The results depicted in Figs. 1–4 show that (a) the feeding time
needed to achieve satisfactory cell growth increased with R, (b) all
the cultivations performed at the longest feeding times reached
satisfactory Xm values irrespective of R, and (c) the maximum cell
concentration progressively decreased from one cycle to the successive
one when tf was insufficient to sustain complete growth.
Therefore, a premature stationary phase could have taken place
owing to interruption of urea feeding and then to nitrogen limitation
(i.e., run 4; for run conditions and numbering see Table 1).
On the other hand, runs carried out at excess feeding times with
respect to the requirements of a given R value (i.e., runs 3, 6 and 9)
exhibited nearly constant Xm values during the three cycles, likely
due to the detrimental effect of shading on biomass growth [41