Nutritional requirements for the hyperproduction of bioactive exopolysaccharides by submerged fermentation of the edible medicinal fungus Cordyceps taii

Exopolysaccharides (EPSs) from Cordyceps taii have potent pharmacological effects and present an unparalleled resource for drug discovery, and optimization of EPS yield through systematic analysis and refinement of fermentation parameters was therefore sought. A one-variable-at-a-time method identified xylose (NH4)2SO4, and vitamins A and D as the optimum source of carbon, nitrogen, and growth-stimulating factor, respectively. Ammonium salts led to acidification of the culture medium, and the combination of (NH4)2SO4 with soybean steep liquor, the best organic nitrogen source, was therefore selected for subsequent optimization. Further experiments identified only xylose, soybean steep liquor and (NH4)2SO4 as having statistically significant effects on EPS production by a Plackett–Burman design. Re-optimization of these factors employed a path of steepest ascent approach followed by a central composite design modeling the response surface as a function of the experimental conditions. This predicted that optimum production of EPS would be achieved using a combination of xylose 31.27 g/l, soybean steep liquor 4.85 g/l, and (NH4)2SO4 0.15 g/l. Experimental validation revealed that the actual yield of EPS (43.87 ± 0.28 g/l) was accurately predicted by the model. This level of production represented an increase of over 8-fold vs. the highest yield (5.21 ± 0.14 g/l) obtained in preliminary experiments.