Inhibition by secondary fermentation products may limit the ultimate productivity of new glucose
to ethanol fermentation processes. New processes are under development whereby ethanol is
selectively removed from the fermenting broth to eliminate ethanol inhibition effects. These processes
can concentrate minor secondary products to the point where they become toxic to the yeast.
Vacuum fermentation selectively concentrates nonvolatile products in the fermentation broth.
Membrane fermentation systems may concentrate large molecules which are sterically blocked
from membrane transport. Extractive fermentation systems, employing nonpolar solvents, may
concentrate small organic acids. By-product production rates and inhibition levels in continuous
fermentation with Succharomyces cerevisiae have been determined for acetaldehyde, glycerol,
formic, lactic, and acetic acids, 1-propanol, 2-methyl-l-butanol, and 2,3-butanediol to assess the
potential effects of these by-products on new fermentation processes. Mechanisms are proposed
for the various inhibition effects observed.