Characterization of oxygen transfer conditions and their effects on Phaffia rhodozyma growth and carotenoid production in shake-flask cultures

This study characterizes the effects of oxygen transfer on the growth and carotenoid (astaxanthin) production of the red yeast Phaffia rhodozyma in liquid cultures. The yeast culture on a liquid medium at 20 °C had a critical dissolved oxygen (DO) concentration of 10% air-saturation (0.8 mg/L) and a maximum specific oxygen uptake rate of 60 mg O2/g cell h. In 250-mL shake-flasks with 50 ml liquid medium, poor oxygen transfer resulted in constantly lower DO level than the critical DO in the culture. Both the biomass and carotenoid yields increased with the decrease in liquid volume in flasks and the increase in shaker speed, e.g., 8.0 g dw/L biomass and 7.4 mg/L carotenoid with 50 mL liquid at 200 rpm versus 10.7 g dw/L biomass and 15.5 mg/L carotenoid with 20 mL liquid at 250 rpm. The oxygen transfer coefficient (KLa) was correlated to shaker speed (N) and liquid volume (VL) by KLa = 0.141 N0.88 (VL/Vo)−0.80 based on linear regression of experimental data (R2 = 0.997, Vo = flask volume). The carotenoid yield showed a strong linear correlation with the oxygen transfer rate (R2 = 0.989). These results show that oxygen supply is crucial for carotenoid production in P. rhodozyma liquid cultures.