Application of Dielectric Heating to Kill Human Pathogenic Bacteria On Alfalfa Seeds

Application of dielectric heating to kill human pathogenic bacteria on alfalfa seeds

2003 Progress Report of the National Alliance for Food Safety

1Dr. Larry R. Beuchat and 2Dr. Stuart O. Nelson

1Center for Food Safety, University of Georgia and 2USDA-ARS, Russell Agricultural Research Center,

Athens, GA

 

Major issue

Several documented outbreaks of enterohemorrhagicEscherichia coli O157:H7 and Salmonella infections

in the U.S. have been associated with consumption of alfalfa and clover sprouts in recent years. Treatment

of seeds intended for sprouting with chlorine (up to 20,000 ppm) and other chemicals reduces populations

of these pathogens but does not eliminate all viable cells. The hypothesis is that cells of pathogens are

lodged in cracks and crevices of damaged seeds, making them inaccessible to chemical treatments.

 

Relevance of problem

Human listeriosis associated with consumption of seed sprouts has yet to be documented. However,

Listeria monocytogenes is not uncommonly found in decaying vegetation and on healthy plant materials, so

its presence in some lots of seeds intended for producing sprouts is likely. When present on alfalfa seeds,

this pathogen is known to grow to populations exceeding 10 million per gram of sprouts produced. The

ability of L. monocytogenes to grow at temperatures as low as 3°C (37°F) on a wide range of foods raises

concern about its behavior on seed sprouts exposed to temperatures routinely used during distribution,

marketing, and preparation in foodservice and home settings.

 

Dielectric heating, i.e., radio-frequency (RF) electric energy, has been successfully used to kill insects and

molds in grains. Another important beneficial effect of dielectric heat treatment is that it stimulates germination

of some types of seeds, including alfalfa seeds. In fact, much of the work done on dielectric heating

treatment of alfalfa seeds has been directed toward determining treatment conditions necessary to

decrease the "hard" seed content, with the objective of increasing germination percentage and enhancing

vigor of plants in the early stages of development. Hard seed is a common condition found in alfalfa and

many other legumes. Although seeds are viable, an impermeable seedcoat prevents the entry of water

necessary to initiate germination. Hard seeds lie dormant in the soil or, in the case of sprout production, fail

to germinate, thus reducing yield. Mechanical scarification, an abrasion of the seedcoat, is often used to

reduce the hard seed condition. This process, however, undoubtedly exacerbates the problem of killing

human pathogenic bacteria by application of chemical solutions because the active components of these

solutions may not reach cells lodged at subsurface locations. The use of dielectric heating as a disinfection

treatment would eliminate this problem, since temperatures at subsurface locations exceeding those

required to kill human bacterial pathogens can be achieved within seconds.

 

Parameters influencing the effectiveness of dielectric heating in reducing the percentage of hard seeds

include the radio frequency (MHZ) applied, field indensity (Kv/cm), temperature, moisture content of seeds,

and time of exposure to treatment. High radio frequencies result in more rapid heating, thus influencing the

time needed to achieve the desired reduction in number of hard seeds. The field intensity doesn't appear

to affect changes in percentage of hard seeds, all other conditions being the same, but higher seed moisture

content tends to diminish the effectiveness of dielectric heating. It was hypothesized that treatment of

alfalfa seeds at a radio frequency of 39 MHZ for times sufficient to achieve temperatures in the range of 70

- 90°C (158 - 194°F), which greatly enhances the germination percentage of alfalfa seeds, would also substantially

reduce or eliminate E. coli O157:H7, Salmonella, and L. monocytogenes that may be present on

seeds.

 

Significant achievement

The objective of this project was to reduce populations of E. coli O157:H7, Salmonella and L. monocytogenes

on alfalfa seeds by at least 100,000-fold while simultaneously enhancing germination and not

adversely affecting vigor or sensory quality of the sprouts. The efficacy of dielectric heating in killing

pathogens, as affected by moisture content of seeds and various treatment time/temperature combinations,

was studied. The effect of treatments on seed germination percentage and vigor (appearance, turgor,

color) was also investigated.

 

Expected accomplishment

The potential for controlling human bacterial pathogens on alfalfa seed used in the production of sprouts

by dielectric heating was studied by experimental exposure of alfalfa seed artificially contaminated with

Salmonella,E. coli O157:H7, and L. monocytogenes to RF dielectric heating treatments at 39 MHz and different

electric field intensities for varying times of exposure. Moisture content of alfalfa seed and final temperatures

produced by the RF exposures were determined, and control and treated seed samples were

analyzed in the laboratory for reduction of bacterial populations and affects on seed germination.

 

Significant reductions in populations of all three pathogens were achieved without reductions in seed germination,

but desired levels of pathogen reduction were not achieved without significant damage to seed germination.

However, treatments effective in significantly reducing bacterial pathogen populations also

increased alfalfa seed germination through reductions in hard seed percentages, so the combined benefits

should be considered in evaluating dielectric heating treatments for practical use.