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We all know oxidation is bad, and cooling hot wort is particularly susceptible to this off-flavor-producing problem. the question is; does oxidation matter for starters?

I use a stirplate to aerate the starter during fermentation - pretty common really. This last batch I put the starter on the stirplate while cooling, which decreased the cooling time, but certainly oxidized the starter wort. I pour off most of the starter beer, leaving just enough to swirl and break up the yeast for pitching. Will off flavors be likely to make it to my main batch, or should I have washed the yeast?

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2 Answers 2

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In short, yes it does oxidize, but it's not usually a problem for your beer, since it's heavily diluted.

With no airlock (to allow gas transfer) and constant stirring, oxidization is inevitable once the yeast have come out of the lag phase and start fermenting. During the lag phase, most of the oxygen is taken up by the yeast as part of propagation, but after then, the yeast prefer to produce energy anerobically and they start to ferment the wort. Fermentation (Wikipedia) says,

even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect)

So, once the fermentation starts, there's little additional oxygen takeup by the yeast so long as there are sugars around. The constant agitation will speed up the oxidization of the wort, and so some oxidization will have occurred before the yeast have consumed all the sugars and switch back to aerobic energy production.

But, is the oxidization a problem for the final beer? I'd guess not. If you pour off the starter wort (as is good practice), then what is left in the slurry is probably diluted about 50-100 times when pitched. With that amount of dilution, off flavours are going to be much less pronounced, but of course it depends upon how oxidized it is and how much flavour will be in the final beer. If you wanted to play it safe, you could make up a dilution the starter wort to 1/50 strength and see how it tastes, but I'm guessing washing will not be needed.

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While I agree with you that dilution will make the oxidization of the starter essentially nil in the whole beer. Your explanation of fermentation and yeast growth is not correct. The Crabtree effect, which states that yeast in high sugar undergo fermentation as opposed to aerobic respiration, doesn't mean the yeast aren't consuming oxygen. Actually it uses all the oxygen it can find to make sterols so that it can make more yeast (think cell membranes). Looked this up a while ago and it went against everything I learned about mammalian cells, but yeast aren't mammalian. ;-) –  Chris Plaisier May 1 '12 at 14:17
    
I agree that propagation requires oxygen to build the cell membranes, which happens primarily during the lag phase. During the bulk of the fermentation phase the yeast have already done most of the propagation that's going to occur, so oxygen consumption is reduced. Put another way, although the yeast's primary goal is to propagate, the yeast cannot simply go on budding forever - there's just not enough raw material in the wort to continue building more yeast, and so far less O2 is required compared to when the yeast grows exponentially. –  mdma May 1 '12 at 14:43
    
Based on this White labs article on the Yeast Life Cycle (whitelabs.com/beer/Yeast_Life_Cycle.pdf) the lag phase is the time taken by the yeast to acclimate to their new environment, and then they turn to exponential growth for the next 1-4 days. All during this time the yeast are undergoing fermentation as their primary metabolic pathway. Think this is semantics, but basically the yeast are using O2 during this "Exponential Growth Phase" which can last from 1-4 days. And during this time most of the sugars are converted to ethanol. –  Chris Plaisier May 2 '12 at 18:10

Using a stir plate does oxidize starer wort very much, but as long as you decant most of the starter wort, there isn't enough left to have much, if any, flavor impact on your beer.

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