Hoppy beers are incredibly sensitive to loss of flavor from oxidation. Several members of HBT have reported that the storage life, especially of the hop flavor and aroma, of their hoppy brews has improved significantly since they have gotten really anal about minimizing O2 exposure after fermentation. Oxidation comes from these areas:
- Opening the fermenter after fermentation has slowed/stopped. Diffusion and convection will allow significant O2 into the headspace. There is no protective effect from a CO2 blanket. You will get O2 into the headspace.
- Cold crashing. Reducing the temp of the fermenter causes the pressure in the headspace to drop, which in turn causes air (O2) suck back into the headspace. The longer you cold crash, the more oxidation will occur.
- Racking. This process also increases the O2 content of the headspace due to backfill of the emptied volume. But a bigger concern is any splashing or turbulence of the beer in the receiving vessel. And an air leak into the racking plumbing is absolutely the worst as far as oxidation is concerned.
- Not adequately purging the keg headspace of O2. Eventually, much (or all) of the O2 in the headspace will make it into the beer, causing oxidation in a mater of weeks or days, depending on O2 concentration and temperature.
So, what to do to minimize O2 pickup?
- Avoid opening the fermenter. If you can afford one, the electronic, in fermenter, hydrometers can eliminate most of the requirements to open a fermenter.
- Limit cold crashing before packaging to 2 - 3 days max. Or, use a closed system that eliminates air suck back during cold crashing (not an easy thing to implement correctly.)
- Fill the keg completely (to overflow) with sanitizer, and then push the sanitizer out with CO2. Leave the keg sealed until racking.
- Rack to the keg thru the liquid out post with the PRV open, or a QD on the gas post for pressure relief. Even better if you can do a totally closed transfer by pushing from the fermenter with CO2 (to prevent O2 suck back into the fermenter headspace during racking.)
- Pressurize and vent the keg headspace five cycles or so to minimize any O2 remaining (due to incomplete keg fill during the CO2 fill.)
- If you can't rack to a closed, CO2 filled keg, then rack to the bottom of the keg, while minimizing splashing. When the keg is full, do 13 pressurize/vent cycles at 30 psi to get the O2 content in the headspace down to 0.1 - 0.2 ppm.
It really does take 13 purge cycles to get the O2 content to what's required if you don't do a closed transfer into a CO2 filled keg. See the chart and table below.
View attachment 372528
View attachment 372529
Racking to a keg via an open lid is NOT closed transfer, and there is no CO2 blanket to keep O2 away from your beer.
Headspace purge cycles in quick succession will not significantly reduce hop aroma/flavor, as there is insufficient time between purges for the aroma compounds to diffuse out of the beer into the headspace. You do want to minimize purging the headspace after the keg has been put into service, as in this case there will be time for hop compounds to have diffused into the headspace.
Finally, most of the gelatin "how-to" write-ups I have seen recommend adding the gelatin after chilling the beer, not while it is warm.
Brew on