Laluce et al. (2002) constructed a special five-stage continuous
fermentation system with cell recycling and with different
temperatures in each stage. With this system, they
experimentally assessed the effect of temperature fluctuations
on fermentation performance. These fluctuations
produced variations in both cell concentration and cell
viability. Hojo et al. (1999) showed that microaeration
plays an important role in the stabilization of ethanol,
substrate and cell concentrations during the continuous
cultivation of sugar cane syrup with cell recirculation of
S. cerevisiae. Without air addition at low rates
(0.05 vvm), these concentrations had significant fluctuations.
One source of fluctuations leading to oscillatory
behavior of continuous ethanolic fermentation using
either S. cerevisiae or Z. mobilis is the high ethanol content
in the broth as in the case of very high gravity fermentations
(see below). Wide variations in ethanol, cell
and substrate concentrations are observed under these
conditions. Bai et al. (2004) showed that the utilization
of packed-bed reactors attenuates these oscillations and
quasi-steady states were attained, but the causes and
mechanism of such attenuation require further research.