Effect of nitrogen limitation on the ergosterol production by fed-batch culture of Saccharomyces cerevisiae

The diversity and content of available nitrogen sources in the growth medium both are very important in the accumulation of ergosterol in the yeast cell membrane. Growth on the good nitrogen sources such as ammonia can harvest more yeast cells than on poor ones, but ergosterol content in those yeast cells is relatively lower. Ergosterol content, one of the most variable parameters in ergosterol production by yeast cultivation, is greatly influenced by nitrogen limitation. The aim of our work was to study how the nitrogen sources affected the membrane ergosterol content and increase the total ergosterol yield. On the premise of keeping high ergosterol content in yeast cell, the ergosterol yield was enhanced by increasing the yeast biomass. Direct feed back control of glucose using an on-line ethanol concentration monitor was introduced to achieve high cell density. Ammonia, which acted as nitrogen source, was added to adjust pH during fermentation process, but its addition needed careful control. Cultivation in 5 L bioreactor was carried out under following conditions: culture temperature 30+/-1 degrees C, pH 5.5+/-0.1, agitation speed 600 rpm, controlling ethanol concentration below 1% and controlling ammonium ion concentration below 0.1 mol/L. Under these conditions the yeast dry weight reached 95.0+/-2.6 g/L and the ergosterol yield reached 1981+/-34 mg/L.