Reiterated Mashing - Brew Your Own

During an episode of the Brew Strong podcast, the topic was raised about brewing really high-gravity beers where mash tun space was limited. This is actually a very common question: What is the highest gravity beer I can brew in a specific size mash tun? Whether using a cooler or a kettle, we’ve all tried to stuff as much malt as possible in it to achieve over-the-top starting gravities for that perfect wee heavy, barleywine, or other unique high-gravity beer. The boilerplate answer to that question has always been that it depends on your water-to-grist ratio, the grain weight, and the volume of your mash tun (or kettle size for brew in a bag mashing). For traditional fly sparging, it also depends on how long you’re willing to boil to evaporate the extra water to concentrate the wort to the desired specific gravity. That said, the easiest solution is to brew the highest reasonable gravity you can in your system, and add malt extract to make up the difference. There is no inherent shame in that solution, but if you’re a purist, or simply want to try a new technique, keep reading!
During the conversation on Brew Strong, co-host Jamil Zainasheff offered a thought about mashing an additional amount of grist utilizing the wort from the previous mash as the strike water for the second mash. While not a new technique, the calculations and techniques aren’t common knowledge. Co-host John Palmer mentioned an interesting chart he’d seen while doing research for the new edition of his book How to Brew. Specifically, how starting specific gravity is a direct relationship to water-to-grist ratio. He also mentioned that he’d love to validate that graph and include it in his new edition. So it was time for some test brewing by John Blichmann, a guest on that podcast episode. But before we get into the details of how to accomplish this on a homebrew scale let’s talk about the theory of what we call sequential mashing.

The Relationship between Wort GravIty and Water-to-grist Ratio

The first wort gravity from a mash is directly related to the potential soluble extract of the malt and the water-to-grist ratio of the mash. The percentage of soluble extract in wort (°Plato) can be converted to specific gravity using an equation from DeClerck1.
The potential extract for a malt can vary season to season, or by maltster, but it should be fair to say that the typical soluble extract of a 2-row barley base malt is about 80% by weight as measured by ASBC Malt Method3, coarse grind, dry basis2. The moisture level for malt is typically about 4%, so that drops the typical extract to a conservative value of about 75% by weight, give or take. The soluble extract in the wort, measured as degrees Plato, is essentially equal to the weight of sugar in solution divided by the weight of the wort. The water-to-grist ratio, R, is the weight of water per weight of grain in the mash, and typically ranges from 5:1 (thin) to 2:1 (thick). Please note that the weight-to-weight ratio, R, is approximately twice that of the water volume-to-grist ratio, Rv in quarts per pound. The weight of the wort is the weight of the water, plus the soluble extract percentage (X) of the grist. The weight fraction in °Plato as a function of the percentage of soluble extract and water-to-grist ratio (R) would be:

(Equation 1)
°P = 100X / (R + X)
The specific gravity (SG) of wort can be estimated from °Plato according to the equation below, from DeClerck3:

(Equation 2)
SG at 17.5 °C = 260 / (260 – °P)
While this conversion to specific gravity is based on measurements taken at 63.5 °F (17.5 °C), the conversion is also valid at 68 °F (20 °C) to the third decimal place, according to the tables and polynomial equation published by the ASBC3. Therefore, we can substitute equation 1 into equation 2 and this becomes:

(Equation 3)
SG at 20 °C = 260 / (260 – (100X /
(R + X)))
R, as weight to weight, is roughly equal to the volume to grist ratio, Rv, in liters per kilogram, if you assume that 1 liter of water weighs 1 kilogram. The water volume-to-grist ratio (Rv) can be calculated by dividing R by the density of water (ρ) at a given temperature. The weight of water is actually less at mashing temperatures, about 0.985 kilogram per liter. In quarts and pounds, that weight is about 2.055 pounds per quart, or roughly a factor of 2, such that an R of 4 pounds per pound is roughly equal to and Rv of 2 quarts per pound.

(Equation 4)
Rv = R/ρ
Table 1 lists some of the wort gravity results for typical R values along with associated water volume-to-grist ratios at 63.5 °F (17.5 °C) and 149 °F
(65 °C). Note that the difference between R and Rv is small.
Table 1 shows the resultant wort gravity (equation 3) plotted against the water-to-grist ratio. The four curves presented are for pure sucrose (100% soluble extract), 80%, 75%, and 70% extract-by-weight. As discussed above, 75% is a more typical value for most commercial brewers. Homebrewers may often achieve results closer to the 70% extract line, depending on their crush and mashing/lautering conditions.

Sequential Mashing

In a perfect world, it follows that the combined wort gravity of a second mash, mashed with wort from a previous mash, would simply be additive if the wort gravity of a mash is simply a function of the water-to-grist ratio. More experiments were done to test this idea.
Two different sets of experiments were run. Palmer ran a small scale lab type test to validate the model in an analytical setting. Blichmann performed an evaluation in a homebrew setting to gauge the model in a setting more appropriate for a homebrewer, and how to perform the practical matters of recipe formulation and hitting the final gravities.

Palmer’s Lab testing

Several small mashes were conducted in 1.5 liter (1.6 quart) containers placed in a recirculating water bath, maintained at 155 °F (68 °C). The mashes typically consisted of 200 grams (7 oz.) of coarsely ground malt, each weighed and ground separately in a two roller mill. The grist samples were placed in tight weave, nylon mesh bags that were the same size as the containers, allowing the grist to move freely when stirred, yet provided easy draining at the end of the mash. Each of the samples was mashed at 155 °F (68 °C) at a water-to-grist ratio of 3 liters per kilogram for one hour with occasional stirring. Afterwards, the grain bag was lifted out and drained to collect all of the wort. The wort was cooled to room temperature, and measured with a refractometer. The accuracy of the refractometer and hydrometer were verified with 10 °P and 30 °P sucrose solutions before testing.
The first mash measured at the anticipated gravity of 1.083 (20 °P), and the second mash using the wort from the first measured at about 38 °P. The wort samples were diluted 50/50 with distilled water to enable measurement with the standard 0-30 °P scale refractometer. The third mash made with the 38 °P wort from the first two measured at 25 °P when diluted 50/50 and had the consistency of syrup. While the experimental results did not match the predicted linear relationship for combined wort gravity, it is easy to see that the results were not far off and can be attributed to the rising sugar concentrations inhibiting extraction. The experiment with these extremely high-gravity worts indicate it is plausible to consistently achieve a more typical
high-gravity wort of 1.120 using a two-step sequential mash.

Blichmann’s homebrew scale validation

Two 5-gallon (19-L) batch sequential mashes were performed. The goal was to obtain enough wort to perform a second mash with the runnings. Equipment used was a 5-gallon (19-L) batch size Blichmann Engineering BrewEasy. The BrewEasy, like Brew-In-A-Bag (BIAB) are both no-sparge methods. This avoids diluting the wort gravity by sparging. BeerSmith recipe software includes the equipment profile and water calculator for the BrewEasy, BIAB, and other similar no-sparge systems. The drawback of no-sparge processes and systems is that significant extract is retained in the wort held by the wet grain, and that loss increases as the specific gravity in the mash increases. So with these systems it is always best to start with the full dose of brewing water at one time, such that the volume and gravity of wort that is drained from the grain bed is sufficient to achieve your target batch size and original gravity (OG) after a standard one-hour boil. That said, many boil kettles won’t hold all the water and half of the malt for the batch, so just try and fill it as full as possible. Then for the second mash you’ll need to add the remaining water. The recipe brewed was Old Monster Barley Wine from the book Brewing Classic Styles, with an OG target of 1.115. It used a total of 30 lbs. (14 kg) of malt. The 30 lbs. (14 kg) of malt was divided into two identical 15 lb. (7 kg) batches. To create this sequential mash-type of recipe in BeerSmith, first create a recipe for the full 30 lbs. (14 kg) of grain. This will give you the total water usage for the brew day. In this case for 30 lbs. (14 kg) of malt it required 10.6 gallons (40 L) accounting for losses and water absorption of the spent grains. Second, create a recipe with all the water from above, and only half of the grain to calculate the first mash specific gravity (SG) and volume. Then the full recipe will allow you to calculate the final volume including boil off.
Some tips: You will need to do a brew or two before you dial in your net mash efficiency and brewhouse efficiency. You’ll be impressed with the amount of trub that develops! So for the first trial shoot for a low mash efficiency, you can always dilute the wort later if needed. And of course, take good notes as you go.

Blichmann’s Results

Mash 1: Infusion at 148 °F (64 °C) for 90 minutes. Water-to-grist ratio: (10.6)(4)/15 = 2.83 qt/lb (5.94 L/Kg). Achieved 5.15 pH (measured at mash temp). Did NOT perform a mash-out. Collected 9.1 gal of 1.044 wort for a total of 9.1*44 = 400 points. This yields a mash efficiency (based on 37 points per pound max extraction) of 400/(37*15) = 72%. The prediction from Equation 3 for Rv of 2.83 (i.e., R = 5.8) at 70% extract is 1.043, so well within expectations.
Mash 2: Infusion mash using wort from previous mash 148 °F (64 °C) for 90 minutes. Water-to-grist ratio: (9.1gal)(4)/15lb = 2.42 qt/lb (2.55 L/Kg). A mash-out at 168 °F (75 °C) was done to reduce the viscosity for better collection efficiency. Achieved 5.10 pH (measured at mash temperature). Collected 7.4 gallons (28 L) of wort at 1.094 SG so a total of 94*7.4 = 696 points. Therefore the net points gained in the second mash was 696-400 = 296 points. 104 points less than the first mash! The efficiency of the second mash was 296/(15*37) = 53%. (Yikes!) And lastly, the aggregate mash efficiency is 696/(30*37) = 63%. Pretty low, but for a no-sparge process for that high of a starting gravity it isn’t unexpected.
That said, you could definitely sparge the remaining grains and then make a nice small beer with the second runnings!
The Palmer prediction for the final mash: Taking the total recipe water and grain bill as the net effective water to grist ratio we get 9.1 gal.(4)/30 lbs. = 1.21 qt./lb. (2.55 L/Kg). The prediction from Equation 3 at 1.21 qt./lb. (2.55 L/Kg) is 1.095 (seriously, we didn’t make up this data). Therefore, this conclusively confirms that the two sequential mashes are identical to having done a single mash using the full amount of water and malt!
After the boil the final numbers were 1.120 SG and 6 gallons (23 L) post boil. The wort losses were high from the big hop bill and the massive amounts of trub — only about 5 gallons (19 L) of wort were run into the fermenter after cooling. It was a long brew day, but the beer was great and it was fun working together on this testing! Blichmann later did a triple-sequential mash and reconfirmed the results above. THAT was a long brew day. And we have a couple kegs of huge beers to age and enjoy for years to come.

Summary

To summarize, the first-wort gravity of a mash is a function of the water-to-grist ratio of the mash and the maximum soluble extract of the malt. The maximum soluble extract that you will achieve from a malt will depend on your mashing conditions and the degree of crush, but in general, the data shows that it should be 70–75% of the weight of the malt. The calculations and methods for sequential mashing allow you to better predict your wort gravity and conduct a sequential mash to reliably achieve a high-gravity wort in a smaller vessel. There is some loss of wort, but it is
no different than if you were to mash the entire grain bill in a much larger mash tun. Sequential mashing lets you brew bigger all-grain beers on your current equipment.
references:
1 DeClerck, J., A Textbook of Brewing, Volume 2, Chapman and Hall, London, 1958, p. 33.
2 ASBC. Malt Method 4. Extract. American Society of Brewing Chemists, St. Paul, MN.
3 ASBC. Determination on Wort, Beer, and Brewing Sugars and Syrups, Table 1 Specific Gravity and Degrees Plato of Sugar Solutions or Percent Extract by Weight, American Society of Brewing Chemists, St. Paul, MN.