High Cell Density Fermentation For Ergosgerol Production By Saccharomyces Cerevisiae

Ergosterol, an important chemical to be used as a pharmaceutical intermediate, is the precursor of liposoluble Vitamin D2and cortisone. It is also a main sterol in yeast cell and is responsible for structural membrane features such as integrality, fluidity, permeability and activity of membrane-bound enzymes. Ergosterol is mainly produced by two different ways. First, ergosterol is extracted from the waste mycelium in penicillin or in citric acid fermentation process. Second, ergosterol can also be produced by yeast fermentation. Fed-batch fermentation is generally applied to achieve a more efficient production of intracellular products with Saccharomyces cerevisiae. The limiting substrate concentration of carbohydrate must be maintained below the value which enables the maximum respiratory capacity of Saccharomyces cerevisiae to overcome the repression caused by over-supply of carbohydrate.In our paper, monitoring ethanol concentration for feedback control of glucose feeding rate was applied to achieve high cell density fermentation for ergosterol production by Saccharomyces cerevisiae. The relationships between the direct fermentation parameters (pH, DO, ethanol concentration and glucose feeding rate) and indirect parameters (biomass, ergosterol yield and content, OUR, CER and RQ) were discussed. Moreover, the culture medium optimization of ergosterol fermentation by Saccharomyces cerevisiae was studied with shake flask culture method. Cultivation in5L fermentor was carried out under following conditions:culture temperature30℃, pH5.5, agitation speed600rpm, fermentation time60h and controlling ethanol concentration bellow1%. Under these conditions, the yeast dry weight reached120g/L and the ergosterol yield reached1500mg/L. For improvement ergosterol yield, different nitrogen control strategy was discussed. Controlling nitrogen feeding according to yeast cell density was applied to improve ergosterol yield over2000mg/L. For further cutting the cost, co-production of glutathione and ergosterol by Saccharomyces cerevisiae fermentation was established. Feedforward control of glucose feeding rate by gradually descending ethanol concentration was applied to co-product glutathione and ergosterol. After40h fermentation, glutathione and ergosterol yield were reached2000mg/L and1400mg/L, respectively.