Multi-Rest Mashing: How-To

In my last post, I discussed the theory behind multi-rest mashing.  Today I’ll be discussing the more practical how-to side of things.

There are many ways to add heat to your mash tun; I’ll go into a few common methods here.

Direct Heating

The most straightforward way to move from rest to rest is to conduct your mash in a pot on the stove.  Heating slowly and stirring constantly (but avoiding oxidation) while watching the temperature is a must.  More advanced temperature control systems exist, but chances are if you have one of these, you don’t need me telling you how to use it.

If you don’t have a pot set up with a false bottom or similar, you’ll need to ladle the mash into a separate lauter tun.

Infusion

mash infusion

Raising my mash temperature with a boiling water infusion.

Most brewers use infusion for mashing already.  This infusion is the strike water either added to grain or vice versa.  For the record, I prefer adding grain to water—I can overshoot my strike temperature and add it to my mash/lauter tun (MLT) to cool to my calculated strike temperature, heating the MLT in the process.  I can consistently hit my first rest temperature this way without any fudge factor for how much heat my MLT will suck up.

When multiple rests are required, the initial strike water should bring you to your first rest, and additional infusions of hot water are added to bring the mash to each additional rest.  Since water is added with rest, it is important to start with a stiff mash; Palmer recommends a minimum of 0.75  qt/lb or 1.5 L/kg (though this is very stiff).  Practically, only two or three rests can be achieved with infusion mashing.

The initial strike water temperature can be calculated as:

T_s=\frac{C_TW_g}{V_s} (T_2-T_1)+T_2

and additional infusion volumes can be calculated as:

V_i=\frac{(T_2-T_1)(C_TW_g+V_m)}{T_i-T_2}

where:

  • T_s = temperature of the strike water
  • C_T =thermodynamic constant; 0.2 for English units, 0.41 for metric units
  • W_g = weight of the grain, lb or kg
  • V_s = volume of strike water, qt or L
  • T_1 = current temperature of the mash or grain, F or C
  • T_2 = desired mash temperature, F or C
  • V_i = volume of infusion
  • T_i = temperature of water added during the infusion
  • V_m = volume of water currently in the mash, qt or L.

You can use the infusion calculation to get to another rest temperature with any temperature of hot water; boiling water is preferred for large temperature changes, but many people use sparge temperatures.  These equations are nicely automated in this web-based calculator.

You may have noticed that there is any easy way to account for the heat lost to your MLT for the first addition by adding water first, but this isn’t an option for additional infusions.  I deal with this by using a lower infusion temperature in my calculations.  For example, even though I am adding boiling water, I use a lower value (about 192F/88.9C) in the equation (this value will be different on your system).  Note that some calculators attempt to account for this for you with a default fudge factor, but each system will be different, so its best to find your own fudge factor.

I recommend having more water than you calculate is needed warmed and ready in case you undershoot your next rest temperature, and some cold water or ice in case you go over.  A missed temperature (even if corrected with extra water) will throw off the amount of water needed for your next infusion, so it can get tricky to do more than two total rests until you really know your system.  Also, keep in mind that your mash may loose a bit of heat during a given rest, so account for this in your infusion calculations.

If possible, add the infusion slowly while stirring, measuring the temperature as you go.  This will help avoid hot spots that will denature enzymes and cold spots where conversion won’t take place.  I like to use a spare kettle set a bit higher than my MLT with an attached brass ball valve and silicone tubing for ease of adding and to keep splashing down.

Decoction

Decoction mashing was historically employed to deal with less modified malts and, some say, give the beer a certain malty je ne sais quoi that cannot be replicated otherwise.  But, it can also get you to your next rest without diluting the mash, or quickly fix a missed infusion target.

Draw off a very stiff portion of the mash, and first raise and it to saccrification temperature for 10 to 15 minutes; stir constantly to prevent scorching.  Then, raise it to boiling, still stirring.  If you are decocting only to raise mash temperature, you can stir it back into the mash now.  If you would like to take advantage of the Maillard reactions (bready, toasty, biscuity, etc.) decoctions are known for, Noonan’s The Seven Barrel Brewery Brewer’s Handbook recommends boiling one of the decoctions for 20 to 45 minutes.

If you are decocting to reach mash-out, you can instead draw a thinner portion of the mash, raise it to 160F/ 70C for 20 minutes, then boil and add back.  This thin portion will be a bit more resistant to scorching.

Finding the necessary decoction volume is a bit trickier than infusions, because you are pulling water and grain out of your mash tun in an unknown ratio.  You can estimate the volume using the equation:

V_d=\frac{(T_2-T_1)(0.4W_g+2V_w)}{(T_d-T_1)(0.4W'_g+V'_w)}

where:

  • W_g = weight of the grain in the mash, lb or kg
  • W'_g=\frac{1}{R_d+0.32} = lb of grain per qt of decoction drawn
  • W'_g=\frac{1}{R_d+0.667} = kg of grain per L of decoction drawn
  • T_1 = current temperature of the mash, F or C
  • T_2 = desired mash temperature, F or C
  • T_d = decoction temperature (usually boiling), F or C
  • V_d = volume of the decoction, qt or L
  • V_w = volume of water in the mash, qt or L
  • V'_w=2W'_gR_d = volume of water per qt of decoction
  • V'_w=W'_gR_d = volume of water per L of decoction
  • R_d = ratio of grain to water in the decoction; choose based on how thick you think your decoction is (English: usually 0.6 to 1 qt/lb; Metric: usually 1.25 to 2 L/kg)

You’ll also have to add some sort of fudge factor to account for MLT thermal loss; you can do this by dropping the decoction temperature in the calculations below what you actually use like with infusions.

Hot Rocks

I have no idea where this method originated, but I once saw a clip (I thought on BrewingTV, but I can’t find it) of some brewers heating their mash and boil kettle entirely with hot rocks from a fire.  You’ll want clean rocks, and they must be dense, closed grain rocks like granite.  Other rock has a tendency to absorb water which vaporizes and causes the rock to explode.  They mentioned a bit of a smoky flavor from the fire, and some caramelization as the extremely hot rock contacts the wort, forming localized vigorous boiling for a few seconds.  You’ll have to come up with your own calculations for temperature increase based on the temperature and thermal mass of whatever rocks you are using (which I think you can find by consulting some geology texts…).

Just so you don’t think I’m crazy, here’s a BYO article with more on the method, and even a commercial beer from Port Brewing.  And make sure to have some heavy-duty tongs and oven mitts on hand for brew day.

[Again, much of this information is from Palmer’s How to Brew, 3rd ed.  I renamed some of the variables in the equations so they would make more sense to me.]

– Dennis,
Life, Fermented

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About Dennis
Home brewer, home chef, garage tinkerer. Author of Life Fermented blog.

2 Responses to Multi-Rest Mashing: How-To

  1. beeryben says:

    Hi Dennis,

    Firstly, good blog; it’s nice to see a fellow engineer brewer!
    Secondly, I can add a little bit more info on the hot rocks methods. This apparently was used where brewing vessels were wooden so couldn’t be heated directly, also known as Stein (stone) fermentation or more usually rauchenfels (translates roughly to smoked rock). It was still being done in Germany until about 10 years or so ago. On a trip a few years back I traced the place that made it but they had stopped doing it a few years previously – I can’t remember whcih trip this was but it was probably either Bamburg (of smoked beer fame) or Nurenburg.
    Might give it a go one day!

    Ben

    • Dennis says:

      Interesting. I was picturing something more colonial. I know a number of N/S American cultures used the technique to keep water warm when forming wood objects such as canoes and things.
      – Dennis

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