Feel the Mash Heat

When I look at my mash tun, I feel a sense of pride. When I withdraw my floating thermometer from the infusion of water and malt and it reads 150° F, I feel like a true brewmeister. Oh, but there were rocky times. I’ve overshot, underheated, scorched, and burned. I’ve been down with the iodine test blues. Looking back now, I can see why hitting my target mash temperature was so hard. As a naive young brewer, I had too little patience, insufficient equipment and, worst of all, no propensity for heat transfer calculations. Now I know that with some minor advance planning any brewer can mash with style and accuracy. All you need is a little understanding — of energy flow.

Understanding Heat

Heat is the transfer of energy between two objects of different temperatures. If you touch your finger to a hot kettle, heat is immediately transferred from the metal to your skin. Heat will only flow from a higher temperature to a lower one. Because your finger is at a lower temperature, heat will flow to it until you finally have the sense to pull your finger away. Either that or the two will reach equilibrium. In this case your finger will increase in temperature and the kettle will decrease in temperature until the two reach the same point. (And your poor finger will be burned.)

This is basically what happens when the brewer mashes room-temperature grains into hot water. The water loses heat to the atmosphere and to the grains the same way the kettle lost heat to the finger. The grains in turn increase in temperature as they are heated until finally the water and the grains reach an equilibrium that is (hopefully) somewhere around starch conversion temperature.

If this makes sense to you, then you understand the first law of thermodynamics. “The energy of an isolated system remains constant.” Or more aptly put, the energy you put into your mash tun in the form of heat is conserved. With that said we know that by adding grain at a certain temperature to water at a certain temperature, we can predict the final temperature. What we don’t know is how to do it with precision. That requires understanding (or at least using) heat transfer calculations.

Heat Transfer

Everything transfers heat at a different rate. A homebrewer’s mash tun is a perfect example of this. Imagine you have a fully insulated picnic cooler and a steel pot of the same volume. An equal amount of hot water is placed in each and their respective lids are placed over the top. An hour later the temperature of the water in the stainless steel pot has dropped by more than 10 degrees while the water in the picnic cooler has dropped only two or three degrees. Why did this happen? Steel conducts heat faster than the insulated material of the picnic cooler.

Every material, including malted barley and water, conducts heat differently. The amount of energy in the form of heat needed to raise a kilogram of malt by one degree is represented by a number called the specific heat. Likewise, there is a specific heat for water, wort, steel, air, and just about any other material you can imagine. By using the specific heat of malt and water along with their weights and temperatures, we can predict fairly accurately what temperature will result when the two are mixed in an insulated mash tun.

Infusion Mashing

Traditional British brewers make their beers by infusion mashing. Infusion mashing involves heating water to a temperature higher than the desired mash conversion temperature, called the strike temperature.

In an insulated vessel the grains and the hot water are mixed. The heat of the water transfers to the malt and the mash reaches a temperature of 150° to 155° F. Using specific heat values for water and malt makes hitting the desired conversion temperature in an infusion mash easy.

Hm = heat capacity of malt = 0.3822 Btu/lb. ° F
Hw = heat capacity of water = 1 Btu/lb. ° F
Tmt= temp. of dry malt, ° F
Tw = temp. of water, ° F
Tma= temp. of mash, ° F
M = weight of malt, lbs.
W = weight of water, lbs.
(1 gallon water = approx. 8.3 lbs.)

The equation:

W x Hw x (Tw-Tma) = M x Hm x (Tma-Tmt)

Now apply the equation to the following Irish-style stout recipe.

Irish Stout
(5 gallons, all-grain)


• 6 lbs. two-row pale malt
• 1 lb. flaked barley
• 1 lb. roasted barley
• 0.5 lb. crystal malt, 40° Lovibond
• 0.9 oz. Northern Brewer hops (8% alpha acid), for 60 min.
• Ale yeast

Boil time = 1.5 hours
IBU = 25   OG = 1.049   FG = 1.012

Step by Step:

First, calculate the volume of mash water using 0.33 gals./lb. malt. 8.5 lbs. malt x 0.33 gals. = 2.8 gallons mash water

Now, calculate the desired strike temperature.
Hm = heat capacity of malt = .3822 Btu/lb. ° F
Hw = heat capacity of water = 1 Btu/lb. ° F
Tmt= temp. of dry malt= 74° F
Tw = temp. of water = ?
Tma= temp. of mash = 150° F
M = weight of malt in lbs = 8.5 lbs.
W = weight of water = 2.8 gallons x 8.5 lbs = 23.8 lbs.

Now, substitute values into the equation and solve for Tw, the strike temperature.

W x Hw x (Tw-Tma) = M x Hm x (Tma-Tmt)

23.8 x 1 x (Tw-150°) = 8.5 x .3822 x (150°-74°)

Tw = 160.4° F

For this brew a strike temperature of 160.4° F will drop to 150° F when the malt is mixed in.

When infusion mashing, use an insulated mash tun such as a picnic cooler or a pot wrapped in a sleeping bag. If you use a cooler, heat the mash water in a pot to a temperature slightly above your calculated strike temperature. Then transfer it to your mash tun. Close the lid and allow the temperature to come to equilibrium with the mash tun for two or three minutes. If the temperature is too high, add cold water by the cup until it reaches the strike temperature. If the temperature is too low, add boiling water. When the strike temperature is right, mix the grains in well. Cover with a lid and allow the solution to come to equilibrium for a few minutes. Then check to make sure the mash temperature is correct. Every time you open the lid, heat is lost to the atmosphere. So once you are satisfied that you have reached the right temperature, leave the mash alone for an hour and allow the magic of conversion to happen.

Decoction Mashing

In Germany and continental Europe undermodified malt made decoction mashing a necessity. The heat-intensive mashing and the boiling process help degrade the starches so that enzymes can react more easily. In contrast the British infusion mash requires well-modified malt and needs no additional decomposition beyond the simple single-temperature conversion. Today’s malts are generally sufficiently modified to make decoction mashing unnecessary. Still, many beer-style purists insist the boiling of the grains contributes important flavors unique to many German-style beers.

Decoction is a mechanical process in which steam vapor breaks down starch molecules. In the decoction mashing process water and malt may be combined at a lower temperature, such as 127° F, to allow protein-degrading enzymes to act. A large portion of the mash is then immediately removed from the main mash and heated to 150° F to allow for a 15-minute starch conversion. The decoction is then boiled for another 15 minutes. Still boiling hot, the decoction is mixed into the main mash to increase the total mash temperature to 150° F. A second decoction can then be performed to increase the temperature of the main mash to 165° F, the mash-out temperature. These figures are just a rough guide.

Once again the homebrewer can turn to the world of heat transfer to make this rigorous mash schedule a simple task. The following recipe is for a medium-bodied German alt-style beer. If you prefer a partial mash, substitute 3.5 pounds of liquid malt extract for four pounds of pale malt and adjust the mash water volume accordingly.

German Alt
(5 gallons, all-grain)


• 4 lbs. pale two-row malt
• 2 lbs. wheat malt
• 2 lbs. Munich malt
• 0.5 lb. crystal malt, 20° Lovibond
• 1.5 oz. Perle hops (10% alpha acid), for 60 min.
• 1 oz. Perle hops (10% alpha acid), for 30 min.
• 2 oz. Mt. Hood hops (4.5% alpha acid), for 15 min.
• Alt or kolsh yeast

Boil time = 1.5 hours
IBU = 28  OG = 1.049  FG = 1.010

Step by Step:

Calculate the volume of mash water:

8.5 lbs. x 0.33 gallons/lb. = 2.8 gallons

Then calculate the initial strike temperature Tw for a protein rest at 127° F.

Hm = heat capacity of malt = 0.3822 Btu/lb.° F
Hw = heat capacity of water = 1 Btu/lb.° F
Tmt= temp. of dry malt = 74° F
Tw = temp. of water = ?
Tma= temp. of mash = 127° F
M = weight of malt = 8.5 lbs.
W = weight of water = 2.8 gallons x 8.5 lbs. = 23.8 lbs.

W x Hw x (Tw-Tma) = M x Hm x (Tma-Tmt)
23.8 x 1 x (Tw-127°) = 8.5 x .3822 x (127°-74°)

Tw = 134° F

The mash water should be at 134° F when the malt is added. After the malt is mixed in, the temperature should drop to 127° F. At this point a decoction will be pulled from the main mash. To calculate how much boiling decoction mash is needed to raise the main mash from 127° F to 150° F, apply the following equation. The specific heat of the mash does not apply to this calculation, since the decoction and the main mash have about the same specific heat value.

Td = Temp. of decoction = 212° F
T1 = Initial main mash temp.= 127° F
T2 = New main mash temp. = 150° F
D = Weight of decoction mash = ?
Z = Weight of main mash = W + M = 23.8 + 8.5 = 32.3 lbs.
D = [Z x (T2-T1)]/(Td-T1)
D = [32.3 x (150°-127°)]/(212°-127°) = 8.74 = 9 lbs.

About nine pounds of decoction mash will be needed to raise the temperature of the main mash to 150° F. The same process can then be applied to the second decoction. In this case T1 would be 150° F and T2 would be 165° F.

I use a three-gallon pot for the decoction mash. The pot is weighed on a kitchen scale before I start. When the decoction is pulled from the main mash, the weight is immediately taken and adjusted to the calculated value. During the 15-minute starch conversion, I let the pot sit in the oven on low heat to keep the temperature up.

After boiling the decoction mash for 15 minutes, carefully stir the hot mixture into the main mash. Cover the mash tun and allow the temperature to come to equilibrium for a few minutes before checking. The heat calculation requires additional parts when dealing with decoction mashing due to the increased complexity of the process. However, I have had good results with the calculation and believe it takes most of the guess work out of decoction mashing.

The heat transfer calculations, one for combining water and malt and the other for mixing two mashes at different temperatures, are adaptable to a number of mash regimens. You just need the temperatures, the weights, and perhaps a little understanding.

Issue: September 1997