Ask the Alchemist #177


Level: Alchemist

Reading time: 13 min

Hi, John - I just watched your roasting video. I am one of those people (for the last 1.5 years since I moved to Seattle) who doesn't currently have an oven. I hope to move in the next few months, but until then I've been taking my beans to my son's house to roast them in his oven. After watching this video, I'd like your opinion.

The oven is a Wisco - 1300 Watts.

https://www.amazon.com/Wisco-Wisco-620-Commercial-Convection-Counter/dp/B013SF411M

Somehow I made the assumption that using this for my beans wouldn't be a good idea because the air circulation is so strong inside it; much stronger than a traditional convection oven. When using it for typical baking I need to drop the temp about 25* from what recipes recommend. Now I'm not so sure. This thing is small - about 15in wide, so these are the Pyrex dishes I have that will fit. Of course, I don't want to take a chance on wrecking a batch of beans. Given what I've described, do you see any problem using this for cacao?

 

Let us get this out of the way early.

I am going to lie to you today.

That is a lie.

What I mean by that is that in a binary world, something is either the truth or a lie, and if I am not telling you the whole truth, I therefore must be lying to you.

This is basically I joke I tell to get the attention of a group of 8th graders when I talk about the science of chocolate making.  Go watch my youtube video (https://www.youtube.com/watch?v=fi2RY8zqy9g ) where I tell the whole story.  Suffice it to say, I am NOT going to actually lie to you (on purpose) but I am going to leave a lot out since what I am going to talk about is full of maths and thermodynamics that are going to make many people’s eyes glaze over.  My goal is to get you to understand what is basically going on and why I am making the statements and assertions I make.

truth

And it’s very possible many of you CAN handle the truth.

With that, hold tight, and let’s jump in.

Hard.

First the answer.  Yes, you can use that oven.  For 1 to 2 lbs of beans.

Now I will tell you why I say that.  But I need you to be somewhat conversant in some basic terms and how I think.

From Google – “The British thermal unit (BTU or Btu) is a traditional unit of work equal to about 1055 joules. It is the amount of work needed to raise the temperature of one pound of water by one degree Fahrenheit.”

I am starting there because I know from empirical experience that my Royal #5 coffee roaster produces about 42000 btu/hour with propane.  How do I know that?  I measured the orifice of the propane outlet and looked up the BTU/hour connected to standard liquid propane.

Multiplying that out, that translates to 44310000 joules/hour.

I can roast 35 lbs in my Royal in about 20 minutes or one third of an hour.  That means I need  14770000 joules of energy for 35 lbs regardless of time it takes me to get it there.  Dividing by 35, that means 422000 joules per pound are required.

Part of this kind of number crunching is verifying I am in the ballpark.  I’m going to do that with checking my pre-heat requirements against how long I know it takes me to pre-heat my roaster.

Iron has a specific heat capacity of 0.45 j/g C.

I know my roaster weights about 300 lbs, but estimate the portion I heat up to 350 F is only about one third of it or 100 lbs.

100 lbs = 45 kg iron =45000 grams

350 F is a change in temperature of about 150 C.

0.45 x 45000 x 150 = 3037500 joules to pre-heat.  If I divide by the heat input (44310000 j/hr) and convert to minutes (multiply by 60)

3037500/44310000 * 60 = 4.11

4.1 minutes to pre-heat.  Yep, that is not a lie.  Some days it’s a touch longer, but it’s in the ballpark, so I am counting that as confirmation that my heat input and roasting requirements are about right.

Next, we can use those numbers to see what a 1300 watt convection oven should be able to do.  But we need to get everything in the same unit.

Again, from Google:

“One watt of power converted to joule per second equals to 1.00 J/s. How many joules per second of power are in 1 watt? The answer is: The change of 1 W ( watt ) unit of power measure equals = to 1.00 J/s ( joule per second ) as the equivalent measure for the same power type.”

That just means that 1300 watts = 1300 j/s

We know we need   422000 j/lb.  A little quick math canceling out joules and the minimum time needed in seconds falls out.

422000/1300 = 324.7 seconds.

Dividing by 60 seconds/min tells up how many minutes we need to roast a pound with perfect efficiency.

5.4 minutes per pound.

On the surface, that sounds great.  Except you really don’t want to roast that fast.  If you try, you are going to over roast the outside and under roast the interior.  So you need a slower roast.  No faster than 12 minutes for a roast is what my experience tells me.  But you don’t want to turn the heat down and not use the oven to its capability so that means you should put in more than 1 lb of beans.  How much?  Just divide 12 by the 5.4 minutes for a reasonable estimate.

12/5.4 = 2.22 lbs = 1 kg

Which amazingly enough is right what I said at the top of the article.

“First the answer.  Yes, you can use that oven.  For 1 to 2 lbs of beans.  “

But you will note I said 1-2 lbs.  Why 1 lb?

Well, that delves into modes of heat transfer.  That means when you add heat to a system, there are three ways for it to get from the source to the item you want to heat up.

Convection, conduction and radiation.

This is a convection oven.  That basically means air heating up by the heat source, circulating to the air to what you are roasting and that heat transfers to the cool items (cocoa beans).   It’s pretty efficient since hot air can surround the beans except where it is touch the pan or other beans.  In a thin layer (what I advocate, and why I advocate it) 70-80% of the surface of the bean can be exposed to the hot air.

Conduction happens with contact.  From the surface the beans are on to the beans.  It’s actually pretty inefficient for anything that isn’t perfect flat.  In the case of a cocoa bean, there is probably only 10-20% of the bean touching the surface of the pan.  That is why it doesn’t do well.

Radiation is basically direct line of sight.  When the sun comes out and shines on your skin and you feel warm, that is radiation.  You are not touching the sun and it isn’t because it caused air to move and warm you.  So if there are elements shining on the beans from above, then they are being heated that way too.  If they are heating from the bottom, then very little radiative heating is happening.  Instead the elements are heating the pan and the pan is conducting the heat (per conduction – above) and heating the beans.  In this case, it’s probably the least efficient way to heat.

Ok, deep calming breath.  I know that was a lot.  We are almost done.  And I can now use those terms to talk in a more efficient manner about why you might only be able to roast 1 pound.

Even though this is a convection oven it doesn’t mean it is efficient convection or convection as good as it can be.  Also, conduction is going to be very low due to the very low percentage of bean surface area actually touching the pan.  And where radiative heat might be effective in some cases, the convection in a way circumvents it by distributing the heat around the chamber.  In this case though, that is not a terrible thing and is actually pretty good or the top of the beans might get scorched.

After all is said and done there is probably only 50-60% efficiency going on here.  The rest of the energy is being lost.  Either through the non-insulated walls and glass door or through direct energy loss when you open up the door to stir the beans every 5 minutes or so.  Remember, you still have to stir. If you don’t, the top of the beans will be over roasted and the underside that is in contact with the pan will be under roasted.

And how do I know so much is being lost?  Again experience and puzzle solving.

Years ago I had a drum roaster that I built.  Over time I modified it for efficiency.   It started off at 2000 watts, with a slow 6 rpm motor and no insulation.

If we do the same calculations, recalling we need 422000 joules per pound  and have 2000 watts we find we, in theory, could roast  a pound of beans in 3.5 min.

422000 / 2000 * 60 = 3.5 min/lb.

The drum contained 5.5 lbs of beans.  So if this roaster was working as well as my Royal, I can apply this math:

3.5 (min/lb) x 5.5 (lb) = 19.25 min

And predict it should have taken me a touch over 19 minutes to roast 5.5 lbs of bean.  But the reality of the situation was that I could only roast about 3.5 lbs of beans in that time.  Doing that efficiency check :

3.5 lb / 5.5 lb = 63.6%

This showed me my system was inefficient.   A little over 35%.

It was not until my 6 rpm motor died and I replaced it with a 45 rpm motor did I discover how important REAL convection is.  As soon as I did it, my roast time dropped to 14 minutes. It was like a 35% boost in power.  Efficiency really.   Consequently I was able to add more beans to my roaster.  When I put in 5.5 lbs, my roast time went back to 19-20 minutes – exactly where a good efficient system should be.

See, it was not enough that the beans were tumbling.  Too many were touching and protecting each other from the heat.  Just like in a table top convection oven with beans on a try.  It was not until I got them lofting did real, full convection kick in.  And it is worth noting that after I insulated my roaster, the roast time dropped to about 17 minutes showing again how much heat gets wasted out the walls and over time.  Just like when you open the door to stir.

So, sure, it is a convection oven, but the beans are only partly benefiting from the moving air.  Sure, if it was not convection, then the roast times would go way out to 30 minutes like a classic oven.    But you really can’t have too much convection in a non-tumbling roasting situation.

That is where I get the 50% efficiency count from.  30% or so from non-ideal convection (no loft) and 20% loss from continuously having to open the oven to stir and there you go.

A bunch of lies.  Well, half truths of omission.  And it was STILL a ton of reading.

Yes, you can use that oven.  For 1 to 2 lbs of beans.

And  now , hopefully, you know why and how to work it out yourself if you need.

Go have some chocolate.  You deserve it.

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