. Nobody likes that.

If we have a shorter reaction we have less molecular weight. In terms of taste, this is great for coffees that have inherently fruity qualities – getting a light body and pronounced acidity still requires the MR, just in a different way to what you’d first think.

Quality control is often focused on the Maillard recation.

Finding the right level of body is a balancing act, as is choosing mouthfeel versus clarity of flavour & delicateness. It’s something that we keep a close eye on, and we are constantly assessing roasts to see if those little adjustments can make a big difference – they often do.

We track when the MR starts when we see the coffee beans turn a yellowish hue instead of the light green they were at the start of the roast. We stop tracking the MR when first crack happens and from there, we start looking at development time and calculate what’s known as DTR. That, however, is a topic for another post…

I hope you enjoyed nerding out over the Maillard reaction with me!

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