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NOVEMBER | DECEMBER 2020 4 3

HEATING

THINGS

U P

A n I n t r od u c t i o n to T h e r m od y n a m i c s

in Coffee Roasting

by Candice Madison

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W H A T A L C H E M Y T U R N S C O F F E E

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.

D E F I N I N G T H E R M O DY N A M I C S

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.”

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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

THE FOUR LAWS of

THERMODYNAMICS

—

T H E Z E R O T H L A W of thermodynamics

states that if two systems are in thermodynamic

equilibrium with a third system, the two original

systems are in thermal equilibrium with each

other. Basically, if system A is in thermal

equilibrium with system C and system B is also

in thermal equilibrium with system C, system A

and system B are in thermal equilibrium with each

other.

THE FIRST LAW of thermodynamics

states that energy can be converted from one form

to another with the interaction of heat, work and

internal energy, but it cannot be created nor can

it be destroyed, under any circumstances.

THE SECOND LAW of thermodynamics

states that the state of entropy of the entire

universe, as an isolated system, will always

increase over time. The second law also states

that the changes in the entropy in the universe

can never be negative.

THE THIRD LAW of thermodynamics

will essentially allow us to quality the absolute

amplitude of entropies. It says that when we are

considering a totally perfect (100 percent pure)

crystalline structure, at absolute zero (zero

kelvins), it will have no entropy. If the system is

not pure, (i.e., it is a mixture), there is an “entropy

of mixing” and thus a non-zero entropy.

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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

HEATING THINGS UP

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

“

IT TOOK ME so long to understand why a smaller batch and faster roast would yield such 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