Simply put, you don’t have to know a lot about mash thickness to brew good beer. Understanding it just gives you more control over the brewing process. If all variables remain constant, then the effects of mash thickness are fairly straightforward.

But most brewers find it worthwhile to consider the issues of mash thickness when they make changes in their brewing process. They know, for instance, that different beers call for different recipes and that different growing conditions create yearly variations in the barley crop. Important issues include mashing in, mash target temperatures, and the interplay among enzyme activities, mash temperature, and mash thickness.

Mechanics

Professional brewers tend to communicate with each other on the subject of mash thickness by using a value called "liquor-to-grist ratio." This is merely the volume of strike water (liters) divided by the mass of grist (kilograms). Its practical range is 2 to 4 and most often is around 2.5 to 3.2. Most homebrewers know this as a ration of quarts per pound, often 1.25 quarts of water per pound of grain (1.2 liters).

Typically, infusion mashes run a little thicker, while temperature program (step) and decoction mashes usually are thinner to make it easier for mash mixing and mash transfer.

The liquor-to-grist ratio determines what strike temperature you need for hitting a target mash temperature. To accurately predict the mash temperature, you must consider several variables in what engineers refer to as a "mass and energy balance." This is an equation that relates the temperatures and masses of everything coming into the mash to the temperature and mass of the mash itself. The fundamental principle at work is the first law of thermodynamics: Energy can neither be created nor destroyed.

To proceed with the calculation of a mash temperature, you must define certain characteristics:

• The target mash temperature (°C)
• Volume of strike water (liters)
• Temperature of strike water (°C)
• Heat capacity of water (kiloJoules/kilogram/°C)
• Mass of grist (kilograms)
• Temperature of grist (°C)
• "Heat capacity" of grist (kiloJoules/kilogram/°C)

These parameters seem straightforward, except the last one. In a laboratory setting this would actually be the real heat capacity of the grist, but in a practical setting this becomes a fudge factor. It combines the heat capacity of the grist and the mashing vessel, as well as the effect of slight evaporative cooling as the strike water enters the vessel. Because every system is different, this parameter needs to be determined experimentally. This means that if you have detailed records of past brews, you can actually estimate what your system would typically have as a heat capacity.

Once you know this value, you can accurately hit a desired mash temperature for a given thickness by calculating what your strike water temperature ought to be.

If you are just starting out in all-grain brewing, you need to trust the recipe you are using and make careful notes on how your system behaves differently than what the recipe describes. Over time you will be able to adjust your mashing with a great deal of control. A very important point to consider is the inherent imprecision in your measurement of strike water volume and temperature, as well as the grist weight. When making calculations as described here, do not bother to take your values to the 20th decimal place, because your calculated quantities can be no more precise than your methods of measurement. (See Calculations at end of article)

Mash Thickness and Mash Temperature

The characteristics of carbohydrate in wort are determined in large part by the critical interplay between mash thickness and mash temperature. Two enzymes in malt are primarily responsible for the degradation of starch, the most important carbohydrate in the brewing process, to fermentable sugars and dextrins, which are non-fermentable sugars.

These two enzymes, alpha-amylase and beta-amylase, each have a very specific role in the mash. The alpha-amylase cuts up starch molecules at many different points along its length. This produces dextrins and also some fermentable sugars. The beta-amylase takes these fragments of the original starch molecules and clips off maltose units from one end. It does so in a very precise manner, one maltose molecule at a time. So over time during the mash, as alpha-amylase yields more and more dextrins, the beta-amylase has more substrate on which to work.

These two enzymes, though they work in concert, behave differently in response to changes in mash thickness and mash temperature. This is because of the difference in their stability at high temperatures. Alpha-amylase has an optimal range from 149° to 158° F. The optimal range for beta-amylase is 126° to 144° F.

High mash temperatures favor a less fermentable wort because alpha-amylase is a lot more stable than beta-amylase is at higher temperatures. This means that there will be less production of maltose as the activity of beta-amylase diminishes. It is hard to say that beta-amylase activity will be expected to drop off at a particular temperature, because the thickness will determine what temperature activates maximum beta-amylase activity. Thicker mashes tend to retain more beta-amylase activity at high mash temperatures than do thin mashes. This is because beta-amylase is more stable when joined with its substrate than when it is not.

Because beta-amylase encounters substrate less frequently in a thin mash, there is more opportunity for it to be destabilized and inactivated.

This discussion is simplified because it is in the context of constant pH and ion concentration. The topic gets more complex when you consider that pH profoundly affects alpha-amylase and beta-amylase activity, and that calcium plays a role in stabilizing alpha-amylase at high mash temperatures.

Thickness in Practice

One problem involving mash thickness that occurred at a professional brewery involved brewing a weizen beer using an infusion mash tun. To get the right balance of clove spiciness in the beer, the brewer wanted to have a low temperature rest to release ferulic acid, a precursor that the yeast needs to create the clove character. (The necessity of a ferulic rest is hotly debated by brewers.) However, the brewer was faced with the challenge of reaching conversion temperature after this low temperature stand without the ability to heat the mash tun directly or to transfer the mash for a decoction. So he used calculations very similar to the ones described here to come up with a known quantity of near-boiling water that he could add to the mash after the low temperature stand.

By starting with a very thick mash, he ended up with a reasonable thickness for the conversion stand. He had to be very careful to keep the mash vigorously stirred during the transfer of the water to ensure that there were no "hot spots" produced, and this is a very sweaty job! But on the stove top, this should be a little easier.

Thick mashes usually are encountered in the brewing of high- gravity beers. A concentrated wort produced in a thick mash makes it easier to attain the original gravity needed for a beer like a doppelbock. But it could be less efficient. The more concentrated the wort, the harder it is to dissolve more into it.

Just as important is the ability of the end-products of starch degradation to inhibit amylase activity (enzymologists refer to this as product inhibition). This means that a thin mash can lead to better brewhouse yield, assuming that lautering is optimized.

If you feel adventurous — and in the mood for a marathon — you can try brewing two beers from a single mash the next time you do a high-gravity brew. Use the first runnings from a thick mash for the high- gravity brew. If you have two places to boil, you can then take the second and last runnings together to make a "small beer," or you can wait until you have cast out the first boil and then use the kettle to finish the small beer. This way you have brewed a light and easy drinking beer to enjoy while you are waiting for the big and hearty beer to finish aging!

When considering what mash thickness you want for a brew, know that the thickness you choose determines the range of temperatures needed for the correct degree of fermentability in your wort. The characteristics of your wort are determined by a balance of mash thickness, mash temperature, mash pH, and ion concentration. Using detailed notes and time, you can find the right balance in your brewing system to give you exactly the wort you need to make your best possible beers.

Calculations

To illustrate the calculation of mash parameters, a couple of examples are provided below.

Finding Grist Heat Capacity

Given: To get a mash temperature of 158° F (70° C), you might have had:

3.1 gallons strike water (11.7 liters, which is effectively 11.7 kilograms)
Strike water temperature = 170° F (77° C)
Heat capacity of water (which never changes) = 4.2 kiloJoules/kilogram/°C
8.1 pounds grist (3.7 kilograms)
Grist temperature (which is essentially room temperature) = 85° F (29° C)

What is the heat capacity of the grist in this system?

Solve for X:

strike water + grist =
(11.7 kg)(4.2 kJ/kg/°C)(77°C) + (3.7 kg)(X kJ/kg/°C)(29°C)
=
water in mash + grain in mash:
(11.7 kg)(4.2 kJ/kg/°C)(70°C) + (3.7 kg)(X kJ/kg/°C)(70°C)

This yields a value of 2.3 kJ/kg/°C for the heat capacity of the grist, which is higher than the real heat capacity of typical grist as found in a laboratory setting. This value reflects not only that the grist is absorbing heat from the strike water, but so is the mashing vessel.
Strike Temperature

The next example is what you would do with the information you just calculated. Suppose you wanted to try the same recipe, but you would like the beer to have a lower finishing gravity.

If you selected 148° F (64° C), then what should your new strike temperature be for this particular mash thickness? To calculate a strike temperature, given:
Target mash temperature = 148° F (64° C)
3.1 gallons strike water (11.7 liters, which is effectively 11.7 kilograms)
Heat capacity of water (which never changes) = 4.2 kiloJoules/kilogram/°C
8.1 pounds grist (3.7 kilograms)
Grist temperature (which is essentially room temperature) = 85° F (29° C)
"Heat capacity" of grist = 2.3 kiloJoules/kilogram/° C

What should the strike temperature be?

Solve for X:

strike water + grist =
(11.7 kg)(4.2 kJ/kg/°C)(X°C) + (3.7 kg)(2.3 kJ/kg/°C)(29°C) =
water in mash + grain in mash:
(11.7 kg)(4.2 kJ/kg/°C)(64°C) + (3.7 kg)(2.3kJ/kg/°C)(64°C)

This yields a value of 70° C or 158° F. So for a 10° F (5°-6° C) drop in mash temperature, the strike temperature had to be dropped 12° F (6°-7° C), in this system. This makes sense, because there is more strike water in the system than grist.

Tom Flores is masterbrewer at Brewer’s Alley, Frederick, Md. He holds a master’s degree in brewing science from the University of California, Davis.