Can't go for a chilled out walk as I have a boil going.
The bit dutto raised about boiling wort interested me. I only do small boils and i top up with chilled bottled water. So that would fit in with the fact that some people don't bother to aerate and don't have problems. Wherry which is a kit wouldn't require boiling of the wort. I say apart from the temp of the ferment the amount and condition of the yeast are the biggest culprits.
I have seen cans of wherry stored high up on the shelves where its quite warm in a certain store and often thought.... that's not the best way to keep the yeast happy. :-(
Nail hit squarely on head.
Oxygen is only needed for the growth phase, so as long as there is enough heathly yeast with the kit, dry yeast shouldn't need aeration, as it manufactured to not need it. However aeration is recommend as a way to get the yeast mixed in the wort.
From Fermentis
Does the wort need Oxygenation / aeration?
As the yeast is grown aerobically, the yeast is less sensitive on first pitch. Aeration is recommended to ensure full mixing of the wort and yeast.
Poor yeast health, insufficient yeast cells or inadequate mixing in wort can all lead to a longer lag phase and slow start to fermentation. The problem here is that yeast is flocculant and wants to clump together and sink to the bottom of the FV. Yeast also consumes sugars in wort in the order of simpliest tp complex, so it ends up a race against time after a slow start with a highly flocculant English yeast to get the job done before the yeast flocculates and impedes the conversion of the more complex sugars into alcohol and Co2. The kicker here is that sugars in wort inhibate flocculation, and so as fermentation progresses the more the yeast flocculates.
Fermentation temperature has an influence also, with yeast fermenting at warmer temperatures, but more critcal to yeast performance is stable temperature. I'll wager most stuck Wherry fermentations have no temperature control.
Yeast performance will also be affected by the fermentation environment. Conditions that are too warm will not impair fermentation and attenuation, but will affect beer flavor, generally creating more fusel alcohols, esters and phenols than desired. Conditions that are too cool will generally slow the fermentation, which may impact attenuation, but ultimately the beer should finish if the temperature is stable and all other yeast factors are being met. Fluctuating conditions, though, can definitely impact yeast performance, and cause them to stop before the beer is finished.
https://byo.com/body/item/1895-attenuation-advanced-brewing
And then we come to wort pH, in all grain brewing a wort pH of 5.0-5.2 is desired for good fermentation. I'm sceptical that when rehydrating kits that this is always achieved in areas with a highly alkaline water supply.
Here is an interesting thread regarding the importance of pH on yeast performance.
https://chemistry.stackexchange.com/questions/11160/why-does-ph-affect-fermentation
So in short, avoid stuck fermentations in Wherry (or any other beer) by:
1) Using sufficient fresh and healthy yeast.
2) Thoroughly mix your yeast and wort.
3) Control the fermentation temperature.
:lol:
