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Oscillatory changes in the growth rate were observed in undisturbed continuous culture of Saccharomyces cerevisiae on sugar-cane molasses media when nitrogen sources (2.56 to 6.17 g/liter of ammonium sulfate or 1.22 g/liter of urea) were added to the feeding mash and when the air rate was 1.3 to 1.6 v/v/m. The oscillations were not affected by the addition of yeast extract. The suppression of the nitrogen source during the continous test leads to a nonoscillatory transient state. No oscillations occured at all when no nitrogen source was added to the medium and/or the air rate was equal to zero or equal to about 3.3 v/v/m. The oscillatory responses of the system were affected by a previous anaerobic continuous cultivation of the yeast.

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In previous articles'-3 we reported the results obtained in tests carried out in order to study the effect of variation of the feeding mash concentration, dilution rate, and temperature on continuous culture of Saccharomyces cerevisiae in sugarcane molasses media, with the main purpose to simulate transient states usually observed in biological treatments of domestic sewage. The influence of mash concentration' and of the dilution rate' was studied without air bubbling, while the temperature-disturbed tests3 were carried out with and without air bubbling.The above experiments lead to the conclusion that the disturbed fermentor can be considered as a steady-state system if the average values of the parameters (mash concentration, cell concentration, dilution rate, temperature, yield factor, and specific growth rate) were adopted to characterize the system.In this article we present the results obtained in pH-disturbed continuous culture experiments, with no air bubbling, carried out in order to compare the average parameters calculated from experimental data obtained in disturbed tests with the corresponding parameters evaluated from undisturbed experiments.The experiments were carried out as described previously' with the following modifications: I ) The concentrated mash, prior to clarification, was prepared diluting 5.0 liter of molasses in 5.0 liter of tap water containing 22.5 g of NaH2P04.H20. 2) The media pH were adjusted after sterilization by adding concentrated sulfuric acid or 4.ON NaOH solution.3) The volume of culture medium in the fermentor was 4.3 liter (3.5 liter of sterilized mash and 800 ml of inoculum). 4) The dilution rate was 0.09-0.12 hr-'. 5) The pH of the medium during the fermentation test was automatically controlled by means of 4.ON H'S04 or 4.ON NaOH solutions. 6) The inoculum was prepared in 1000-ml Erlenmeyer flasks containing 200 ml of clarified diluted mash, incubated at 30°C for 20-22 hr in a rotatory shaker. 7) The steady states were maintained for no less than 24 hr. 8) The cell concentration was measured by filtration of 25.0 ml of the medium through a Millipore membrane (pore diameter, 1.2 pm); the cells were washed with 50 ml of distilled water and dried (100-1 10°C; 2 hr). 9) The time interval necessary to change the pH of the fermenting medium in each disturbance was 5-15 min.The disturbances were produced by changing the pH as presented in Figures Table I. Figure 8 shows a typical result obtained in undisturbed experiments. The values of the yield factor obtained in the undisturbed tests and in the disturbed experiments are presented in Figure 9.During the disturbed tests, when the pH was suddenly increased to 9.0-10.0, the cell concentration unexpectedly increased and, after 3-6 hr, decreased. The percent increment of the cell concentration due to the cited pH disturbance depends on the cell concentration value at the moment when the disturbance was produced, as presented in Figure 10. Table I shows that & = DThis result is similar to that obtained in our previous article...