When brewing, how little yeast can you get away with? If I use 1 packet of dried yeast for a bathtub of apple wine, won't it just multiply until either the sugar runs out or the 14% alcohol kills the yeast? If not, what's stopping it?
Yeast only reproduces during the aerobic phase. Once the O2 is used up the yeast stop reproducing and start consuming the sugars. There's a limited amount of O2 that can be dissolved in liquid so once it's used up that's it, no more reproduction.
Xylose utilization is of commercial interest for efficient conversion of abundant plant material to ethanol. Perhaps the most important ethanol-producing organism, Saccharomyces cerevisiae, however, is incapable of xylose utilization. While S. cerevisiae strains have been metabolically engineered to utilize xylose, none of the recombinant strains or any other naturally occurring yeast has been able to grow anaerobically on xylose. Starting with the recombinant S. cerevisiae strain TMB3001 that overexpresses the xylose utilization pathway from Pichia stipitis, in this study we developed a selection procedure for the evolution of strains that are capable of anaerobic growth on xylose alone. Selection was successful only when organisms were first selected for efficient aerobic growth on xylose alone and then slowly adapted to microaerobic conditions and finally anaerobic conditions, which indicated that multiple mutations were necessary. After a total of 460 generations or 266 days of selection, the culture reproduced stably under anaerobic conditions on xylose and consisted primarily of two subpopulations with distinct phenotypes. Clones in the larger subpopulation grew anaerobically on xylose and utilized both xylose and glucose simultaneously in batch culture, but they exhibited impaired growth on glucose. Surprisingly, clones in the smaller subpopulation were incapable of anaerobic growth on xylose. However, as a consequence of their improved xylose catabolism, these clones produced up to 19% more ethanol than the parental TMB3001 strain produced under process-like conditions from a mixture of glucose and xylose.
Don't confuse reproduction with respiration. There are 3 basic types of cell respiration; Aerobic respiration, Fermentation, and Anaerobic respiration. It's all about who's accepting electrons. Remember the Glycolysis->Oxidative decarboxylation of pyruvate->Kreb Cycle from biology class? Without oxygen, pyruvate goes through fermentation, the waste products being ethanol and CO2...and NAD+ to re-enter the glycolysis pathway...not a very efficient path for energy production but we're after the ethanol, not ATP
Thanks for the insightful thoughts! I'll experiment with some extremely low doses of yeast to see what i'll get.
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