FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
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Contents:
Malt Analysis (Thomas Rohner)
Mash temperature calculations (specifically "slaking heat") ("Bill Pierce")
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Date: Wed, 25 Feb 2009 13:07:14 +0100
From: Thomas Rohner <t.rohner at bluewin.ch>
Subject: Malt Analysis
I think it would be a good idea for a maltster to put the lot data of
the products on their Website. This way the ones interested in the data
could get it without the maltster having to reply phone calls or emails.
I'm also using Weyermann almost exclusively. (the easiest to get around
here)
Cheers Thomas
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Date: Wed, 25 Feb 2009 11:26:01 -0500
From: "Bill Pierce" <BillPierce at aol.com>
Subject: Mash temperature calculations (specifically "slaking heat")
Thanks to all for the replies to my earlier question about the
thermal calculations for accurate mashing temperatures. It takes me
some time to digest all of the information (I'm not a scientist by
training), and of course brewing is not the only thing in my life.
As a couple of people pointed out, the difference between the
results of my experiment with hot water and the calculated thermal
mass of my converted keg mash tun based on the heat capacity of 304
stainless steel is due to the fact that the keg is not full; only a
portion of it is in direct contact with the strike water. No doubt
a truly accurate formula would have to take the geometry of the
vessel into account, as well as the thermal conductivity of the
vessel's material as it conducts and radiates heat into the
environment. I was also supplied with an interesting article by Dan
Morey that I had not seen before, which includes formulas for
predicting the temperature drop over time when mashing. My thoughts
on that subject are that additional factors such as humidity and
wind velocity (if mashing outdoors) have an influence as well.
I'll retreat to a position that the best way to determine a vessel's
thermal mass is to calculate it on the basis of measurement by
experiment. It's simple enough to do, and I trust my formula:
ThM = ((Ts-Td)*2.08635*Vw)/(Td-Tm)
Where:
ThM = Thermal mass of vessel (lbs. or kg)
Ts = Strike water temperature (degrees F or C)
Td = Water temperature achieved after settling (degrees F or C)
Vw = Volume of strike water (quarts or liters)
Tm = Mash tun temperature (degrees F or C--typically ambient
temperature)
The formula works for metric brewers as well, as long as they use
metric values throughout and omit the 2.08635 coefficient, which
converts the water volume in quarts to pounds. Do the measurement
with water only; I have eliminated the grain variables from the
equation.
However, several people pointed out one other variable I had not
considered, nor does this seem to be mentioned in the standard
homebrewing literature. This is the notion of "slaking heat";
apparently the hydrolysis of the malt starches is an exothermic
reaction that releases measurable heat. Various professional
brewing references, including "Brewing: Science and Practice" by
Briggs, Boulton, Brookes and Stevens, include the following formula:
F = (St+RT+0.5h)/(S+R)
Where:
F = Final temperature of mash after settling
S = Specific heat of malt
t = Temperature of malt
R = Water/grain ratio of mash
T = Strike water temperature
h = Slaking heat of malt
Solving the equation for h (slaking heat), results in this formula:
h = -2*(R+S)*(((St+RT)/(R+S))-F)
The literature includes a table of slaking heat based on a mash
temperature of 150 degrees F. If I plug this temperature and my own
typical values (malt temperature of 70 F, strike water at 160 F, 0.4
for the specific heat of malt, 1.35 quarts per pound water/grain
ratio) into the formula, I arrive at a the same value (18.8
gram-calories per degree F) as the table. Using the formula and
converting 18.8 gram-calories/degree F) to the value used in my
strike water temperature formula, the result is 0.07460 BTUs (metric
brewers will excuse me here). In a hypothetical mash with 10 lbs.
of grain, again at a water/grain ratio of 1.35 qts/lb, the result is
a total of 111.45 BTUs at 150 degrees F.
The effect on the calculated strike water temperature is
significant, a total of 4.0 degrees F in my hypothetical example.
This is consistent with what Briggs et al. report. However, as I
have said, this is nowhere mentioned in the standard homebrewing
literature nor apparently included in the software most homebrewers
use.
There is a suggestion that the slaking heat is offset by the thermal
mass of the mash tun. Indeed if I eliminate both the vessel's
thermal mash and the slaking heat from my strike water calculation
formula, the difference is reduced to 0.8 degrees F, likely within
the measurement resolution of most thermometers.
Still, it seems there is considerable margin for error in the
homebrew strike water temperature formulas, and those of us who wish
to be precise (some would say anal) about these matters are
disturbed. Is the slaking heat of a typical homebrew mash as great
as 4 degrees F as the formulas and literature suggest, or is this
largely much ado about nothing?
Any input from the technically minded would be greatly appreciated.
Brew on!
Bill Pierce
Cellar Door Homebrewery
Burlington, Ontario
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End of HOMEBREW Digest #5513, 02/25/09
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