Same Yeast, Different Attenuation

[whovous]

I've made three beers with WLP 075 and all attenuated in the 87 to 90+ range. The first was a Centennial Blonde with fresh yeast that came out of the kettle at 1.052 that I diluted to 1.047. It finished at 1.006, or 87%.

I just made it again with a dubious slurry of the same yeast and did not oxygenate this time. I did not oxygenate this time. I transferred to a keg for my first attempt at spunding and it seems to have stopped at 1.013, or 73%. My reading suggests that underpitching causes slow starts and may result in unusual esters, but it does not tell me it will lead to underattenuation.

Could spunding explain the difference? Or unhealthy yeast? I'd like to crash this keg soon, but not if it is not finished. Then again, what are the downsides of crashing too soon?

 

[GoeHaarden]

Unhealthy yeast can do it, and not oxygenating may played a part especially when using an old slurry. Honestly unless you brewed everything exactly the same (I mean exactly) then FG are bound to change. I'd take 1.013...

Yeast also perform different under pressure, so the spunding may have had an impact.

Regardless, since you keg you really don't have to worry about when to cold crash since you will be supplying CO2 anyways.

 

[whovous]

Yeah, I am going to let it sit for a few days in any event, as I do not need to serve the brew until the 29th.

But the brewing part was exactly the same with the exception of diluting one batch from 1.052 to 1.047 at the end.

 

[kh54s10]

All three things you mentioned will change attenuation. OG of the wort, fresh yeast or slurry, aeration. There are also many other things that will affect attenuation, among them, pitch rate of the yeast, age of the yeast, number of generations of using slurry, recipe changes, fermentation temperatures....

The effects of crashing too soon is also attenuation, if the yeast have not finished and you chill them they will go dormant.

 

[whovous]

Thanks all. Looks like I can crash when the mood strikes me.