Ferment speed variation - Anyone else surprised by this?

[AtlChief]

I brewed a 10g batch of a smoked pale ale last Wednesday... Split it into 2 fementors (5.5 g into a plastic bigmouth bubbler... only 4.5 into SS brew bucket.)
Took a sample of each last night (5 days in) and was surprised...

OG: 1.047
Current gravity in plastic fermentor: 1.010
Current gravity in SS bucket: 1.019

Everything is identical except vessel and volume.
- They are In the same temp controlled fridge...
- Split from same boil
- both had 1 rehydrated packet of SAF-05 pitched

The temp probe is in the plastic vessel w/ 5.5 gals... so my only theory is that the 4.5 gals may be having temp suppressed a little more, due to less mass?

I'm not worried... (I'm assuming both will finish, just may take the SS one a little longer) - I'm just Surprised! .009 seems like a huge difference on a 1.047 OG beer. One is ~93% to expected FG of 1.007... the other is only about ~70% of the way there.

Any thoughts on this, or is this type of variation within your typical experience?

View attachment ImageUploadedByHome Brew1496166086.196505.jpg

 

[Pappers_]

That is a mystery!

 

[HausBrauerei_Harvey]

weird indeed. Are you sure you dont have an infection in the plastic fermentor, some brett or something?

 

[AtlChief]

I didn't taste any funk in either sample. They actually tasted fairly similar too, despite the gravity difference. Also, I expecting both of them to be in that 1.010 range after 5 days. I'm typically only getting a point or two of further attenuation in the second week...so I don't think there is an infection in the plastic one.

I stared out for the first 3 days at 18.5C (so around 65F). If the 4.5 gal one was a few degrees lower than that, I could see that potentially inhibiting fermentation a bit. I'm up to 20C (68F) at this point, so I'm hoping the SS vessel will catch up. I just found it interesting/surprising...

 

[stevehaun]

I think your theory of the smaller batch being kept cooler is the likely culprit. The smaller batch is 18% smaller than the larger one with the thermowell. Stainless would also conduct heat more readily than plastic.

 

[AtlChief]

[emoji106]
I'll report back in a week w/ FGs...

 

[Schreiner]

Was the packet of yeast that went in the SS out of date? Just grasping at straws.

 

[AtlChief]

Did not check the dates, but had gotten them in the same 3-pack, so assumed they were same lot.

 

[AtlChief]

Update: Did a FG reading while kegging last night. Both finished out to 1.008... both taste the same.
I'm concluding it was the temperature issue, just giving the lower volume SS batch a slower start.

 

[SanPancho]

ditto on the vote for temp difference. that stainless will chill alot more readily than plastic. smaller batch size amplifies the discrepancy. could easily be enough to keep one batch 24 hours behind the other, especially if you ferment on the cool side.