Can mash get hotter after turning off the heat source?

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Brownyard

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This stems from another thread, but thought it was worth inquiring in this forum...

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 think it's possible. Either you didn't mix your heated water well enough before you mashed in, or you left the induction burner on (simmer maybe?).
 
I dunno, I think this happening could be explained.

If the pot had a thick enough heat spreader I bet a substantial portion of its mass would be hotter than the contents of the pot from a standing start and for some time thereafter until the whole "system" reaches equilibrium. Lidding and wrapping it cut down on a substantial thermal loss path...

Cheers!
 
I have a brew kettle that has a super thick bottom and have made the mistake of stopping at my target temp before shutting down the jet burner. What I learned is I really have to sneak up on my strike temp. What I mean is when I get within 10 degrees I back the burner way down and stir and carefully monitor then let stabilize before adding grains. I think the thick bottom absorbing heat that hadn’t quite radiated up before turning heat off and covering was my issue. Thin bottom keggle never happened.
 
I use a 10G pot on a cast iron burner stand and the thermal mass of the red hot cast iron will put significant heat into the strike water after shutting off the propane. But that's a pretty extreme example.
 
I have a brew kettle that has a super thick bottom and have made the mistake of stopping at my target temp before shutting down the jet burner. What I learned is I really have to sneak up on my strike temp. What I mean is when I get within 10 degrees I back the burner way down and stir and carefully monitor then let stabilize before adding grains. I think the thick bottom absorbing heat that hadn’t quite radiated up before turning heat off and covering was my issue. Thin bottom keggle never happened.
This makes the most sense to me.
 
Where was the thermometer placed and was it used for both measurements? I'll give an example where this is very important. My HLT has its temperature probe in a tee right before the valve. If I don't recirculate the water with my pump, I can set the PID for 168F but the water will end up boiling as the probe is recessed and a little lower than the heating element in the keggle. That's more extreme than I can imagine your situation as you are applying heat at the bottom of the pot.

Perhaps an operator error of some kind, like not letting the thermometer sit long enough? What kind of thermometer? Dial thermometers for example need a little time to reach the correct temperature on the dial. Or you might be recalling the order of events out of sync, like potentially you took a reading for the water, got the grains, stirred for a bit, and then turned off the burner. Not trying to gaslight you, but I sometimes have mistaken my recollection of a process (hungover for instance 🙂 .

I also know that with my own 3V 2P system that the reading on the MT temp probe requires about 5+ minutes to stabilize after striking, even after stirring. A little bit of recirculation time plus I have to stabilize the HLT temperature at a lower temp by dumping in a gallon or so of colder water as the HLT is at strike temperature. I'm trying to think which way mine is off but I have learned to just wait a few minutes for stabilization. Again, maybe not your situation though as you are expressing confidence that your strike water was 155F.

From a physics standpoint, you are putting in a set amount of energy to the system. I've never used an induction burner, but an electric burner will add some heat for about a minute or two after turning off as the burner cools down. (I sometimes turn the burner off about a minute before a boil is due to finish on my stove.) Not nearly enough though for what you are reporting but mentioning it as a small addition to what @balrog is saying in that there are multiple components to the total energy going into the system. However, once you turn off the heat that will end the energy input, potentially with a lag.
 
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A side question, what type of brewing exactly are you doing? I use Beersmith to do my mash calculations. I would say my strike temps are always higher than that for a target mash temp of 152. More than 3 degrees difference between the strike water and mash temp.
 
This was a 3 gallon, BIAB Porter in a 5 gallon kettle. The same dial thermometer in the same placement for both measurements. I made notes as I went along. After turning off the cook top at 155º, I added and stirred the grains. I watched it at 152 for at least 5 minutes after stirring in the grains (very well) to make sure it was holding, then I covered it with a towel. My kettle has a thick bottom, and holds heat very well, so I'm going to assume there was enough residual heat left to heat it up further once covering, although 30º seems ridiculous. I guess I need to stir outside of the bag as well? I have no other explanation for it. I do know that I will be removing the pot from the cook top for future mashes. I'm also going to make use of my remote grill thermometer with an alarm in the future.
 
I do always remove my kettle from burner, but I know there would be heat transfer after burner was off. Of course, I wanted to meticulously chart the amount of heat and eliminate the "move the pot off then back on" steps, but I knew that since I brew in the garage, I would have seasonal variations. I pretty quickly decided there was no return on that investment of time.
 
A 22º rise in temperature when you should have been seeing a possible drop in temp by a degree or 2 does not sound like it was from lack of circulation or residual heat rise. I suspect that your induction burner was placed on a low setting or it's faulty and didn't turn off when it should have. I would test it out with just water to see how it behaves. You can also unplug it when it's turned off to ensure the plate is not continuing to heat.
 
I did tests with water only prior to my first batch. The results differed from the wort. I'm going to test again with the pot removed from the burner.
 
Did you also change your pot when switching to the induction burner? If not, the thermal mass of the pot has not changed. The induction burner should not be providing heat once turned off, so removing the pot should have zero affect. Maybe @Silver_Is_Money is on the right track with the burner throwing the thermometer off kilter.
 
Did you also change your pot when switching to the induction burner? If not, the thermal mass of the pot has not changed. The induction burner should not be providing heat once turned off, so removing the pot should have zero affect. Maybe @Silver_Is_Money is on the right track with the burner throwing the thermometer off kilter.
But I'm not using a digital thermometer.
 
Just read this.
"Because induction magnetics have some "reach" above the cooking surface, they'll often heat the thermometer directly so you don't get an accurate temp reading."
I guess I could turn the cooker off to take temps while heating up, but doesn't do much for constant monitoring.
 
Along the lines of maybe the burner was still on, one of the dials on my stovetop will be turned to where it looks off but isn't. It's a flat top electric (not induction) and the knob needs to pop out to be completely off. If I am moving quickly in the kitchen sometimes I go to turn it off and I don't get it all the way around and it will look like it's off. If you thought it was off but you doublechecked it at some point you may have moved it to completely off. It's pretty obvious in my case that it is still on as this burner has a small inside burner and the control knob controls that on one side of the dial and the outer ring + the small ring on the other. I am generally using the full power side of the dial and turning it off approaches full power plus the element gets red.
 
Along the lines of maybe the burner was still on, one of the dials on my stovetop will be turned to where it looks off but isn't. It's a flat top electric (not induction) and the knob needs to pop out to be completely off. If I am moving quickly in the kitchen sometimes I go to turn it off and I don't get it all the way around and it will look like it's off. If you thought it was off but you doublechecked it at some point you may have moved it to completely off. It's pretty obvious in my case that it is still on as this burner has a small inside burner and the control knob controls that on one side of the dial and the outer ring + the small ring on the other. I am generally using the full power side of the dial and turning it off approaches full power plus the element gets red.
I just saw the other thread identifying your model which doesn't have a knob so no help with this suggestion!
 
Thermal mass of the pot had absorbed the heat but not distributed it to the wort yet. Simple.
The thermal mass is the mass multiplied by the specific heat.

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

(Yes, I sought an answer at physicsforums.com)
 
The same dial thermometer

Perhaps dial thermometers have improved since I last used one, some twenty years ago, but it didn't take me long to realize that lab thermometers used over the course of several minutes of slow, steady stirring was the way to go...then the clouds parted, the angels sang, and the glorious light of Thermapen was revealed to me. Despite the Thermapen's speed and accuracy, it still takes several minutes of stirring to reach anything that approaches a range where you can safely say, I'll hit equilibrium within my target mash range. You're dealing with a huge amount of matter at wildly different temperatures. It takes a significant portion of the mash duration to achieve your maximum, stable mash temp. Until then, you have wild temperature gradients.

Personally, I think you accidentally added energy into your system, but the simpler answer is that you didn't stir enough and your thermometer was reading a cold spot--or perhaps it botched the strike temp?

Is your thermometer calibrated? Do you trust it? If so, can you demonstrate why?

Our brewing software is amazing and it's so very easy to stop thinking on brew day because the software is so incredibly good. Nevertheless, as brewers, it's our responsibility to be vigilant and skeptical observers. It's our job to rigorously fact-check our software, never trusting it.
 
Anything is possible. I stirred it for a good 5 minutes at least, but that may not be enough for some. It was only 3.5 gallons of water. I'll be doing another test soon. The thermometer always worked great on my standard electric stove top. Maybe once I killed the power on the induction top, the thermometer resorted to proper temp reading, if interference was a factor. I'll try to determine this.
 
Anything is possible. I stirred it for a good 5 minutes at least, but that may not be enough for some. It was only 3.5 gallons of water. I'll be doing another test soon. The thermometer always worked great on my standard electric stove top. Maybe once I killed the power on the induction top, the thermometer resorted to proper temp reading, if interference was a factor. I'll try to determine this.
How much grain, about 5 lbs? I'm not really familiar with BIAB brewing specifics but I plugged in a basic BIAB medium body recipe at 3 gallons into Beersmith. The mash calculation came out about the same temperature as your strike temp so I see now where that is closer in temperature to my usual calculations (More water as compared to the grain weight). Something is wildly untrustworthy in the actual measurements, I don't think an answer lies in a potential software input such as not accounting for your equipment or an error in an input field. And that's if you used a software program or brewing calculator. To arrive at 180F, assuming the induction burner was off off, the water would have to have been upwards of 183F to start with. Also, a five gallon pot doesn't weigh much 5 pounds maybe(?) as opposed to 3.5 gallons of water plus about 5 lbs of grain, that's over 30 pounds. It's not boiling but did you notice anything indicating the strike water was that hot (183F)? Getting a little steamy or the pot making creaky noises? What was the specific gravity at the end? I think you would have cooked the **** out of the mash at that temperature.

Is your dial thermometer mounted on the pot? If not, you could try different size pots and see if you get unusual readings based on thermometer distance from the induction burner. Definitely use a small pot as at least one test.
 
Well, after doing another test today, I determined that it had to be my error. I tested 4 different thermometers, the 12" dial thermometer I originally used, two new temp probes attached to my ThermoPro TP-08s digital, and a folding instant read digital. All were within 2º of each other throughout the test. Water boils at 211.5º at my elevation, and the two ThermoPro probes registered 211º and 212º, so I reckon that's a .5 degree error. I can live with that, and will use these in the future. I was glad to find there was no detectable interference with any of the thermometers while the induction top was turned on or off. Also, I didn't remove the pot from the cook top at any time. Upon cutting the power at 156º, one of the probes registered a 1º increase in temp, before dropping back. Temp then fell from 156º to 149º over an hour, while cook top was turned off. The pot and lid were covered in 1 layer of Reflectix, with a towel draped over the top.

Possible causes for the original perceived jump from 152º to 180º:

- I found the clip that holds the 12" dial thermometer was moved when the lid was put on, which could have raised the tip out of the wort during one of the readings. It could have been measuring air temp above the wort, or even the side of the pot...

- Most likely though - When I added the grain to the 155º strike water, I stirred it in (really good), resulting in a temp of about 149º. I then turned the cook top up to bring it up to 152º, then I turned it off. There is a real possibility that I failed to stir the wort again after turning the cook top off. If that's the case, I guess the water at the bottom of the pot heated up quite a bit, and when I stirred the wort again at 20 minutes, it brought the temp up to 180º.

Regardless, I'm glad to know I can make the induction burner work. I just need to really pay attention when I add the grains, and stir every time I add heat.
 

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I think you would have cooked the **** out of the mash at that temperature.
I was afraid of that, but the wort did taste good, and sweet, and OG was 1.059, .003 above target. I only had Windsor yeast, which is apparently a notoriously low attenuator, but after 4 days, gravity had dropped to 1.025. Hopefully it will continue downward over the next couple of weeks. Tastes good though!
 
I guess I need to stir outside of the bag as well?
The mash bag should fill the whole kettle space, wall to wall, or at least very close to filling it. It's counterproductive for mash efficiency having water/wort on the outside of the bag, between the bag and wall, similar to having (too much) deadspace underneath the bag/mash. I can recommend buying a custom made-to-size BIAB bag from @wilserbrewer.

Wrapping a double layer of Reflectix and/or a couple thick, folded-over towels around the kettle during heating, mashing and boiling reduces heat loss. You can't (and shouldn't!) do that on a regular coil or flat top stove, as flammables should be kept well away from the hot glowing coils/elements.
That way your 1800W induction plate will be working much more efficiently.

When mashing place a couple thick, folded-over towels on top of the lid too, for the same purpose.
 
- Most likely though - When I added the grain to the 155º strike water, I stirred it in (really good), resulting in a temp of about 149º. I then turned the cook top up to bring it up to 152º, then I turned it off. There is a real possibility that I failed to stir the wort again after turning the cook top off. If that's the case, I guess the water at the bottom of the pot heated up quite a bit, and when I stirred the wort again at 20 minutes, it brought the temp up to 180º.

Yes, this! We were all stumped by the source of the additional heat. The original post did not mention the fact that the burner was turned back on after mashing in. This is definitely where that extra heat came from.
 
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