Vince Burns
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In a brewery they open the fermenter at the top. You can do this with a pressure fermenter too. CO2 is heavier than air, so little if any air will enter. Depending on the brewery, some only add from a freshly opened packet (e.g. the Australian Brewery). The packets they buy are 5kg bags and they have an oxygen free environment inside and have been steralised. So no need to worry about germs or oxygen. You could do the same.
If the packet has been opened (or routinely in some breweries, e.g. The Malt Shovel Brewery) they mix the hop pellets with boiling water first, remove some of the fermenting beer from lower down on the fermenter (but above the yeast bed) to cool the steralised hop mixture, then force it back in. You could do something similar by letting your now sterile hop tea cool (cover while cooling) and then pouring that into the top of your pressure fermenter.
In either case, you can purge any air that was introduced into the pressure fermenter after you close it again by pulling on the PRV a few times over the next few minutes. Air (and the oxygen it contains) floats on the top of CO2 so it will be the first gas to leave via the PRV. This is if you are dry hopping after reaching terminal gravity and the yeast has stopped producing CO2. You can give it a hand by attaching a CO2 cylinder to the gas post to pressurise it, then pulling on the PRV a few times. This is when most breweries will dry hop. It ensures a constant contact time without having to worry about how long the diacetyl rest will take.
However, if you are dry-hopping around day 3, the beer will still be fermenting so it will purge naturally within minutes through the spunding valve (or an air-lock even) without you needing to do anything.
You can then use a hop blocker attached to the end of the dip tube (which you attached prior to fermentation) to transfer the beer into a new keg if you want to remove the hops and yeast before chilling and force carbonating. No need to open it again to remove the hop sock. In fact, putting it in a hop sock in the first place will prevent you from adding steralised hop pellets. The hop sock will not be steralised. Even if you try to steralise it you will then need to manipulate sterile hop pellets into a sterile sock, tie it up, position it, later remove it, all under strict aseptic technique, and that's very unlikely to be the case.
If the packet has been opened (or routinely in some breweries, e.g. The Malt Shovel Brewery) they mix the hop pellets with boiling water first, remove some of the fermenting beer from lower down on the fermenter (but above the yeast bed) to cool the steralised hop mixture, then force it back in. You could do something similar by letting your now sterile hop tea cool (cover while cooling) and then pouring that into the top of your pressure fermenter.
In either case, you can purge any air that was introduced into the pressure fermenter after you close it again by pulling on the PRV a few times over the next few minutes. Air (and the oxygen it contains) floats on the top of CO2 so it will be the first gas to leave via the PRV. This is if you are dry hopping after reaching terminal gravity and the yeast has stopped producing CO2. You can give it a hand by attaching a CO2 cylinder to the gas post to pressurise it, then pulling on the PRV a few times. This is when most breweries will dry hop. It ensures a constant contact time without having to worry about how long the diacetyl rest will take.
However, if you are dry-hopping around day 3, the beer will still be fermenting so it will purge naturally within minutes through the spunding valve (or an air-lock even) without you needing to do anything.
You can then use a hop blocker attached to the end of the dip tube (which you attached prior to fermentation) to transfer the beer into a new keg if you want to remove the hops and yeast before chilling and force carbonating. No need to open it again to remove the hop sock. In fact, putting it in a hop sock in the first place will prevent you from adding steralised hop pellets. The hop sock will not be steralised. Even if you try to steralise it you will then need to manipulate sterile hop pellets into a sterile sock, tie it up, position it, later remove it, all under strict aseptic technique, and that's very unlikely to be the case.
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