Thursday, 19 July 2018

Fermenter Geometry

It's a while since I've done a post about technical beer nerdery, what with more important things getting in the way. But I think I've slowly reached an understanding about why fermenter geometry is significant so it's time for another.

It's a term used in text books, usually shortly before launching into descriptions of the various weird and wonderful types. Yorkshire Squares and Burton Unions often get a mention, and what the hell was going on Spherical fermenters? Most beer is made in cylindro-conical fermenters though nowadays.

Obviously yeast collection techniques will differ with different shaped fermenters, Ale yeast traditionally skimmed off the top, lager yeast scrapped out the bottom. With cylindro-conical fermenters the yeast sediment can be drawn off from the bottom of the cone and they are now commonly used for both ale and lager brewing. 

But that's not really what I was interested in. I wanted to know what does the fermenter shape do to the beer flavour. Having started out as a microbiologist I know that bugs don't read the text books so will at times behave strangely, but how does the shape of a fermenter affect this? It seems the main difference between different fermenter types is not so much due to the shape as due to the depth of the wort.

Traditional fermenters are quite shallow and lager maturation was carried out in horizontal tanks so not much depth of wort.

Not deep
As cylindro-conical fermenters are often big...very big (6000hl)...they can get very tall too. Up to 15m apparently (and even 40m if used for maturation/conditioning).


Greater depth means greater hydrostatic pressure which affects how the fermentation progresses. Carbon dioxide concentration increases resulting in greater mixing of wort due to gas lift. As the height to diameter ratio increases, so does natural agitation and the fermentation rate. This rapid fermentation can mean medium gravity ales can be produced with unseemly haste (30 to 49 hours). Beers made rapidly in tall fermenters with higher pressure contain less esters, as these figures for ethyl acetate show:

Ethyl acetate content of beer resulting from shallow and deep fermentation
Wort height (m) Ethyl acetate (mg/l)
3                        20
18                      12

This demonstrates how putting the same wort into different fermenters can make different beers. There are other factors that affect ester production during fermentation so things such as oxgenation rate, temperature and pressure can be adjusted to compensate.

I also recently learnt that greater surface area at the top of the fermenter means greater loss of hop volatiles if dry hopping in tank. As one of the purposes of traditional lagering is to get rid of unwanted volatiles will it also have an effect there? Certainly horizontal tanks were favoured over vertical ones. There may be more mysteries to fermenter geometry yet.


  1. Curious to see copper lined fermenting vessels still in use (?), since most brewers stopped using them a century ago. Can I ask what brewery that picture is from?

    That said, I've always wondered if the Burton brewers intentionally used copper pipes/troughs in their union systems to aid in the removal of sulfur, or if it was just coincidental.

    1. That was at Arkell's. Not sure if they still actually use it. I know some breweries do use copper electrodes to remove sulphur though.

  2. Heard some good talks on fermenter dimensions in last few brewcons but don't remember which speakers. Was definitely somethinf thought sifnifisign when fermenting lagers. Also tank design for dry hopping both area in contact and hydrostatic pressure. Yeast performance often specific to fermenter and switching can rwsulr in complete change of behaviour

    1. Some yeasts definitely work best in specific fermenters, e.g. highly flocculent yeasts that need a lot of rousing in Yorkshire Squares.

  3. I think the reason to use horizontal tanks for lagering instead of vertical is simply for more efficient use of cellar the space. It surely evolved from barrels, that were stacked several levels high.

    1. Horizontal conditioning tanks take the advantage of height to volume ratio. There is less distance that the yeast has to floc out, therefore it takes less time to clarify between a vessel that is 3m hight than 10m high.