Ethanol production by nitrogen-deficient yeast cells immobilized in a hollow-fiber membrane bioreactor
Cells ofSaccharomyces cerevisiae ATCC 4126, immobilized within the macroporous walls of asymmetric hollow-fiber membranes, were alternately perfused with 10% glucose complex medium and with 10% glucose defined medium which was deficient in nitrogen. Using complex growth medium, ethanol productivities during the initial 10 h of culture attained a maximum level of 133 g/l-h based on the total fiber volume (3% ethanol). Productivities during nitrogen deficiency stabilized at 10 g/l-h (0.5 ethanol). In subsequent growth phases, ethanol production rates increased to levels 40–70% of initial growth-phase values, but the ability to regenerate the fermentation activity decreased with culture age. During nitrogen deficiency, the fermentation efficiency declined with a concomitant reduction in the total protein concentration of immobilized cells within the hollow-fiber membranes. The molar ratio of acetaldehyde to ethanol increased seven-fold during nitrogen deficiency, indicating that the overall decline in glycolytic activity was accompanied by preferential reduction in alcohol dehydrogenase activity. The molar ratio of glycerol to ethanol increased two-fold during nitrogen deficiency, and large lipid-like droplets accumulated within the nitrogen-deficient cells. In addition to these findings, we conclude that current hollow-fiber membrane reactors should be limited to cell cultures having low growth rates, low O2 requirements, and low CO2 production rates.
KeywordsEthanol Production Membrane Reactor Nitrogen Deficiency Ethanol Production Rate Alcohol Dehydrogenase Activity
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