3.5. Effect of phosphate concentration on cell growth and kefiran
production
Phosphate ions are important for cell growth and kefiran production.
Different concentrations of K2HPO4 were tested
ranging between 0.0 and 2.0 g L1. The addition of phosphate
showed a significant effect on kefiran production, cell growth
and pH. In this study, the absence of phosphate resulted in a
very poor growth and kefiran production. Increasing the phosphate
concentration to 0.25 g L1, resulted in a significant
increase in kefiran production (1.187 g L1), corresponding
to a 25.2% increase. However, further increase in the concentration
of phosphate up to 0.5 g L1 of phosphate; significantly
decreased the production of kefiran concentration to
0.867 g L1 (Fig. 5). The addition of higher phosphate concentrations,
i.e. above 0.5 g L1, showed no effect on both cell
growth and the concentration of kefiran. These results were
in agreement with those obtained by Hsieh et al. (2006), who
investigated the production of polysaccharides under different
limitations. They found that phosphate concentration greatly
affects the production of the polysaccharide as well as cell biomass.
Their results revealed that the absence of phosphate
adversely affected both polysaccharide production and cell
biomass. Moreover, they also found that increasing the phosphate
concentration above the optimal concentration favors
cell growth rather than polysaccharide production, which they
attributed to the decrease in the activity of enzymes catalyzing
the biosynthesis of the polysaccharide. They concluded that
lower molecular weight polysaccharides (56–152 KDa) require
phosphate concentrations lower than 1 g/L1, while higher molecular weight polysaccharides (92–277 KDa) are mostly
produced at phosphate concentrations above 2 g/L1.