Arrhenius

Activated Roasting

Brazil nut effect
Transforming green beans into the coffee we all recognise. Maillard reactions are behind some of the chemistry involved in coffee roasting. But how can we determine how fast a reaction will occur?

Coffee roasting is a complex process involving chemistry, physics and art. The experience and skill of the roaster turns the unpromising looking green beans into fragrant coffee beans that we can appreciate. Activated by the heat, many chemical processes occur as the aromatic volatiles are formed, compounds in the bean are transformed and the bean changes colour to that deep brown appearance with the smell that we associate with coffee. One of these processes are the Maillard reactions.

Maillard reactions transform “reducing sugars” such as glucose and fructose into the browning melanoidins (via a couple of intermediary steps). They are responsible not just for the colour and aroma of coffee, but also for the crust of a freshly baked loaf of bread, the transformation of a steak or just browned (not caramelised) onions and all manner of culinary processes. In coffee, the Maillard reactions usually start to become noticeable above 140C. At higher temperatures you also get caramelisation. But even at room temperature, or at body temperature, some Maillard reactions occur, just very slowly. Maillard reactions have even been implicated in the formation of certain cataracts. What is it that determines how fast the Maillard reactions occur?

The rate at which a chemical reaction takes place is determined by an energy known as the activation energy. The activation energy is the energy that the molecules would need to overcome in order to react together. It may be the result of having to overcome a repulsion between the molecules getting close together, or it may describe an energy needed to transfer electrons from one chemical to the next. Molecules can gain this energy from heat which means that at higher temperatures, more molecules have the energy for the reactions to occur. We could rephrase this to say that the rate of the reaction is greater at higher temperatures. This is expressed mathematically with the Arrhenius relation. In a fantastic illustration of the connectedness of things, this same Arrhenius relation can be used to describe many other phenomena such as how fast water evaporates from a coffee cup, how quickly milk goes off and even how long semiconducting devices will last before failing.

The Arrhenius equation also describes how quickly steam will evaporate from a coffee cup. As you can see above the cup here at Carbon Kopi

Although the reactions are faster at higher temperatures, there is no defined temperature below which they stop. Instead, the rate just decreases to such a point that the reactions happen rarely. Perhaps you could observe some of the chemical changes of roasting coffee at room temperature if you waited long enough. But before that point, other reactions with lower activation energies would occur or fungal growth may happen that would turn the beans rancid. Best to follow the roasting recipes.

Yet for coffee there is an additional complication before the Maillard reactions can happen. Unlike the situation where all the chemicals are together and able to react, the chemicals in the coffee bean exist within a structure. The molecules are not necessarily in the same place as each other; they need to move across the bean, including perhaps through the cell walls. And as the bean is heated, there are structural transitions that make it easier (in some cases) and harder (in others) for the chemicals to meet each other in order to react. What exactly happens when coffee is roasted?

To track what was going on Loong-Tak Lim and colleagues at the University of Guelph looked at how parameters such as the lightness of the roast or the weight of the bean varied as a function of roasting time. They roasted a lot of (small batch) coffee. Impressively, they also managed to put a thermometer right into the middle of a green coffee bean to track the temperature of the interior of the bean rather than the atmosphere in the roaster. The unfortunate detail was that they had to glue the thermometer in place.

Roasting coffee at four temperatures (220, 230, 240 and 250C), they showed how the degree of roast (indicated by the lightness of the bean) varied with roasting time and temperature. Unsurprisingly, a higher roast temperature produced a darker roast more quickly. But there were surprises too.

When they plotted the lightness of the roast as a function of time, they saw not one reaction with one activation energy but two. The two regions were quite distinct indicating that something chemically significant happened to the roasting process at around the point indicated by a “medium” roast. The activation energy of the first stage was 59.7 kJ/mol while the second stage had an activation energy of 170.2 kJ/mol. Whereas the first stage was over pretty quickly, the higher activation energy of the second stage meant that it happened far more slowly.

Don’t they look great? Roasting coffee connects to a vast range of concepts in physics and chemistry. Perhaps now is just a time to appreciate them.

The same sort of two step process was seen when they looked at how much mass the bean was losing as it was roasted. A lot of mass was lost early in the roast but as the roast degree went on, so the reaction slowed.

What caused the rapid slowing down of the second stage? One of the suggestions was that it was associated with the moisture loss as the green beans dried. A second suggestion was that a structural transition in the bean (of which there are many at these temperatures) hindered the reaction dynamics. This highlights a difference between coffee roasting in the lab and in the cafe. In the lab, the beans were rapidly heated to a set temperature at which they were held until the end of the experimental roasting time. In contrast, to produce great tasting coffee, many roasters will tweak the temperature-time profile of the roast so that a lot of the drying occurs before the Maillard reactions are allowed to ramp up. In a sense, the science is behind the experience here. To find out what is going on most parameters have to be kept constant while only one or two are varied. It doesn’t make great coffee but, hopefully, it is the start of a journey to understanding what is really happening as the beans seem to magically transform into something we can drink.

Meanwhile for those of us who neither roast nor experiment with the coffee but rather just enjoy the results of other people’s work, we can admire the connections that are being illustrated through working out what exactly happens as we roast the coffee. From the vastly disparate subjects covered by the Arrhenius equation, to the fact that the structural transitions that affect the coffee roast also occur in ceramics and magnetic materials. You will often hear it said that “everything is connected”. For coffee at least, this is yet another case where that appears to be true.