All-Grain Brew Day Walkthrough (IV: The Mash)

This is the fourth installment in the All-Grain Brew Day Walkthough, which started with a post on strike water preparation



The mash — hot brewing liquor mixed with crushed malt.

This walkthrough is going to focus mostly on the practical. However, at this point, let’s stop and think about what’s going on in the mash. While the mash is progressing, the homebrewer typically does fairly little. He might stir the mash, or his rig may circulate the wort through the grain bed for him, but it’s mostly downtime for the brewer. At the end of the article, I’ll focus on the things you can be doing during the mash, but first, let’s examine what’s happening in the mash tun.

What’s in the Mash?

The mash is a mixture of hot brewing liquor and crushed grain. Your treated brewing water has dissolved minerals in it, with calcium salts (such as calcium chloride or calcium sulfate) being among the most important for brewers.

The crushed grains supply starch granules to the mash, in a range from fairly small particles to flour. Husk particles are also a part of the mash. The flavor active molecules from the malt are located in the husk. The husks also contain tannins. There is also bacteria clinging to the husk pieces. (Pale malt acquires a fair amount of bacteria in the malting process. This does not have many consequences at the high temperatures and short times it takes for a single infusion mash. However, if you were to mash at 120 °F/49 °C overnight, these bacteria would transform the mash into a sour mash.) The malt also supplies a variety of other molecules to the mash, such as proteins, gums (including beta-glucans) and other biological molecules, although these are far less abundant than the starch molecules.


What Happens In The Mash?

A variety of things happen in the mash while the brewer waits for the rest to finish. Heat is transferred from the hot brewing liquor to the colder crushed grains, and also to the interior surfaces of the mash tun (if it is not pre-heated). Simple chemical processes such as the dissolving of starch and the diffusion of water-soluble molecules out of the husk particles occurs. Chemical reactions occur that alter the pH of the mash. For example, calcium and phosphate react to lower the mash pH. On the other hand, if bicarbonate ions are present, they will react with hydrogen ions, slowing the drop in pH. Other chemical reactions such as calcium ions reacting with oxalate to form calcium oxalate are occurring. Calcium oxalate is a component of beerstone, a common scale found in brewery vessels. (It’s also a major component of some human kidney stones.) And of course, biochemical reactions occur. Most importantly for brewers, amylases and other enzymes in the wort breakdown the starch into simple sugars and smaller carbohydrates.

The high temperature of the mash also kills off much of the bacterial load of the malt, and destroys many malt enzymes with temperature optima far below mash temperatures.

The major reason I’ve listed these things is to point out that all of these processes take time. Some of the things I’ve listed take place very quickly. For example, if you’ve stirred your mash well, the heat transfer from hot water to grain should be done, or just about, when you’ve stopped stirring. Likewise the dissolution of water soluble substances from the grain husks happens almost instantaneously. If you took wort samples from your mash every 5 minutes or so, your first sample would likely have the full color of your wort and the samples would not get progressively darker over time. (On the other hand, if you took a density reading with your hydrometer or refractometer every 5 minutes, you’d likely see the specific gravity rise for the first 20 minutes.) The starch-degrading reactions can take anywhere between 5 and 40 minutes with modern malts. And, if your water has a lot of bicarbonate ions in it, your mash pH may rise slowly as you rest.

Of all the things that happen in the mash (that a brewer is concerned with), starch conversion is usually the slowest. So, by the time start conversion is complete, you can be pretty certain that everything else is taken care of. (For example, you won’t have to continue mashing to extract the full amount of color, flavor, or aroma from your malts.)

So, with all this going on in the mash, what should the brewer be doing?


Heat Your Sparge Water

While the mash is resting, prepare your sparge water. Heat it so that it is ready to go after the mash has been mashed out and the wort recirculated. The amount of sparge water required depends on your expected pre-boil wort volume, the amount of first wort in your mash, dead space in your system, and amount of water you plan to leave behind in your lauter tun. (See my article on how to plan your sparge water needs and end up with no water in your lauter tun when you finish wort collection.)

There is no need to treat your sparge water as you do your brewing liquor. Remove any chloramines, and it doesn’t hurt to add some calcium (~50 ppm), but beyond that you do not need to fine-tune it as you do your brewing liquor. Your mash conditions have already been set and all you need the sparge water for is to rinse the grain bed. If possible, avoid using sparge water that is high in carbonates. In general, there’s no need to add acid to your sparge water. There are some times when this may be beneficial, but it is not needed as a general course of action.


Stir the Mash

Stirring helps even out the temperature in a mash and mixes the liquids and solids more thoroughly. If you can manage it, you should always stir your mash at least a few times during the saccharification rest. On a homebrew scale, stirring your mash involves opening up the mash tun, and exposing the mash to cooler environmental temperatures. As such, you will likely need to add heat, or a small amount of boiling water, to bring the mash temperature back up to your target. If you are unable to heat your mash tun directly, and lack the space for a boiling water addition, you will not be able to stir the mash without losing heat. This is not a fatal drawback, however, you may lose some extract efficiency. (And always remember that you can mash in your kettle and transfer to your lauter tun later.)


When Should You Stop?

Your recipe will likely specify a mash duration. Often, homebrew recipes give one hour as a length of time for the mash. However, your mash is likely to be converted before 60 minutes has elapsed. If you are looking to save time, or are brewing a full-bodied beer and don’t want the mash to go beyond the bare minimum, you can always perform an iodine test to indicate when the mash is over. (See more in my article, “How Long Should You Mash?”)

If you are attempting to brew a dry beer, letting the mash go beyond a negative iodine test can be beneficial. Carbohydrates not large enough to be detected as starch, but big enough to potentially add body to your beer, can be degraded in this excess time.

The 60-minute mash time specified in most homebrew recipes is a failsafe instruction. There is almost no chance that your match will not have converted in that time. If you need to shave time from your brew day, however, this is one place to look.


Mash Out

Mashing out has several benefits, and you should always mash out if possible. First and foremost, raising the temperature of the mash lowers the viscosity of the wort and makes it easier to separate the work from the spent grains. In addition, if you are trying to brew a full-bodied beer, a mash out also speeds the destruction of enzymes that could keep working on wort carbohydrates. Whether mashing out by heating the mash or adding boiling water, stir well during this step.

Next up, mash out and recirculation.


Related articles

Tannins in the Mash

Reiterated Mashing

Fossil Cove Sour Mash Experiment


  1. Thanks Chris,
    a question: should mash out be done prior to collecting first wort or as part of the the sparge process?
    I have seen both methods recommended in brewing lirerature and have used both methods. They both seem to work well.

    • Chris Colby says:

      I’ve never heard of mashing out after collecting the first wort. That doesn’t mean it wouldn’t work, but I think it would be simpler to mash out before any wort collection.

  2. You’ve talked a lot about stirring after initial mash. I’ve been taught to not stir after the initial mash in because it’s good to set up a solid grain bed. Have I learned the wrong way and should I stir frequently throughout mashing?

    • Chris Colby says:

      It only takes about 5 minutes to set up a grain bed for lautering. Stirring evens out the temperature in the mash and, more importantly, increases your extract efficiency. Most commercial brewers have stirring paddles or rakes that stir in their mash constantly. As I homebrewer, if you can stir your mash several times during the main rest, you’ll be doing well.

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