Aeration of wort: doesn't it depend...

Aeration of wort: doesn't it depend...

Post by Gary Be » Mon, 07 Nov 1994 06:22:00



There have been a couple of threads recently about getting oxygen into
your wort by stirring, shaking, or using a bubbler rig from an aquarium
pump. The purpose for getting air into your wort is to provide the few
lonely yeast cells that you've suddenly plunged into all that food with
the means (aerobic respiration) to do lots of reproducing really fast.
They do this respiring the same way you and I do, to whit:

        C6H12O6 + 6 O2 ------> 6 CO2 + 6 H2O

The products, in this case, do not include ***, CH3CH2OH.

It's only under anaerobic conditions that you produce ***:

        C6H12O6 ------> 2 CH3CH2OH + 2 CO2

So, my question is this: If you are priming your wort with a big slurry
of actively reproducing yeast, for example using a one or two quart
yeast starter in a 5 gallon batch, do you really need to get a lot of
air into your wort? In fact, if you use a big starter, are you actually
wasting your wort in making more yeast *fast* instead of making ***?
What would be the break even point in this regard -- a quart, 2 quarts,
a gallon of starter? Or is the amount of oxygen you get into the wort
insignificant in terms of *** production?

Comments? Answers? Inquiring minds want to know!

Cheers,

 
 
 

Aeration of wort: doesn't it depend...

Post by Spencer W. Thom » Wed, 09 Nov 1994 05:56:22


Gary asks whether you still want to aerate if you're using a big
starter.  Especially since the products of aerobic yeast respiration
don't include ***.

Point 1: A starter isn't *big* until it's more than 10% of the batch
        size.  Thus, a 2 quart starter in a 5 gallon batch just MIGHT
        qualify as big.

Point 2: The amount of O2 you can force into your wort is pretty much
        insignificant in reducing *** production.  Consider the
        equation he posted:

        C6H12O6 + 6 O2 ------> 6 CO2 + 6 H2O

        One sugar molecule requires 6 oxygen molecules to be
        "consumed".  A sugar molecule weighs 180 units.  6 oxygen
        molecules weigh 192 units.  In "back-of-the-envelope"
        precision, 1 mass unit of sugar is combined with 1 mass unit
        of oxygen during respiration.

        Ok.  How much oxygen might there be?  At 8ppm (a commonly
        stated goal of aeration), there will be about 160 mg of O2 in
        5 gallons of wort.  Thus, about .16 grams of sugar will be
        consumed during aerobic respiration.  I wouldn't worry about it.

--
=Spencer W. Thomas              |  Med Ctr Info Tech, B1911 CFOB, 0704
   "Genome Informatician"     |  Univ of Michigan, Ann Arbor, MI 48109


 
 
 

Aeration of wort: doesn't it depend...

Post by Tim Bus » Wed, 09 Nov 1994 09:19:46


Aeration of wort: doesn't it depend...

Quote:

>What would be the break even point in this regard -- a quart, 2 quarts,
>a gallon of starter? Or is the amount of oxygen you get into the wort
>insignificant in terms of *** production?

I think your last sentence hit the nail on its head!

Obviously in a home environment it is virtually impossible to measure the
oxygen content of your fermenting beer. But I do a fair number of
fermentations at work (E.coli not yeast-hopefully) and there I do measure
and control oxygen levels. Obviouly I am growing the cells to a much
higher density in the lab, but the principle is the same. When I cut the
flow of air into the fermenter, the O2 level drops in SECONDS to
baseline. So I can imagine in my beer the level dropping back in, maybe,
tens of seconds. In a fermentation lasting say 4 days, this isn't going
to significantly (or measurably) affect the amount of ***   produced.
I give my beer a GOOD rousing twice a day for the first couple of days.

Tim.