Ask the Alchemist #163


Level: Alchemist

Read time: 9 minutes

We roast 300g (10 oz or so) in our Genecafe.

We are trying a new batch at 350 F for 15 minutes, and one for 20 minutes. The beans start to pop really quickly after we start (around 3 minutes) though, is this unusual? Also, after roasting, some beans have their skin cracked open, and others don't. Should they all be cracked open, or maybe it does not matter at all?

We get a chocolate cake/brownie smell quickly as well, which we've heard is a good sign during roasting. 

Regarding the 30 minutes at 300F , we decided on that from what we read online and what we saw in various videos... 

You are bringing back memories of me using the Genecafe.  I too seem to recall popping really, really early on.  3 minutes in is the sign that the heat is quite a bit high.  And that does not correlate with 300 F unless the beans are that hot.

Let’s get a couple simple things out of the way.  You don’t have to have pops or cracks in a roast, but can.  You absolutely don’t and won’t ever have them all pop like popcorn.  At most I don’t think you will ever get more than 2% popping. And maybe only 0.1%.  That said, I personally have found I like chocolate made from cocoa that has popped.  But that is comparing against the same beans that did not pop and are vetted beans that I know are good.

So it looks like we are talking roasting again. That is good. In my opinion, it can be the make or break step after fermentation.  And it seems to be the hardest to grasp in it’s totality.

Let’s see if we can make some connections.  Have a look at this graph.  It is a version of the oven profiles I put up a few weeks ago.  We will talk about it in a moment.

profile

Here is the thing with what you are finding for roasting profiles and what you are using.  300 for 30 minutes sounds like oven roasting.  Not drum roasting with forced air, which is basically what the Genecafe is. You have control over heat and profiles and should be trying to use them if you can.  That means not using one temperature for a set amount of time.  I like to call that baking, not roasting.

At first I thought you were under roasting with a 30 minute 300 F roast.  I am going to back up and say that it may indeed be that you are/were over roasting.

Why?  The key here is understanding how the heat is applied and absorbed by the beans.  In an oven (even convection) the heat transfer is very slow.  Only the top of bean on the top layer of beans is really getting heat.  It means most are not in contact with heat so they can’t heat up quickly.  It’s why it can take 40 minutes at 400 F to get the beans to 250 F.  In a drum roaster, and even more so in one with blowing hot air, the heat transfer is MUCH faster.  And that is what you are seeing in your roaster.

I recall having trouble stretching roasts out in the Genecafe (one reason I never pursued it or recommend it).  Actually, if your bean temperature was indeed 300 F then you over roasted.  250-260 if as hot as you want for  a bean temperature.

Keeping in mind I've not used this roaster in nearly 13 years, here is where I would start.

I would first start at looking at that graph.  I’m going to decide I want about a 15-18 minute roast and find the curve/line that corresponds to it.  The 400 F OVEN temperature looks good.  We picked a line.  Now forget the 400 part.  It is for an oven.  We don’t care.  It is immaterial.  What is important is the bean temperature and the time it takes.

Shoot for this profile. I'm mostly just taking numbers off the chart.  Meaning adjust your temperature knob as you go, keeping an eye on your bean vs set point.

65 C/150 F for 5 minutes 80 C/175 F  4 minutes 96 C/205 F 3 min 110C/230 F 2 min 120C/250 F 2 min

In general, those are the set point and I'm trying to estimate where the beans will be at those time points.  So, in this case, if everything goes right, those are also the bean temperatures.  Or to give it its technical name, that is my “roasting profile”.  To say it again, a roasting profile is the plot of bean temperature vs time.  NOT  ambient temperature vs time.  Yes, sometimes you will be given an ‘oven profile’ because it is easier, but as you can see it isn’t that useful whereas a bean profile is.  It is transferable to ANY method of roasting if you can get bean surface temperatures.

And that is where you went wrong.  I mistook an oven temperature profile for a bean profile.  The later is transferable to any roaster, the prior almost never so.

So what is going on up there in my suggested profile is that I hope to hit those bean temperature markers in that allotted time.  I’m not just putting the ambient temperature at 300 F because this roaster transfers heat to quickly and you can indeed burn or scorch the beans.  I’m effectively assuming heat transfer is going to be very fast and so I keep the difference between my target (say 150 F in the first step) and the initial temperature (ambient in this case) pretty small (only about 80 F).  I stretch it out to 5 minutes so there is plenty of time for the heat to soak into the beans before I start my next step or ramp.

A couple final things.  This roaster has a bean temperature thermocouple also.  You get actual feedback.  If at 5 minutes the temperature is at 120 F, then I want to increase my temperature set point to something a little higher.  Press on the gas pedal more as it were to go faster.  If they hit 150 in 2 minutes, the I need to back off the temperature a bit so they are not just hanging out at 150 so long.

I realize this may be way more that many people want or can use, but I also rather hope that if you read it enough, look at the situation different ways, that eventually there is be a click or ah-ha moment where you get it.  With that, see if these make sense, and if not, give them some thought.

  • The more your beans move, the quicker the beans will heat up.
  • The faster beans heat up, the smaller the difference ambient temperature has to be to the bean surface temperature. This can be as little as 50 F in a good drum roaster or as much as 300 F in the oven
  • The faster your beans move, the cooler you can keep your ambient temperature.
  • You can only heat up beans so fast before the outside scorches.
  • If your cocoa beans are not moving much you have to have a higher ambient temperature to keep the heat transfer moving at a reasonable pace.  Make it as big as you can without scorching the beans.
  • Cocoa beans can scorch if the difference in temperatures is more than about 300 F.  A 200 F difference is safer.

Give that a try and report back.

Oh, and one last thing.  I don’t sell or really suggest the Genecafe roaster. It’s not because it can’t do a good job.  It’s because I find it expensive to the amount of beans you can roast.  It’s kind of off putting to my mind and my goal is all about approachability and affordability wherever possible.  If you have one, go ahead and use it, but otherwise don’t go out and get one just for cocoa.

 

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