Study of fermentation behavior and formulation of a semidefined nutrient medium based on acid‐production measurements in Z. mobilis cultures

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Abstract
Batch fermentations of glucose to ethanol by Z. Mobilis.(ATCC 10988) were examined in several semidefined nutrient media. The measurement of acid produced by the microorganism was used to study its transient fermentation characteristics. Limitation of nitrogen source in the semidefined medium of Rogers and coworkers2 was found to limit the growth of this microorganism in the late stages of batch fermentations, when the initial glucose concentration was 75 g/L and higher. The microorganism exhibits a preference for inorganic nitrogen over preformed organic nitrogen provided by yeast extract. The microbial growth occurs exponentially in the presence of ammonium sulfate and yeast extract. However, in the absence of ammonium sulfate, the growth occurs in a linear fashion. The “linear” growth phase is characterized by poor cell‐mass yields, and during this phase, growth and ethanol production are decoupled. An improved semi‐defined growth medium is established which supports better growth rate and cellular yield, without affecting the ethanol yield.