Viability of sprout seeds as affected by treatment with aqueous chlorine dioxide and dry heat, and reduction of Escherichia coli O157:H7 and Salmonella enterica on pak choi seeds by sequential treatment with chlorine dioxide, drying, and dry heat
Introduction
Seed sprout is a generic term used to describe germinated forms of vegetable seeds which are usually harvested in 2–7 days (Kurtzweil, 1999). Seed sprouts are low in calories and fat, and contain substantial amounts of vitamin C, folate, and fiber (Bari et al., 2008, Kurtzweil, 1999). Additionally, some types of seed sprouts are rich in anticarcinogenic and antioxidant compounds (Fahey et al., 1997, Soengas et al., 2011).
While seed sprouts provide high nutritional value, consumption of raw sprouts has also been associated with outbreaks of foodborne illnesses (Dechet et al., 2014). These outbreaks have been caused largely by Salmonella and pathogenic Escherichia coli (Health Canada, 2011). A total of 33 outbreaks associated with the consumption of sprouts were reported from 1998 to 2010 in the United States, resulting in causing 1330 illnesses, 123 hospitalizations, and 2 deaths (Dechet et al., 2014). Twenty-eight of these outbreaks were caused by Salmonella, four were caused by shiga toxin-producing E. coli O157, and one was caused by Listeria monocytogenes (Dechet et al., 2014). In the European Union, Salmonella was the leading cause and pathogenic E. coli was the second leading cause of outbreaks associated with sprouts from 2004 to 2012 (Callejón et al., 2015). The largest outbreak associated with consumption of seed sprouts occurred at an elementary school in Sakai, Japan in 1996. School children consumed white radish sprouts contaminated with E. coli O157:H7, and approximately 12,000 patients were infected; among them, 121 patients developed hemolytic uremic syndrome and 12 died (Fukushima et al., 1999, Michino et al., 1999). In Germany in 2011, approximately 4000 people became ill, 54 of whom died, due to the consumption of sprouts contaminated with E. coli O104:H4 (Frank et al., 2011).
Microorganisms capable of causing foodborne illnesses can contaminate seeds during harvesting, handling, preparation, sprouting, and distribution of final products (Taormina et al., 1999). Contaminated seeds used to produce sprouts have been suspected as the major source in most of the outbreaks linked to consumption of seed sprouts (Breuer et al., 2001, Mahon et al., 1997, United States Food and Drug Administration, 2014, Winthrop et al., 2003). Even if populations of foodborne pathogens on seeds are low, growth can occur during sprouting. Conditions of sprouting are generally favourable (warm temperature, high humidity, and high nutrients provided by seeds) for the microbial growth (National Advisory Committee on Microbiological Criteria for Foods, 1999, Zhang et al., 2011). The ultimate solution to prevent outbreaks of foodborne diseases associated with the consumption of sprouts is to eliminate foodborne pathogens present on sprout seeds and prevent contamination during sprouting and subsequent handling.
Decontamination studies have focused on reduction of foodborne pathogens on seeds using sanitizers, heat treatment, irradiation, and high pressure treatment, but very few studies have reported complete inactivation. In a previous study (Bang et al., 2011b), we reported that E. coli O157:H7 was inactivated on radish seeds without decreasing the germination rate by sequentially applying aqueous chlorine dioxide (ClO2 [500 μg/ml, 5 min]), drying (45 °C/23% relative humidity [RH], 24 h), and dry-heat (70 °C/23% RH, 48 h) treatments. In the present study, we applied the sequential treatments to other types of sprout seeds using lower concentrations of aqueous ClO2 and higher dry-heat temperatures to shorten the total treatment time. Additionally, we determined the resistance of several types of sprout seeds having a wide range of surface morphologies against aqueous ClO2 and dry-heat treatments. The resistance of foodborne pathogens and viability of sprout seeds against these stresses have not been reported.
The objectives of this study were to determine the viability of various sprout seeds as affected by aqueous ClO2 (200 μg/ml, 5 min) or dry-heat (80 °C/23% RH, 24 h) treatment, evaluate the lethality of sequential treatments with ClO2 (200 μg/ml, 5 min), drying (45 °C/23% RH, 24 h), and dry heat (80 °C/23% RH, up to 48 h) against E. coli O157:H7 and Salmonella enterica on pak choi seeds, and estimate safety risks of sprouts produced from pak choi seeds on which these pathogens were not completely eliminated by ClO2, drying, and dry-heat treatments.
Section snippets
Bacterial strains and preparation of inocula
Five strains of nalidixic acid-adapted E. coli O157:H7 were used: ATCC 43895 (isolated from raw hamburger meat), E0018 (isolated from bovine faeces), F4546 (isolated from a patient in an alfalfa sprout-associated outbreak), H1730 (isolated from a patient in an outbreak associated with lettuce), and 932 (isolated from a patient with hemorrhagic colitis). Five serovars of S. enterica subsp. enterica were used: Salmonella Enteritidis (isolated from human faeces), Salmonella Typhimurium (isolated
Comparison of seed viability as affected by aqueous chlorine dioxide or dry-heat treatment
Table 1 shows the changes in the germination rates of sprout seeds after treatment with aqueous ClO2 (200 μg/ml, 5 min) or dry-heat (80 °C/23% RH, 24 h). Before treatment with aqueous ClO2 or dry heat (controls), seeds had germination rates of 82.0–99.3%. Germination rates of alfalfa, kohlrabi, kyona, mustard, pak choi, red radish, and tatsoi seeds did not decrease significantly (P > 0.05) after treatment with aqueous ClO2. Although the germination rate of red radish seeds was not significantly
Acknowledgments
This study was supported by Youlchon Foundation and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068475)
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