Milking of microalgae: Production and extraction of β-carotene in two-phase bioreactors

The low productivity of photobioreactors used for production of high-value compounds from microalgae is a big bottleneck in commercialization. “Milking” of microalgae for the production of high-value compounds in which the produced biomass is reused for production can be a solution to overcome this bottleneck. We chose β-carotene extraction from Dunaliella salina in a two-phase bioreactor with an aqueous phase and an organic solvent phase as the model system. The goal was to develop an alternative and more efficient process than the commercial production process of β-carotene.Biocompatibility of different solvents with values of log Poctanol ranging from 3 to 9 for the cells of D. salina was investigated. Results showed that solvents having log Poctanol > 6 can be considered biocompatible for this alga. On the basis of the results the “milking” process for β-carotene production was developed. Growth of the cells was performed at low light intensity after which the cells were transferred to the production bioreactor, which was illuminated at a higher light intensity. The second bioreactor was a two-phase bioreactor consisting out of an aqueous and a biocompatible organic phase. In this bioreactor mixing and extraction were performed by re-circulation of the organic phase. The results showed that D. salina stayed viable for a long period (>47 d) in the presence of a biocompatible organic phase at high light intensity. The cell growth, however, was very slow in this situation. β–Carotene could be continuously extracted to the organic phase. The cells kept producing β-carotene and the extracted molecules were substituted by the cells. As a result β-carotene was continuously milked from the cells. The β-carotene extraction efficiency in this system was more than 55%. The productivity of the system was 2.45 mg. m-2.d-1 which is much higher than obtained in commercial plants for β-carotene production.