Left kegged beers hot, no good :(

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So it's somewhat normal practice for me to leave a freshly racked keg out of refrigeration for a week to let it really clean up before lagering or at least not have to worry about doing so if there's no room in the kegerator, but the past two brews I've done (pale ale and pilsner) have both had some really interesting things happen in the extreme heat we've been experiencing these past few weeks to the point they're pretty well ruined

The results are difficult to describe, but diacetyl and acetaldehyde come to mind, a good after hint of fusel alcohol and a general diminishing overall quality of the total character of these otherwise normally fresh crisp beers. It took me a little while to clue on that this is what's happened because it's a first for me. Almost makes me think it's an infection but it's not quite as overpowering as that. Only thing that's changed is the heat is extreme day and night at the moment, you just sweat standing up

Can anyone explain the science behind what's likely actually occurred here? I was under the impression that finished beers fermented properly were no longer prone to yeast producing off flavours


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Flavour stability probably attracts more R&D dollars than anything else in brewing. Yeast produced off flavours are only a very small part of what could be happening to your beer.
Remember that there are different types of flavours produced by yeast, hot early in the ferment will help produce fusels and a bunch of esters, late yeast breaking down can produce a bunch of other totally unrelated harm.
Obviously having little to no yeast in the stored beer will help reduce the harm from yeast breakdown, this is the path chosen by most large brewers.

Yeast is just one of the factors, probably not even one of the major cause of change.
There is a pretty standard test commercial brewers use to determine flavour and haze (Colloidal Stability) stability.
it involves heating and cooling a sample of the beer, its possible to get a year or more of aging in a week or so.
The test only heats beer to 37oC or so, here in Oz we can do that indoors for a fair fraction of the year, which makes it a pretty tough place to keep good beer out of the fridge.

Colloidal stability is mainly the increase of haze in the beer, haze hasn't got all that big an impact on flavour in itself but its a great indicator of what is going on in the beer. It is also quick, cheap and easy to measure.

Sadly you cant un-age beer, if its past drinking its a write off.
There are things you can do to minimise the speed beer ages at, big ones are: -
Keeping it cool in storage, around 10-12oC is ideal.
Reduce or eliminate yeast transferred to package - cold store (lagering), racking, filtering all help.
Reducing Polyphenol and high molecular weight proteins - starts with choice of ingredients, mash temperatures/times, boil times (short mashes and boils don't help), proper pH during mashing and storage. Critical is good hot break separation and to some extent cold break if there is an excess.
Reduce Oxygen pickup - this starts with mashing in and goes all the way to packaging
Healthy yeast and lots of - good pitches with lots of healthy yeast will help in a number of ways, including the cleaning up of products produced early in the ferment, good attenuation leaving less sugars that can contribute to ageing if some bacteria are involved and on and on.

If you make good beer, that requires following good brewing practice, it will last longer and develop better. Not even the best of beers is going to last long at 40oC, really might be worth looking for some more fridge space.
Remember that if you are storing beer at 10-12oC it takes a lot less energy than at say 4oC and at least you aren't writing off whole brews.


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