Ask Mr. Wizard

Mashing Out


Paul Kempkes • Buffalo, New York asks,

I use single infusion mashing with a modified picnic cooler. When attempting to mash out I use a good deal of near-boiling water just to raise the temperature from the mash to 170 ºF (77 ºC). Not having the ability to apply direct heat to the mash tun, I was wondering if I could, like in decoction mashing, remove the thick portion of the mash, boil it and return it to raise the temperature to mash out, thus not having to add additional water. Would this extract tannins and “grainy” flavors since the beer was made via the infusion method? If this is feasible how much would I pull off of the main mash to boil in order to achieve the mash out temperature?


The easiest way to go about solving this problem is to not mash out. Breweries who have infusion mash tuns have no way of doing a decoction
and do not add a bunch of hot water to the mash to increase its temperature. Instead, infusion mash brewers usually start collecting wort and sparge with water that is around 168 ºF (76 ºC) and allow the temperature to slowly rise during wort collection. While mashing out does stop enzyme activity and “fixes” the carbohydrate profile of the wort, the main reason for mashing-out is to reduce wort viscosity and improve wort flow through the grain bed. This is more important when doing stirred, multi-temperature mashes because the mash bed becomes denser. Infusion mashes are usually easy to run-off and this is not so much of an issue.

You can do a single decoction to increase mash temperature if you really want to mash out. A little extra malty or grainy flavor is typically associated with beer made with decoction. How much mash to boil is a weighted average calculation. In this case you want 100% of the mash to be at 170ºF and have (100 – x)% at 155 ºF, for example, and (x)% at 212 ºF. The equation to solve is: (1) x (170 ºF) = [(1 – x) x 212 ºF] + [(x) x 155 ºF]. Solving this equation for x results in 0.74, meaning that 74% of the mash should be left in the mash tun and 26% of the mash should be boiled.

You probably will find that this will undershoot the desired temperature of 170 ºF (77 ºC) because some heat is lost when the two portions are mixed. This can be accounted for by deflating the decoction temperature used in the calculation based on empirical data. You can solve the equation in reverse to determine what temperature to use in the future. If the temperature only increased to 165 ºF (74 ºC), solving the equation (1) x (165 ºF) = [(0.26) x (y ºF)] + (0.74 x 155 ºF) indicates that 193 ºF (89 ºC) was the effective decoction temperature. Using this value for future brews suggests boiling 39% of the mash.

For the sake of simplicity, I have assumed that there is no difference in the thermal properties between the thin and thick mashes because there is no easy way to measure the mash thickness in the portion of the mash removed for boiling. I apologize to those readers who wish to consider every possible variable in brewing calculation and am sure those who wish to make my simple algebra more complex will figure a way to do so!

So there you have the answer to your question. If you really want my opinion — you are getting it whether you want it or not! — I suggest keeping things simple and only adding levels of complexity to your routine if you have a real requirement. If you have a problem getting the mash easily flowing through your mash tun or have a flavor-related concern that this will address I give the added complexity two thumbs up, otherwise keep it simple!

Response by Ashton Lewis.