Calculation - Mash and Sparge Water Volume

This article discusses the recipe creator's process for calculating mash and sparge water volumes to get a specific batch volume given the use of single infusion equipment and specific water to grist ratio.

Mash Water

The mash water is the volume of water needed in the vessel to be able to adequately saturate the grains so that the enzymes can freely move between starch molecules to break them down into fermentable sugars. The factors that affect mash water are:

• Grain weight
• Water to grist ratio
• Volume parameters of the equipment used e.g. dead space and mash tun maximum volume
• Grain absorption.

Water to Grist Ratio (Mash thickness)

The water to grist ratio is one of the least significant factors in influences mash performance the typical range of these mash ratios is 1-2 US qt/lb (2-4L/kg). Within this range, the effect of the mash ratio of fermentability is less than 5% (Palmer, 2017). However, at ranges outside the speed at which conversion occurs can be greatly reduced eg very thick mashes of less than 1 US qt/lb (2 L/kg) took twice as long to convert, 40 minutes as opposed to 20 minutes and very thin mashes like no sparge methods also take longer due to enzyme dilution.

Due to the above, most brewing systems have a recommended water to grist ratio for best performance and the ease of use.

Sparge Water

Is the volume of water used to rinse the grains of as much fermentable sugar as possible while limiting the effect on pH. The factors that affect sparge water volume are:

• Mash water volume
• Sparging vessel parameters e.g. volume and dead space (if applicable)  
• Preboil volume
• Boil-off rates
• Finished batch (fermenter volume)
• Losses during transfer (from the boiler to fermenter)

Calculating Water Volumes

For this calculation, we’ll start with the typical parameters for a 23 L (6 US Gal) G30 brew session

First is to work backwards from the batch size to the preboil volume:

Batch Volume + Boil Losses + (Boil Length * Boil Off Rate) = Preboil Volume

23 L + 2 L + (1 x 3 L) = 28 L

Next to calculate mash volume:

Grain weight x Mash thickness + Mash Tun Dead Space = Mash Volume

5 x 2.7 + 3.5 = 17 L of Mash Water

There are a couple of assumptions with this calculation;

• The first is that the water is measured out at room temperature.
• The next is that the room temperature water is then heated to the strike or first mash temperature.

With commercial breweries and homebrewers that use three-vessel systems, where a large hot liquor tank is used to preheat water to strike or first mash temp. then the change in density needs to be considered when measuring out the water. Eg density of pure water at 4°C (39.2°F) is 1.000 the density at 20°C (68°F) is 0.998 and at 60°C (140^°F) it is 0.983. This is an example of thermal expansion and as a result, if 1 L (1.06 US qt) of water is measure out at each of the temperatures and then the volume is remeasured when they all reach room temperature (20°C/68°F) then:

1L @ 4^°C = 1.002L @20^°C

1L @ 20^°C = 1L @20^°C

1L @ 60^°C = 0.983L @20^°C

This does not seem like a significant difference when looking at 1 L (1.06 US qt) but when you apply this to each litre measured for a homebrew batch or a commercial batch then it can become a significant error. When using brewing software ensure infusion or decoction mash settings are not used, as these require further calculations based on thermal mass.

Now to calculate the sparge water so that the volume of wort in the boiler is the same as our calculated preboil volume.

Therefore, 28 L (7.4 US Gal) is the preboil volume we want then:

Preboil Volume –  Mash Water + (Grain Weight x Grain Absorption) = Sparge Volume  (Daniels, 1996)

28L – 17 L + ( 5kg x 0.8 L/kg) = 15 L

As with the mash water, the same assumptions of measuring and heating the sparge water apply due to thermal expansion. This especially applies when measuring the preboil volume.  If your sparge vessel has a dead space where the water cannot be recovered, most hot water urns can be manually emptied by tipping the remaining contents into the grain basket. But if you cannot recover the volume this volume will need to be added to the sparge volume.  

When working with other software:

1. The recoverable space in the unit below the grain basket and the boiler, the current equipment profile for many other software is currently set to 0. But for the G30 this is 3.5 L (0.9 US Gal) and for the G70 it is 6.8 L (1.8 US Gal). Without these set the volumes of mash water predicted for these other softwares will be short ~3.5 and 6.7 L respectively. What this means for the mash is that the mash water even with the displacement of the grain basket this could result in water to grist ratio in the mash basket that is too thick and results in lowered efficiency.
2. The wort/ water shrinkage. This is a much lower effect, other software are designed for 3 vessel systems which use a preheated vessel of water to fill their mash tun and to sparge with. However due to thermal expansion water at 20°C (68°F) fills a smaller volume than water at 65°C (149°F) (see below)

So when measuring out hot water by volume (say sparge at 75°C/167°F) then the actual volume of water is 2.5% less then the same volume of water measured at 4°C (39.2°F). Since the Grainfather system uses vessels that are designed to be filled with ground temperature water then heated to the temperature needed this effect doesn’t come into play. So for our software we don’t take this into account, whereas other software use a wort shrinkage factor of 4% to account for the thermal expansion at near boil.

3. Water to grist / mash thickness, the mash thickness we believe is best for the maximum efficiency for the Grainfather system is 2.7 L (2.85 US qt) per 1 kg (2.2 lb) of grain and this needs to be matched in the other softwares as this is not part of the equipment profile parameters.

 Grain weight x Mash thickness + Mash tun dead space = Mash Volume

If this is different this will result in a different volume for the mash and a resulting different volume for the mash water

4. The next is grain absorption. Based on the malt we commonly get in NZ we have experienced that the typical absorption is 0.8 L per kg of grain. Other software have this as somewhere between 0.7-1.2 L (0.74-1.27 US qt) per 1 kg (2.2 lb). Unfortunately, this can be a seasonable variable of a agricultural product that can change depending upon the maltster, the country of origin and the barely for that season.

Preboil volume –  Mash water + (grain weight x grain absorption) = Sparge Volume  (Daniels, 1996)

5. Sparge and mash tun vessel volume, depending on the volume of water your sparge/mash vessel this will affect the maximum water volume for each the mash water and sparge water. So for the G30 mash tun, the max volume of mash water is 30 L if the mash water exceeds this then it will max out at 30L and add the remaining to the sparge water and the same for the sparge vessel. If you set your sparge vessle volume to 16.5 L (4.34 US Gal) like our sparge water heater, then if the sparge water exceeds this then it will add this excess water to the mash which for either of these examples it will change the water to grain ratio.

References

Daniels, R. (1996). Designing Great Beers. Boulder: Brewers Publications.

Palmer, J. J. (2017). How to Brew 4th Edition. Boulder: Kristi Switzer.