Lactic Acid Fermentation And Structure Variation Of Rhizopus Oryzae Under Dense CO₂

Compressed or supercritical fluids are excellent extraction solvents. They have got wide applications in food processing and storage, medicine making, as well as biomaterial processing. As an environmentally benign solvent, supercritical carbon dioxide possesses many advantages due to its low toxicity, nonflammability, high availability and ambient critical temperature. It could be used as an alternative to replace traditional solvents in the extraction of many fermentation products.According to experimental investigations, it has shown that high-pressure carbon dioxide has the negative effect on lactic acid production. To find out the effect of high-pressure carbon dioxide on lactic acid fermentation, the Rhizopus oryzae cells were treated at the different pressures for 4 hours followed by fermentation at atmospheric pressure for 24 hours and measurements lactic acid concentration. It has found that the prohibitive effect is more obvious with pressure increase. Rhizopus oryzae ferment under atmospheric pressure for 6,8 hours after disposed under different pressures for 4 hours, the pH of the fermentation broth decreased. It has proves that the Rhizopus oryzae is still activated.The SEM, TEM, DSC and EDX was used to evaluate the structure change of Rhizopus oryzae in fermentation broth under atmospheric pressure and the Rhizopus oryzae treated under 5, 6.5 and 7.5 MPa CO₂ for 4 hours before 24h fermentation under atmospheric pressure. SEM was used to detect the surface changes of the Rhizopus oryzae, and the results revealed that the cells in the atmospheric fermentation broth were thin, columned and single. But the Rhizopus oryzae cells become thick, compressed and conglutinate, and the phenomena become more obvious with pressure increase, when the pressure was 7.5MPa the Rhizopus oryzae could not aggregate. TEM was used to detect the structure changes of the inner Rhizopus oryzae. The results revealed that the nucleolus and the cell membrane were still there, although Rhizopus oryzae was disposed under high pressures (5, 6.5 and 7.5 MPa) for 4 hours. And also we have found that the nucleolus has the phenomenon of fissiparism, although Rhizopus oryzae cells are not fissional. DSC was used to evaluated the inner composition changes of Rhizopus oryzae cells in four differentfermentation broth, the results were analyzed, and it could be concluded that cell membrane, protein and some kind of Iipid had obvious denaturation. It can be concluded that the Rhizopus oryzae cells do not increase after ferment under atmospheric pressure for 48 hours.