Effect of ammonium sulphate concentration and agitation speed on the kinetics of alginate production by Azotobacter vinelandii DSM576 in batch fermentation

Growth and alginate production by Azotobacter vinelandii DSM576 as a functi on of initial ammonium sulphate concentration (0.45-1.05 g l(-1)) and agita tion speed (300-700 rpm) were studied in batch fermentations at controlled pH. The time course of growth, alginate production and substrate consumptio n and the effect of nitrogen concentration and agitation speed on kinetic p arameters and on maximum alginate molecular weight (MW) was modelled using empirical equations. The kinetics of growth, alginate production and polyme rization were deeply affected by agitation speed and, to a lesser extent, b y inorganic nitrogen concentration. Average and maximum specific growth rat e and maximum alginate MW all increased with agitation speed, and were high er at intermediate ammonium sulphate concentration, Maximum alginate MW (>2 50,000) was obtained at high agitation speed (600-700 rpm) and alginate dep olymerization was limited or did not occur at all when the agitation speed was higher than 500 rpm, while at 400 rpm depolymerization significantly re duced the MW. However, alginate yield was negatively affected by increasing agitation speed. A good compromise between alginate yield (>2 g l(-1)) and quality (MW >250,000) was obtained with agitation speed of 500-600 rpm and 0.75-0.90 g l(-1) of ammonium sulphate.