A bulletin for the Australian Food Industry August 1999
Contents: Refrigerated retail cabinets | Cheese from unpasteurised milk and listeriosis | Alicyclobacillus spp. - a source of flavour taints in acidic foods | Dioxins in food | Volatile nitrosamines in food and drink | Disinfection of seeds for sprouting
Disinfection of seeds for sprouting
The Australia New Zealand Food Authority in its Proposal P178, Microbiological Limits for Foods, has nominated cultured seeds and grains as a new category of foods for which microbiological standards may be appropriate. The organisms for which standards are nominated are Escherichia coli and Salmonella spp.
Australia does not have any documented incidents of food poisoning caused by seed sprouts. However in 1994, Australia was the source of alfalfa seeds which caused hundreds of cases of salmonellosis in Sweden and Finland when sold as sprouts. In this incident, routine antimicrobial treatment of seeds at the Finnish production plant with 0.5% sodium hypochlorite for 45 minutes failed to prevent the outbreak (Lancet 345 1995 462). The pathogen was isolated from germinated sprout but not seeds by investigating authorities.
Since that time, at least 13 foodborne disease outbreaks worldwide have been attributed to sprouts. Four of the outbreaks were caused by E. coli and three of these involved the particularly dangerous E. coli O157:H7. The largest outbreak occurred in Japan in 1996 when about 9000 people suffered illness and 17 died after eating radish sprouts. White radish sprouts were again implicated epidemiologically in an outbreak of E. coli O157:H7 the following year in Japan (J. Food Protection 62 1999 318). It is now generally recognized that microorganisms, including potential pathogens, are protected in cracks and crevices in seeds for sprouting from what would normally be lethal concentrations of chlorine and other sanitizers. Nevertheless a sanitizing step for seed should be an essential part of a HACCP plan for sprout production to minimize the risk of subsequent development of pathogens on the sprouts, many of which are now consumed without cooking.
The International Sprout Growers Association based in California recommends a presoak of seeds for sprout production in either sodium hypoclorite (2000 mg/L), calcium hypochlorite (1800 mg/L) or hydrogen peroxide (6%), for up to 10 minutes followed by a fresh water rinse. These recommendations are based in part on work published by Beuchat in 1997 (International Journal of Food Microbiology 34 1997 329). This investigation studied the efficacy of these chemicals in eliminating Salmonella from seeds and found that the time-concentration procedures outlined reduced inoculated Salmonella populations by more than 1000 fold. However, viable Salmonella were detected in seeds treated for 10 minutes in these solutions. The inoculated seeds were dried at 21°C for 26 hours before the sanitizers were applied.
Beuchat, together with his colleague Peter Taormina, has now extended these studies to E. coli O157:H7. Again using inoculated seeds which were dried before being subjected to a sanitizing step, these workers have endeavoured to find a chemical treatment or a combination of heat and chemical treatment to ensure elimination of E. coli O157:H7 from alfalfa seeds. Any treatment recommended must of course not seriously affect the ability of the seeds to germinate.
They report that while some of the treatments trialled look promising, no treatment eliminated E. coli O157:H7 from seeds initially contaminated with 100 to 1000 organisms per gram. They regard a reduction in numbers by a factor of 10 to 100 of minimal practical significance because surviving cells would be expected to grow during sprouting.
Significant reductions in numbers of E. coli O157:H7 were obtained, however, with at least one test concentration of each chemical tested at times ranging from three to 10 minutes. Solutions tested included calcium hypochlorite (2000 mg/L), acidified chlorine dioxide (100 mg/ L), acidified sodium chlorite (500 mg/L), hydrogen peroxide (0.2%), and trisodium phosphate (1%). The authors concluded that the high concentration of chlorine remaining available throughout the test period from the chlorine based compounds simply does not come in contact with bacteria protected by seed structures.
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