Aerobic vs. Anaerobic yeast culture

[The Editor]

Hey everyone, I've been thinking today...

When we set up DIY fermenter's we deliberately seal the fermenter bottle to prevent loss of CO2 and to try and prevent unwanted bacteria setting up shop in our fermentation vessel.

Anaerobic respiration results in CO2 and ethanol being produced, while the CO2 is great, the ethanol build up in concentration until its at a level toxic to the yeast at which point it starts to die and we get no more CO2 so we re-charge the fermenter and go again...

Aerobic respiration produces water and CO2, and as long as there's food the yeast should be happy right?

The only way (that I can think of) of running a reactor aerobically is to use and air pump to bubble air through the yeast broth. This will result in greater agitation in the yeast broth, and the production of a CO2 rich stream of air...

This as I understand will be less desirable than pure CO2 as it will be harder to diffuse the same amount of CO2 into the water when it's in a smaller concentration in a bubble, however there will be a higher CO2 concentration in the water than had the air not have been bubbled through the yeast broth initially.

The main advantage I see in running this method is the lack of culture dying off due to ethanol toxicity. This would allow a humble aquarist to simply dose the yeast broth with sugar regularly to maintain a stable production of the CO2 rich stream.

Now I'm in no way experienced with planted tanks, so I may have completely missed some negatives to this method. In which case please point them out to me!

But as a complete novice looking to tread upon the first rung on the way to a fully planted high light tank, aerobic DIY fermentation doesn't seem like a bad idea...

Has anyone ever considered or even used this before?

 

[bonsimon]

Hi,
I know this is a very old post, but I wanted to mention that I have been thinking about the same thing lately. Have you given it a try?

One thing that would be difficult to know is exactly how much CO2 the aerobic setup produces. A possible test would be to bubble the expelled gas through clear limewater and compare the rate of change in milky-ness to that when pure CO2 is bubbled through.

 

[The Editor]

Wow, holey thread resurrection Batman!

As for the idea, no I never gave it a go... I only had a small tank at the time and never really thought it worth doing.

However as luck wouht have it I got involved with a yeast + algae research project at uni earlier this year, this involved amongst other things bubbling air through a yeast culture then into a separate algae culture (same principle really).

We didn't attempt to quantify the CO2 conc in the outlet stream from the yeast culture, but what we did find was a marked increase in the growth rate of the algae attached to the aerobic fermenter. This surprised us as the resulting yeast effluent stream would still only be a low conc. of CO2, but apparently it's enough to give a boos to the algae.

We used a yeast media with 10g/L glucose as the carbon source, 5g/L Bacto Peptone, and 3g/L of yeast extract and malt extract, and had each experiment running for about 2 weeks.

What I guess is that if you were to set up a similar system at home you could operate it as a fed batch. Just keep topping up the sugar every few weeks as it's used up.

As it happens I've now got a much larger planted tank set up with a classic DIY yeast fermenter up and running. I used a 5L water bottle as the fermenter, about 300-500g of sugar (didn't bother measuring), 1 heaped teaspoon of marmite, tip of a teaspoon of bicarbonate of soda, and 1 crushed aspirin. It's been running for about 2 weeks now, with daily top ups of sugar (about 1 tsb).

Another way of getting some indication of the CO2 conc. in a mixed air stream would be to use the pH. Since you'd only expect the CO2 to be fairly low conc coming out of the aerobic fermenter you could bubble the air through a water solution (ideally some known buffer standard, 4dKH for example) with a few drops of bromethyle blue (the pH indicator solution in the API master test kit). The more CO2 in the air stream the more yellow the water will go, the less CO2 the more blue. Without any sort of calibration data though you couldn't use this method quantitatively, just an easy visual indicator of production rate.

You've got me thinking about it again now! I may have to dip back into this idea.

 

[bonsimon]

Well if it worked for the algae it's certainly worth a try in an aquarium! I'm going to give it a go.

The aerobic technique definitely sounds more sustainable, but I'm not sure it would last very long if only the sugar was topped up. There would still be accumulation of waste products etc. Perhaps replacing 50% with new medium every few weeks would do.

Its also great to see that you're using a more complex growth medium than the usual sugar in water. This seems like an obvious limitation to growth that many have overlooked.

 

[bonsimon]

I asked a friend who has some experience working with yeast about all this and he had the following to say:

Yeast can still produce a lot of Ethanol under aerobic conditions... This is to gain competitive advantage against any other organism that cant tolerate it. No matter how aerobic you make it, you will end up with ethanol somehow. A more efficient way to keep your yeast happy is to run a system where you continuously pump in nutrients. This dilutes the waste, dead, inactive yeast etc and replaces the media. I am not sure at what rate you have to set it at, but there is an optimal replacement rate, say 1 volume every day. I think its called a chemostat. What will happen is that your yeast will evolve through accelerated selection to become more efficient on your food supply and rate of nutrient renewal. If you can keep your yeast in logarithmic growth phase, they will produce more goodies. If your flow rate is too fast, your yeast will get too diluted and will be too young. Too slow, your yeast will start with the lagging phase.