This is carbon dioxide, not air.
Coming out of the fermenter, your beer is very lightly carbonated. Yeast has been happily munching the fermentables in the wort, producing alcohol and carbon dioxide (CO2). Some of that gets expelled through your air-lock, but some of that also goes into solution in your beer. If you don’t believe, taste a little, and you’ll feel the slight carbonation on your tongue. You’ll also find that when using the Bouncer out of kettle, you won’t see any bubbles in the filter.
When you pass your beer through the stainless steel mesh on the bouncer, the slight turbulence causes that CO2 to come out of solution. And since the bouncer is a closed filter, that CO2 is trapped in the filter.
Depending on the beer (gravity, yeast, malt bill, hops, etc.) this can be quite noticeable, and on a slowly flowing siphon, can actually cause you to lose the siphon. This happens more often when the CO2 bubble gets large. To lessen those chances, periodically lift the Bouncer so that the inlet/outlet are horizontal, and the CO2 bubble will be drawn through the siphon.
What about oxidation? Well, since CO2 is getting trapped in the closed filter, it also means that air is NOT getting into the filter. So, using the Bouncer will not increase oxygen in your beer.
However, oxidation of your beer out of the fermenter is a real thing, and can significantly change the taste of your beer. You should still try to minimize it by:
- Try not to splash the beer as it flows into your bottling bucket, keg, secondary, or other target vessel. It helps to have a tube that extends to the bottom of your target vessel.
- If you have a CO2 tank, add some CO2 to your target vessel. It is heavier than air and will create a layer of CO2 on the beer which will act as a barrier to the oxygen in the air.
- If you are racking into a keg, “burp” the keg by adding CO2, then using the pressure release. Do this several times. This will help purge the keg of the remaining air, replacing it with Co2.