However, not all metabolites of the micro-organisms during fermentation
could penetrate the membranes and been removed
from the broth. Some of the secondary metabolites, such as acetic
acid, lactic acid, propionic acid, glycerol and so on, could not been
extracted by PDMS membrane, and therefore would accumulate in
the broth. The concentration of several secondary metabolites in
the residual broth at the end of the CCCF process and a set of classical
data of batch fermentation from literature (Atkinson and
Mavituna, 1991) were listed in Table 2. Besides ethanol and the
permeable other substances, those impermeable secondary metabolites
had been thought as inhibitors against yeast cell growth
(Maiorella et al., 1983). Although further behaviors of those inhibitors
were not explored in this work, it could be imagined that
those substances listed in Table 2 deteriorated the broth and then
deteriorated the cultural environment for the microbial cells indeed.
The cell growth feature during the later period (stationary
stage and decline stage in Fig. 3) supported this imagination. From
viewpoint of transient process, the breeding and lysis of the yeast
cell was attaining the dynamic balance during stationary stage,
while the number of the lysed cells exceeded that of the newborn