Heat diffusion through grain mash to reach equilibrium

[majorjc]

I was reading posts this morning on another forum and came across a question that made me start thinking about physics. Since I've always tried to satisfy my curiosity through reading and trying to learn new things, or trying to remember the things that I've forgotten years ago, I went to google and started searching for an answer. That led me to this forum.

My question relates to this post on homebrewtalk.com forums.

I used an induction cook top for the first time this past weekend. I brought my mash water up to temp (155º), turned off the cook top, and added my grains. After stirring well, the temp was now down to 152º. I covered my pot, which was wrapped and topped in one layer of Reflectix, with a large towel. I set a timer for 30 minutes. I came back about 20 minutes later, and decided to give it another stir. The temp now registered at 180º! How could this be? The cook top had been turned off. Is it even physically possible for the temp to rise after killing the heat source?

I don't own an induction cooktop, but I understand that it creates a magnetic field that causes the cookpot itself to generate heat. I don't know what type of brewing pot this home brewer was using, but he did say that it was "a 3 gallon, BIAB Porter in a 5 gallon kettle." I don't know the water/grain ratio for his mash.

So, my question is: Can a heavy bottomed brew pot hold enough heat to raise the temperature of a grain mash by 28°F over a 20 minute period after the induction burner is turned off?

I would love to see the formulas that would be used to calculate the heat diffusion --> equilibrium in this type of situation even if there is not enough known information in the above example to reach an answer.

Thanks for your time.
Major

 

[jrmichler]

Was the home brewer stirring the mash while originally heating it? If not, the temperature would not be the same throughout. If the thermometer was in a less hot location, the heat from hotter locations would conduct through and raise the temperature after turning the stove off. It is unlikely that the pot was the source of the heat because metals have low specific heats, so the thermal mass of the pot is normally much less than the thermal mass of the contents. The thermal mass is the mass multiplied by the specific heat.

Note that any stove heats the pot, and heat conducts from the pot to the contents. If the contents circulate freely, the temperature variation could be minimal, while an unstirred mash could have large temperature variation.

 

[majorjc]

Was the home brewer stirring the mash while originally heating it?

The way this is done is the water for mashing is brought up to a specific temperature based on the Mash Infusion, Strike Water, and Rest Schedule Calculator on https://www.brewersfriend.com/ or some other brewing software. This calculator determines the temperature of your "Strike Water," which is simply the water you will be adding your grains to. In the calculator you enter the weight of your grains, the water/grain ratio and the temperature of you grains and what you want your Mash Temperature to be. It will output the volume and temperature that your strike water needs to be so that when you stir in your grains the final temperature will be your mash temperature.
In his case, he heated his strike water to 155°F and then turned off his induction burner.
At this point he stirred in his grains completely, and measured his temperature again at 152°F, which was his desired mash temperature. When he returned to check it 20 minutes later, the temperature had risen to 180°F.

So, if we assume that the mash was stirred well and was evenly heated throughout, and the heat retained by the metal pot after the burner was turned off could not have caused the mash to increase 28°F, then the most likely explanation is that the induction burner was either not turned off all the way or that it is faulty.

 

[majorjc]

It is unlikely that the pot was the source of the heat because metals have low specific heats, so the thermal mass of the pot is normally much less than the thermal mass of the contents. The thermal mass is the mass multiplied by the specific heat.

I see what you mean if these numbers I found on google are correct.

Water's specific heat power is 4.2 joules per gram per Celsius degree.
The specific heat of malt is about 0.44 times the specific heat of water. (so 1.8J/g/°C)
304 Stainless Steel Specific Heat Capacity 0.500 J/g-°C @Temperature 0.000 - 100 °C