I’ve been reading about old-fashioned open fermenters lately and I think they are due a come back in the microbrewery world.
The downsides of open fermenters for big breweries are:-
- They need to be cleaned by people. People are expensive. People don’t consistently do a good job of cleaning and fermenters are full of dangerous CO2 gas.
- Open fermenters make it hard to collect CO2, which is a wasted resource.
- Open fermenters are more subject to infection, because they are harder to clean, and because they are open to the atmosphere, which means there is greater potential for lost product.
- Yeast collection (for bottom fermenting yeasts) is harder.
- Open fermenters need to be housed in an air-conditioned room, whereas closed fermenters can be sited outside.
Collectively, these downsides mean that there is no way a large brewery would opt for open fermenters now. They simply aren’t efficient enough.
Most, perhaps all, microbreweries seem to have accepted that closed, vertical, cylindroconical fermenters are the way to go. They have the following advantages:-
- They take up less floorspace in proportion to their volume. Vertical dimensions mean efficient convection currents inside tank
- They can be CIP’d.
- They can be used as fermentation and storage/lager tanks.
- They make it easy to collect yeast of any kind.
- They can be bunged to allow the beer to build up natural carbonation.
- The CO2 produced in fermentation can be collected (e.g. it is possible to purge an empty tank with CO2 produced by an active fermentation simply by connecting the headspace of the fermenting tank to the bottom of the receiving tank, perhaps bubbling through sanitiser on the way to ensure no infection carries over this way).
- Completely closed system makes it easier to manage multiple different strains of yeast.
- Completely closed system helps to satisfy supermarkets’ Global Food Standards requirements.
That said, CCVs are dangerous in their own way. They quickly get very tall, which means you are working at height when you add finings or dry-hops to them (or else you need special equipment to fire the additions in). This can be really dangerous. I once had a nasty accident at the top of a ladder adding dry-hops to a beer at the end of fermentation when the beer had a lot of CO2 in solution. The surface of the beer began to swirl and as if it was about to come to the boil and then a huge wave of CO2 came out of the tank and cascaded over me. I couldn’t breathe. I heard rushing in my ears and began to see stars. I stumbled down the ladder quickly before I passed out, but I easily could have fallen off the ladder. When there’s too much CO2 in solution the beer can geyser out of the fermenter as in this video.
What are the advantages of open fermenters.
- High surface area in proportion to their volume leads to an efficient purging of volatiles. No “egg salad” beer.
- Easy to see what’s happening to the beer and to monitor it.
- Shallowness of fermenters lends to a rapid settling time and promotes beer clarity.
- Easy to collect top-fermenting yeast (by skimming)
- Easy to dry-hop and fine beers.
Open fermenters can be cooled either by cooling panels in the walls or by cooling pipes submerged in the beer.
It is worth noting that open fermenters used to be how everyone used to do it. Even lager breweries. Beer can be carbonated naturally by moving beer before it has completely fermented out into a closed tank. Open fermenters can be given hoods that will direct the CO2 somewhere for collection. Yeast can be cautiously scraped from the bottom of the FV and washed or made into a slurry with fresh cold water and collected after the beer has been moved.
Open fermentation, however, can only be carried out commercially successfully if the fermentation cellar is adequately air-conditioned. There needs to be regular refreshing of the air because the CO2 produced from fermentation can rapidly become toxic. Wikipedia currently puts the content of co2 in the air at 0.04%. The safe operating maximum is 0.5%, or for brief periods up to 1.0% (Kunze Technology: Brewing and Malting 5/e 2014, p.553). It can easily go above these levels in closed rooms. The air needs to be cool and dry and preferrably filtered. The room can’t have any overhead pipes that might serves as dust traps or places where drips might form. The floor has to be easy to clean and the walls and floor of the room and the sides of the fermenters should be smooth.
There’s a nice video and some photos of open fermenters in use at Edinburgh’s Caldonian Brewery here.