Kegging half my ferm and leaving the rest

[Northcoast]

Sorry if someone has posted/answered this already but I couldn’t find it on the forum. I currently have ~10L of [a hopefully tasty] oatmeal stout in my primary fermenter. Once I hit terminal gravity, I am thinking about kegging only ~half of it into my 4L mini-keg and leaving the rest in the fermenter - after force carbing and serving the 4L keg, a week later I am planning on pulling another 4L from the fermenter into the same keg (cleaned and sanitized, of course) and force carbing/serving. This is all because I only have the single 4L keg available right now with my other mini-keg and bottles all currently full.

Feels like it shouldn’t be an issue because I’ll be pulling from the bottom valve on the fermenter and won’t ever open the fermenter or allow any oxygen ingress. Any advice on if or how I should do this?

NC

 

[VikeMan]

What kind of fermenter do you have? Assuming the pressure in your fermenter's headspace is at about one atmosphere, I doubt you'll be able to rack half of the beer to a keg without introducing gas of some kind, intentionally or not.

 

[Malticulous]

You could toss a little sugar in to purge the headspace.

 

[VikeMan]

You could toss a little sugar in to purge the headspace.

Large commercial breweries target dissolved O2 limits in low parts per billion at packaging, even for bottle conditioned beers. A couple of pro anecdotal tidbits regarding yeast's ability to use O2 in bottle conditioned beers (which is similar to, but not as severe as, the problem of introducing O2 to the dormant fermenter 1/2 volume headspace):

John Mallet (Bells) said that the yeast will use about a third of the headspace O2. (Bells also CO2 purges and caps on foam, so one would have to assume John is talking about a much smaller amount of starting O2 than a homebrewer would typically get in the headspace.)

Garrett Oliver (Brooklyn) said "Yeast can remove small amounts of dissolved oxygen from beer, but very little from the bottle headspace air, which means these benefits will only attain to breweries using sophisticated brewing methods and very good packaging equipment."

I think a lot of folks have unreasonable expectations about what yeast will do with O2 when a little sugar is added. The main thing yeast does with O2 in the presence of sugar is make sterols, so that they can build cell walls, allowing for exponential growth. But when there's not much sugar (and thus little potential for growth), they don't need much O2, because they won't need to build sterols.

 

[IslandLizard]

I’ll be pulling from the bottom valve on the fermenter and won’t ever open the fermenter or allow any oxygen ingress.

Is this a closed, pressurized fermenter? If so, you can do that, yes, and leave the rest of the beer in there, under slight pressure.
But if it's a bucket, carboy, etc., when beer is transferring out of your fermenter, air will get sucked in.

 

[wsmith1625]

Like @VikeMan said, as the volume of beer leaves the fermenter it will be replaced with a volume of gas. If your fermenter fills with air, the amount of oxygen is going to be very detrimental to remaining beer. If you can replace the volume of beer with co2 you should be fine. If you have a fermenter designed to do closed transfers you'll be fine. If not, you may want to try filling a mylar balloon with co2 and connecting it to your airlock when you do the transfer. As the beer fills your mini keg, the fermenter should pull the co2 from the balloon into your fermenter instead of pulling in air. It's probably the simplest option available to you.

 

[Northcoast]

Sorry for the delayed reply and thanks for all the helpful answers! My fermenter is a glorified bucket with a sealed tap at the bottom, I was thinking keeping the top sealed would avoid O2 ingress but I clearly forgot my high school physics! I like the idea about attaching the CO2 filled mylar balloon to the airlock prior to the transfer, that is doable for me and sounds like will get the trick done. Will also see if I can scrounge up some empty growlers from friends to keg + bottle all the beer in one go. Thanks again!

 

[IslandLizard]

My fermenter is a glorified bucket with a sealed tap at the bottom,

Keep that spigot clean and sanitize well before and after using it to avoid infections.

attaching the CO2 filled mylar balloon to the airlock prior to the transfer,

Some buckets with lids are better sealed than others.
The balloon can also be inflated/filled with fermentation gas. Regardless, it will always be at atmospheric pressure. When tapping from the bottom the headspace pressure drops and will be filled with gas from the balloon, and/or air from any leaks.

scrounge up some empty growlers

Glass growlers cannot be pressurized.

 

[wsmith1625]

Most spigots come apart for deep cleaning. I always break then down and give a good wash between brews. If you take samples from the spigot while fermenting, spray it down with StarSan afterwards to keep it sanitary.

 

[Northcoast]

@IslandLizard - I did not know that about growlers. I have filled and carbonated swing top 2L glass growlers in the past to take on the go, is there risk of those growlers exploding?

 

[IslandLizard]

@IslandLizard - I did not know that about growlers. I have filled and carbonated swing top 2L glass growlers in the past to take on the go, is there risk of those growlers exploding?

Yes, they can explode when "bottle conditioning" them. The heavier/thicker the glass the more pressure they can take, such as the original wide mouth ones with the flip top and large handle, may fare better.
The cheap brown 1/2 gallon "wine jugs" basically, with the little ear, they sell at growler filling stations cannot take much pressure. I had one of those explode inside a cooler with ice/water, out of the blue, when I picked it up to remove it.

Also, the bigger the bottle the higher the risk of exploding.

 

[scrap iron]

Good advice on the balloon method. Another is to use a co2 catcher during fermentation. I use one of those and it also works for a cold crash. Always sanitize the spigot well before and after use to keep them from becoming sticky. I cover the spigot with a Zip-lock sandwich bag to keep out bugs and flies.