Interventions Seeds 831 Chemical and Physical

Sanitizing sprout seed presents a unique challenge in the arena of produce safety in that even a low residual pathogen population remaining on contaminated seed after treatment appears capable of growing to very high levels (up to 8 log10 CFU/g) due to favorable conditions of moisture, relative humidity, temperature, and nutrient availability during seed germination and subsequent sprout growth [51,52]. In addition, after a sanitizing procedure seed germination as well as sprout yield and quality need to be maintained at commercially acceptable levels. In 1999, based on research available at the time, the FDA published guidance documents recommending that commercial sprout growers treat sprout seed with one or more antimicrobial treatments such as 20,000 ppm of Ca(OCl)2 that have been approved for reduction of pathogens on seeds or sprouts, with at least one approved antimicrobial treatment applied immediately before sprouting [53]. Also, in 2000 the FDA and the California Department of Health Services, Food and Drug Branch jointly released a food safety training video [54] for use by commercial sprout growers. The video, based on the FDA guidance documents, contains a recommendation to treat sprout seed with 20,000 ppm available chlorine from Ca(OCl)2 for 15 minutes (continuous mixing) with potable water rinses both before and after seed treatment. Since sprout seed is considered a raw agricultural product, chemical seed treatments are subject to approval by the U.S. Environmental Protection Agency and not the FDA.

Population reductions reported after treatment of alfalfa seed artificially inoculated with salmonella or E. coli O157:H7 using 16,000 to 20,000 ppm of available chlorine has varied considerably among different laboratories, but usually are in the range of 2 to 4 log10 (Table 8.2). Lesser reductions were achieved after treatments with lower amounts of chlorine. A number of factors likely contribute to the variability in results. Such factors include the percentage of treated inoculated seed with broken, cracked, or wrinkled seed coats [81], differences in the initial pathogen population on the seed, the extent of mixing of sanitizer during treatment, the initial organic load on the seed, and the use of rinse steps before and after seed treatment. Some studies have been done with relatively low initial pathogen populations on the seed allowing for maximum population reductions of 2 to 3 log10. One consistent finding among the various laboratories is that the two pathogens when artificially inoculated onto sprout seed are not eliminated even by treatment with 16,000 to 20,000 ppm of available chlorine for 10 to 15 minutes.

The findings for similar studies with naturally contaminated seed are not consistent among laboratories (see below).

Investigations of recent foodborne outbreaks of salmonellosis due to contaminated sprouts indicates that treatment of sprout seed with high levels of chlorine by commercial growers reduces, but may not always eliminate, the risk of human illness [16-18,20]. The inability of seed treatments with high levels of chlorine to always ensure a pathogen-free seed under commercial practice may be due to several factors including the use of differing protocols for administering seed treatments at grower locations. Also, the particular seed treated, if naturally contaminated, may differ in the level of contamination present and the location of the pathogens on the contaminated seed (e.g., deep in cracks, crevices, and/or natural openings) (Figure 8.2). The ability of bacterial human pathogens to be internalized in seed under natural conditions in the field is not known, but seeds in general can harbor internalized native bacteria [82]. If present in internal tissues of the seed, pathogens may escape contact with chemical sanitizers.

Numerous chemical treatments in addition to chlorine as well as several physical treatments have been tested individually or in combination for eliminating pathogens from artificially inoculated sprout seed. To date there are few reports of stand alone chemical or physical interventions capable of eliminating pathogens from artificially inoculated sprout seed or consistently achieving the recommended 5 log10 reductions [4] without significant adverse affects on seed germination and/or sprout yield (Table 8.2). Most of the interventions included in Table 8.2 have been tested using more than the single set of conditions listed. Additional chemicals tested in the references cited, but not included in Table 8.2, are aqueous acetic acid, calcinated calcium, carvacrol, cinnamic aldehyde, citric acid, Citricidal® (NutriTeam, Inc., Reston, VT), CitroBio™ (= Pangermex) (CitroBio, Inc., Sarasota, FL), Environne Fruit and Vegetable WashTM (Consumer Health Research, Inc., Brandon, OR), ethanol, eugenol, linalool, methyl jasmonate, sodium carbonate, sodium hypochlorite, thymol, trans-anethole, trisodium phosphate, Tsunami 200® (Ecolab, Mendota Heights,

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