Roasting volumes

[Shane Lewis]

I am currently looking at your roasting drums.

I have been using a “coretto” method with good results, but can only produce 500 grams of roasted beans at a time. I would ideally like to be able to roast up to 2.5 kgs per roast, as this is the bag size of green beans my supplier sells.

I have looked at some feedback and a tutorial on the RK, and have a few questions.

Would a 6lb or 8 lb drum be the best option for nice even roasts of 2.5 kgs of green beans?
-Shane

[Shane Lewis]

Definitely see where you would want this. Currently it’s not a very convenient thing to accomplish with our drum. You can see where it would be challenging. Something I’ve meant to solve for years. It’s, however, not cost effective related to its benefit as it would be about $200 USD in equipment to pull it off. Something I might design in the future but, for now I feel very comfortable with our alternative. Let me explain it and hopefully it will make sense.

Ideally of course, it would be preferable, as you suggest to have your probe in your beans to know what temperature your beans are “feeling”. Can’t disagree with you here. That would be ideal. Because we can’t easily get a temperature probe into a rotating drum at 52RPM without all kinds of complicated things, we have to settle for having a temperature probe right in the chamber, as close to bean level as possible. Clearly the coffee will be sitting closer to the heat than the probe a few inches away. Okay, fine. So let’s say, for the sake of the discussion, the barbecue is running at 500 degrees F within it’s chamber as registered on the lid-mounted probe. We can assume, whether it is true or not, that the coffee is running slightly higher. Let’s say it’s at 530 F. (I assume you use C, you’ll adjust your temps accordingly, but for this discussion it’s not relevant). Ok, so is this a problem? The coffee is hotter than we think it is! I say it is not. Here is why.

Coffee roasting is a linear process, a simple operation. A bit like boiling water. You inject enough heat into water, it will eventually have enough heat energy input into it where it begins to boil, convert to a gas and boil away right? Coffee is the same principle. It is essentially a seed with water in it. Between 9% and 14% ideally inside the bean. Now, if we inject enough heat into the bean, the water inside will begin to boil, the sugars will begin to caramelize, and will enter first crack. (the evaporation point of the water, the sound of the water escaping is first crack). Eventually as the bean caramelizes, it will eventually fracture its outer shell, (2nd crack). All of these happen at a predictable, linear rate once the coffee has received enough heat energy. Just like once all the water has reached 212F it will begin to boil and evaporate every time from our pot of water. We can predict it. If we run the same amount of heat into our pot of water, we can pretty well set a watch to it and within 15 seconds or so, predict boiling every time.

So let’s put that to action. When roasting on the RK drum, (or with any coffee roasting method) you always want to roast your coffee towards your prescribed roasting time referenced in the chart above. In some systems, it is computer controlled so you never think about this. With manual roasting, we do want to consider what is happening in the system.

So let’s take this info to an example.

Let’s say you want (per the above chart) to roast 6LB of green coffee to produce your 5LB (2.5KG)

Our estimated temperature will be between 550 and 575 F. That is the temperature of the chamber, not necessarily the beans as you suggest. But, at this estimated temperature, (and whatever temperature the beans happen to be at) it will cause the water to boil and go into 1st crack at around 18 minutes, and then enter 2nd at 19-20 minutes.

Now, there will be variables, your barbecue might be different. Your temperature probe might be closer (and therefore feel hotter) to the flame (or it could be farther away) than my probe, so your numbers will be different than the chart. But, the chart is close. It is close enough. So let’s look at one final piece.

Since, again, coffee roasting is a linear operation. You inject more heat it roasts faster. You inject less heat it roasts slower. We can adjust our heat to target our time to get that perfect coffee.

In our example, we are targeting an 18 minute first crack. Lets say we choose 560F for our roast. We start our roast. We want to make sure we are at 560 and holding at no later than 4 minutes after the start of the roast. This should, produce a 1st crack at around 18 minutes. Ok, fine. If we are at 17 minutes and we see now steam/smoke (telling us we are approaching 1st crack). This tells us what? That we aren’t close to first crack right? We are looking for an 18 minute first right? So what must we do? Accelerate the roast. Inject more heat. More heat means faster roast. So we bump up the heat from 560 to 580 or 590. We are going to try and get the roast to fire first crack at 18 minutes. We adjust our heat accordingly. Do we care what the beans feel? Not really. We have a relative reading from right next to them. We are roasting to target our time, not our temperature anyway.

So we adjust the heat to 590F. This accelerates the roast and we begin to enter first crack quickly now and we achieve first by 18:15 because we injected more heat. If we keep this trend we will be assured of a 2nd within 2 more minutes which keeps us on our time profile.

Conversely, if we are seeing signs of 1st crack at 16 minutes, that tells us what? We are too hot. We need to back down to 530 degrees to slow things down to stay on our target time. So, you start with the chart on the website as a starting point and you adjust your temperature to achieve your time profile and you will come out with the same roast every time (for each coffee) without seeing the coffee, or knowing what the beans are feeling. You can set your watch to it and you can even predict 1st or 2nd crack by your clock, to within about 15-20 seconds of accuracy. It’s pretty cool. This is what I mean by adjusting your temperature to achieve your roast time. It works every time. Hope that makes sense. I may post some of this on the website if that’s ok with you. Might help if it were explained a bit better. Anyhow, feel free to ask anything else you might like.

There are many RK roasters in AU, closest that I recall, is James Axisa somewhere up near Sydney? North Bay Maybe, maybe I have that wrong. Contact him and ask him how he’s doing. He’s been roasting a few years now.

Anyhow, thanks for the email and let me know if you have any other questions.
-Shane

[Shane Lewis]

I am used to using a temp probe directly in the bean mass for tight roast profile control. How hard is it to achieve consistent results with a simple air temp probe as the source of temp data?
-Alan Snow

[Shane Lewis]

Not at all. It, in fact, is a ratio of the volume of coffee to heat input. You are roasting 500G in a nearly full container in your coretto method. In order to fully saturate that amount of coffee, being somewhat confined and bunched together, with enough heat to get it through second crack, it takes 16 minutes of heat at your temperature. If you tried only 100G of coffee I bet it would take it much less time?

The same principle applies here. 500G in a 6LB drum would roast in about 6 minutes as it would be very thin within the drum and would require very little heat energy input into the system to complete. Conversely, a fully loaded drum (6LB) at approx 70% volume (you need space to mix the coffee) will take 21 minutes to fully saturate the coffee with enough heat energy to bring it to 2nd crack. This is our target roast time.

have a look at the following roast table.

https://blackbalsam.co/rkdrums/index.php/how-to/roasting-guide/item/roaster-prep?category_id=22

You will notice that the time increases on each drum as the load increases. This is that question of time to saturate the “more dense” volume of coffee.

Truthfully, your “baked” taste won’t begin to appear until you have exceeded the prescribed roast times generally by 30-40% Hope that makes sense.
-Shane

[Shane Lewis]

It seems a roast with the RK takes longer than I am used to (22 mins vs 16). Does the coffee taste “baked” rather than roasted with this technique?
-Alan Snow

[Shane Lewis]

If all you are looking to do is a max of 2.5KG, which is approx 5LB, then the 6LB drum would handle that job at its approximate max capacity. For example. You will want to add 3.2OZ of coffee extra for every 1LB (approx .5KG) of coffee. So with this, if you do the math, 6LB of green will net you 5LB of finished roasted coffee every 20 minutes. (or approx 2.5Kg). The 8LB or the 12Lb drum will give you even more overhead and I would steer you in that direction right away if you have any immediate ideas about growing your business. When I initially started my business, I started with the 4LB drum and within 90 days I was standing out there a lot longer than I wanted to be. Now, I’ll admit it was laziness. Didn’t want to be out there roasting for 3 hours to complete 12LB of coffee, so I ordered another 6LB drum and started roasting on that one side-by-side on two grills.

I wished I’d gone bigger to start with.

All of that said, the 8LB is slightly longer and a bit more challenging to roast on as it’s length means you must make sure you have even heating. The 12LB drum is a bit easier as it is huge and mixes well.
-Shane

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