Radio Frequency Dielectric Heating of Alfalfa Seed for Reduction of Human Pathogens

Radio-Frequency Dielectric Heating of Alfalfa Seed for Reduction of Human Pathogens

American Society Of Agricultural Engineers Annual International Meeting, Chicago, Illinois. Paper No. 026001. 2002. Nelson, Stuart, Lu, C-Y, Beuchat, L,  Harrison, M, University of Georgia  

Interpretive Summary: There have been some outbreaks of human illness associated with eating vegetable sprouts that were contaminated with Salmonella and E. coli. The likely source of the bacterial contamination has been identified as the sprouting seed used for sprout production. Several chemical treatments and produce washes have been tested, but none has so far been found completely effective in eliminating the bacterial contamination from the seed. Radio frequency and microwave dielectric heating treatments were earlier found effective for increasing the germination of alfalfa seed lots that contain percentages of hard seed, i.e., seed with naturally impermeable seed coats that protect the seed, but also prevent the entrance of moisture needed for germination. Such seed lots in the commercial seed trade are scarified, i.e., subjected to an abrasive process that scratches the seed coat to permit the entry of moisture so they can germinate. Therefore, research was conducted to determine whether such dielectric heating treatments might be useful in reducing bacterial populations on alfalfa seed. Treatments of a few seconds that raised the temperature of alfalfa seeds, artificially contaminated with Salmonella, E. coli, and Listeria, were found to provide significant reductions in the bacterial populations without significantly lowering the seed germination. However, treatments that reduced bacterial populations to desirable levels also reduced the seed germination. Those treatments that lowered bacterial populations also increased the seed germination percentages, thus improving sprout yield. Thus, dielectric heating treatments that produce moderate reductions in bacterial populations and improve sprout yield might be considered for application, but are not likely to provide the desired control of bacteria that may contaminate alfalfa seed stocks intended for sprout production. 

Technical Abstract: The problem of bacterial contamination of vegetable seed sprouts produced for human consumption is reviewed briefly. 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, Escherichia coli O157:H7, and Listeria monocytogenes to radio frequency (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 effects 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 reducing bacterial pathogen populations also increase alfalfa seed germination through reductions in hard seed percentages, so the combined benefits should be considered in evaluating dielectric heating treatments for practical use.