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).

Deep

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.