Reintroducing Oxygen During Fermentation

For those of you who read our mash a couple of weeks ago on the methods of aerating wort, you will know that there are very few stages in the brewing process where the introduction of oxygen is desirable and that fermentation in particular is a stage where the introduction of oxygen should be avoided. This is because oxygen in a finished beer can reduce its shelf life and cause it to go stale.

However, we also know that yeast requires oxygen to properly synthesise sterols and unsaturated fatty acids in order to grow and that after 3-9 hours of fermentation there is unlikely to be oxygen present in the wort. We also know that the ‘lag phase’ of the yeast growth cycle, when this absorption of available oxygen occurs, can occur for up to 15 hours after pitching your yeast.

This means there is potentially 6 hours where the yeast could be absorbing oxygen and producing the nutrients required for healthy and vigorous fermentation but there is no oxygen available to the yeasts. We caught up with Dave in the UK who has been experimenting with techniques to maximise yeast growth in his fermentations.

“Reintroducing oxygen into fermentation is an idea that’s been floating around on the homebrew forums for longer than I’ve been brewing. It’s one of those typical homebrew techniques that seems such a bad idea that there must be something to it and as a result it splits opinion fairly evenly between those who swear by it and those who promise it will ruin your beer.

After doing some reading into it though I did feel there was something to the idea. Yeast needs oxygen to grow and if you can reintroduce oxygen early enough the yeast will continue to absorb it and use it for growth meaning it won’t spoil your beer. Theoretically this should lead to more, healthier yeast which means a better fermentation.

I decided I was going to have to test this out to see if it made any real difference. I decided to brew a breakfast stout which I would split ferment, one with oxygen reintroduced after 12 hours and one fermented as normal with no extra added oxygen.

The recipe for my stout was;

5kg Maris Otter (59.2%)
0.7kg Munich (8.3%)
0.6kg Flaked Oats (7.1%)
0.45kg Brown Malt (5.3%)
0.38kg Brown Sugar (4.5%)
0.38kg Peated Malt (4.5%)
0.34kg Crystal (4.0%)
0.34kg Roasted Barley (4.0%)
0.25kg Carafa III (3.0%)

50g Simcoe at 90 minutes (BU/GU: 0.68)

100g Dark Chocolate and 57g Nicaraguan Coffee @15 minutes

I also stirred two lots of 28.5g of coffee into two beakers with 200ml of boiling water in each, allowed this to cool and poured into the fermenter around 3 days before the end of fermentation.

In terms of reintroducing oxygen I have an oxygen stone which I placed into one of the fermenters after 12 hours and switched on for 30 minutes, which is how long the instructions say to run the air pump for when oxygenating wort prior to pitching.

My start gravity was 1.083. On my first check the re-aerated wort was down to 1.071 and the control wort was at 1.075. This continued for pretty much the duration of fermentation with the re-aerated wort always being a few points lower than the control. The biggest difference was on day 6 when the re-aerated wort reached 1.019 and the control was at 1.027. It took 2 more days to get a steady reading of 1.019 from the control.

I found it quite interesting that the re-aerated wort came down quicker than the one with no re-aerated wort but they both finished at exactly the same gravity. My theory was that the re-aerated version would come down a little lower.

I bottled the beers as usual and marked which batch they were from.

When it came to tasting the results were interesting. I informed all the tasters of the experiment prior to them trying the beer and everyone independently said they thought it was very close but ultimately the beer with reintroduced oxygen was better with most tasters citing a more complex flavour profile and a ‘fuller’ body.

I’ll have to go back to these beers in a few months to see if reintroducing oxygen has had any negative effect on the stability of the packaged beer but based on this experiment it seemed like the oxygen helped the beer to ferment quicker and possibly helped to improve the flavour in the end beer.”

Have you experimented with re-introducing oxygen during fermentation? If so, what were the results? Let us know by emailing [email protected]

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