Ferment small batch in flask?

[Rob2010SS]

Im going to be brewing up a couple of test batches this week. Im doing half gallon batches. Half gallon ends up being almost 1900mL. I have a 2L flask and a 5L flask.

1. Anyone ever used a flask as a fermentation vessel?

2. Is 2L flask to tight of a space to ferment 1900mL if wort? Should i go with the 5L flask instead?

 

[day_trippr]

1. Never, aside from starters.
2. Definitely go with the 5 liter flask. It'd be crazy to leave it on the sideline while your 2L overflows

Cheers!

 

[Sadu]

I've done this a LOT for test batches. I scored 9x half gallon glass flasks for $2 each but they had no lids. So I fitted bungs and airlocks and used them as fermentors.
I was making a 5 liter / 1.25 gallon brew and splitting that across 3 flasks. That is 5 liters into 5.7 liters total capacity. I never had a blowoff issue. For some reason these tiny batches never generate more than an inch of krauesen, but 1/2 inch is more typical.
After primary I would then transfer 3x flasks into 2x flasks for secondary, this gets rid of all the headspace. Don't want to start a discussion on whether to secondary or not, but I liked that it freed up a flask for the next brew. Also, these little flasks tuck nicely into the fermentation fridge alongside your big fermentor.
To answer your question, there is no way you can ferment 1900ml in a 2000ml flask. But 1650ml works.
If you are cool with the amount of work and cleaning for such a small batch size, go for it.

 

[ESBrewer]

As a 1.2 gallon brewer, I agree with Sadu. I have mainly used 5 liter glass carboys. 2 liters of space is not sufficient for 1.9 liters of wort but 3 liters or 1 gallon would be enough. 5 liters can be used but be careful not to let oxygen to the head space post-fermentation or you'll see plenty of oxidation. You want to have at least 1/4 head space to feel relatively safe.

 

[Rob2010SS]

Thanks for the info! Appreciate it. Yeah i thought the 2L flask was too small. I may drop the volume to like 1600mL or something like that. We'll see. Thanks again