Does aeration really matter?

I succeeded in hitting my target of brewing 1000 bottles of beer last year. Now that my challenge is complete, I’m going to concentrate on doing a few homebrewing experiments. Rather than blogging about every brew I do, the plan is to carry out some nicely controlled experiments and write up the results here.

Ever since dramatically improving the efficiency of my mash tun with a new homemade false bottom (I had no idea the geometry of the mash tun would make such a difference – drawing the wort from the centre of the bottom rather than the edge had a huge effect) I’ve been brewing beers with high final gravities, that is, lots of unfermented sugar left in the beer. The improved mash tun efficiency means I’m getting a lot more sugar out the grain, but not much more sugar actually turning into alcohol. I plan to figure out what’s going on over the next few months.

The first idea to test is aeration. Yeast needs oxygen for good fermentation. And boiling removes oxygen from liquids. So, after you’ve cooled down your wort you’re supposed to vigorously stir it, shake it about (or if you have the right kit, inject oxygen into it). I’ve always gone with the stirring technique but have got a bit lazy recently. Maybe this is why I’m ending up with unfermented sugar?…

So, using one of my favourite bitter recipes, my experiment was to brew a batch as usual, then dived the wort into two fermenters, one would be treated gently (bitter A) the other would be vigorously stirred (bitter B).

Bitter A on the left, Bitter B on the right (all frothy from vigorous stirring)

Bitter A on the left, Bitter B on the right (all frothy from vigorous stirring)

I measured the specific gravity (sugariness) regularly during fermentation to track what was happening. After just 24 hours it was clear that the aerated wort was fermenting faster. After 3 days I did something that you’re not supposed to do. I gave Bitter B another good stir. If you add oxygen later in the fermentation process you can oxidize the beer. And that tastes bad. What I didn’t realise is that it also appears to upset the yeast. In the end, the beer without any aeration (Bitter A) achieved a lower final gravity that the one that was over-aerated.

aeration chart

Two lessons here. 1) Aeration does matter. 2) you really shouldn’t aerate after fermentation has started. I’m waiting to taste whether Bitter B with be noticeably oxidised, but even if it doesn’t I now know it’s a bad idea.

And the next thing to test…. How much aeration is needed? I should be aiming for 8 to 10pmm of dissolved oxygen but there is no easy way for me to measure that. I’ll do the same experiment, but this time beer A will be stirred a bit and beer B will be stirred a lot. Do I really need to waste time stirring for several minutes or is “just a bit” ok?


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