Fed-batch cultivation of Saccharomyces cerevisae is the dominating technique
in high cell density cultures of processes such as the production of Baker's yeast
[l] and recombinant proteins [2]. Due to the high oxygen demand of these
cultures, the oxygen supply to the culture is an important and difficult task. The
use of hyperbaric air, e.g., air at increased pressure, for oxygen mass transfer
improvement has been proved to be applicable to several microbial strains [3,4].
In this study, the effects of hyperbaric air up to 1.5 MPa on the viability and
morphology of S. cerevisiae cells grown in fed-batch cultures were investigated.
Fed-batch experiments were performed in a stainless steel stirred tank reactor.
Exponential feeding at dilution rates up to 0.1 h-1 was used, in order to ensure
full respiratory metabolism. The ethanol production due to oxygen _limitation at
atmospheric pressure was reduced by the bioreactor pressurization up to 1.0
MPa. No differences on the fraction of viable cells, size of the cells and
genealogical age were observed in this range of total pressure. Moreover, best
results were obtained for experiments where pressure was increased gradually
throughout time. This observation indicates the existence of an adaptation
period of the cells to hyperbaric conditions.
However, a strong inhibition of cell activity was observed for the operation at
1.5 MPa total air pressure. This effect was due to the increase on the oxygen
partial pressure because similar cell behaviour was found using pure oxygen at
the same partial pressure (0.32 MPa). Oxygen toxicity resulted in a drastic
decrease of cell viability, inhibition of ATP synthesis and morphologic changes,
mainly, cell size decrease
