Why do different size batches ferment at different rates?

[Ryue]

This might be a stupid question, so please for give my high school drop out knowledge of chemistry...
So I started a batch of mead on 1-13-18. My recipie produced 5.5 gallons of must, so I filled up my 5 gallon carboy and put the other half gallon in a 1 gallon carboy (I know, too much headspace, but its all I had available).
Since they are from the same must, their starting contents should be exactly the same...
Now, bear with me, today (1-18-18) I checked the gravity in both carboys. Here are my notes. I apologize if they don't make sense to everyone, I wasn't trying to write a book.. Lol!

Notes -
5 gallon vs. .5 gallon started at same time:
Yeast pitched 1-13-18
Target Gravity: 1.007
Starting Gravity: 1.158
Checked: 1-18-18 = 5 days.
5th day Gravity 5G: 1.120 apx. 4%ABV
5th day Gravity .5G: 1.070 apx. 11%ABV
Difference: 0.050


So I had read and expected them to ferment at different rates, but I was surprised at the extent there of in less than a week.

Does it make sense? It does.. But at the same time it doesnt... If that makes sense? In theory all ingredients should be exactly the same and I would think that the yeast would convert the sugar at the same rate.. So I would Think they should be the same.. But I guess not? lol

Okay, long winded question over. I appreciate you if you are still reading this and thank you in advance for any answers!

 

[brewshki]

Did you pitch the same rate of yeast into both batches? In your situation, there are 2 carboys with must of the same gravity, but of different volumes. If you were to pitch, as an example, one pouch of dry yeast into each one, they would definitely ferment at different speeds because the smaller batch has a smaller volume and proportionately more yeast. You could make them ferment the same if they had the same amount of yeast in proportion to their volume. If you had a 5 gallon batch and a 2.5 gallon batch, it is easy to see the proportions. To give each batch the same amount of yeast per the volume of must, the 5 gallon batch would get 1 whole pack of yeast and the 2.5 gallon batch would get a half pack of yeast.

 

[Ryue]

Did you pitch the same rate of yeast into both batches? In your situation, there are 2 carboys with must of the same gravity, but of different volumes. If you were to pitch, as an example, one pouch of dry yeast into each one, they would definitely ferment at different speeds because the smaller batch has a smaller volume and proportionately more yeast. You could make them ferment the same if they had the same amount of yeast in proportion to their volume. If you had a 5 gallon batch and a 2.5 gallon batch, it is easy to see the proportions. To give each batch the same amount of yeast per the volume of must, the 5 gallon batch would get 1 whole pack of yeast and the 2.5 gallon batch would get a half pack of yeast.

I made the must and pitched the yeast in a 15 gallon brew pot, then racked into the two carboys. So while in theory they should have the same amount of yeast, I suppose its possible more of them to have ended up in one than the other?

 

[brewshki]

Totally possible. I would think that would be fairly likely unless you mixed it pretty well.

 

[Ryue]

I feel like I did, but maybe not as well as I thought.
On the plus side, the half gallon should be done fairly quick, and even at 11%, only 5 days old and still fermenting it is delicious and surprisingly smooth! The 5 gallon tasted good too, but its hard tp compare with so much C02 still being produced.

 

[bernardsmith]

I am a social scientist not a physical scientist so my thinking may not make much sense to the physical scientists. Where did you pitch the yeast? How do you know that 1/5.5 of the yeast count went into the .5 gallon batch? Did you count the number of yeast cells in each fermenter? If the smaller batch had significantly more cells then that is one reason for the difference. But
Let's suppose that the cells are equally distributed. Is it possible that when you filled the smaller fermenter you provided the yeast with more O2? (did you pour the excess or rack it bottom up?) If you provided more O2 that might have enabled the yeast count in the smaller fermenter to exponentially increase in a way that oxygen starved (or nutritionally hungry) yeast in the larger container could not bud (reproduce).

 

[Yooper]

The other thing that comes to mind is that the smaller batch simply has more oxygen contact, so it can ferment faster. C02 is poisonous to yeast, and with the larger headspace that can dissipate easier.

Mead is very often stirred daily or two times a day in the initial stages to degas it for a healthier fermentation.

 

[Ryue]

Without pitching the yeast into the carboys themselves its impossible to know, though I figured their number would gave at least been pretty close to each other. I racked both carboys through a siphon from the bottom up.
I have been stirring both daily. Oddly enough, just buy loiking at the two, the 5 gallon looked extremely active while the half never seems to get too crazy (I posted an earlier thread about my blow out all over the bathroom with the 5 gal). I guess thats from the lack of pressure in the one gallin carboy so it doesnt have to fight so hard l?