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Heat treatments enhance the safety of mung bean seeds.

E. coli O157:H7 has been associated in the past with outbreaks of contaminated seed sprouts. Many of these outbreaks have been traced to sprout seeds contaminated with low levels of pathogen. E. coli O157:H7 strains can grow an average of 2.3 [log.sub.10] in a two-day period during seed germination. So, it is important to find an effective method that can reduce possible E. coli populations on seeds before they sprout.

The goal of scientists at Cornell University was to assess the effectiveness of various heat treatments in reducing E. coli O157:H7 populations on mung bean seeds intended for sprout production. They also wanted to see how the treatments affect the way the seeds germinate. They found that levels of pathogenic E. coli on mung bean seeds could be significantly reduced by heat treatments without affecting the germination rate of the seeds.

Mung bean seeds were inoculated with five different E. coli O157:H7 strains that harbored the green fluorescent protein gene. These were air-dried overnight. Heat treatments were performed by incubating the seeds at 55 C for varying periods of time, although four days of treatment appeared to give the best results. One-gram samples of the heated seeds were taken by the scientists, who diluted them in peptone water and mascerated them.

Appropriate dilutions were plated onto tryptic soy agar. These were incubated at 37 C for 18 hours to 24 hours, and green fluorescent colonies were enumerated. Researchers soaked 5 g of dried seeds in distilled water for six hours at ambient temperature. The water was then poured off, and the seeds were transferred onto wet Whatman filter paper and incubated at 22 C for two days to allow them to germinate. Germination rates were calculated as the number of germinated seeds per the total number of seeds after a two-day period.

After the inoculation and the drying process, 5.26 logs per g of E. coli were detected on the seeds. But the amount of E. coli on the seeds was under detectable levels--less than 1 log per g--after the seeds underwent heat treatment, while the germination rate was 100%.

Further information. Randy Worobo, Department of Food Science and Technology, Cornell University, New York State Agricultural Experiment Station, 630 W. North St., Geneva, NY 14456; phone: 315-787-2279; fax: 315-787-2284; email:
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Publication:Microbial Update International
Date:Oct 1, 2005
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