forcing air out of plastic bottles before capping

[big handsome Archie]

Read somewhere about this practice. Could someone tell me what the advantages are in doing this, or is it a waste of time. Archie.

[Mark D Pirate]

Not worth the effort .

Fill bottles to almost brim and the volume the bottling wand takes up is about perfect for headspace .

Think about how much extra space you're asking the yeast to fill with farts !

You'll end up with flat beer by the time they fill bottle properly and generate enough pressure to force CO2 into dissolving in the beer .

 

Let your beer get to a good stable FG and bottle .

Only reason I can think of is excluding oxygen which will help beer last longer but then you have an unpredictable priming rate to get carbed beer .

[Otto Von Blotto]

It's generally done to stop oxidation. The consensus these days seems to be that the yeast don't do much of a job at mopping up the oxygen in the headspace of bottles, so it's free to get into the beer and ruin it. The flipside is that most batches of home brew probably don't last long enough for oxidation to really be a concern if due care is taken throughout the process, so I personally probably wouldn't bother squeezing out air.

 

From accounts of those who do it, it does work though. The PET bottles simply return to their normal shape under the pressure generated by the CO2 farts and this creates the headspace inside the bottle which is almost all CO2, instead of CO2 mixed with air.