Contamination was a consideration in the poor take-up of continuous fermentation technologies by potable alcohol producers, especially because holding prepared wort for sometimes lengthy periods without yeast inoculation provided an excellent growth medium for adventitious microbes in the brewery.177 With the accumulation of operating experience in fuel alcohol facilities, bacterial populations have been identified that not only reduce yield but also can prove difficult to eradicate; some bacteria (including lactic acid producers) form biofilms under laboratory conditions and can colonize many (perhaps every) available surface in complex sequences of linked fermentors and the associated pipe work.190

TABLE 4.2

Immobilized and Free Cell Systems for Fuel Ethanol Production: Critical Parameters for Process Efficiency

When bacterial contaminants reach 106-107 cells/ml, the economic losses for ethanol production can reach 3% of volumetric capacity; if profitability is marginal, this will have a serious impact, and antibiotic regimes have been devised to pulse controlling agents through continuous processes.191 This prophylactic approach has been applied to continuous ethanol facilities where the total losses will be greater because continuous operations have begun to dominate the larger (>40 million gal — lons/year) production plants — an antibiotic such as penicillin G is not metabolized and degraded by S. cerevisiae, and its addition rate can be poised against its expected chemical degradation at the low pH of the fermentation broth.[40] Outside the spectrum of known antibiotics, a useful alternative is the curious (and little known) chemical adjunct between urea and hydrogen peroxide; this bacteriocidal agent can effectively control lactobacilli in wheat mash and provides useful levels of readily assimilable nitrogen and O2 (by enzyme-catalyzed decomposition of the peroxide) to enhance yeast growth and fermentative capacity.192