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I plan to scale a 20 litre batch recipe down to just 4 litres so that i can try the recipe out before committing to a full batch.

Problem is I'm unsure if my 5 litre demijohn is large enough to use as a fermentation vessel.

If its important its Kolcsh style, using Kolsch yeast [i.e. WLP029 or WYEAST 2565]

Thanks

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    It seems the best way forward would be to just use a blow off tube regardless, just to be safe. – AlexS Jul 6 '18 at 23:57
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That white labs kölsch yeast is very vigorous and in my experience produced a large krausen during the first couple of days (I pitched a whole pack into 10L of wort at around 17°c). I'd advise using a blow off tube at least until it calms down.

  • No matter which yeast you use, in my experience using a demi-john as primary, because of its shape, will always blow off. Using a blow off tube is the best. Otherwise you will need to monitor the airlock closely. – Philippe Jul 5 '18 at 16:18
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Usually, as a rule of thumb, you keep 20-30% of empty volume on top of your wort for safe fermentation.

edit: as mentioned in another answer, using a blow-off tube instead of a regular bubbler can help prevent overflow of krausen.

  • I’d say that would work typical home brew scenarios, but in terms of scaling a recipe to 300L you would need much less. – Martin Jul 25 '18 at 19:58
  • yes ofc, but here we are speaking 20L to 4L, but yeah, good point – JeanMi Jul 31 '18 at 6:55
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I use a 5L flask for 4L brews. Usually going for pale ales or belgian style ales. But a 1L head room is plenty for those. Similarly, a Kölsch is top-fermenting ale (brewed in Cologne). Remember that the yeast cake will still occupy 200-400mL, which is lost when racking the primary. So I would not expect more than a 3,6L yield.

Bottom line: you should be fine.

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20% headspace should be adequate for your test brew. I would add a blow off tube just to be safe rather than sorry.

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