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post #1 of 4 (permalink) Old 05-08-2006, 04:34 PM Thread Starter
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yeast and ph

Doing a test in school today about yeast and bacteria, it turned out that yeast is doing a lot better in a ph of 4 than in a ph of 7. Maybe this is common knowledge, but it was new to me So I was thinking about trying it out, to set up two identical bottles except for the ph. Anyone know what to use to lower the ph? Maybe lemon juice? Would it be safe? (I'm guessing it wouldn't affect the co2?). How much to use to lower the ph from 7 to 4 in say 1 litre of water? Or maybe someone has already tried this and figured out it's no point?
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post #2 of 4 (permalink) Old 05-09-2006, 01:35 AM
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I haven't tried this, but my logic tells me that the DIY yeast/sugar solution would already be a low PH solution, due to the CO2 dissolved in the solution. If you add bicarbonate of soda to your mix you would be buffering it to a higher PH. It might make an interesting project to take samples from a DIY bottle and check the PH, both with and without bicarbonate of soda in it. (And, you could try it with a bit of white vinegar too, if you wished, to see if you could lower the PH even more.) If you do this, use a simple bubble counter to measure the CO2 production.

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post #3 of 4 (permalink) Old 05-09-2006, 07:42 PM
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Not an expert on yeast biology (but I play one on TV), but I'd like to ask a couple of questions about the school experiment:
Do you know if your yeast are growing aerobically or anaerobically, i.e. are they getting fresh air unlike our DIY bottles that use up the available oxygen in a few hours?
Is there any pressure build up happening or is the CO2 allowed to escape freely?
How long did your experiment run? Great yeast production that kills itself in a day or two is not as valuable to us as moderate yeast production for a week.
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post #4 of 4 (permalink) Old 05-09-2006, 10:35 PM Thread Starter
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Of course, I'll explain the experiment.

The point was to briefly examine what conditions that gives best growth of yeast and bacteria (E. coli and P. fluorescens). Variables were:
Glukose as carbon and energysource: G (0,5% yeastextract) and G5G (0,5% yeastextract and 5% glucose)
Temperature: 25 and 37 degrees C.
pH: pH 7 (G5G) and pH4 (G5G, 0,3% tartaric acid)
Oxygen: aerobic / anaerobic

we transferred a suspension of yeast and bacteria to testtubes containing:
- G
- G5G
- G5G and 0,3% tartaric acid
- G5G in anaerob bottle to be incubated at 25 C
- G5G in anaerob bottle to be incubated at 37 C
all was incubated for 4 days.

We use a simple scale from 0 - +++ to estimate content of yeast and bacteria. The culture before incubation was yeast (++) and bacteria (+++).

I don't have all the results now. What I do know is that while the testtube with G5G contained some yeast and some bacteria, the bottle with G5G and tartaric acid contained only yeast. And lots of it. We will finish the experiment next monday, and I will have some better results then.

The point here is that it seems like the yeast outcompeted the bacteria in the culture with added tartaric acid. This is probably not because the bacteria has intolerance to low pH, as E. coli has both respiratory and fermentative metabolism. Also, it should not matter if it was aerob ar anaerob conditions, as of course yeast has fermentativ metabolism. It would be good though to try anaerob conditions with different pH, but as this is just a small part and time is very limited in the labs, I haven't had a chance to try. Maybe some other time.

It also needs some testing. As Ransom says, not much point if it only lasts for a few days. However, the yeast will most likely last as long as it gets nourishment. It might also be a good thing to kickstart bottles, especially when using glass diffusors (mine takes hours to build up enough pressure).

I'll post back with more conclusive results when I have them. Meanwhile I'd love more comments on this, as it might be a waste of time (so, someone will probably say "go pressurized". Which is true. But not as much fun )
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