SummaryCitric acid fermentation on pilot plant scale was studied, using two strains of Aspergillus niger of different productivity. Growth and product formation were analyzed from a high number of data points. Several phases could be distinguished which were described by a sequence of logarithmic, cube-root, and linear equations. Growth and product formation could be related by a modified Luedeking-Piret equation, taking into consideration a lag time for the organism to shift into the physiological state ready for product formation. The observed differences in productivity of the two strains are reflected by the different nongrowth-associated terms of the equation. In light of these findings the classical "type I1 fermentation" is redefined as a mixed type-resembling type I during trophophase and type 111 during idiophase.

SummaryA highly instrumented computer-coupled bioreactor is used to investigate metabolic changes of Sncchnrornyces cerevisine in aerobic fed-batch systems which are generally applied in bakers' yeast manufacture. The four types of metabolism (oxidation of glucose, aerobic fermentation, oxidation of glucose and ethanol, and oxidation of ethanol) appearing in such systems are characterized by four significant fermentation parameters: respiratory quotient (RQ), glucose uptake rate (On). ethanol turnover rate (&OH), and growth yield on glucose (YE). Below the critical glucose concentration glucose and ethanol are utilized simultaneously. The shift from aerobic fermentation to nondiauxic growth on glucose and ethanol is not only dependent on glucose concentration, but also on the precultivation of the cells. The uptake of ethanol is controlled by the glucose supply except in the case when ethanol is limiting; the oxygen uptake rate (Qo,), however, is unaffected by the ratio of Q, and QE~OH. Critical glucose concentration is not a constant value for a particular strain, but varies corresponding to the nutritional state of the cells.

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