Optimization of microbial cell disruption using pressurized CO2 for improving lipid recovery from wet biomass.

Microbial cell disruption using pressurized gases (e.g., CO2) is a promising approach to improve the lipid recovery from wet oleaginous microorganisms by eliminating the energy-intensive drying required for conventional methods. In this study, we perform cell disruption of Rhodotorula glutinis using pressurized CH4, N2, and Ar where we find the efficacy of these gases on cell viability is minimal. Since CO2 is found to be the only viable gas for microbial cell disruption among these four gases, we use a combination of Box-Behnken design and response surface methodology (RSM) to find the optimal cell disruption by tuning different parameters such as pressure (P), temperature (T), exposure time (t), and agitation (a). From RSM, we find 6 log reduction of viable cells at optimized conditions, which corresponds to more than 99% cell death at P = 4000 kPa, T = 296.5 K, t = 360 min, and a = 325 rpm. Furthermore, from the scanning electron microscope (SEM), we find a complete morphological change in the cell structure when treated with pressurized CO2 compared to the untreated cells. Finally, we find that up to 85% of total lipid can be recovered using optimized pressurized CO2 from wet biomass compared to the untreated wet cells where up to 73% lipid can be recovered.

Microbial cell disruption for improving lipid recovery using pressurized CO2 : Role of CO2 solubility in cell suspension, sugar broth, and spent media.

The study of in situ gas explosion to lyse the triglyceride-rich cells involves the solubilization of gas (e.g., carbon dioxide, CO2 ) in lipid-rich cells under pressure followed by a rapid decompression, which allows the gas inside the cell to rapidly expand and rupture the cell from inside out. The aim of this study was to perform the cell disruption using pressurized CO2 as well as to determine the solubility of CO2 in Rhodotorula glutinis cell suspension, sugar broth media, and spent media. Cell disruption of R. glutinis was performed at two pressures of 2,000 and 3,500 kPa, respectively, at 295.2 K, and it was found from both scanning electron microscopy (SEM) and plate count that a substantial amount of R. glutinis was disrupted due to the pressurized CO2 . We also found a considerable portion of lipid present in the aqueous phase after the disruption at P = 3,500 kPa compared to control (no pressure) and P = 2,000 kPa, which implied that more intracellular lipid was released due to the pressurized CO2 . Solubility of CO2 in R. glutinis cell suspension was found to be higher than the solubility of CO2 in both sugar broth media and spent media. Experimental solubility was correlated using the extended Henry's law, which showed a good agreement with the experimental data. Enthalpy and entropy of dissolution of CO2 were found to be -14.22 kJ mol-1 and 48.10 kJ mol-1  K-1 , 9.64 kJ mol-1 and 32.52 kJ mol-1  K-1 , and 7.50 kJ mol-1 and 25.22 kJ mol-1  K-1 in R. glutinis, spent media, and sugar broth media, respectively. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:737-748, 2017.

Effect of different C/N ratios on carotenoid and lipid production by Rhodotorula glutinis.

Due to the increasing demand for sustainable biofuels, microbial oils as feedstock for the transesterification into biodiesel have gained scientific and commercial interest. Also, microbial carotenoids have a considerable market potential as natural colorants. The carbon to nitrogen (C/N) ratio of the respective cultivation media is one of the most important parameters that influence the production of microbial lipids and carotenoids. Thus, in the present experiment, the influence of different C/N ratios, initial glucose loadings, and ammonium concentrations of the cultivation medium on microbial cell growth and lipid and carotenoid production by the oleaginous red yeast Rhodotorula glutinis has been assessed. As a general trend, both lipid and carotenoid production increased at high C/N ratios. It was shown that not only the final C/N ratio but also the respectively applied initial carbon and nitrogen contents influenced the observed parameters. The lipid yield was not affected by different ammonium contents, while the carotenoid production significantly decreased both at low and high levels of ammonium supply. A glucose-based increase from C/N 70 to 120 did not lead to an increased lipid production, while carotenoid synthesis was positively affected. Generally, it can be asserted that lipid and carotenoid synthesis are stimulated at higher C/N ratios.