... I'm not sure what the protein is for, but I had a container of really crappy tasting stuff laying around that I bought when they were out of my usual stuff. You can buy a cheap container for like $15, and it will last "forever". Apparently soy powder works just as well and is cheaper, but I don't know.
I will try to find a link to where I got the recipe, but I can't remember right now, and can't find it again.
The protein recommendation comes from a Tara Nyberg PowerPoint presentation from the AGA2K3 convention.
http://www.aquatic-gardeners.org/Nyberg_yeast.ppt
Yeast Growth/metabolism
Yeast can either produce energy by fermentation or oxidative phosphorylation (ox/phos)
Yeast greatly prefer fermentation over ox/phos and will not start ox/phos until all the sugars are converted to ethanol.
Ethanol is a good source of energy and in the presence of oxygen yeast use it up as well.
How to live on the cheap --CO2
I have used yeast CO2 on tanks up to 180 gal. Also 75s, 65s etc.
2 gallons of yeast changed once every 3wks to month depending on the temperature works well. I don’t rotate the bottles either.
The key to long lasting cultures is allowing them to grow and be happy. Also a little basic knowledge of yeast growth helps.
Fermentation also produces 2 CO2 molecules per molecule of sugar-- as a by product. =)
Why is just sugar and water not ideal?
When you just add sugar and water to yeast, they are essentially starving to death.
However the enzymes for converting sugar to ethanol are still in the cell and will work for a limited amount of time.
Since the cells do not have what they need to make new enzymes as cells starve/run out of enzymes the culture produces less and less CO2.
Also, we add too much sugar.
Ethanol and sugar are increasingly toxic to yeast at greater than 10% concentrations.
Therefore 2cups (~500ml) of sugar in 2L of water (~25% sugar) is unhealthy for the yeast.
It is also a waste of sugar because 10% sugar will yield roughly 10% Ethanol at which point the yeast stop growing anyway.
Special strains of yeast, like champagne and wine yeast, have stronger cell walls that protect them from the Ethanol -- so they grow longer.
Happy yeast give you long, productive cultures.
The solution is to give yeast less sugar and also supply them with the nutrients they need to grow.
This will give you a long lived culture that produces a consistent amount of CO2, (they are not challenged by toxic conditions at the beginning and end)
Also your yeast mass at the end will be alive and well and able to quickly start growing again when you add more sugar.
The Recipe!
Improvise at will, but here is a good start:
Use 1 cup sugar per 2L H2O (tank water is great or dechlorinated tap -- chlorine, but not ammonia, kill yeast. ) (they like ammonia)
Add 1-2 tsp of a protein drink mix
(optional) Add 1 tsp of ammonium sulfate, otherwise use 1 T molasses. (or both)
1 tsp baking soda is also nice to keep the pH from crashing (they like it >pH3-4)
Leave yeast from previous mix in the bottom.
Benefits/conclusions
Growing yeast this way saves you time (less re-starting) and money on sugar and yeast.
The only real extra cost is the protein mix, but you can use really old/cheap stuff.
The key thing is that it has protein and vitamin/minerals, Anything that has “yeast extract” in it is perfect.
(Even non-fat powdered milk will do the trick, but it will smell funny)
