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We have a batch of beer that has been in primary for about a week and we are getting ready to rack to a secondary fermenter. We want to experiment with the difference between doing primary only and both primary and secondary fermentation. We have 5G total and are planning to transfer about half into a 3.5G carboy and bottling the rest now.

However, for primary-only we've seen recommendations to leave it for two weeks, so I'm a little concerned our test isn't really going to contrast the two approaches. Will there be a significant difference between:

  • bottling the primary-only batch after a week but leaving bottles for the extra week to let the primary-secondary batch catch up and
  • leaving the batch in primary for an extra week (assuming we weren't disturbing it by transferring a lot of it for secondary) and having it bottled for a week less?
  • @Philippe Sorry, it's beer. I've edited the first sentence to make it clear. – thesquaregroot Oct 22 '15 at 17:09
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That depends on the question you want to answer.

bottling the primary-only batch after a week but leaving bottles for the extra week to let the primary-secondary batch catch up and

How does the beer taste if we bottle a week early?

leaving the batch in primary for an extra week (assuming we weren't disturbing it by transferring a lot of it for secondary) and having it bottled for a week less?

Do we really need to transfer the beer before bottling?

My advice

After a week the beer should have reached terminal gravity. At this point the yeast spends some time cleaning up some fermentation by-products, primarily diacetyl and acetaldehyde. At New Republic Brewing we start taste testing for diacetyl at terminal gravity. Once it is no longer perceptible we prepare the beer for packaging.

You will likely end up with a better beer if you test your second question by transferring some of it to a secondary vessel, leaving the remainder in the original fermenter. Bottling now may not give the yeast a chance to clean up after itself.

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  • Out of curiosity do you do VDK testing on samples straight off the fermenter, or do you treat them in any way first? – Franklin P Combs Oct 5 '15 at 0:17
  • Take two samples from the fermenter. Heat one in a covered container to 60°C for 10 minutes. Chill both samples and compare for diacetyl. – Dean Brundage Oct 10 '15 at 2:05
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Here is a good test of what you are looking to do.

http://brulosophy.com/2014/08/12/primary-only-vs-transfer-to-secondary-exbeeriment-results/

Here is the conclusion of the test:

Once all the data was collected and I wasn’t worried about blowing through these kegs of beer, I started serving it to people stopping by. On a few occasions, with folks who hadn’t completed the survey and knew nothing of this exBEERiment, I served the 2 beers one after the other. Once the second glass was empty, I’d ask, “Which one did you like better?” The response 100% of the time was some form of, “I didn’t know they were different.”

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If you want to experiment with the difference between "primary-only" and "primary-secondary", then rack half of your batch into a new fermentor, and bottle both halves at the same time.

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    Will it be a problem that we had to open the primary fermenter in order to transfer to the secondary? I imagine we'd be introducing a lot of oxygen with the added headspace in the primary. What difference would we see from not leaving it in primary another week? – thesquaregroot Oct 3 '15 at 14:45
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    Nope, not a problem. Like jsled says, you want to give them both the same amount of time in the fermenter in order to be a fair test. – Denny Conn Oct 3 '15 at 16:50
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    @DennyConn I think thesquaregroot is entirely right in pointing to this as a potential weakness of the test. The problem isn't opening the fermenter as much as it is the large volume of air pulled in during racking (the same volume as the beer removed). In this manner you really would be testing primary+secondary vs. primary+oxygen that wouldn't otherwise be there. Sure the difference may be small (especially if the beer is consumed quickly), but I think it's a good point to consider. – Franklin P Combs Oct 4 '15 at 0:57
  • Franklin, exactly what difference would it make? – Denny Conn Oct 4 '15 at 16:46
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    Assuming primary was in a 6.5 gal. fermenter racking would introduce 2.5 gal. of air (at ~20% oxygen), meaning you'd be aging the primary-only beer under ~125,000ppm oxygen. At room temp. especially (which is where I imagine most brewers would be doing this step) that could easily have noticeable staling effects on the beer. I imagine it's probably the same exact reason so many brewers here advocate not racking to secondary at all. Of course it makes no difference if the OP is not purging before racking, but I doubt many around here would recommend doing that. – Franklin P Combs Oct 4 '15 at 19:18

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