3.4. Effect of yeast extract concentration on cell growth and
kefiran production
Yeast extract is generally known as a source required for nitrogen
and growth factors. In order to determine the effect of
yeast extract on kefiran production, L. kefiranofaciens cells
were cultivated on medium containing 20 g L1 sucrose and
different concentrations of yeast extract ranging from 0.0 to
14 g L1. As shown in Fig. 4, yeast extract exhibited a significant
effect on cell growth and kefiran production. Less growth
and kefiran production was observed when none yeast extract
added to the production medium. Increasing the concentration
of yeast extract increased the production of kefiran, where it
increased from 0.377 to 0.948 g L1 as the concentration
increased from 2.0 to 6.0 g L1, respectively. This corresponds
to about 151.5% improvement in the produced kefiran. These
results were in agreement with those reported by Taniguchi
et al. (2001), who found that the addition of 5 g L1 yeast
extract is essential for kefiran production and resulted in the
production of 650 mg L1 of kefiran in the culture medium.
Further increase in the concentration of yeast extract adversely
affected the production of kefiran. The concentration of cell
biomass reached 0.933 g L1 at 6 g L1 of yeast extract, where
it remained more or less constant at higher yeast extract concentrations.
Taniguchi et al. (2001) showed that cultivating
the cells without yeast extract resulted in poor growth and kefiran
was hardly produced. According to Hsieh et al. (2006), the
production of polysaccharides increased with lower concentration
of nitrogen sources. Accordingly 6 g L1 yeast extract was
chosen as the optimal concentration for further optimization
steps.