Tips to improve roast consistency

The following is an article on improving roast consistency on the RK Drum with focus on the 8LB drum.

In some grills you may have difficulty, particularly with the 8LB Drum and very minimally present on the 6LB Drum with uneven roasts.  This is a result of trying to roast coffee evenly over an 18" or 24" span over varying heat sources.  Specifically, your burners may not be distributing heat evenly across the entire length of the drum.  This is particularly present on 8LB Drums and on grills that don't have adequate burner diffusion.

To clarify, off of each burner you have a column of heat rising up off of the burner directly, which is controlled by your burner knob.  It can be difficult to tell exactly how hot that flame is.  Just because your knob on burner 1 is set to 75% and burner 2 is set to 75% and burner 3 is set to 75% does NOT mean that each burner is actually producing the same amount of heat.  The valves in the burner knobs on these grills are just not manufactured with fine tolerances, and may have slightly different gas flows depending on their internal structure.  Furthermore, your burners themselves may have slightly differing hole sizes which translates to more or less gas getting through and thus more or less heat.

The longer a drum gets, the more pronounced this becomes as you are stretching the coffee out over 24" (in the case of the 8LB) and it is quite easy to get the right side feeling hotter then the left, or to have cold spots here or there (relatively speaking).  This is almost invisible in the 4LB 12" drum as the coffee is compacted into such a concise area.

So, to solve this, some experimentation is necessary.  The objective we are trying to achieve here is to even out the heat that the drum "feels" across its entire length.

So how can we even out the heat?  Here are a few approaches. Please apply these points in order as the first suggestions will be the most likely and easiest solutions.

• Leveling the Drum: Keep in mind, that your drum must be level when roasting.  Use a level to see that the drum is completely level to the ground.  This will prevent coffee from piling up more in one end than the other.  If this happens then you will get  a larger bean mass on one side vs. the other and that heavier side will roast slower than the lighter side.  This will cause unevenness.  Try this as a first approach.

• Heat or Rotational Speed: Another reason that could explain uneven roasts is rotational speed.  If you are roasting at less than 6RPM's you will probably get uneven roasts.  The closer your approach to 40-60 RPM's the more even your roasts will be.   Finally you may not be roasting hot enough.  If you have done all these things and still getting uneven roasts, you may not be getting enough heat penetration.  Try increasing your heat by 25 degrees and see how the roast turns out.

• Burner Balancing:  Adjust your burners up or down to achieve equal heating on all burners.  This is a bit tricky and may involve some raw roast time.  You may notice that coffee in one end of the drum is darker than the other.  This should indicate to you that burners on that side are running a hair too high.  They can only be 10 degrees off and that 10 degree difference over a 20 minute roast can cause the coffee on one side to be predominantly darker to give an example.  So backing that burner down to a lower setting for the entire roast may be a solution.  I have heard of some people getting to know their grill so well, they know what each burner needs to be set at to  get an even roast.  An infrared thermometer may help here.  You can take the laser pointer of the infrared thermometer and hold it on the bottom of the drum to see what areas of the drum are hotter than others and then adjust it accordingly.

• Burner Diffusion: Another approach and possibly a better approach in my opinion is simply to diffuse or blend the heat coming from all the burners into one heat source.  The basic way to do this is to install a piece of metal over the burners.  It is true that most grills do indeed come with a heat diffuser over the burners.  However, if you look at some of these diffusers (particularly in grills whose burners run front to back) you will see that the diffuser only covers the burner tube itself in a sort of upside down "V" profile and then allows the heat to boil straight up.  You can see that this really does very little to break up that heat column coming up off of the burner.  The idea here is to keep the drum from feeling the direct heat off the burner and blend all the heat together.  Some grills have a great diffuser plate which rather covers the majority of the bottom of the grill thereby blending the heat very well.  Some of the new infrared grills are better at this as they use an entire plate to actually separate the heat column from the coffee.  (Be careful to watch BTU requirements on the infrared grills).    So what you need to do is look at your diffuser and decide if the diffuser is doing a good job of breaking up and blending the heat rising off the burners.  If not, throw out the diffuser you have and install a piece of aluminum or steel.  Usually 1/16" steel works well.  The piece should be big enough to fit in the entire bottom of the grill leaving only 1 and 1/2" of clearance between the edge of the plate and the wall of the grill on all sides.  So what we want is for the burners to heat the plate, and then for the plate to radiate heat towards your coffee and secondly for the heat to collect under the plate and then boil up around the edges of the plate to fill the space in the grill where the drum sits.  (So you should have heat rising up around the edges of your square plate on all four sides of the rectangular plate).  Some people have drilled holes in this plate to allow for more heat to trickle up through the holes.  Lastly, please understand that your heat up and cool down timeframes will be extended as the majority of your heat will go towards heating the plate initially extending your heat up time, but conversely the plate will hold heat between roasts reducing your heat up time for roast #2.  Remember steel retains heat longer than aluminum so keep that in mind.  It doesn't matter either way however.  See the two images for further clarification