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February 15
(1) Fermentation Vessels Shape and (2) Oxygen During Tank Transfers

Question

​I've been looking and asking around for insight into a couple of issues we're encountering in our multi-location brewpub operation:

1) how does fermentation vessel shape/design impact fermentation rate/degree of attenuation? We've noticed significant differences between the times when we completely fill our 20 bbl unitanks and when we only fill them half way with 10 bbl. batches (faster/further attenuation during 1/2 fills).

2) is there a safe, effective, and efficient way to ensure minimum oxygen uptake during transfers between tank? I understand the theory of filling a tank with water and pushing water out with co2, but that is not terribly practical for many small breweries w/o dumping most of the water. I've heard about meters that measure outflow of gas from tanks during purging (http://www.analox.net/product-docs/manual-20.pdf / they measure in o2% down to 0.1%), but have not come across any info that points in a definitive direction based on quantifiable measurements of o2 pickup between tanks. Sniffing the tube at the end of the CIP arm is often employed, but does not seem terribly safe or verifiable between users' sniffers.

Answer

1) There are several factors affecting fermentations that you are describing:

  • Geometry of the FV tank affects fermentation and flavor. Very tall tanks suffer from CO2 scrubbing of aroma compounds during active feremntation, some larger breweries blend beer made from tall CCV’s with beer made from smaller tanks in order to retain these aromatic properties. 
  • Rate of fermentation in single brew vs multi brew fermentations can be affected somewhat by tank geometry but will have more to do with the fact that the single brew will have full aeration and yeast pitch to start fermentation without the addition of sugars from added brews which will temporarily slow the fermentation down and may even cause stratification in multi brew fermentations. 
  • The attenuation rate is controlled in the brewhouse by the grist composition, mash temperature, rest time, etc. and not the geometry of the tank or the number of brews added unless there is incomplete aeration, pitch rate or some other fermentation problem. An easy bench top test to find out where the fermentation should stop is a forced attenuation test. Take 500 mls of oxygenated wort during wort cooling into a 1 liter flask, pitch with 50 mls of your yeast and set on a bench at room temperature, 68F (20C) with a cotton top, rouse this regularly or better yet use a stir bar to keep it continuously stirring. The wort should attenuate fully in 24- 36 hours and let you know the “attenuation limit” of that batch of beer to compare with your FV results.

2) The best way to de-aerate a tank is the water push method you mentioned but this does waste water unless it can be captured and used for something else like tank rinsing. To de-aerate small vertical beer tanks, ie 5-100 bbls its pretty easy to gas them with CO2, this can be done by adding CO2 gas at a slow rate through the bottom valve tank inlet after sanitizing for about 10-15 minutes and letting the gas exit through the vent line, preferably out of the building. Although this is not as efficient a way to de-aerate the tank as the water push and will not completely eliminate the oxygen present, it will substantially reduce the oxygen level for your transfer and is certainly better than leaving it with an normal atmospheric environment. 

!!CO2 WARNING: Serious Care must be taken that the room in which you gas a tank has excellent ventilation as CO2 gas accumulation can cause asphyxiation! Vent the tank out of the building while gassing with CO2!!

Gassing the tank should provide enough reduction of oxygen for a safe tank transfer. Some other factors to consider for low oxygen pick up:

  • Pack the hoses, pumps and lines with water and then push the water out with beer at a tee located at the tank inlet at the start of the transfer. This will reduce oxygen pick up from the empty transfer line/hose.
  • Pump the beer slowly at first to build  a level that will cover the inlet and avoid “fountaining” the beer in the tank
  • CO2 “dusting” or addition of a small amount of CO2 gas through a carbonating apparatus can help flush out oxygen, especially post filter. The dusting though can also flush out beer aromatics like hop aroma so use only a minimal amount.
  • Gas the tank again post transfer to push out any air in the tank head space and bung it closed under a slight positive pressure.

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