I've been doing quite a bit of my own research and experimentation; I'm a real chemistry nerd and this stuff is exciting. :roll: Anyway this is what I have discovered:
Originally Posted by ninoboy
The prod. of CO2 also dropped quite noticeably after a week when I use Fleishmann yeast so I don't really think it's the lack of baking soda. My KH is 6.7 and my Redstar mixture is still running strong after 12 days although I forgot to put baking soda in that bottle.
-Don't use baking soda.
Ninoboy, as you clearly pointed out, your setup WITHOUT baking soda is running longer than the setup WITH baking soda. Although you used different brands of yeast and do not know if that is the reason of better longevity, I have used both Redstar and Fleishmanns and IME both are pretty equal in production. Therefore the baking soda is the reason of poorer production.
I'm not sure where the myth developed about acids killing yeast, because actually, yeasts are far better off in acidic conditions. Yeast can tolerate alkaline conditions, but what they can not tolerate is Na+ ions from baking soda that kills them(Na
HCO3). Simply, don't use baking soda.
I have narrowed down the problem of short lived production to high alcohol concentration. I did some of my own stoichiometric calculations of the common recipe of "2 cups sugar per 2 liter bottle" which ends up to let us use 1.20 liters water (when filled to the point the bottle starts to narrow).
The paragraph below contains chemistry jargon and just shows all the reasoning and calculations for the answer I have found...skip to the bottom if you wish to pass.
By the balanced equation: C12H22O11 + H2O ---> 4 CH3CH2OH + 4 CO2. That is 1 mol of sucrose (sugar) reacts with 1 mol water to make 4 mol ethanol (alcohol) and 4 mol carbon dioxide. 2 cups of sugar about = 2 mol and 1.20 liters water @ 70F is about = 67 mol (many various conversions to reach these mol answers.) Therefore, sucrose by far is the limiting reactant and in the process, 8 mol of alcohol are made (2 mol sugar x 4 mol alcohol/1 mol sugar = 8 mol alcohol). Some of this alcohol is lost to vaporization, though since the vapor pressure of alcohol is not overly appreciable, most stays in the solution - and alcohol is lethal to yeast. Over the course of this reaction, the alcohol concentration increases to under 30% by mass. (under because some has vaporized). I've have read that bread yeast has a tolerance of about 5% but none of them specify if this is by mass, though it probably is - does anyone know? Anyway, as we can see, they are dieing of early because of the alcohol.
In conclusion, the best solution is to use a yeast with higher alcohol tolerance. Wine and Champagne yeasts have levels of 17-22% tolerance, though again I don't know what the percentages are of (mass, mols, volume, widgets...). Or as wendyv was thinking of doing, use a bucket of water so the alcohol doesn't become too concentrated. If this is the path taken, we don't want to increase our sugar dosing. Another solution is just to use regular 2 liter bottles, but to use allot less sugar.
I personally like to use more water with the same amount of sugar, but not a whole bucket. I used to use one 2 liter bottle with 2 cups of sugar, but now I use three 2 liter bottles and still only use 2 cups of sugar. The production has been extended from 1 week to 3 weeks.