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Inactivation of Escherichia Coli 0157:H7
on Inoculated Alfalfa Seeds with Ozonated Water and Heat Treatment
March 2002
Journal of Food Protection: Vol. 65, No. 3, pp. 447451.
Ratna R. Sharma, Department of Agricultural and Biological Engineering, The
Pennsylvania State University, University Park, Pennsylvania 16802; Ali Demirci,
Department of Agricultural and Biological Engineering and Life Sciences
Consortium, The Pennsylvania State University, University Park, Pennsylvania
16802; Larry R. Beuchat, Center for Food Safety and Department of Food Science
and Technology, University of Georgia, 1109 Experiment Street, Griffin, Georgia
30223-1797; William F. Fett, U.S. Department of Agriculture, Agricultural
Research Service, Eastern Regional Research Center, Plant Science and Technology
Research Unit, 600 East Mermaid Lane,
Wyndmoor, Pennsylvania 19038, USA
Abstract
Alfalfa seeds inoculated with a five-strain mixture of Escherichia coli O157:H7
were immersed in water containing 4, 8, 10, and 21 ppm of ozone for 2, 4, 8, 16,
32, and 64 min at 4°C. Direct ozone sparging of seeds in water was used as an
alternative mode of ozone treatment. Ozone-sparged
seeds were also subsequently exposed to heat treatment at 40, 50, and 60°C for
3 h. Populations of E. coli O157:H7
on untreated and treated seeds were determined by spread plating diluted samples
on tryptic soy agar supplemented with 50 g/ml of nalidixic acid. Since E. coli O157:H7 was released from inoculated seeds
during treatment with ozone, the rate of release of cells from inoculated seeds
soaked in 0.1% peptone water for up to 64 min was also determined. The overall
reduction of E. coli O157:H7 on seeds treated with ozonated water without
continuous sparging ranged from 0.40 to 1.75 log10 CFU/g (59.6 to 98.2%),
whereas reductions for control seeds were 0.32 to 1.03 log10 CFU/g (51.7 to
90.5%). Treatment with higher ozone concentrations enhanced inactivation, but
contact time longer than 8 min did not result in significantly higher reductions
(P > 0.05). For seeds treated by
ozone sparging, a 1.12-log10 CFU/g (92.1%) reduction was achieved using a 2-min
contact time, and a 2.21-log10 CFU/g (99.4%) reduction was achieved with a
64-min contact time. The corresponding reductions for control seeds were 0.71
log10 CFU/g (79.5%) and 2.21 log10 CFU/g (99.4%), respectively.
Treatment of ozone-sparged seeds at 60°C for 3 h reduced the population
to an undetectable level by direct plating (4 to 4.8 log10 CFU/g), although
survivors were detected by enrichment. Ozone did not have a detrimental effect
on seed germination percentage.
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