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Old 08-19-2013, 01:42 AM   #1
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Default Crazy-low mash temperature when making my pumpkin beer!

Hi,

I was brewing my Southern Tier Imperial Pumking clone today. My grain bill was:

14 lbs - Briess 2-row
3 lbs - Pumpkin puree
1 lbs - Caramel 80L
1 lbs - Rice hulls
Some crushed up graham crackers

Based on all my calculations, both in Brewtarget and on several websites, my strike temp should have been right around 170 to hit a mash of 156 (yes, mashing high). I transferred the strike water to my 10 gallon cooler and allowed it to warm the cooler for a few minutes.

I then added the 3 lbs of pumpkin puree (temperature 67~ degrees). I then added my grain and stirred vigorously. After 15 minutes, I returned to ensure I was hitting my mash temp. 146!!!!! WTF???? I thought my floating thermometer was wrong, so I grabbed another. Ditto.

How can this be? I only had the lid off for about 2 minutes to add the pumpkin and the grain. This makes no sense. I've never had trouble with hitting my mash temp before. If anything, I usually overshoot by a degree.

Any thoughts? I don't get it, unless I REALLY didn't read my HLT thermometer right when I transferred, but I really don't think that's the case.

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Old 08-19-2013, 01:50 AM   #2
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Easy answer, but I can understand your dismay: 10° down... Now what?

The thermal mass of pumpkin puree is quite high. Do the calculators really have a "dense pulp" function built in?

I'm ready to brew a pumpkin ale and intend to just use the absolute minimum of spices, so it doesn't taste like pumpkin pie. Pumpkin has a very delicate flavor and I've made ice cream out of it. Without all the over-the-top spice it tastes really good. I'd like a pumpkin ale similar to that.

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Old 08-19-2013, 01:58 AM   #3
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Quote:
Originally Posted by IslandLizard View Post
Easy answer, but I can understand your dismay: 10° down... Now what?

The thermal mass of pumpkin puree is quite high. Do the calculators really have a "dense pulp" function built in?

Haha, dense pulp function - that falls into the statistically improbable phrase category. I wish. I didn't think of that. Most "things" you can add have low density. Pumpkin is basically a big heat sink in the mash tun. I managed to adjust on the fly and still hit my target gravity, albeit at the sacrifice of some mouthfeel I would imagine. Now, the stuck lauter and sparge was a whole 'nother beast. I plan to brew this again in a few weeks. I think, based on my experience, I'm going to raise my strike temp about 10 degrees.
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Old 08-19-2013, 02:05 AM   #4
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I did a pumpkin this afternoon also. My recipe called for the pumpkin to be cooked in the oven at 350 for an hour. I took it out and let it cool for about 15 min so mash temp wouldn't go crazy high. It still was 2 degrees hotter than I wanted.

And I also got a stuck sparge....

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Old 08-19-2013, 01:48 PM   #5
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So I did some googling to find the thermal conductivity value for pumpkin. Some university physics course website in New Zealand had the value at .83 J/ m-1 s-1 deg. C -1, which is actually equivalent to the more standard Watts/m-1 deg K-1. That's really high, especially among the fruit and vegetable group (see here).

So, with Fourier's heat transfer law, you are looking at a very large temperature gradient and relatively high thermal conductivity compared to grain. Solutions --- overheat the water to compensate, or preheat the pumpkin puree to close to mash temperature. Both would reduce the temperature gradient, and subsequently, the thermal transfer.

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Old 08-19-2013, 02:09 PM   #6
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Quote:
Originally Posted by waldzinator View Post
So I did some googling to find the thermal conductivity value for pumpkin. Some university physics course website in New Zealand had the value at .83 J/ m-1 s-1 deg. C -1, which is actually equivalent to the more standard Watts/m-1 deg K-1. That's really high, especially among the fruit and vegetable group (see here).

So, with Fourier's heat transfer law, you are looking at a very large temperature gradient and relatively high thermal conductivity compared to grain. Solutions --- overheat the water to compensate, or preheat the pumpkin puree to close to mash temperature. Both would reduce the temperature gradient, and subsequently, the thermal transfer.
The OP used pumpkin puree so the thermal conductivity of chunks of pumpkin is not the issue here. Rather it's the high specific heat of the pumpkin compared to grain that's the problem. It takes a lot of energy to warm up that three pounds which results in a lower mash temp.
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Old 08-19-2013, 02:16 PM   #7
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The OP used pumpkin puree so the thermal conductivity of chunks of pumpkin is not the issue here. Rather it's the high specific heat of the pumpkin compared to grain that's the problem. It takes a lot of energy to warm up that three pounds which results in a lower mash temp.
Agree. I was the OP. I was thinking of pumpkin chunks because that was my ideal situation --- I just couldn't find fresh pumpkins yet. But yeah, the specific heat of the pumpkin puree is the issue here.

Either way, both situations require adjusting either the strike water temperature of the starting temperature of the pumpkin.

I use brewtarget since I'm a Linux guy. Does any other brew software handle pumpkin and other popular fruit/vegetable adjuncts better? There was nothing in my fermentables category that I could add that even comes close to pumpkin in terms of conductivity or specific heat.
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Old 08-19-2013, 02:21 PM   #8
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Maybe next time you could take the pumpkin puree, add it to a big bowl, add some water to it that is around 190 degrees stir it around, and then dump it all in.

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Old 08-20-2013, 11:36 AM   #9
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Quote:
Originally Posted by waldzinator View Post
Agree. I was the OP. I was thinking of pumpkin chunks because that was my ideal situation --- I just couldn't find fresh pumpkins yet. But yeah, the specific heat of the pumpkin puree is the issue here.

Either way, both situations require adjusting either the strike water temperature of the starting temperature of the pumpkin.

I use brewtarget since I'm a Linux guy. Does any other brew software handle pumpkin and other popular fruit/vegetable adjuncts better? There was nothing in my fermentables category that I could add that even comes close to pumpkin in terms of conductivity or specific heat.
I would assume that the specific heat of pumpkin is about that of water. Therefore, you could treat the pumpkin as if it was additional water in your strike. Let's say the software said you needed 4g at 170F and you're using 3lb pumpkin (about 3/8 gallon) at a temperature of 70F. Your 3.625 gallons of strike water then would need to have a temperature of T:

3.625*T + 0.375*70 = 4*170

Therefore:

T = (4*170 - 0.375*70)/3.625 = 180.3F

Of course I could be totally wrong here but the results seem reasonable.
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Old 08-20-2013, 12:45 PM   #10
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Quote:
Originally Posted by BackAlley View Post
I would assume that the specific heat of pumpkin is about that of water. Therefore, you could treat the pumpkin as if it was additional water in your strike. Let's say the software said you needed 4g at 170F and you're using 3lb pumpkin (about 3/8 gallon) at a temperature of 70F. Your 3.625 gallons of strike water then would need to have a temperature of T:

3.625*T + 0.375*70 = 4*170

Therefore:

T = (4*170 - 0.375*70)/3.625 = 180.3F

Of course I could be totally wrong here but the results seem reasonable.
One problem I'm running into is finding a good specific heat of pumpkin. So far, the only source I've found is the link above which indicates it is 3.97 kJ kg-1 degK -1. I believe this is somewhat less than water, right? Still, pretty close. So the algebra to me seems right. It also jives with the empirical data...I fell about 10 degF short.
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