From BYO magazine-
Step Mashing in a Picnic Cooler
Multirest mashes require heat additions to step the mash temperature through the various enzyme rests. This process can be tricky in a picnic cooler because rather than simply heating the vessel to the desired temperature, you must instead add precisely calculated quantities of boiling water to achieve the desired temperatures (see below). A further complication is that the thermal mass of the mash increases with each addition, and more and more water is needed at higher temperatures to continually raise the temperature.
Therefore, if your cooler is moderately sized for your mash, you need to start out with a stiff mash (perhaps even as low as 3Ž4 qt/lb of grain) to leave yourself enough volume for the additional water. Even then, only two temperature rests are usually possible, but you can achieve a third rest if the change in temperature is only a few degrees.
You need to decide whether the additional work is desirable, or even necessary, for your recipes. Review Jim Busch's article on step mashing on page 26 to help make the determination. It's probably best to get a real handle on the single-infusion mash before diving into further manipulations.
Calculating Water Additions for a Step Mash
This calculation is based on calorimetry and thermal equilibrium. By determining the amount of heat provided by a volume of hot water we can predict how much that heat will change the temperature of the mash. The basis for this calculation is the first law of thermodynamics, which assumes that no heat will be lost to the surroundings.
The factors used in the following equation are rounded to single digits to make the math simpler. The difference between these and more precise figures is at most a cup of hot water and less than 1 °F. The equation presented here has been algebraically simplified, including conversion of the mass of hot water to volume. All temperatures must be in degrees Fahrenheit. Experience has shown the equation to be fairly reliable, even if it may be a few degrees off in its prediction, depending on the mash tun. It will be consistent if the mash tun is preheated in the same manner for each batch.
Performing your step mash:
You can tackle the initial infusion in two ways. You could use the seat-of-the-pants infusion approach described in the main text for the initial wetting (that is, guessing the proper strike water temperature to be 10-15 °F above the target mash temperature). Measure your resulting temperature and proceed with the infusion equations from there.
Or, use the simplified equation provided here to arrive at the proper strike water temperature. When mixing hot water with dry grain, the amount of grain does not matter, only its temperature.
Initial infusion equation:
Strike water temperature (Tw) = (0.2 ÷ R) X (T2 - T1) + T2
Mash infusion equation:
Wa = (T2 - T1) X (0.2G + Wm) ÷ (Tw - T2)
where:
Tw = the actual temperature of the infusion water
R = the ratio of water to grain in quarts per pound
T1 = the initial temperature of the mash (or dry grain)
T2 = the target temperature of the mash
Wa = the amount of boiling water added (in quarts)
Wm = the total amount of water in the mash (in quarts)
G = the amount of grain in the mash (in pounds)
The infusion water does not have to be boiling; the nominal sparge water temperature of 170 °F (77 °C) will also work, which means that the Tw becomes 170 °F, and more water (Wa) will be needed to make up the additional quantity of heat.
Example
This example pushes the envelope with three rests. Suppose we plan to mash 8 lb of grain through a 104 °F, 140 °F, and 158 °F (40 °C, 60 °C, and 70 °C) multirest mash schedule. For the purposes of this example, we will assume that the temperature of the dry grain is 70 °F (21 °C). The first infusion will need to bring the temperature of the mash from 70 °F to 104 °F. We will start with an initial water ratio of 1 qt/lb. Using the initial infusion equation, the strike water temperature is:
Tw = (0.2 ÷ R) X (T2 - T1) + T2
Tw = (0.2 ÷ 1) X (104 - 70) + 104 = 110.8, or 111 °F
For the second infusion, to bring the temperature to 140 °F, we need to use the mash infusion equation. At 1 qt/lb, Wm is 8 qt. We will assume that our boiling water for the infusions has cooled somewhat to 210 °F.
Wa = (T2 - T1) X (0.2G + Wm) ÷ (Tw - T2)
Wa = (140 - 104) X (1.6 + 8) ÷ (210 - 140)
Wa = 36 X 9.6 ÷ 70 = 4.9 qt
For the third infusion, the total water volume is now 8 + 4.9 = 12.9 qt.
Wa = (158 - 140) X (1.6 + 12.9) ÷ (210 - 158)
Wa = 18 X 15.1 ÷ 52 = 5.2 qt
The total volume of water required to perform this schedule is 8 + 4.9 + 5.2 = 18.1 qt, or 4.525 gallons). The final water-to-grain ratio has increased to 17.9 ÷ 8 = 2.2 qt/lb.
