[CoryMathews]

This is not at all common in beer brewing. I cannot speak for the wine world but most of them have variable height lids.

No brewery would ever put glass marbles into a primary fermenter. As a professional brewer we routinely try to have the beer touch as few items as possible, post chill, due to sanitation risks. Not only would marbles be a nightmare to sanitize (tiny chips or cracks would not be properly sanitized), its a logistical nightmare to later get them out to clean the tank. Tanks are cleaned in place with a pump and spray ball method without ever opening the tank. Getting excessive hops out is enough work much less marbles.

Breweries purge any air out of a tank with Co2 before filling, for multiple reasons, but since Co2 is heavier than air it will settle on top of the unfermented wort as it is gently transferred into the fermentation tank thus removing the need for any kind of marbles or headspace reduction.

A second major reason this would not be done is that when beer ferments it needs extra headspace as the yeast in an ale ferments on top of the beer. A hefeweizen will routinely create a yeast layer about 10-15% of the height on top of the liquid. So if this space was blocked it would be forced out the top, which for most modern breweries would create a large mess into the floor drains and lead to the yeast count being too low reuse.

On the flip side I would be very interested to know of a brewery doing this and why.

[rsync]

Sorry - I am speaking specifically of the home brewing community. You can see them using, and recommending, glass marbles / glass balls all the time.

Yes, I cannot imagine this method being used in serious retail production.