Part 1: Understanding the roast process….Heat vs. Mass

“As simple as boiling a pot of water.”

One of the most common subjects we teach new roasters about is understanding Heat vs. Mass.  A common approach for folks excited to start producing coffee on their new roasting toys within minutes of the roaster being assembled, is to throw a pound of coffee inside, crank up the temperature and roast away until something happens.  More often than not, it results in something drinkable, but not necessarily exceptional, and in some cases a fire whence the coffee has gotten too hot and catches flame.

In order to get the whole process under precise and predictable control, I find it necessary to go back to basics on something that we all understand very well.

 

So to begin, I want to take us through a few scenarios, and if you will be patient through the end of this series, you will understand the whole process with mastery.

Let’s have a look at a pot of boiling water on the stove.  Something that we are closely familiar with, whether we boil water for cooking or making a pot of tea.

Let’s have a look at our experiment and the variables:

Variables:

Mass to be heated: Amount of Water and the pot itself.

Heat: The amount of heat coming from the stove.

The conditions for all experiments: lukewarm water at “X” temperature and the room at “X” temperature.

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Experiment #1.  How long does it take water to boil? (1 cup, medium heat)

1.  Take one random pot from your cupboard, and place the pot on your stove, and set the following conditions.

  • Conditions: Room and Pot at room temperature.
  • Heat Setting: (On a scale of 1-9 with 1 being low and 9 being high) :  5 (medium heat)
  • Place 1 cup of lukewarm water in the pot.

Discussion:  Under these conditions, let’s say for the sake of our discussion that 1 cup of water at medium heat will take 5 minutes to reach boiling.  You might say, “well,  my stove boils in 8 minutes or 3 minutes.”  Maybe so, but, that point is not relevant for this discussion, let’s just assume for the moment that 5 minutes, is what it takes 1 cup to boil under medium heat in this experiment.

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Experiment #2.  How long does it take water to boil? (1 cup, high heat)

1.  Take one random pot from your cupboard, and place the pot on your stove, and set the following conditions.

  • Conditions: Room and Pot at room temperature.
  • Heat Setting: (On a scale of 1-9 with 1 being low and 9 being high) :  9 (high heat)
  • Place 1 cup of lukewarm water in the pot.

Discussion:  Now, in this repeated experiment, nothing has changed except for the heat setting.  Here we have 1 cup of water in the same pot at the same temperature, and the only thing we change is we move the heat to 100%, high setting.  If that same water at medium boiled in 5 minutes, we would expect this high setting to boil that same water in (let’s take a guess) say 2 minutes.  The point isn’t really in the exact numbers, it’s understanding that adding more heat to a given mass (pot+1 cup of water) will cause the water to boil faster right?  Good.  We are adding more heat to the system and things now happen faster.  All on the same page?  Great, let’s move on to Experiment #3

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Experiment #3.  How long does it take water to boil? (1 cup, low heat)

1.  Take one random pot from your cupboard, and place the pot on your stove, and set the following conditions.

  • Conditions: Room and Pot at room temperature.
  • Heat Setting: (On a scale of 1-9 with 1 being low and 9 being high) :  2 (low/simmer heat)
  • Place 1 cup of lukewarm water in the pot.

Discussion:  Now, in this third experiment, nothing has changed again except for the heat setting.  Here we have 1 cup of water in the same pot at the same temperature, and the only thing we change is we move the heat to 15% (setting 2 on the stove, low heat/simmer setting).  If that same water at medium boiled in 5 minutes, we would now expect this low setting to boil that same water in (let’s take a guess again) say 15 minutes.  Again, the point isn’t really in the numbers, it’s understanding that addding less heat to this given mass (pot+1 cup of water) will cause the water to boil much slower now?  We are adding LESS heat to the system and things now happen much SLOWER.  

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Experiment #4.  How long does it take water to boil? (2 cups, medium heat)

1.  Take one random pot from your cupboard, and place the pot on your stove, and set the following conditions.

  • Conditions: Room and Pot at room temperature.
  • Heat Setting: (On a scale of 1-9 with 1 being low and 9 being high) :  5 (medium heat)
  • Place 2 cups of lukewarm water in the pot. 

Discussion:  In this 4th experiment, we are repeating experiment number 1, and the only thing we’re changing is the MASS.  We’re now doing 2 cups and the same pot instead of 1 cup.  The mass doubled. (2cups+ pot=mass)  So now, just to restate, 1 cup of water boiled in 5 minutes at medium heat and so we would expect 2 cups with the same setting to do what?  Take longer right?  Let’s call it 10 minutes.  More mass, means the same heat will take longer to convert.  Conversely, if you were to half the mass in the pot to 1/2 cup it would boil much faster.  More mass=more heat,  less mass=less heat to boil the water at a specified time.

So let’s put an end to the experiments….  So here is the final point:

THE POINT:  If you increase the mass (pot+water) with a given heat setting, it will take longer to boil the water.  If you decrease the mass, it will boil the water faster.  If you add heat things happen faster, if you take away heat then things happen slower.  Finally, if you repeat the experiments over and over, you will predict the moment the boiling will happen time and time again, if the mass and heat does not change.  As a matter of fact you can set a watch to it and predict the boiling moment to within 15 seconds of accuracy every time.  You can set your watch to it.

Let’s look at the above 4 experiments in a chart for clarity

HEAT Mass (the pot is assumed in the mass) Time to boil
Exp. 1 Medium(5) 1 cup 5 mins
Exp. 2 High (9) 1 cup 3 mins
Exp. 3 Low (2) 1 cup 15 mins
Exp. 4 Medium (5) 2 cups 10 min.

 

 So what does this have to do with coffee roasting?  EVERYTHING!  Understanding the above points is critical to good coffee roasting.  We’re going to translate this knowledge of boiling water to roasting coffee.

 

 CONTINUE TO PART 2