Category: Roast Training Articles

What happens if your grill isn’t powerful enough for roasting, BTU vs. Mass explained

The case of the thimble, a 5 gallon bucket and what any of this has to do with coffee roasting.

One of the most commonly misunderstood concepts is the heat required to roast a <insert LB’s of coffee here> pound mass of coffee.  While we have come up with the following guidelines, some people don’t understand why we have arrived at those numbers.  It’s true the numbers are slightly arbitrary in the sense that we don’t have measured science behind it, but they’re also based on a significant amount of testing, and are designed to give you plenty of extra heat\speed in roasting if you need it.

I recall at one point one guy saying… “Hey, my grill is 28,000 BTU and I have no problem getting to 650F, so why can’t I roast 8LB?”  We in fact, recommend 45,000 BTU, so what’s wrong with his claim?

See, BTU is a measurement of heat output, not necessarily temperature output.  Let me provide the following analogy to best illustrate the point.

“A candle produces approximately 100 BTU\hour.”  

 

THE EXPERIMENT:

Imagine if you took a thimble, the little thing you use for sewing, placing it onto your finger.  I use this as an example of a little tiny cup.  Imagine if this cup were filled with water and placed over a common candle.  Do you think this water in the thimble is capable of boiling?

 

 

“Imagine you have a thimble filled with water, do you think it would boil placed over a candle?”

 

What’s interesting about this experiment is that you have an analogy that’s easy for all of us to understand based on common everyday experience.  I imagine in certain cases, the water might not boil, but I think most of us would agree the answer is “yes, the water in the thimble would boil”.  After all, we’re talking about a very small amount of material that is heating up.  This material, what we will call the “mass” is the thimble and the water.

 

“1 BTU is the amount of heat energy needed to raise 1LB of water by 1 degree Fahrenheit.”

 

 

Now imagine if you take 5 gallons of water and you place it over the same candle in the previous thimble experiment, do you think the water would boil?

 

“Do you think a 5 gallon bucket of water would boil placed over a candle?”

 

 

Most of us would say “no” this water will not boil in a 5 gallon bucket.  Why do you suppose this is true?  In both cases the candle, which burns an average 2372-2552 °F is the same in both experiments?  So what’s happening here and what does it have to do with coffee roasting?

 

In the second experiment, the mass that we are heating up is the bucket and the 5 gallons of water.  If the water heats up, it will heat up relatively little.  What happens is that the entire mass (the bucket and the 5 gallons of water) is absorbing the relatively small amount of heat input from the candle, then what heat energy is absorbed dissipates into the environment before the water heats up to any appreciable degree.  You can imagine an even more exaggerated experiment by trying to heat up a swimming pool with a candle.

 

So you see, as mass increases…. you need more and more heat input to be able to bring the mass to the desired temperature.  How then does this translate when roasting coffee?  Well, it’s exactly the same.  Instead of a thimble or a bucket, we have a drum.  Instead of water we have beans that contain water.  The larger the mass grows (2LB drum up to 12LB drums) the more heat input you have to have.  The flame temperature might be the same, but the capacity to input heat increases.  This is the BTU number.  Think of this as 50 candles under the 5 gallon bucket of water, and now we might have something to talk about.

 

In a real life example, we tried to get an 8LB drum to roast 8 pounds in a 28,000 BTU grill.  This grill, when empty had no problem heating up to 650-700F.  But the moment you add a bunch of mass to the grill, (much like going to the 5 gallon bucket of water over the candle) that same grill with 8LB of coffee, struggled to get above 175F.  You could roast 1-2LB in this scenario, but not 8LB.

 

We then later realized we needed to trap as much heat as possible and keep it in the grill.  We used sheet metal to cap off every hole we could find in the grill.  The result was that we barely achieved 600F by trapping every ounce of heat.  The roast finished at 22 minutes, which is a pinch longer than our ideal.  This could be considered a success, but this was literally with 100% gas on the burners.  If one needed to go faster, the heat simply isn’t there.  At 45,000 BTU you’ll have more than enough head room, even at 8LB to speed up the roast by increasing heat if you should need to.

 

This is a real life example of how BTU plays out when dealing with increased mass.    The more mass you have (the larger your roast)..the more heat output (BTU) you need to finish the roast on time.  Now this “on time” is key.  In the above example of the 8LB roast, where we capped off all the holes, this was trapping enough heat to get the roast to finish on time.  We could have left the vents uncovered andlost a bunch of heat, roasted at 350 for 45 minutes and finished the roast.  The roast would have tasted terrible, but it would have finished.  The trapping of the heat allows us to finish on time and speed up the whole reaction.

 

So in summary, More mass requires more BTU to get the coffee to finish with sufficiently short time.  If you sacrifice any of these, the variables change and you can find greater degrees of success.   For example, if you struggle with BTU, you can lower your mass…or lengthen your roast (not ideal), or you can supply more heat to compensate.  Hopefully this sheds light on a complicated subject, which hopefully now makes a little more sense.

Shane-RK

 

 

Roast Development in an RK Drum

Roast Development in an RK Drum

Article borrowed with permission from Seth Newsome. Please visit his excellent Blog. Thanks Seth!

We’ve celebrated our third month in business and are staring our fourth right straight square in the face.  During this time, I’ve had to overcome the learning curve of roasting coffee on a five burner gas grill converted into a coffee roaster while nailing down the roast development necessary to make great coffee.

This learning curve has been helped considerably by Shane Lewis at RK Drums who I can’t speak more highly of.  Shane has been instrumental in helping me understand the how to roast coffee on a somewhat less than conventional and often quite finicky device intended for burning burgers and charring chicken breasts.

Nonetheless, I’ve come across some recent discoveries that may assist other RK Drum roasters in creating a more consistent product from batch to batch while eliminating some of the less desirable qualities of that somewhat finicky device.

Time and Temperature

The essential function of all coffee roasting is time and temperature.  Pro roasters will incinerate me (and probably Shane) for saying that internal bean temperatures are not necessary for producing an excellent, quality, and consistent roast.  Nevertheless Shane, along with the company’s creator Ron Kyle, developed a simple time and temperature chart for their drums where under a given temperature and green coffee load a coffee should reach first and second crack at a certain time.

 

roast development rk drum

A snippet of the 12 pound drum roast profile chart from RK Drums. You can find the full chart as well as the charts for all of the RK Drums here.

However I, along with many other roasters, find that the time and temperature chart is merely a guideline and more often than not does not align in the slightest with what my temperamental roaster wants to do in any given day.  With that said, I did some research and developed a system that seems to work really well for our machine and our coffees.

Roast Development

While I can’t with any confidence express that I’m an expert in roasting, roasting techniques, nor roasting coffee beyond a brown, grindable, drinkable substance we call “coffee”, I’ve sufficiently studied some roasting techniques in order to better understand how professional roasters roast great coffee time and time again.

 

We’ve celebrated our third month in business and are staring our fourth right straight square in the face.  During this time, I’ve had to overcome the learning curve of roasting coffee on a five burner gas grill converted into a coffee roaster while nailing down the roast development necessary to make great coffee.

This learning curve has been helped considerably by Shane Lewis at RK Drums who I can’t speak more highly of.  Shane has been instrumental in helping me understand the how to roast coffee on a somewhat less than conventional and often quite finicky device intended for burning burgers and charring chicken breasts.

Nonetheless, I’ve come across some recent discoveries that may assist other RK Drum roasters in creating a more consistent product from batch to batch while eliminating some of the less desirable qualities of that somewhat finicky device.

Time and Temperature

The essential function of all coffee roasting is time and temperature.  Pro roasters will incinerate me (and probably Shane) for saying that internal bean temperatures are not necessary for producing an excellent, quality, and consistent roast.  Nevertheless Shane, along with the company’s creator Ron Kyle, developed a simple time and temperature chart for their drums where under a given temperature and green coffee load a coffee should reach first and second crack at a certain time.

roast development rk drum

A snippet of the 12 pound drum roast profile chart from RK Drums. You can find the full chart as well as the charts for all of the RK Drums here.

However I, along with many other roasters, find that the time and temperature chart is merely a guideline and more often than not does not align in the slightest with what my temperamental roaster wants to do in any given day.  With that said, I did some research and developed a system that seems to work really well for our machine and our coffees.

Roast Development

While I can’t with any confidence express that I’m an expert in roasting, roasting techniques, nor roasting coffee beyond a brown, grindable, drinkable substance we call “coffee”, I’ve sufficiently studied some roasting techniques in order to better understand how professional roasters roast great coffee time and time again.

I stumbled across the Cat & Cloud podcast above with the Czar of modern coffee, Scott Rao, which really turned me on to exploring more technical aspects of our roast.  While Rao’s “textbook” on coffee, The Coffee Roaster’s Companionis one of the predominant texts for coffee roasters and enthusiasts, his article, Roast Development Time and the Prerequisite for Any Successful Roast, published on the Daily Coffee News really shines a clean albeit simple light onto the process of roast development.

In Rao’s article, he mentions that great, memorable coffee has two consistent characteristics that stand out from roast to roast.  One, Rao says, is that coffee hits first crack at 75-80% of the roast and the coffee develops 20-25% of the total roast time.  In layman’s terms, the time between roast start to first crack should consume 75-80% of the roast while the time after first crack starts should be 20-25% of the roast.

https://soundcloud.com/catandcloud

Rao’s Roast Development on an RK Drum

While Shane’s chart is a great starting place, my numbers almost never line up.  Depending on how the roaster decides to heat on any given day and various ambient factors, I’ve had to adjust accordingly on our RK setup.

For roaster’s looking to improve and measure a roast on an RK Drum, the simple way of doing so is recording the time of first crack’s start* and using some simple division to calculate an approximate development time and when to pull a roast to optimize origin character and roastiness.  Using this method will negate any external factors that may be affecting your roast and allows the roaster using an RK Drum to make realtime calculations on their roast.

rk drum roast development

Our 12 lb. RK Drums setup on a 5 burner propane grill.

Let’s use an example from Shane’s chart in the paragraph’s above.

Take a look at the 4 lb. example in Shane’s chart.  According to his chart, we should be hitting first crack at approximately 10 minutes.  Assuming that we are hitting first crack at 10 minutes, using Rao’s methodology, we can divide our first crack time by .80 and .75 respectively to give us a range for when to pull our roast.

Using the formula 10 ÷ .80 and 10 ÷ .75 we get a range of 12:30 and 13:20.  Meaning, our coffee has developed for 20% of the roast at 12:30 and 25% at 13:20.  But if we look at Shane’s chart, a 4 lb. roast should hit 2nd crack at 12:00 which is a little too long for an evenly developed roast and at risk for hitting second crack.  Ipso facto, I generally adjust my constants in my equation to .85 and .80 respectively.

Using the constants of .85 and .80, assuming we hit our first crack at 10:00 as per Shane’s chart, we can create the following formulas for our development range: 10 ÷ .85 and 10 ÷ .80 giving us a range of 11:45 and 12:30.  With that said, we can be watching to pull our roast within the timeframe of 11:45 – 12:30 total roast time.  If everything stays on Shane’s schedule, you would be pulling your coffee a fuzz before 2nd crack making a very developed, well-roasted, flavorful coffee while eliminating some of the guesswork involved in your roast.

*For the longest time, I marked the start of first crack when first crack started “rolling”.  In other words – when the beans were popping and cracking in rapid succession.  While measuring first crack’s start is somewhat arbitrary, Rao suggests measuring first crack, “the moment [you] hear more than one or two isolated cracks.”  If you measure differently, you may need to adjust your constants accordingly.

Some Notes on Roast Development

While this formula works well for most applications and adapts to your roast for the day, it isn’t the end-all be-all for home roasters.  There is certainly room for variation, but this formula is adaptable to any roast.  If your roaster is running fast, the formula adapts accordingly.  If the roaster is running slow, that is accounted for as well.

Roast development really doesn’t have to be rocket science, but far too often we make it just that.  The problem I’ve always run into is mastering the art of “guesstimation”.  I’ve always played guessing games with the development of our coffee in the roaster and made assumptions as to when and how well it’s developed.  With this method, I’ve seen far more consistent roasts and far more consistency in the cup.

As for Shane and RK, I can’t say enough good about the quality of the equipment, the personal service, and dedication to perfection that Shane displays even months after my initial purchase.  Shane even called me on a Sunday afternoon prior to a family obligation!  Now that’s service of the best kind!

I’m interested to know how others have been able applied this or similar tactics to help predict their roasts on RK equipment, so please leave comments down below.  If needed, I can always publish a followup piece that explores additional methodology.

Part 3: Putting it all together during the roast.

Before diving into this article, please have a read of the previous two prerequisite sections, Part 1 (Understanding the roast process….Heat vs. Mass) and Part 2 (What happens during the coffee roast process…).

So much like in the 1st part of this article, understanding heat vs mass, is critical to understanding how to properly roast.  Roasting coffee is exactly like boiling a pot of water, except instead of a pot, we have a drum and instead of water, we have beans with water in them.  If you understand the pot-water-boiling experiments in the 1st article, then you can understand easily what is happening inside the bean.  

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Part 2: What happens during the coffee roast process…

Let’s look at what coffee roasting is, and should be…

What happens during the ideal roast…

First of all coffee is a seed, of the arabica or robusta tree….  it generally has between 9-14% water inside of it.  Yes, the same kind of water in our pots in the 4 experiments above.  When we roast the coffee, at a certain point, that water reaches the boiling point and begins to escape the bean with an audible low-volume “POP”.  This commonly sounds the same as popcorn in the microwave popping, just lower volume.  This is commonly what people refer to as 1st crack.  It’s generally one of two indicators of the status of our roast.  As the water boils away from the bean, and as we keep heating it, the bean begins to caramelize and the structure of the bean begins to expand, and stretch as the internal sugars begin to caramelize.  This builds up stress in the bean to at which point the bean’s shell fractures and cracks.  This cracking can be

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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.

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