Regardless of the type of fermentation vessel used for commercial scale fermentation, the one process step which they all have in common is the requirement to thoroughly clean and sanitise the vessel between batches.  A lot of effort and time goes into cleaning equipment after use, hence the old adage that a brewer is a glorified janitor. From a sanitary processing perspective the most critical equipment in a brewery is from the wort heat exchanger onwards.  After the wort has been cooled it is susceptible to contamination so the equipment which it comes in contact with must be thoroughly cleaned and sanitised.  Residual yeast, dried on fermentation foam (krausen) and fermented beer must be completely removed from the fermenter before the next batch of wort can be added to the vessel.  All of this residual material is an ideal medium for bacterial growth which will readily contaminate the next batch of beer if not removed by means of cleaning.  This cleaning is referred to as Clean In Place or CIP as the vessels are not disassembled and placed in a remote washer for cleaning but are cleaned as an integral unit in their production location.  Additionally, the vessel must be sanitised with a suitable solution to minimise the starting bioburden (bacteria) load in the vessel before the next batch of cooled wort and yeast can be added.
In addition to the requirement to use strong cleaning agents, the cleaning process is water and energy intensive. A cleaning regime is always more effective at elevated temperatures so this process is performed hot, typically >65C.

A typical unitank cleaning regime includes;

  1. Ambient water prerinse to flush out residual yeast and loose gross dirt. This generally requires manual intervention to remove the dried on krausen which sticks to the side wall of the fermentation vessel and is very difficult to remove without some mechanical scrubbing.
  2. Hot cleaning which involves the recirculation of the cleaning solution form an ancillary vessel via a pump to the vessel spray balls before exiting the vessel via the bottom outlet valve and flowing back to the wash vessel. The wash vessel and pump are typically part of a CIP skid which will has its own control system, instrumentation, water rinse tank, cleaning solution dosing pump, etc..
  3. Final water rinse: performed to flush out the residual cleaning solution. This is typically a time-based operation with clean water pumped into the vessel via the spray balls at a target pressure. A conductivity or pH check is frequently performed on the final rinse water to confirm removal of the cleaning solution. The amount of water required to flush out the residual cleaning solution is not insignificant.

Vessel sanitation is similar to the CIP chemical wash process but in this instance an ambient sanitising solution, such as peracetic acid, is pumped around the unitank.  After a minimum contact time the residual sanitising solution is drained down and the vessel is allowed to air dry.

Improper cleaning & sanitation leads to complete contamination of beer before it is even packaged or best case a reduced shelf life which is expensive and to be avoided.  A brewer strives to achieve a so called axenic or monoculture in the fermenting beer.  In practice, there is always some minimal amount of so called adventitious bacterial present in the fermenter so prior to starting a fermentation the equipment is considered “low bioburden” as opposed to sterile.  Most microbreweries perform so called “briefly exposed” operations, e.g. connecting hoses, adding hops during dry hopping, etc..  This invariably leads to the exposure of the open hoses and vessel pipework to some bacteria.  This is not a significant risk as the high starting concentration of yeast effectively out-grows any bacteria which may be present.   Many breweries also perform “open” fermentation, in this case the fermenter is completely open to the environment.  In some cases, the yeast comes from the surrounding environment, e.g. Belgian style Lambic.  However, the vast majority of craft brewers ferment their beer in closed vessels and strive to minimise exposure to bacteria and wild yeast.  If a significant amount of the “right” bacteria or wild yeast is present at the start of the fermentation, these microorganisms will compete with the yeast and can effectively destroy the beer by producing nasty off-flavours.