What is Decoction Mashing

Mashing strike temperatureWhat is decoction mashing?

In simple terms decoction mashing is the process by which the mash temperature is increased by removing a portion of the mash, heating it to boiling point and then adding it back to the original mash.  By adding back the boiling mash the temperature of the whole mash is increased.  This contrasts with the traditional single temperature infusion mash, typically employed by home brewers, where the mashing process is carried out at a single temperature.

So why do brewers use the decoction mashing process and are there any advantages to be gained by the home brewer?

There has been an argument put forward that British ales were traditionally produced using infusion mashing at a single temperature and Continental brewers had to use decoction mashing because our Continental cousins had poorer quality malt and therefore had to work harder during the mash.  I have not seen any evidence to either prove or disprove this argument but to this day the decoction mashing process has been largely identified as a continental mashing process.  As such there are some home brewers who would suggest that to brew a more “authentic” continental style lager beer you have to use the decoction mashing process.  I am yet to be convinced of this argument and would suggest that perfectly decent lager style beers can be brewed using a single temperature infusion mash.  In fact I would even go as far as to suggest that many continental brewers have switched away from using the decoction mashing process because they believe there is a negative impact of using decoction mashing on beer flavour


As with any process there are a number of positives and negatives of using decoction mashing which are worth considering and include:

  1. There is less protein breakdown in the boiled mash because of the rapid heating destroying the heat sensitive proteases.  This could impact upon fermentation performance if not enough free amino nitrogen (FAN) is available for the yeast.
  2. There is more extensive gelatinisation and saccharification of the mash due to the use of a boiling step.  This is certainly positive as it can lead to an increased brewhouse yield and some brewers have recorded a higher brewhouse yield from using the decoction mashing process.
  3. There is a likelihood of an increased extraction of husk material which can cause some harsh off-flavours in the final beer and lead to a greater risk of haze formation.
  4. Because the mash is boiled there is an increased formation of melanoidins which are dark coloured compounds.  Therefore the brewing of particularly low coloured beers can be difficult.
  5. There is an increased removal of dimethyl sulphide (DMS).  DMS is a highly flavour active compound found in beer which gives the finished beer a sweetcorn flavour.
  6. There is an overall reduction in the amount of active enzymes in the mash due to the boiling step.

Can a home brewer replicate decoction mashing?

Certainly it is possible to use decoction mashing at home but it requires additional brewing equipment.  For example a brewhouse using decoction mashing will have a mashing vessel, a mash boiler and a lauter tun for wort separation once mashing is complete.  A home brewer can get away with having just two mashing vessels, one for mashing and wort separation and one for mash boiling.

A distinction can be made as to the type of decoction mash employed and this is defined by the number of times a portion of mash is removed and boiled.  Typically this can be done 1 – 3 times but generally brewers will try to include the least number of boiling steps as possible.  If you are going to try this at home my advice would be to only use one boiling step initially.

Decoction mashing is started by mashing the complete grist charge into the mashing vessel and I would usually try to achieve a mashing temperature of 45ºC – 52ºC.  Once mashed-in a portion of mash is removed and put into the mash boiler for heating.  The amount removed to the mash boiler is a matter of trial and error but usually represents about a third to a quarter of the mash.  Once the mash is boiled transfer it back to the mashing vessel with continuous stirring of the mashing vessel to achieve an adequate mixing of the two mashes.  It is important to stir well when adding the boiling mash to the mash vessel to prevent the hot mash destroying the enzymes in the cooler mash.  Extreme care must be taken if doing this at home as manually transferring boiling mashes can be very dangerous and serious burns can result if anything is spilled.  The idea is to increase the mash temperature from 45ºC – 52ºC to approximately 65ºC by the addition of the boiling mash.  Once this is achieved leave the mash until starch conversion is complete and then carry out wort separation as normal.  As mentioned earlier a more complicated mashing profile can be attempted by including more boiling stages.  So for example a three boil decoction mash may follow the following profile:

  1. Mash in at 35ºC.
  2. Remove a portion of mash and boil.
  3. Add the boiling mash to the mash vessel to raise the temperature from 35ºC to 45ºC – 52ºC
  4. Leave the mash for a 25 minute protein rest
  5. Remove a portion of mash and boil
  6. Add the boiling mash to the mash vessel to raise the temperature to 65ºC
  7. Leave the mash for a 40 minute stand to allow for maltose formation
  8. Remove a portion of mash and boil
  9. Add the boiling mash to the mash vessel to raise the temperature to 72ºC
  10. Leave the mash for a 15 minute saccharification stand.

This profile is perhaps the most complicated decoction mashing profile to achieve requiring as it does 3 separate boiling stages.  However, with some experimentation it should be possible to achieve at home.

Decoction mashing is an interesting process which for most commercial brewers has been consigned to the brewing history books.  However, for the more adventurous home brewer decoction mashing can be an interesting experiment in mastering a traditional brewing technique but please be careful boiling mash can cause serious burns.

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