99E-13 |
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E. E. UGARTE-ROMERO1, H. Feng2, and S. E. Martin2. (1) Dept. of Food Science and Human Nutrition, Univ. of Illinois at Urbana-Champaign, 382D Agricultural Engineering Sciences Building, MC-646, 1304 West Pennsylvania Avenue, Urbana, IL 61801, (2) Dept. of Food Science and Human Nutrition, Univ. of Illinois, Urbana-Champaign, 1304 W. Pennsylvania Ave., 382-F Agricultural Engineering Sciences Bldg., MC-646, Urbana, IL 61801 Ultrasound is a promising alternative food processing technology for preservation of pumpable foods. Although several studies have been conducted to examine the inactivation kinetics of microorganisms in ultrasound treatment, very few have studied the mechanism of inactivation. The cavitation phenomenon is believed to be responsible for microbial inactivation, but no physical characterization of cell damages has been reported. The objective of this work was to understand the inactivation mechanism of Escherichia coli K12 cells treated with power ultrasound by examining morphology changes with electron microscopy. Stationary phase E. coli K12 cells were inoculated into sterile, irradiated (2.5 kGy) apple cider. Thermal and ultrasound inactivation tests were conducted at 40, 55, and 60o C. The ultrasound treatments were carried out using a VC-750 ultrasound unit (20 kHz) with 100 mL sample volume. Temperature was controlled using a set point control mechanism. Changes in cell morphology were examined with scanning electron microscopy. Three mL samples were taken at 3 min for 40 and 55o C treatments and at 1, 2, and 3 min for 60o C, and filtered through an Anodisc filter. Filtered cells were fixated with 1.25% glutaraldehyde solution in 0.1 mol liter sodium cacodylate buffer for 4 hrs. Fixation was conducted in 1 % osmium in 0.1 mol liter sodium cacodylate buffer for 90 min. Samples were rinsed and dehydrated with a graded ethanol series. After critical point drying, cells were mounted in a stub and coated with gold/palladium. Compared to thermal treatment at 1 min, ultrasound treated E. coli K12 cells showed extensive changes in cell surface topography and morphology, with less granular texture and damages ranging from localized surface pitting, partial collapse, to complete integration. Ultrasound treatment at 60o C (thermosonication) caused extensive rupture of cells with obvious pitting on the cell surface. Ultrasound treatment resulted in irreversible cells damages.
Session 99E, Nonthermal Processing: General II
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |