61C-31 |
Effect of sublethal stresses on yeast inactivation by high voltage pulsed electric fields (PEF) |
B. ALMASI1, P. Cerrutti, M. A. Galvagno1, and A. M. R. Pilosof3. (1) Dept. of Chemical Engineering- Faculty of Engineering, Buenos Aires University, Pabellón de Industrias- Ciudad Universitaria, (1428) Buenos Aires, Argentina, (2) Department of Industry, University of Buenos Aires PEF is a prospective technique for inactivating microorganisms in pumpeable foods without significant adverse effects on food quality. However, food and environmental factors (such as composition, pH, temperature), genetic (strain dependent) and physiological condition of the yeast at the time of PEF imposition, can modify the effectiveness of the treatment. Our objective was to study survival of yeast cells after being submitted successively to sublethal stresses (heat, alcohol, freezing) and PEF treatment . A beer industrial strain of Saccharomyces cerevisiae and S. cerevisiae CBS 1171 were incubated overnight at 27°C with agitation. Cells were harvested by centrifugation, resuspended and then submitted to different “stresses” (heat at 40°C and 52°C; 20% v/v ethanol ; freezing in N2 and at –20°C) . Afterwards, 3 pulses of 5kV/cm or 10kV/cm with 50 mF capacitance were applied (Gene Pulser II electroporation System). Our results showed that survival of yeasts depended on the severity of stresses imposed as well as on the type of strain studied. The industrial strain was less resistant to all the stresses applied. Heating at 52°C and freezing with N2 appeared to be the most severe stresses for this strain (viable count reduction of 2 log cycles and 6 log cycles respectively). In contrast, ethanol stress appeared very moderate for both strains (viable count reduction < 1 log cycle) as it could be expected for this yeast. Nevertheless, both strains showed to be more resistant to PEF treatment at 10 kV/cm after being subjected to the sublethal stresses assayed. Efficiency of PEF treatments were dependent on the yeast strain studied. Previous application of sublethal stresses could also play a significative role, as tolerance of yeast cells to higher electrical fields (10 kV/cm) was increased. These findings may be useful to determine the operating conditions for the treatment of real foods.
Session 61C, Food Microbiology: General I
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