Oh Vienna! Double Decoction Day

On Saturday 14 January 2017, we brewed a one-off special Vienna-style lager using a double decoction mashing process (Zweimaischverfahren). This will be the first in a series of limited edition labels we will be doing this year. We had a few friends over to help out and keep us on on our game for the day and a lot of fun it was.

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The decoction process involves taking a portion of the mash out and boiling it separately and adding it back to the main rest to raise the temperature of the whole. It is seldom seen these days (outside the Czech Republic) because it is energy-intensive and time consuming.

Vienna is an also seldom seen lager style known for its complex malt character. We wanted something interestingly different from our excellent Dortmunder style lager. And something that would showcase a technique that, more than anything else, is all about the malt.

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As a process, it makes most sense if you ask yourself how you would find that mash temperature window if you did not have access to a thermometer (which was not invented until 1612). (In the very early days) maybe you learnt through trial and error how many stones you need to heat in the fire to heat your mash to get the best result. Or, maybe you learnt what proportions to mix boiling water and cold water from your water supply. Another way is to learn what size decoctions to draw. By blending volumes of different fixed temperatures you could reliably find the mashing sweet spot. Boiling wort or boiling water is one fixed temperature. Cold well water is another.

But there’s more to it. Decoction lets you follow a stepped mashing programme hitting multiple mashing temperature sweetspots 35˚C to acidify the mash, 40–45˚C to break down gummy cell material called betaglucans to make starch more accessible, 50–55˚C to break down proteins, to improve beer foam, and promote beer clarity, 60–65˚C to break down amylose and amylopectin starch molecules into simple sugars, and 70–75˚C to break starch down into more complex sugars.

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These days with improvements in malting, there’s a legitimate question as to whether good beer requires anything more elaborate than a single temperature infusion, but back in the middle ages it certainly did. Even in modern times natural seasonal variation in malt quality can throw up challenges that the decoction process was designed to meet.

2014 was a bad year for malt in Ireland and many brewers noticed a drop in brewhouse efficiency. There was a lot of partially gelatinised starch going out with the spent grains. Somehow the starch proved resistant to hydrolysis and enzymatic breakdown.

Decoction helps in cases like this because boiling the decoction thoroughly breaks open the starch granules to make them accessible to enzymes left behind in the rest of the mash.

To prepare for the brewday we carefully measured water into the kettle to find ‘landmarks’ that would tell us on the day that we had the volume we needed. Helpfully there was a stainless steel bracket holding the temperature probe to the wall about 1cm above our desired decoction volume. (The desired decoction volume was determined by calculation based on the temperature changes we were hoping to make).

The decoction went very smoothly and one positive thing was the heating times, which were much quicker than usual because we were heating only a portion of the mash at one time (Decoction mashing uses more energy over all, however, because you heat the malt all the way to boiling and then keep it there).

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One thing that interested me as a brewer was the averaging out of the different treatment received by the malt. We mashed in low and held the whole mash at 45˚C for ten minutes to break down betaglucans to reduce wort viscosity and give a smooth lauter run off. Then the temperature was raised to 50˚C before the mash was divided into two. We sent some to the lauter tun and held some back which we heated first to 76˚C for 5 minutes to convert all the starch and then to 100˚C to spring open the starch molecules. As the decoction was being heated and boiled, the rest of the mash was being subjected to a protein rest at 50˚C which is potentially quite damaging for beer foam because too much protease activity can break protein down too much. We will see how this turns out, but this is my biggest worry about the beer. However, because the decoction was reasonably large and was heated very quickly there is room to hope that a sizable portion of protein survived the mashing intact. The decoction wasn’t held long enough at protease temperatures for significant break down and after it was added back to the mash, the temperature would have been outside the comfortable temperature band for protease action. If we do a decoction brew again, it might be worth leaving the main mash to 45˚C rather than 50˚C (i.e. still in the range for beta glucan breakdown but a too cold for significant protease activity. (The trouble for us these days is simply that modern malts are probably too well modified for the rigors of a full decoction).

The fermentation profile was very traditional. The wort went into the fermenter at 8˚C and was allowed to rise naturally to 11˚C. The purpose of the cool initial period is to suppress the formation of esters. Esters are quite an important component of an ale’s flavour profile, but they’re not right for lager. We pitched a large amount of yeast to compensate for the cool start and aerated vigorously with bottled oxygen.

So far everything is progressing beautifully and the beer is really coming together. Updates to follow…

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Andrew Jorgensen, Head Brewer