61C-25 |
Studies on the effect of high hydrostatic pressure (HHP) on cell morphology of Leuconostoc mesenteroides with scanning electron microscopy (SEM) |
H. ALPAS1, F. Bozoglu1, J. Lee2, and G. Kaletunc2. (1) Food Engineering Department, Middle East Technical University, Ankara, 06531, Turkey, (2) Department of Food, Agricultural and Biological Engineering, Ohio State University, Columbus, OH 43210-1057 The factors contributing to inactivation of microbial cells by HHP may range from membrane solubilization at low and moderate pressures to cell rupture at high pressures. Several mechanisms are proposed, however, the exact mechanism of bacterial inactivation by HHP is still not known. The objective of this study is to determine the effect of HHP on the cell structure of Leuconostoc mesenteroides, Gram-positive cocci using SEM and to relate the structural changes to viability of the cells. L. mesenteroides samples are grown in MRS broth at 30oC to their early stationary phase at a final cell concentration of 1.3 ± 0.1 x 108 cfu/ml. The cell suspensions are subjected to pressures of 150, 200, 250 and 500 MPa at 35oC for 5 min. After HHP treatment pressurized samples are plated on MRS agar for viable cell count. Cell suspensions are fixed using glutaraldehyde and examined with SEM. The scanning electron micrographs show that untreated L. mesenteroides is typically arranged in chains of 2 to 5 with smooth surface appearance. As pressure increases from 150 to 500 MPa, the number of cells in a chain decreases in comparison to control samples. Concurrently, the roughness of the cellular surface increases with pressure and at 500 MPa blister like formations appear on cell surfaces. Such formations are localised particularly at the points of new cell-wall synthesis. At 500 MPa more than 8 log cycle cell inactivation was observed-where at 150 MPa the reduction was only 0.14 log cycles. An understanding of the mechanism of microbial inactivation is vital to assess the foodborne disease risk associated with HHP treatment. Comparison of cell morphology by SEM prior and subsequent to HHP allows us to evaluate visually the changes resulting from HHP processing which coincide with inactivation of microorganisms.
Session 61C, Food Microbiology: General I
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