Ethanol Fermentation And Structure Variation Of Yeast Cells Under Dense CO₂

Compressed or supercritical fluids are excellent extraction solvents. They havegot wide applications in food processing and storage, medicine making, as well asbiomaterial processing. As an environmentally benign solvent, supercritical carbondioxide possesses many advantages due to its low toxicity, nonflammability, highavailability and ambient critical temperature. It could be used as an alternative toreplace traditional solvents in the extraction of many fermentation products.High-pressure carbon dioxide has the disadvantage effect on the ethanolproduction. With the pressure increase the ethanol concentration decreased veryquickly. At 1MPa pressure, when the pH value is 8 the ethanol concentration is thelargest. And when using the common inoculation amount (10% v/v) of yeasts, theethanol concentration is almost zero at 4MPa pressure. In order to solve this problem,the larger inoculation amount (4-6 times concentration of the common inoculationamount) was used. The ethanol production has been raised in some extent, but abovethe supercritical pressure the ethanol conenntration is also very low, only about 6% ofthat at the atmospheric pressure fermentation. The viability of yeast cells underdifferent pressure conditions was also investigated. We found that there were a part ofcells inactivated under 1MPa pressure disposed, but almost all of the cells inactivatedunder a higher pressure of 6MPa condition, and when the samples was cultivated for24h under atmospheric pressure nearly all of the inactivated cells became activated.The ethanol production under different pressure conditions, 30 min short timedisposed and to ferment under atmospheric pressure for 24h is nearly equal to theconcentration of atmospheric fermentation. On the same disposed time, under asupercritical pressure (7.5MPa), we found that the ethanol concentration is also nearlyequal to the atmospheric fermentation and almost no concern to the pH value.We also use the SEM (Scanning electron microscopy) AFM (atomic forcemicroscopy) and DSC (differential scanning calorimetry) to evaluate the structurechange of Saccharomyces cerevisiae cells in ethanol fermentation broth underatmospheric, 1, 6, and 8MPa pressure environment. SEM was used to detect thesurface changes of the yeast cells, and the results revealed that the cells in theatmospheric fermentation broth were smooth and full and cells figure were almostround and ellipse. But the coarseness and wrinkle formation on the cell surface andbecame severe with pressure increase, when the pressure was 8MPa, this becamemore severe and there were a little amount of cells broken. AFM was used to detectthe structure changes of yeast cells in the atmospheric pressure fermentation broth, theresults represented a new way: the three-dimensional stereograph combined with theplanar ichnography of signal analysis. It gave us a new perspective for study of cellsstructure. DSC was used to evaluate the inner substance changes of yeast cells in thefour different fermentation broths, the results were analyzed, and it could beconcluded that cells membrane, protein, and the lipid in the cells had obviousdenaturation.