Investigations on the pH increase phenomena during the cultivation of Lipomyces starkeyi in the hemicellulosic hydrolysate of birch wood chips

Oleaginous yeasts produce oils with a lipid profiles similar to edible vegetable oils that can be used in biorefineries for biodiesel production. However, lipid synthesis from pure carbohydrates results in high raw material cost. To overcome this drawback, hemicellulosic hydrolysates can be used as low cost carbon source. Attempts to domesticate the yeast Lipomyces starkeyi CBS 1807 in hemicellulosic hydrolysate of birch wood chips revealed an unexpected pH increase from 6 to 8 instead of pH drop after 24 hours of cultivation. To investigate the mechanism of this pH increase a hemicellulosic fraction from birch wood chips was generated by water extraction at 160 ºC for 90 minutes and a subsequently hydrolysis with 4% sulfuric acid at 121ºC for 60 minutes. Fed batch cultivation with this hydrolysate containing 1.8 g/L glucose, 39.2 g/L xylose, 13.4 g/L acetic acid and 2.2 g/L furfural was done in a bioreactor with controlled pH. For each hydrolysate addition the yeast first consumed the acetic acid. Further experiments of cultivating L. starkeyi in a xylose media with 2.5 g/L of acetic acid at pH 6 indicated that the consumption of acetic acid was responsible for the pH increase to 8, which shows that acetic acid can be utilized as a carbon source and that is interesting to control pH during domestication of microorganism in hemicellulosic hydrolysates.