The only research that has been done is from commercial brewing (unless you count Brulosophy and other anecdotal evidence as research which I don't.)
Two weeks is regarded as the maximum, after that harm can be measured. Most of the damage comes from yeast Autolysing (autolysis literally means self eating).
When yeast autolyses it releases Nitrogen compounds mostly Ammonia based chemicals, apart from not tasting and smelling good they raise the pH of the beer which makes it less palatable. At it's worst these flavours start to taste like burnt rubber.
The other really noticeable damage comes from the release of Protease-A, an enzyme that will given time chop every protein it can find down to peptides. The first sign is a fall in the head holding ability of the beer, certain proteins (LPT1 and Protein Z in particular) are vital to head formation and retention, at extremes a loss of body and mouth feel in the beer. Call it flat and thin - not really a good look in beer.
Commercial brewers tend to pitch way bigger than home brewers do, good rule of thumb is if you haven't hit FG in 7 days you have under pitched.
This is a bit of a two edged solution, more yeast, more that can autolyse, more autolysis products.
Less yeast, slower ferments, the yeast tends to be older by the end of the ferment and more prone to breaking down, but there is less of it to do harm.
I suspect there is no way to give a definitive, perfect solutions, one of these so often brewing answers "well it all depends..."
For mine, pitch a lot of good healthy yeast, control the temperature well, if I'm going to leave the wort in primary for more than 7-10 days - rack (or drop from the cone is you are lucky enough to have a conical).
Mark
Two weeks is regarded as the maximum, after that harm can be measured. Most of the damage comes from yeast Autolysing (autolysis literally means self eating).
When yeast autolyses it releases Nitrogen compounds mostly Ammonia based chemicals, apart from not tasting and smelling good they raise the pH of the beer which makes it less palatable. At it's worst these flavours start to taste like burnt rubber.
The other really noticeable damage comes from the release of Protease-A, an enzyme that will given time chop every protein it can find down to peptides. The first sign is a fall in the head holding ability of the beer, certain proteins (LPT1 and Protein Z in particular) are vital to head formation and retention, at extremes a loss of body and mouth feel in the beer. Call it flat and thin - not really a good look in beer.
Commercial brewers tend to pitch way bigger than home brewers do, good rule of thumb is if you haven't hit FG in 7 days you have under pitched.
This is a bit of a two edged solution, more yeast, more that can autolyse, more autolysis products.
Less yeast, slower ferments, the yeast tends to be older by the end of the ferment and more prone to breaking down, but there is less of it to do harm.
I suspect there is no way to give a definitive, perfect solutions, one of these so often brewing answers "well it all depends..."
For mine, pitch a lot of good healthy yeast, control the temperature well, if I'm going to leave the wort in primary for more than 7-10 days - rack (or drop from the cone is you are lucky enough to have a conical).
Mark
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