Minimizing the Risk of Listeria monocytogenes and Other Pathogens in Dried Foods

Drying is a convenient way of preserving foods for later use. The Cooperative Extension Service has long provided recommendations to consumers on how to dry foods at home. The recent implication of dried/fermented meats, such as beef and venison jerky and fermented sausages, in outbreaks caused by Escherichia coli O157:H7 and other enterohemorrhagic E. coli serotypes and Salmonella of the serovars Enteritidis and Typhimurium indicates that application of current processes, such as drying of meats, may fail to inactivate pathogenic bacteria. Also, the increasing number of foodborne outbreaks associated with fresh produce in recent years suggests that bacterial pathogens may survive in home-dried fruits and vegetables. <P> At present, conflicting data on pathogen destruction in home-dried foods exist, while research has shown that sausage fermentations are insufficient to effect the required 5 log pathogen reduction. Additionally, several drying studies have shown that the initial rapid decline of the target pathogen is followed by survival of a residual population of bacteria in the ready-to-eat product. Furthermore, post-processing contamination of dried foods with enteric pathogens and Listeria monocytogenes may increase food safety risks. <P>The overall goal of the studies is the development and validation of product-specific processes for assuring the safety of home-dried meats, fruits and vegetables, and the development of recommendations for the safe drying and storage of meats and produce. Consumer surveys will be conducted across the United States to understand the range of methods used to dry foods in humid and dry climates and at various elevations. <P>Based on this information, experiments will be designed with the objectives to: 1) Assess the potential for survival of L. monocytogenes, E. coli O157:H7, and Salmonella in foods dried using common home drying methods; 2) Develop effective pre-drying processes and drying procedures for the destruction of the above pathogens in home-dried foods and identify critical control points and critical limits to ensure their safety; 3) Assess the fate of pathogens surviving the drying process or introduced following processing in dried/fermented foods during storage; 4) Apply, when needed, antibacterial agents and/or post-drying processes that have the potential to help control the pathogens during product storage; 5) Conduct consumer taste panels to assess the overall acceptability of uninoculated foods produced by the alternative drying systems that have been validated to destroy the pathogens; 6) Develop product-specific models to predict the risk for pathogen survival in dried foods during processing and storage; 7) Use the information generated to develop extension educational material and training programs to provide food scientists, processors, retailers, Cooperative Extension Agents and Specialists, and consumers with revised recommendations on how to dry, store, distribute and consume dehydrated foods safely. The research will lead to a better understanding of pathogen behavior in foods dried at home or in small-scale operations and will minimize foodborne illness episodes due to such products.
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Current home-drying methods for meats, fruits and vegetables may fail to control foodborne pathogenic bacteria, as indicated by outbreaks associated with beef and venison jerky. Moreover, cross-contamination of dried foods with bacterial pathogens during packaging may increase food safety risks and this may have been the case with pre-sliced dry and semidry fermented sausages implicated in outbreaks. This project will establish procedures to control pathogenic bacteria in dried foods by developing effective pre-drying treatments and drying processes and by applying antimicrobials without adversely affecting product quality. The results will be used to develop educational material and provide training on safe food drying and storage for Cooperative Extension personnel, processors and consumers.
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The target of the research is to assess limitations in the effectiveness of drying procedures and practices currently in use, and to find alternative pre-drying, drying or post-drying treatments to minimize any risk of bacterial pathogen survival in dehydrated meats, fruits and vegetables. To accomplish these goals, Cooperative Extension Specialists with food preservation responsibilities will be surveyed nationwide to identify methods recommended for drying foods at home. Foods intended for testing in pre-drying and drying experiments to determine the behavior of bacterial pathogens will include beef and venison meat (whole muscle or ground to make jerky), apples, peaches, tomatoes and parsley. Pathogens to be examined will include L. monocytogenes, E. coli O157:H7 and Salmonella. Use of selected marker bacterial strains, (e.g., resistant to streptomycin or rifampicin), will allow monitoring of pathogens in the presence of natural flora. Fresh meats and produce will be inoculated with single or mixed pathogenic strains at levels that will permit validation of a 5-log reduction during drying. The efficacy of pre-drying treatments (e.g., acids, marinates, heat) to enhance destruction of pathogens when combined with drying will be evaluated with the objective of minimizing survival of bacteria. Developed critical control points and critical limits will be evaluated for the potential to lead to development of stress-resistant bacterial pathogens. Dried experimental or commercial foods with surviving pathogens or inoculated after drying will be stored at refrigeration or ambient temperatures in air or vacuum to evaluate pathogen survival or growth. Antimicrobials (e.g., chemical additives, bacteriocins, protective cultures) and post-processing interventions (e.g., heat) will be used, singly or as multiple hurdles, to enhance inactivation of surviving or post-process contaminating pathogens in dried foods during storage. Consumer panel tests will be conducted to evaluate the overall acceptability of uninoculated foods dried and stored by alternative pre-drying/drying methods validated to destroy the pathogens. Data obtained will be used to develop and validate product-specific models for predicting the fate of pathogens during processing and storage of dried foods. Model design will be based on monitoring of factors that increase the destructive effect of the pre-drying/drying processes or the storage conditions against pathogens (e.g., decrease in D-values, lowering in aw, increase in salt, nitrite or acid concentration). Potential problems might be the ineffectiveness of treatments to deliver the desired pathogen reduction, and the need to identify and test additional treatments, singly or in combinations, while experimental limitations might arise due to operation unreliability of home-style dehydrators. The information generated will be used to revise current fact sheets and bulletins on home drying of foods, which will be distributed through Cooperative Extension and Agricultural Experiment Stations. To enhance adoption of revised methods, train-the-trainer classes will be conducted with Extension Agents and volunteers.
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The overall aim of the studies was to generate research data and consumer recommendations to enhance the safety of dried meats, fruits and vegetables by controlling through inactivation pathogenic bacteria during processing and storage. The objectives included: (1) survey Cooperative Extension Specialists across the U.S. to identify the range of methods used to dry various foods; (2) assess the potential for survival of Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella in meats, fruits, and vegetables dehydrated using commonly recommended home drying methods; (3) develop effective pre-drying processes and drying procedures for the destruction of the above pathogens in these products; (4) identify critical control points and critical limits to ensure safety when drying products at home or in small-scale operations; (5) assess the fate of pathogens surviving the drying process or introduced following processing in dried/fermented meats, fruits and vegetables during storage; (6) identify and apply, if needed, antibacterial agents and/or post-drying processes that have the potential to help control the pathogens during product storage; (7) conduct consumer taste panels to assess the acceptability of uninoculated foods produced using newly revised or validated alternative drying procedures and systems; (8) develop product-specific models to predict the risk for pathogen survival in dried foods during processing and storage; and, (9) use the information generated to develop, pilot-test and promote extension educational materials and training workshops teaching the new recommendations. All of the objectives have been accomplished. Home food drying recommendations compiled from 27 states showed wide differences among them, including differences in treatments recommended for use before drying. Numerous experiments were conducted with beef, apples, tomatoes, parsley, peaches, carrots and potatoes to assess the effectiveness of various pre- and post-drying treatments. Results generally showed that a blanching or acid-dip pre-treatment enhanced inactivation of L. monocytogenes, E. coli O157:H7, and Salmonella during drying and product storage. Taste panels conducted with consumers showed no differences in acceptance between dried foods produced using traditional methods and those produced using pre-treatments that enhanced inactivation of pathogens. Three CSU Cooperative Extension fact sheets, a 15-page booklet on Drying Foods, and a one-hour training workshop were developed to promote the new recommendations. The workshop was pilot-tested with six groups of consumers and extension educators (n equals 75). Knowledge and attitude scores significantly (P greater than 0.05) improved from pre- to 6-week follow-up evaluation; however, no changes in behavior scores were seen, in part because of the short time between evaluations. The newly developed materials have been made available in electronic and print format. The experimental part of the project involved training of several graduate students and post-doctoral fellows, and resulted in the publication of numerous refereed journal articles and presentations at national scientific meetings.
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In the past, most home-drying procedures were based on empirical evidence and not evaluated for their influence on bacterial destruction during drying and storage. Such procedures may not be as effective as desired for pathogen inactivation and may lead to sublethal stress and development of resistance of pathogens to food preservation methods. Dried food products such as jerky have been implicated as vehicles of transmission of pathogenic bacteria such as E. coli O157:H7. Data published by USDA have indicated that pathogens such as Salmonella and Listeria monocytogenes are found present in commercial jerky at rates of 0.31 percent and 0.52 percent, respectively. The studies reported here showed that currently used marinade formulations may not be very effective in enhancing destruction of pathogenic bacteria during drying of beef jerky. The modified marinades studied, however, included more antimicrobial hurdles and enhanced death of pathogenic bacteria during drying and storage. Other studies showed that the use of blanching or acidic pre-treatments enhanced inactivation of pathogenic bacteria during drying of fruits and vegetables. Results were used to develop research-based recommendations for home food drying published in electronic and print media forms and promoted through one-hour workshops and the electronic media. Significant improvements in knowledge and attitudes were seen pre- to 6-week follow-up evaluation. Consumers can now dry foods at home with the confidence they are using research-based methods that effectively destroy pathogenic bacteria, if present.