Level: Apprentice

I have been following your roasting profile recommendations and I am loving the results.  I am having a bit of trouble though keeping the roast from going too fast.  I know we are driving off water in the first part so I turn the power down 5-10% to account for that but it never seems enough.  I’m afraid to turn it down more and mess up the roast by having it take too long.  How much should I have to turn my roaster down?

I’m really glad to hear you are finding the new profiles helpful.

You are on the right track turning your heat down during the roast.  But you are not turning it down enough.  When I roast I tend to turn the power down 25-30% from the end of the drying phase to the EOR.  In many ways I am just coasting.  Unless you have a really cold environment or a large roaster (that is also not fully heated up) turning the heat/power down only 5-10% is going to have the roast racing along too fast.

Water is an interesting substance.  It expands when it freezes unlike most solids that shrink.  It takes quite a lot of energy to heat it up and way more than you might expect to actually boil it.

It is so interesting and unique it is used to define many units of energy.  For instance a BTU or British Thermal Unit is the amount of energy it takes to raise 1 pound of water 1 degree Fahrenheit.  Similarly in the metric system 1 calorie is the amount of energy it takes to raise one gram of water 1 degree Celsius (1 cal/g C).

Most dry foods don’t take nearly that amount of energy to heat up.  Cocoa beans only need about 1/3 of the energy as water needs to get to the same temperature.  Only 0.35 cal/g C.  Without resorting to a bunch of maths that just means as water is driven off in the drying phase and the weight starts to drop, it acts like a lot more weight was lost because it took so much more energy to increase the temperature of the water.

For every percent of moisture lost (and we only lose 2-3% of the total weight) it is going to behave like we lost 3 times that amount.

But I hear you saying to yourself that we lost 3% water, and it acts like 9%, why do we have to turn it down more than 10%?

It is because of another frankly amazing property of water.  It is called the Heat of vaporization.  All that is is a measure of how much more energy it takes to boil the water.

Say we have a gram ofwater at 100 C (212 F), or 211.9999 F (at sea level of course), what happens if we add 1 more calorie of energy.  It should go to 213 F (or try to) and boil, right?  Nope.  Not even close.  It takes a whopping 533 calories to boil that gram of water.  It takes a lot of energy to actually get the water molecules to let go of one another and the water become steam.

To kind of put that in perspective, when you are working your way through the drying phase you are raising the beans and water about 75 C.

Just to keep the numbers small let’s just say we are roasting 1 cocoa bean which is about 1 gram.  It is going to take about 0.4 calories  (0.07 for the 7% water +  0.33 for the bean (93% remaining weight) for each degree Celcius.  That means about 30 calories to get a bean up to the boiling point of water. (apologies, it is going to take some basic maths)

To go beyond that point we have to start vaporizing some of the water.  I know from taking weights before and after only about half of the water leaves during the roast so we can say 3% for convenience sake.  That means it takes another 16 calories (0.03 * 533) for wat water, so that really means it takes 46 calories to get truly into the Development phase.

Let’s say it took us 10 minutes to get there.  So we have been pumping in 46 calories over that time or 4.6 calories/min.  But suddenly we no longer have need of 4.6 cal/min.  We only need 3.0 cal/min (since much of the water is gone).  You can see pretty easily why we need to turn the heat down 1/3 or so, yes?  If we don’t the roast will proceed too fast, just as you saw and why 10% just isn’t enough to turn it down.

You might note I say I only turn down 25-30%, and in reality sometimes only 15-20%.  The reason is that other 3% of water that is still in the beans and still absorbing energy as I tries to vaporize.

I hope that explains why we have to turn the heat down so much after the drying phase and just how much a little bit of water will contribute to the energy needs of a roast.