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United States Department of Agriculture

Agricultural Research Service

2011 Annual Report

1a.Objectives (from AD-416)
Develop more effective means for decontaminating organic and conventionally grown fresh and minimally processed fruits and vegetables including sprout seed containing human pathogens to ensure food safety and security by assessing the efficacy of new and/or improved intervention technologies. Determine effectiveness of treatment combinations (multiple hurdle approach). Assess factors that might limit treatment efficacy. Transfer effective decontamination technology to the produce industry in order to reduce the risk of foodborne illness.

1b.Approach (from AD-416)
A variety of chemical, physical and biological intervention technologies will be evaluated. Physical and chemical treatments include the use of hot water pasteurization, ultrasound, gaseous chlorine dioxide, cold plasma, hydrogen peroxide vapor, and ionizing radiation alone or in combination. Conduct studies on the use of single or multiple isolates of antagonistic bacteria for inhibiting the outgrowth of bacterial human pathogens on sprouting seed and on sprouts postharvest. Determine the mode of action of effective antagonists. Scale up studies of effective interventions from laboratory scale to pilot plant scale. Investigate changes in composition and structure of indigenous microbial communities in relation to shelf life and hygienic quality of produce while in storage. Study the formation of biofilms by pathogens alone or in combination with native microflora on the surface of selected produce. Evaluate the effects of the various interventions on sensory and nutritional quality attributes, yield, physiology, and shelf-life to ensure acceptable quality of treated foods.

3.Progress Report
The project has completed research in support of NP Component 1D (Pathogen Toxins and Chemical Contaminants-Intervention Strategies). Results of the research (described below) have addressed the project objectives. The impact of post-contamination storage time on the efficacy of irradiation was determined for leaves of Romaine lettuce and baby spinach, inoculated with a cocktail of three strains of Salmonella. Leaves were stored to allow for biofilm formation, then treated with a chlorine wash or increasing doses of irradiation. Chlorine washes resulted in maximal reductions of 1.9 log cfu/g. From 0 to 72h of storage, D10 (the dose required for a 1 log reduction) increased from 0.28 to 0.34 kGy for spinach, and from 0.30 to 0.37 kGy for Romaine, suggesting that the biofilm habitat can reduce the efficacy of irradiation. These results can be used to establish best practices for incorporating irradiation into a lettuce/cut salad processing chain. Cold plasma was tested for ability to remove biofilms from food-contact surfaces. Salmonella and E. coli O157:H7 cultures were allowed to form adherent biofilms on glass slides. Slides were placed on a conveyor belt and passed under a plasma jet emitter. An optimized treatment of 15 seconds was able to reduce the most durable biofilms of E. coli O157:H7 by 3.03 log cfu/cm, and of Salmonella by 2.12 log cfu/cm. Cold plasma shows promise as a rapid treatment of food contact surfaces. In collaboration with industry, large-scale studies with fresh plantains and pineapples showed that ClO2 gas effectively penetrated fruit boxes, delayed mold development, and extended the shelf life of the fruits by a minimum of two weeks. Pilot scale ClO2 gas treatment of green tomatoes and cantaloupes, inoculated with Salmonella Montevideo or Poona was conducted. Produce was treated with ClO2 for 6h at 0.4 or 0.8 mg/l (tomatoes) or 1.0 mg/l (cantaloupes). Reductions of up to 5 logs were seen following 7 or 8 days of storage. The shelf life of produce was extended due to the reduction of spoilage microorganisms on the surface. These results establish the feasibility of incorporating this treatment in the produce shipping chain for enhancing quality and safety. A study was conducted to investigate the effect of modified atmosphere packaging (MAP) and delay of irradiation application on the quality of cut Iceberg lettuce. Results showed that irradiation (0.5 and 1.0 kGy) of cut lettuce induced tissue browning when stored in air; however, when cut-lettuce was stored in MAP, irradiated lettuce had better appearance than the non-irradiated ones. The results suggest that MAP is essential to minimize quality deterioration caused by irradiation. Research program on microbial ecology and the application of biological control cultures for controlling the outgrowth of Salmonella was completely suspended due to the SY’s extended sick leave.

1. Low temperature storage of leafy greens reduces efficacy of irradiation in killing Salmonella. Contamination of leafy green vegetables with human pathogens is a source of ongoing concern for consumers. Bacteria in mature microbial communities such as biofilms are relatively resistant to chemical treatments, but little is known about the response of leaf surface biofilms to radiation. Leaves of Romaine lettuce and baby spinach were dip inoculated in a cocktail of three strains of Salmonella and stored at 4C for various times to allow biofilms to form. After each time, the leaves were treated with either a 3-minute wash with sodium hypochlorite solutions or increasing doses of radiation. ARS researchers at Wyndmoor, PA, recovered and enumerated viable bacteria. Chlorine washes were moderately effective, and resulted in maximal reductions of 98.7% for baby spinach and 98.0% for Romaine. For spinach, storage times up to 48 h had no effect on chlorine efficacy, while after 72 h in storage, 600ppm was significantly more effective than 0 or 300 ppm. For Romaine lettuce, 600ppm chlorine was significantly more effective at 0 and 28 h, but not at 24 or 72 h of storage, indicating a more variable response. For both types of leaves, time in storage reduced the efficacy of irradiation. D10 values (the dose required for a 1 log reduction) significantly increased with increasing storage time. These results indicate that the biofilm habitat can reduce the efficacy of irradiation in eliminating pathogens from leafy vegetables. These results can be used by vegetable processors to produce safer lettuce and bagged salads for consumers.

2. Pilot-scale thermal treatment kills Salmonella on green tomatoes. An economical means of controlling Salmonella on whole tomatoes is urgently needed by industry. Optimization of pilot scale hot water surface pasteurization of green tomatoes was conducted by ARS researchers at Wyndmoor, PA, at the ERRC. Whole tomatoes were inoculated with Salmonella Montevideo to an approximate final concentration of 10,000 log CFU/gm, and stored at 12.5C for 24 h prior to treatment. Tomatoes were treated at 70C for 3.5 minutes and were rapidly cooled in ice water and stored at 18C – 90% relative humidity for up to 7 days. The pilot-scale treatment was less effective than the lab scale treatment, but was capable of reducing the pathogenic cell concentration up to 99.9% on day zero. Pathogen cells regrew during refrigerated storage. Shippers and processors of tomatoes will be able to integrate this approach as a means to control Salmonella in storage and during ripening of tomatoes.

3. Minimizing quality loss of lettuce due to low-dose irradiation is minimal. Recent outbreaks of foodborne illness associated with fresh produce highlight the need for effective intervention technologies. Ionizing radiation is known to effectively eliminate human pathogens such as Escherichia coli O157:H7 on lettuce and other fresh produce. However, the commercial application of irradiation to fresh produce is still limited due to concerns about possible changes in product quality. ARS researchers at Wyndmoor, PA, found that appearance and texture of cut lettuce were not negatively affected by low dose irradiation of 1.0 kGy, providing the lettuce was stored in low oxygen film bags. The information will enable the produce industry to consider irradiation as an intervention technology to improve microbial safety of lettuce.

Review Publications
Jin, Z.T., Niemira, B.A. 2011. Application of polylactic acid coating with antimicrobials in reduction of Escherichia coli O157:H7 and Salmonella stanley on apples. Journal of Food Science. 76(3):184-188.

Keskinen, L.A., Annous, B.A. 2011. Sanitizer solutions containing detergents for inactivation of Escherichia coli O157:H7 on romaine lettuce. International Journal of Food Microbiology. DOI: 10.1016.

Fan, X. 2010. Commercialization of irradiation on fresh fruits and vegetables. In: Doona, C., Kustin, K, Feeherry, F., editors. Case studies in novel food processing technologies. Cambridge, UK: Woodhead Publishing. p. 427-446.

Guan, W., Huang, L., Fan, X. 2010. Acids in combination with sodium dodecyl sulfate caused quality deterioration of fresh-cut iceburg lettuce during storage in modified atmosphere package. Journal of Food Science. 75:S435-S440.

Sampedro, F., Rodrigo, D., Fan, X. 2010. High hydrostatic pressure processing of fruit juices and smoothies: research and commercial application. In: Doona,C., Kusting, K., Feeherry, F., editors. Case studies in novel food processing technologies. Cambridge, UK: Woodhead Publishing. p. 34-72.

Last Modified: 9/3/2015
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