Why don't we stir the fermenter?

So I was talking with a few microbiologists over Christmas while they were trying my homebrew. Apparently the bioreactors that produce cell cultures and enzymes for many genetic drugs are stirred. They were perplexed that beer wasn't stirred - oxidation or outside infection would do at least as much damage to their product as it does to ours. Therefore if it works in a properly controlled bioreactor, it would work in a fermenter.

I'm not convinced this is the case, and keeping the beer properly free of O2 during stirring sounds very expensive. So does sterilization of all those moving parts (sanitation probably wouldn't be sufficient)

Thoughts?

P.S. - you can blame the magic bean for this question ;p.

brewguyver said:

So I was talking with a few microbiologists over Christmas while they were trying my homebrew. Apparently the bioreactors that produce cell cultures and enzymes for many genetic drugs are stirred. They were perplexed that beer wasn't stirred - oxidation or outside infection would do at least as much damage to their product as it does to ours. Therefore if it works in a properly controlled bioreactor, it would work in a fermenter.

I'm not convinced this is the case, and keeping the beer properly free of O2 during stirring sounds very expensive. So does sterilization of all those moving parts (sanitation probably wouldn't be sufficient)

Thoughts?

P.S. - you can blame the magic bean for this question ;p.

Click to expand...

Sure their product would have oxidation concerns just like ours but theirs is not one where tasting oxidation is a major concern. Who cares if their medicine has a wet cardboard taste?

As for why we don't stir the fermenter, what would we gain by it? Not nearly what we would lose. Beer yeast starts out aerobically but most fermentation occurs after all oxygen has been taken up and occurs anaerobically.

Additionally, we want yeast to flocculate for beer clarity. You aren't going to get flocculation if you are stirring the tank.

cluckk said:

Sure their product would have oxidation concerns just like ours but theirs is not one where tasting oxidation is a major concern. Who cares if their medicine has a wet cardboard taste?

As for why we don't stir the fermenter, what would we gain by it? Not nearly what we would lose. Beer yeast starts out aerobically but most fermentation occurs after all oxygen has been taken up and occurs anaerobically.

Click to expand...

Oxidation does more than just make things taste bad - it kills cells. One company I consulted at earned $500k revenue per patient per year. I'd imagine anything that would make beer taste bad produces unwanted chemicals that would destroy their product. Then again I'm assuming they don't WANT oxygenation.

To your point we oxygenate the hell out of starters, and they taste AWEFUL. I'll have to follow up on that one. I'd assumed they ran an anaerobic process that keeps oxygen out (e.g. nitrogen flushed).

Beernik said:

Additionally, we want yeast to flocculate for beer clarity. You aren't going to get flocculation if you are stirring the tank.

Click to expand...

That was my argument too. Made them all huffy, like I was being difficult. For small batches it seems like a waste of time.

Cell culture is all about keeping the cells in the reproductive phase. For beer, we're not too interested in that step for the actual fermentation, at least on a 5-10 gallon scale the yeast reproduces up to a plateau fast enough for it not to matter.

I have a small pet theory that if you stirred the fermentor during active fermentation you might be able to hurry it along faster, but the trick would be stopping it at the right point so you stop oxygenating the beer when the yeast are still consuming the dissolved oxygen so you don't oxidise the beer itself. However you'd need to get it right otherwise you'd ruin a batch very fast.

I'd imagine if we could aggitate without actually introducing any room air or O2, the aggitation process itself would initially speed up the fermentation process, but like it was stated above, it would need to be earlier in the fermentation, since you'd still want to end up with clear beer when you're finished.

I think on a 5 gallon scale, you could build a stir plate & bar strong enough to mix the beer without sloshing it to the point of entraining additional air. The mixing apparatus in a 50 to 100 barrel tank would be have to be big and rigid, made of stainless steel, and require a hefty motor to turn it.

Besides being biologically unnecessary, it's cost prohibitive.

I'm a wine maker and mead maker, and I stir my primaries as a matter of course. In fact, if I had a stir plate that big, I'd probably use it!

The time when I stop stirring is when the SG hits 1.020 or so, in order to avoid oxidation.

Stirring degasses the C02 (poisonous to yeast), as well as aerates. It also breaks up the "cap" that forms and keeps it from drying out (and hence molding). I think of those benefits, aeration is the least important, as the yeast as already multiplied and fermetation is an aerobic activity.

If you drank a Schlitz between the early 1970's and 2007, you've had beer that was stirred while fermenting. In the 1970's Schlitz changed their recipe and started using the continuous fermentation process developed by Morton Coutts. It involves several tanks (usually 4), and fresh wort being added slowly to the first tank at the same rate the partially fermented beer is transferred to the second, and on down the line. The first three tanks are stirred continuously, and it's not until the 4th vessel where fermentation is essentially over that the yeast is allowed to settle out. The main advantages are shorter fermentation time, increased efficiency, and higher production capacity for a given size of system, none of which are all that appealing to the average homebrewer. I've read that brewers using this process have experienced issues with increased diacetyl and shorter product shelf life, but it's unclear if those are related to the constant stirring or to other aspects of the process.