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Home | The Hop Bine | Decoction, concoction

Decoction, concoction

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Charlie Bamforth of the University of California, Davis

The irascible Charlie Bamforth returns to Brewers’ Guardian

Charlie Bamforth returns to Brewers' Guardian with a four-part series on once essential brewing practices that continue to be used yet may no longer be necessary in light of more modern production techniques. First up: decoction mashing.

When I first “went up” to Hull University in 1970 there was one computer on campus. It occupied a building – a whole building. You registered to spend time on it but I didn’t much bother and stuck to my log tables and slide rule. I couldn’t be bothered with the other seemingly interesting (to some) but limited tasks that it could do. No FreeCell or Facebook in those days.

As a calculator it certainly was a major advance on the abacus, first used by the Sumerians - to tally beer sales, perchance? But just as the abacus seems primitive by comparison with Hull’s main frame, so does the latter appear, from this distance of 42 years, and as I type into my laptop, to be almost laughably primitive.

I feel much the same way about decoction mashing and yet there are those who insist that it is the only way to travel in pursuit of an authentic Germanic lager.

It was, of course, a brewer who coined the phrase “if it ain’t broke don’t fix it” and devotees of decoction might defend their beliefs with such a mantra. Indeed, I know several who argue that it is impossible to recreate the je ne sais quoi (or should that be ich weiss nicht, was?) of a continental lager without using this convoluted approach. 

I would contend that decoction certainly can have impacts on the quality of a brew but generally these are negative. There was a rationale for decoction mashing at the time it was invented but, with the huge advances in the understanding of brewing science as well as major leaps forward in technology, it seems that adherence to decoction is as naïve and restrictive as shunning a smart phone in favour of Morse Code.

What is decoction mashing?

Just in case you don’t know what decoction is, it’s a process whereby the brewer takes out portions of a mash, boils them and then adds them back to raise the temperature. 

Let’s be clear about why it started.

First, the brewers were confronted with malt with less than perfect modification. In short, it was inhomogeneous – with lots of residual cell wall materials and undegraded protein. Now, back in the day, the brewers had no idea about β-glucans and hordeins and enzymes. All they knew was what their eyes and temper told them. They had long since worked out that to make beer you needed to warm up the mash with water to a temperature that they could estimate by tricks like sticking their thumbs in and looking for their faces in the steam. 

The results were inconsistent and sometimes the brew would clog up, or at best the wort was collected slowly and yields of good beer weren’t as healthy as they would like. By trial and error, though, they worked out that they could mash in at quite a cool temperature, allow some settling to take place in the mash and then schlep over some of the thicker stuff into another tank and boil it up. They had no thermometers – it took an Englishman called Michael Combrune to come up with that in 1762 – but there is no mistaking when something is boiling. So once they had boiled for a while, they slopped that back into the main mash, thereby raising the temperature.

A typical approach was to have three temperatures for the main mash, arrived at with all this shifting of wort. A very low temperature (we would now quantify it as around 35°C), an intermediate temperature (50°C) and then a higher temperature (65°C). Nowadays a decoction adherent would call them the acidification, cytolytic/proteolytic and conversion stages. There were other variants of the decoction technique, varying in the number of separate boilings.

Surely there’s a better way. 

We have come a long way in our understanding of the chemistry and biochemistry of malting and mashing. We have a model for the structure of the cell wall of the starchy endosperm and have a pretty thorough appreciation of the various enzymes needed to dismantle those walls and the properties of those enzymes, notably their temperature tolerance. The same goes for the proteins and the starch and the enzymes that dismantle those substances.

Barley breeding programs and the ability to select, grow and sprout the choicest grain mean that (allowing for the ongoing competition for land between malting barley and easier, more profitable crops) no brewer need hurt for the availability of homogeneously well-modified malt.

Just in case a brewer wants to lie snug in his or her bed confident that brewhouse performance is optimal, they can invest at no great cost in a wide range of enzyme cocktails to reinforce the activities from the malt.

And for those brewers adamant that they won’t use such additions but who have access to the long-since available jacketed mash mixers as well as a supply of steam, the detailed understanding of what is actually happening molecularly in a mash allows them to perform temperature ramped mashing in a single vessel. No need for sloshing stuff between vessels.

Pros and cons

And still there are those who will point to the supposed benefits of decoction, perhaps distilled into a rather brief list:

  • Improved starch degradation and therefore higher brewhouse yield
  • More removal of dimethyl sulfide (DMS)

They probably would not argue with the major downsides:

  • A much higher energy demand, perhaps as much as 50% more as compared to an infusion mash
  • An increased risk of oxygen uptake 
  • An increased extraction of undesirables from the husk
  • Greater physical damage to the grist, jeopardizing lautering

I am not going to quibble with these negatives. But I do question at least one of the purported advantages, namely the DMS issue. Regarding the starch issue, I do believe that we do need to understand rather better the differences in gelatinization behaviour between starch granules from different varieties and different growth locations – see some of my comments in MBAA Tech Quart, 2003, 40, 89-97 – and I am prepared to accept that there may be some advantages from decoction in this area, although they are likely marginal.

A saga of DMS

Let’s consider DMS though. One of the two precursors of this molecule is S-methylmethionine (SMM), which emerges from the malt. It is a very thermo-sensitive entity and breaks down when subjected to high levels of heat, to produce DMS. It is for this reason that there tends to be more DMS precursor in a lightly kilned pilsner malt than in a more robustly dried pale ale malt.

So let’s focus on one of those pilsner malts and let’s invoke the work of Chris Dickenson, back in the day, at Allied Breweries in Burton-on-Trent, England. Chris had been three years ahead of me at Hull and, as a mature PhD student, did more than anyone else to introduce me to the beauty of enzyme kinetics. 

In the case of SMM degradation, Chris brilliantly turned his hand (in work that earned him the very first Cambridge Prize from the then Institute of Brewing) to a different type of kinetics, namely the modelling of SMM degradation to DMS by heat. He indicated that the half life of SMM in boiling wort at 100°C is 38 minutes. In plain English this means that if you boil wort for 38 minutes half of the SMM is broken down into DMS. For every 6°C drop in temperature the SMM degrades half as quickly. So take a look at the table… 

Temperature (oC)

Half life of SMM (minutes)

















…and then realize that in an infusion mash that won’t get above, say, at most 72°C, there will be precious little production of DMS in anything like a sensible mash duration.

But consider decoction. Let’s say that we take out 20% of the mash and boil it for 20 minutes. It is customary for that boil not to be especially turbulent and so the DMS produced will tend to linger in the boiling wort and be pumped back into the main mash. 

Now let us say that our wort, as mashed in at the lower temperature, contains 500μg per litre of SMM. (It could be a lot more!) So if we take out 20% that’s 125μg/L. If we boil for 20 minutes then we are going to generate about 30 μg/L DMS and that is going to head back into the mash, with more being produced in any other boils. If anything I have understated these values – the real numbers may be at least twice this.

In other words, decoction mashing is increasing the free DMS level in the sweet wort. Of course, if that wort is destined for a very vigorous boil in which turbulence will drive off the free DMS then decoction may not be an issue. 

But I would contend that it ain’t always so – and small wonder that when Clapperton and colleagues back in the early seventies were searching for what it was in German lagers that gave them their “lager character”, it was DMS (which has a flavour threshold of 30 μg per litre). Notwithstanding, in Germany it is considered an off-flavour. 

Either way, I think we know where the extra DMS might be arising. Decoction has a lot to answer for.

Charlie Bamforth is the Anheuser-Busch endowed professor of malting & brewing science at the University of California, Davis. 

Comments regarding this feature are welcome via email to editor Tara Craig:

Next month: the contributions of yeast to beer flavour.


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