How Beer Evolves in Barrel vs in Bottle?

Patrick Dawson addresses this some in his book Vintage Beer. I simply don't remember it well enough to give you an accurate answer. It's a good read, especially since you seem interested in some of the finer technical points. I do recommend picking it up.

 

Once beer goes into the bottle it basically stops evolving unless we're talking about bottle-conditioning. Even then, there's only so much that can go on. Evolving? Or devolving then becomes the question.

In a barrel there are hundreds of chemical reactions going on that are far to complicated to go into here, let alone getting into the differences between fermenting in wood and aging in wood, and working with things like Brett. and Pediococcus. As for the 3 things you've listed I'd say you've got 1 and 2 right, but number 3 is much more complex.

There are a few books out on the subject but information is continually coming out on these concepts as well. Maybe one day there will be a comprehensive source but for right now you'll have to scrabble together what you can and read it all.

 

Yes, oak breathes, expanding and contracting with temperature changes, every single day, causing oxidation, in a more favorable manner than the sort of oxidation you want to avoid in packaged product, but still oxidation all the same. For clean beers especially, it's a risky process and why many breweries still steer away from it.

 

If the beer is bottle-conditioned it will ferment the residual or added sugars creating carbonation and then giving some extra flavor via the yeast itself. (With any bottled beer that has yeast in it, if the yeast has sedimented properly, pour off a clear glass and then rouse the yeast and pour a glass of that and taste them side by side. They will not taste the same.) At that point it will act just like any other bottled beer and basically start to deteriorate due to oxidation. It may be "evolving", but is it getting better or worse? That's a question that a lot of people who age beer have to grapple with. Sometimes there's a pleasant change, perhaps in perceived maltiness and softening of the bitterness, or perhaps in the smoothing of some of the hotter alcohols. Other times the beer just loses life and becomes muddled and dull, or even noticeably oxidized with papery or cardboard notes.

Brettanomyces on the other hand, although also a yeast, is totally different and gives off interesting phenols and esters - it does not eat them although it may transform them. It can eat some additional sugar, however, given time. Brett also takes much longer to ferment than a standard brewers yeast, and will most likely still be working for some time after bottling. Very slowly, but yes, still working. There are other factors at work as well: how much oxygen the Brett has to work with, and the possible changing pH of the beer. What I've referred to so far covers fermenting in stainless steel, fermenting in wood is completely different, and apart from other microflora that's probably in the wood, the environment is quite different and complicated. One example would be that in wood there will be oxygen coming available to the Brett that it won't have in a fermenter, keg, or bottle.

There's a basic article about Brett and souring critters here:
https://draftmag.com/brettanomyces-lactobacillus-pediococcus-beer/

 

Are you just curious about mixed microbe fermentations or about all barrel fermentations?

If we're just talking about mixed microbe fermentations, most of the barrels that are used to carry out those are neutral barrelage. In other words, there will not be very much, if any, barrel character to impart. This is why traditional lambic, although aged for an extended period of time in wood, lacks wood character.

Oxidation, in mixed microbe fermentations, is the least of your worries. Brettanomyces sp. when in an environment with oxygen will produce acetic acid and also esterify that acetic acid, in the presence of ethly alcohol, into ethyl acetate, which, at higher concentrations, give the beer an acetone/nail polish remover character.

Even though Brett sp. can ferment cellobiose due to glucoside enzyme activity, probably not, as most ester formation occurs in the log phase of fermentation and phenol formation happens secondary to phenolic precursors (mostly hydroxycinnamic acids) and has to do with the strength of the species' decarboxylase and reductase enzymes and the amount of cellobiose fermented will not be sufficient to change the ABV of the beer in any but the most miniscule amounts.

IMO, outside of classic lambic and Methode Traditionnelle fermentations, time in the barrel should be limited to the completion of fermentation, which can be as quick as 8 weeks because of the reasons stated above.

 

Cool.

Na. Once you've got the beer into the package, there is a finite amount of oxygen to which the liquid can be exposed. Even though the closure is permeable, it's not THAT permeable.

The hops will give you two major things: inhibition of unwanted microbes like enteric species and some lactic acid bacteria like lactobacillus and phenolic precursors especially caffeic acid which is a precursor to vinyl and ethyl catechol.

The thing that prevents above threshold acetic acid and ethyl acetate production in long term barrel aging is the barrel itself. If it has a low ingress and is closed properly, it is a good vessel for long term aging. If not, well, not so much.