Heating Things Up
An Introduction to Thermodynamics in Coffee Roasting
By Candice Madison
What alchemy turns coffee brown and releases a plethora of flavors and aroma? Heat! Not the steamy heat of a summer night, but the tried and tested application of heat energy, over time, in a controlled environment. This is thermodynamics.
In broad strokes, thermodynamics is the study of the relationships between heat, work, energy and temperature. What we are discussing is the transfer of energy from one place to another and from one form to another.
Coffee roasters must understand how and when to apply heat throughout the roasting process in order to produce the desired flavor profile for a particular coffee.
Defining Thermodynamics Terminology
There are several significant distinctions for roasters to wrap their minds around as early as possible. The first is the difference between heat and thermal energy. It is also important to note the difference between heat transfer and thermodynamics, as well as between heat and temperature. But in order to explain further, let’s take a quick look at a few necessary definitions (all via Wikipedia, britannica.com, and dictionary.com, unless otherwise noted):
- SYSTEM: “A thermodynamic system is a quantity of matter of fixed identity, around which we can draw a boundary. The boundaries may be fixed or moveable. Work or heat can be transferred across the system boundary. Everything outside the boundary is the surroundings.”
- STATE: “For thermodynamics, a thermodynamic state of a system is its condition at a specific time, that is fully identified by values of a suitable set of parameters known as state variables, state parameters or thermodynamic variables.”
- WORK: “In thermodynamics, work performed by a system is energy transferred by the system to its surroundings, by a mechanism through which the system can spontaneously exert macroscopic forces on its surroundings, where those forces, and their external effects, can be measured.”
- ENERGY: “The capacity or power to do work, such as the capacity to move an object (of a given mass) by the application of force. Energy can exist in a variety of forms, such as electrical, mechanical, chemical, thermal, or nuclear, and can be transformed from one form to another.”
- ENTROPY: “Every substance in an equilibrium state has an entropy value that reflects how much internal energy it stores, and how it stores that energy. Entropy change is measured by heating the substance in increments, and summing each added energy increment by the temperature during that heating increment.” (This definition was provided by retired physics professor Harvey S. Leff.)
Now, let’s look at the difference between heat and thermal energy. The difference is that the latter is not in the process of being transferred but remains as part of the internal energy of the system. However, heat describes energy in transit (or energy in the process of being transferred from a hotter system to a cooler one). It is important to note that the flow of energy is always from a higher temperature system to a cooler one. When the two systems reach the same temperature, they are said to be in equilibrium.
This heat transfer can occur in one of three ways—conduction, convection or radiation. In coffee roasting, we mostly concern ourselves with the transfer of heat due to conduction and convection, while acknowledging the unseen role radiation must play when considering the totality of heat transfer.
If heat describes energy in transit, what is thermodynamics? Heat transfer and thermodynamics vary, insofar as thermodynamics is concerned with the amount of heat transfer as a system goes from one equilibrium state to another, while heat transfer describes the length of time it takes for heat to be transferred to or from a system. Thermodynamics is a process, but it is a process that is unconcerned with time as a metric. Heat transfer describes how long the process of thermodynamics takes to occur.
And just before we jump into the roaster, let’s take a look at the difference between heat and temperature. Although in everyday life we use the terms “heat” and “temperature” almost interchangeably, they are not the same thing. Heat, as we have seen, is related to thermal energy; it is measured in increments such as watts, calories or joules. Temperature, however, is a measure of how hot something is, and we measure this in Celsius, Fahrenheit and kelvin.
Heat is the total energy of molecular motion in a substance, and temperature is a measure of the average energy of molecular motion in a substance. The measure of heat energy depends on the speed at which the particles are moving and the number of particles in the system itself, as well as their size or mass. It is also dependent on the type of particles in an object. Temperature does not depend on the size or type of object.
So, for example, the temperature of a mug of water might be the same as the temperature of a tub of water, but the tub is said to contain more heat because it contains more water, and thus more total thermal energy.
In terms of heat transference, heat will raise or lower the temperature of the material or substance in question. If we add heat to a material or system, the temperature will increase. If we remove heat from that material or system, the temperature will decrease. Higher temperatures mean that the molecules are moving, vibrating and rotating with more energy.
Taking two materials that are the same temperature and bringing them into contact with one another will not result in an overall transfer of energy between them because the average energies of the particles in each object are the same. However, if the temperature of one of the materials is higher than the other, there will be a transfer of energy from the hotter material to that of the colder, until both materials reach the same temperature.
“It took me so long to understand why a smaller batch and faster roast would yield such a high drum-retaining temperature in comparison to a big batch size and longer roast. I began to apply [the concept of] thermal energy to roasting, instead of just thinking about it as heat or a number.”
-Izi Aspera, roaster, Wrecking Ball Coffee Roasters
Thermodynamics of Coffee Roasting
Now that we have some of the basic concepts under our belts, we can start to see what they have to do with roasting coffee. Coffee roasting is a thermodynamic process, in which the application of gas or amperage to energize the heat process, and the resulting uptake of that heat (heat transfer) by the beans directly impacts the flavor profile of the coffee being roasted.
In other words, how a roaster applies the heat to a batch of coffee changes over time and, in turn, creates a particular flavor profile. A roaster’s ability to understand and manipulate the heat transfer process is gained through learning and experience, and it affords each roaster a great deal of mastery over this thermodynamic process.
As we talk about the roasting process, we can consider the individual bean, the batch of beans, and the roasting machine itself as distinct systems with boundaries. This helps when talking about the transfer of heat from one system to another.
If we consider the roast drum (or static chamber—roasters, as we know, have evolved; I have retained some terminology for ease of reading) and the beans, the rate at which heat is transferred from the environment to the system depends on the amount of thermal energy present in the environment and the amount of heat necessary to raise the temperature in the system (heat capacity).
The conditions under which heat is applied are affected by environmental factors, including drum capacity, airflow and humidity, barometric pressure, and ambient temperature.
The ability of the bean to take on heat and the rate at which the absorbed heat dissipates throughout the bean are dependent on the metrics of the green coffee, including moisture percentage, density, bean size and—to a degree—water activity.
In the case of coffee roasting, the heat initially moves from the roasting environment into the green bean. This is referred to as an endothermic reaction, when a system absorbs (heat) energy from its surroundings.
“As roasters, we can go years knowing what actions to take as we roast and what results they will create, without knowing why these actions have the effect they do. The explication of the major forces we interact with and manipulate as roasters creates a foundation for understanding the cause and effect within our heat application processes.”
-RJ Joseph, cupper and content strategist, Red Fox Coffee Merchants
As specific chemical processes and reactions take place within the bean, the endothermic process gradually becomes more and more exothermic: the system begins releasing heat into the environment and contributing to the temperature changes in the surroundings.
The chemical changes taking place at any point during a roast depend upon the amount of heat the batch of coffee has already absorbed, the amount of heat available in the roaster, the ability of the mass of beans to conduct heat (in relation to the physical state of the coffee, i.e. those green metrics), and the reactions that have already taken place or those in process.
To put all of this together when considering a roast from start to finish, let’s imagine a generic roast curve. The roaster is heated to and idled around a drop temperature of 370 degrees F (188 degrees C), and the turning point is achieved in a commensurate amount of time for the batch size and composition (blend versus single origin, for example). What occurs next is a (hopefully) steady ascent through stage one of the roast to stage two—often referred to as the Maillard or coloring stage—with significant color and physical changes to the bean. This will be followed by stage three, a resultant, timely first crack, and a desirable post-crack development that results in the “Goldilocks zone” of roasting—neither underdeveloped nor overdeveloped, but just right.
These are stage markers and observable results of the chemical processes taking place during the roast, which are familiar to us as roasters. But now that we’re thermodynamics students, we should consider these changes in light of the Second Law of Thermodynamics.
The Second Law of Thermodynamics posits that the state of entropy of the entire universe, as an isolated system, will always increase over time. It also declares that the changes in the entropy in the universe can never be negative. Essentially, the total entropy of an isolated system can never decrease over time. A thermodynamic driving force (heat, in this case) flows in one direction, from a system or environment that is hot to a system or environment that is cooler.
An example of this is that there is no spontaneous transfer of heat from cold to hot. Say I was to leave a glass of water out at room temperature on a warm day. If I put ice in that glass, after a while the glass (surroundings) would transfer heat to the ice (system), changing its physical properties and melting it in the glass. Once equilibrium has been reached, there would be no ice left in the glass. However, if I put that glass in the same room under the same conditions, ice would not form spontaneously. Even if I added ice to the water, the water (surroundings) wouldn’t turn to ice. The Second Law of Thermodynamics states, in effect, the transfer of heat is always from the warmer temperature medium to the colder one. Always.
Knowing this, we understand that adding cold beans to a drum will make the bean thermocouple record plummeting temperatures. But around 90 seconds into the roast, the closed environment of the roaster and the beans achieve equilibrium.
When the temperature difference between two entities is large, the rate of change of temperatures is greater than when the temperature difference is less. For example, if I put an ice cube in a glass of hot water, it will melt much faster than if I put it in a glass of cold water. Coffee roasters use this delta (or difference between values) to understand whether their roast is going too fast or too slow, and for myriad other reasons.
Breaking Down Thermodynamics By Roast Stage
Stage One: Drying
In order to brown the beans, they must be dry, or—at the very least—drier than they started off. At this early stage of the roast, a great deal of the thermal energy in the drum is required for evaporation, but little of this energy is being used for roasting reactions just yet. The “drying stage” is like an overture to the main event, and the amount of time it takes has a great deal of influence upon the temperature difference between the exterior and interior of each individual bean.
As the bean’s surface moisture is vaporized, the first cell walls begin to collapse and some of the internal moisture of the coffee bean warms, in turn heating the interior of the bean. Roasters will observe this by noting the expansion in size and a change of color to the bean. The state variables—the green metrics of the beans—influence the rate at which this takes place. Therefore, conductivity of the system is a function of the physical state, or, in other words, the metrics of the green coffee have a direct impact on the rate of heat transfer.
How much the bean increases in volume is a sign of how quickly the bean is taking on heat. This is partially dependent on the amount of moisture in the bean, which in specialty coffee typically varies from 10-12 percent at the point of import. The drier the bean, the faster the heat uptake.
During stage one of the roasting process, few observable chemical changes are taking place, but this stage is important in the development of aroma precursors, hence the reason some roasters refer to it as the enzymatic stage.
What occurs during this stage—either due to roaster operator manipulation or the way the coffee itself reacts to being roasted—is also important in determining how quickly the absorbed heat will distribute throughout the bean.
It is possible to overheat the beans at this stage, burning and charring the outside, yet not allowing a gradual enough heat penetration from the surroundings into the system. This defect is called scorching, as scorch marks are evident on the still-green beans. It occurs when the drum is too hot, via conduction when a bean comes into contact with the surface of the drum, the paddles, or the surface of another green bean in the pile.
But, if not enough heat is applied and/or the batch is too large, the beans will not heat evenly. These conditions result in insufficient hot air circulation, and as a result there won’t be enough of the faster-moving molecules in the environment to agitate those in the system and create a steady uptake of heat. The roaster will be left with either a stalled batch (which doesn’t ascend the roast curve at a standard rate of change/rise), or unevenly roasted beans, all of which may crack, but at different times.
Stage Two: Sugar-Browning Reactions
After a few minutes, and with enough applied heat as well as the evaporation of the surface moisture, the sugar browning chemical changes of coffee roasting begin to occur.
At this stage the coffee will turn from light green to yellow, followed by a cinnamon color. These sugar browning reactions (which include Maillard reactions and caramelization, among others) are what move the thermodynamic process incrementally from an endothermic reaction to an exothermic reaction.
The first of these chemical change processes begins around 320 degrees F (160 degrees C), called the Maillard phase. It is a non-pyrolytic reaction between amino acids and reducing sugars, first observed by Louis-Camille Maillard in 1912.
As the temperature in the roaster increases during this stage, caramelization of the sugars occurs. This results from the oxidation of simple sugars such as glucose and sucrose.
The change in the internal heat energy of the batch, or system, during the Maillard stage is not only due to the heat entering the drum; it is also due to the heat being created by the sugar browning reactions themselves.
As they take place, the sugar browning reactions develop momentum, increasingly cascading one after the other. Around 347 degrees F (175 degrees C), the sugars begin to give off heat (exotherm), and depend less on the heat in the environment as the roast progresses, and more on the heat the system (or batch) has already taken on and is producing itself.
At this point, the rate at which caramelization happens and the amount of sugars involved is determined by the concentration and type of sugars present in the bean. For example, higher-quality coffees have a higher concentration of sugar than lower-quality coffees, and past-crop coffees tend to have less sucrose but more glucose present.
Stage two of the roast process, or the Maillard/sugar browning stage, is one of the most important phases of roasting. During this stage, chemical reactions emerge as a result of the amount of heat taken on and sugar browning reactions take place.
It is important to avoid excessive temperatures and heat applications at this point, since the bean can only take on heat at a certain rate and still taste well-roasted and properly developed.
On the other hand, if the surrounding (drum) temperature is too low, the heat within the bean will dissipate, halting the roasting reactions and stalling the roast itself.
Apart from color, aroma is incredibly important to note as the coffee develops. As the coffee moves from yellow- to cinnamon-colored, chemical changes turn a sourish, greenish aroma to a sweet, yeasty, baking aroma. Finally, the more familiar aroma of fresh-roasted coffee begins to develop at around 356 to 374 degrees F (180 to 190 degrees C).
Stage Three: First Crack
All these reactions occur as the steam pressure builds up within the beans and tries to escape the cellulose walls. Eventually, this steam has nowhere to go but out! The steam pressure eventually grows too strong for the weakened cellulose walls, which rupture under the pressure of the escaping vapor—a phenomenon known to roasters as “first crack.” As the cellulose structure is weakest at either end of the bean, along the seam, evidence of first crack can be seen as fissures at one, if not both, of its ends.
At first crack, the coffee becomes truly exothermic. While the sugar browning reactions take place, the system—in this case, the bean—is generating its own heat as well as absorbing heat. So, at first crack, the bean has already absorbed the potential heat energy that will produce those reactions.
First crack not only releases all the built-up heat that the beans have been gathering over the course of the roast, it also heralds the transfer of considerable mass from the beans into the drum in the form of moisture, carbon dioxide and other gases.
The amount of exothermic energy released by the beans not only depends on the green coffee metrics of that particular coffee, it also depends on the location, the temperature, the reaction rate, and the reaction enthalpy (heat loss as the result of the reaction). Additionally, the exothermic energies being released into the drum will increase the temperature of the coffee without any additional heat from the roaster’s source.
At this point, the coffee has lost a lot of its mass and most of its moisture. Because of this, the thermodynamic interaction between the batch of coffee (the system) and the surrounding environment (the drum) will now be different. As the system has less mass, it is able to take on heat more quickly.
However, because the batch is now far less dense and has lost most of its moisture, it can also lose heat more quickly, depending on how heat has been applied previously, as well as during and after first crack.
Experienced roasters will have seen this particular phenomenon: Some batches begin to take on excessive amounts of heat after first crack and can become uncontrollable through normal roaster operations. But others, quite counterintuitively, stall if you try to reduce the heat to counteract the temperature spike caused by cracking coffee.
Under the roaster’s control, the batch of coffee stabilizes briefly after first crack, and—having exhausted most of the exothermic reactants—becomes endothermic once more for a short while, climbing in temperature. At approximately 428 degrees F (220 degrees C), the batch will enter the phase known as “second crack” with the expulsion of carbon dioxide (although that expulsion is quite soft and sounds more like a snap, crackle and pop than like corn kernels exploding in the microwave).
Due to the degradation of cellulose—the result of which is expansion and lost mass—the beans are at their most fragile. Because of this density loss, the beans will be more prone to heat uptake and, as a result, more prone to carbonization after both cracks have occurred, which leads to roasty notes.
There is always more to say about coffee roasting, and coffee in general, but it is here that I leave you to reflect on the concepts discussed in this article and consider the importance of applying them in the roasting process. Developing a thorough understanding of the science behind how heat is transferred in the roasting machine, and the impact of this on the final coffee, is critical to ensuring that the roaster operator is in control of the machine—and not the other way around.
I give my grateful thanks to NASA and Paul Songer, among others, whose clear writings and thorough explanations helped me to roast better, as well as to write this article. We stand on those giants’ shoulders, which enables us to deliver better coffee to our customers and for ourselves—and we should be exceptionally grateful for their work.