Tips to Improve Roast Consistency and Uniformity (Revisited)

If you should experience lack of uniformity or mixed dark and light coffee upon finishing a roast, please apply the following tips in order.  It just happens that the most common culprit for uneven roasts is also the easiest to fix.  Work through these solutions step-by-step in order to fix this issue.  Almost every case of uneven roast has been solved with the below methods.

Please apply these points in order as the first suggestions will be the most likely and easiest solutions.

To clarify this issue,  let me give you an analogy.  Imagine a glass cylinder, half filled with water.  Now turn it on the side, the same way as a drum would be positioned.  The way it would roll off a table.  Now, tilt it at 10 degrees, right or left.  Doesn’t matter which way.  So now you will have a little bit what resembles a swimming pool with a deep end and a shallow end.  That’s exactly what happens to the coffee in a drum on an incline, or with an unlevel drum.  You end up having a shallow end of coffee and a deeper end.  That shallow end will roast faster, and the deeper end will roast slower.  

As the roast progresses, the shallow end enters first crack because it is thin and small massed, and gets more direct heat exposure, and then as the roast progresses it works it’s way cracking from the shallow end to the deep end.  But since there is relatively a short time between 1st and 2nd crack, then by the time the deep end starts on it’s way through 1st, the shallow end is well finished with 1st and begins into 2nd, and then the process repeats with 2nd crack having the 2nd crack roll slowly from the shallow end to the deep end.

 So the end result is this really confusing mess where 1st blends in with 2nd and the crack seems to go on for a strangely long time and the end result will be dark beans mixed with light beans and if you wait until the deep end finishes the 2nd crack, then by that time, the shallow end is well on it’s way to becoming charcoal.  The greater the angle the more pronounced this effect will be.  Thus the importance of leveling the drum.

 It just makes no sense.  What in reality happened here, was that on roast #1, the temperature indicated 500 but was really at 450.  On roast #2, even though our roaster bumps the temperature to 525 to compensate for the late roast, in reality the temperature this time wasn’t 50 degrees to low, but now 75 degree too low or worse, and thus was roasting at 425F.    All this is to say, that if you use the built in thermometer, you have a good chance of chasing a ghost and will end up frustrated in the end.  You won’t know how much you are off, or if you are off at all until you get a good one to compare to.  I love the Tel-Tru brands of thermometers, but any quality thermometer will do.  I’ve seen supposedly high dollar thermocouples be off as well, so just so you know.  Roasting at the wrong temperature can also then be a cause for uneven roasts.

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/12LB) and it is quite easy to get the right side feeling hotter then the left (or vice versa) , 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 compact area.

So, to solve this, some experimentation and trial and error 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.  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 seen 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.

I’ve also seen some individuals install thermometers, one relatively over each burner to see what each burner was doing so they could adjust things in real time.

These approaches, have solved 99% of uneven roast issues. If you’ve thoroughly looked at all of the above solutions and are still having issues, please give us an email or a call and we can step through things together. Thanks

Author: Shane Lewis

Shane is the founder of the green coffee exporter Old Spanish Main, Ltda. in Bogotá, Colombia, having practiced under the renowned grader & cupper Lelly Espitia. He is a cultivator of a #14 Colombian Cup of Excellence coffee. Shane currently designs & manufactures coffee, cacao & grain roasters with RK Drums.