Relationship between Sucrase Activity and Formation of By-products in Sucrose Fermentation by Zymomonas mobilis

The sucrase activity in Zymomonas mobilis B-806 (producing three different sucrases: intracellular sucrase SacA, extracellular levansucrase, SacB and extracellular sucrase, SacC) and a mutant strain LS1A (producing only extracellular sucrase, SacC) at different fermentation conditions was studied. The sucrases were synthesised constitutively in these strains and the carbon sources (glucose, fructose or sucrose) in the growth medium did not influence the synthesis of different sucrases. The strain LS1A produced nearly 2/3 of the sucrase activity (2.2 U ml^-1) of the strain B-806 (3.1 U ml^-1). The increase in concentration of the sucrose in the medium (100-250 g l^-1) did not increase the level of the sucrases but increased the production of levan and sorbitol by the strain B-806. The increase in fermentation temperature (30°C to 39°C) favoured the activity of SacC and the synthesis of sorbitol but decreased the production of levan. The activity of SacB and production of levan were maximum at pH 5.5 of the growth medium while the synthesis of SacC was constant at pH 5 to 6.