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Ultrasound-Assisted Surface Decontamination of Fresh Produce to Reduce Microbial Safety Risk

Feng, Hao
University of Illinois - Urbana-Champaign
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The long-term goal of the proposed research is to explore the use of power ultrasound to enhance the microbial safety and minimize the food safety risk of fresh produce.

This proposal requests funding for a four-year period to implement the following specific objectives:

  1. To test ultrasound and sanitizer combined treatments with selected produce in a small wash tank in batch mode and in a pilot scale continuous ultrasonic washing system to examine the effect of sanitizer, washing time, and produce-to-sanitizer solution ratio on microbial reduction.
  2. To conduct experiments to examine the effect of ultrasound on the quality of produce.
  3. To conduct experiments on the combination of surfactants with sanitizer and ultrasound on the microbial count reduction, which will be based on surfactant screening and HLB values. Also, emphasis will be given to the effect of sanitizer + surfactant + ultrasound on produce quality.
  4. To investigate the distribution of acoustic energy in the washing tank and find means to improve the uniformity.
More information
The recent outbreaks of Escherichia coli O157:H7 and Listeria infections due to consumption of produce reaffirmed the importance and challenge of produce microbial safety. These outbreaks and recalls have brought significant economic losses to the produce industry, and more importantly, recurring produce-related outbreaks erode consumer confidence in fresh produce and could jeopardize the long term development of the produce industry. Currently, commercial operations rely on a wash treatment with antimicrobials, often with chlorine, as the only step to reduce microbial populations on fresh produce. However, chlorinated water containing 50-100 mg/L free chlorine can only achieve a 90% to 99% reduction in microbial population in an industrial scale operation. Obviously, more effective postharvest intervention technologies are needed for assuring the microbial safety of fresh and fresh-cut produce. Several physical approaches have been evaluated for produce pathogen reduction. Some (UV) failed due to lack of efficacy, while others (plasma and irradiation) degrade product quality. Use of ultrasound in produce decontamination is relatively recent, with only a few publications reporting its use in combination with sanitizers to reduce microbial populations. Some studies have used single-frequency ultrasound to decontaminate selected fruit and vegetables. Mixed results have been reported; with some authors concluding that 90% of additional reduction was achieved, while others reported no additional reduction. Recent studies conducted by our lab strongly suggest that ultrasound's microbial reduction efficacy for produce can be significantly improved with higher acoustic energy density and more uniform exposure. A key problem in traditional ultrasonic treatment of food and nonfood products is field nonuniformity. Measures to improve the treatment uniformity in sonication are therefore important. We have shown that ultrasonic washing in a tank with a selected combination of surfactant and sanitizer can improve microbial reduction compared to ultrasound and sanitizer alone. However, product quality was less than satisfactory. Efforts will be made to identify a surfactant that significantly enhances ultrasonic washing performance consistent with produce quality. Anionic and nonionic surfactants will be tested for removing feces and other surface contaminants from lettuce. Hydrophilic and lipophilic balance (HLB) values in the range of 7 to 18 will cover good wetting agents and detergents with good penetrating capacity. Surfactant concentration will be controlled at just above the critical micelle concentration (CMC) to maximize surface tension reduction. Temperature will be maintained just below the cloud point, at which surfactant solution separates into a very dilute aqueous surfactant phase and surfactant-rich micellar phase. It is expected that a significant improvement in the efficacy of fresh produce sanitation will be achieved by carefully studying the combination of ultrasonication, surfactant, and low concentration sanitizers. This will contribute to minimize the microbial food safety risk and fresh produce.

1. Test organisms: In the experiments, a nalidixic acid-resistant derivative of E. coli O157:H7, strain 87-23 (non-pathogenic) will be adopted, which was obtained from the former Produce Quality and Safety Laboratory, USDA-ARS (Beltsville, MD). The cells in tryptic soy agar (TSA) slant will be transferred 3 times to tryptic soy broth (pH 7.3) by loop inoculation at successive 24-h intervals followed by incubation at 37C. Bacterial cells will be harvested, after 24 h of growth, by centrifugation (6000 X g) at 4C for 10 min. The cell pellets will be washed twice in peptone water, and resuspended in 10 mL of peptone water. 2 Preparation of treatment solutions: The concentration of sodium chlorite will be determined by a method developed by Alcide Corporation (Redmond, WA). Acidic electrolyzed water (AEW; 80 mg/L) will be generated using an AEW generator (ROX-20TA) and collected from the anode of the generator with sanitized beakers. The pH and oxidation reduction potential (ORP) of the AEW will be measured with an AR15 pH and ORP meter (Accumet Research), and the residual chlorine concentration will be determined using an EPA-approved chlorine colorimetric test kit. POAA (80 mg/L; Tsunami 100) will be prepared according to the manufacturer's instruction. 3 Quality analysis: Selected produce samples will be placed in a MMM (variable frequency) tubular reactor for the treatment. A sample holder will be used to hold cut produce samples. The treated produce samples will be packaged with film OTR (oxygen transmission rate) of 8.0 pmolO2 s−1m−2 Pa−1, and stored three weeks at 1C. At days 0, 7, 14, and 21, the produce will be sampled and tested for the quality evaluation through the instruments and visual inspection of sensory panelists. A Minolta CR-300 Chroma Meter (Minolta Corporation, Ramsey, NJ) will be used to assess changes in produce color using the CIELAB system. Firmness of the samples will be determined using a Kramer Shear Press with 5 blades (TA-91) attached to a TA-XT2 Texture Analyzer (Texture Technologies Corporation). The electrolyte leakage of the treated fresh produce will be measured immediately after treatment and during storage to determine possible tissue deterioration. 4 Computer simulation of the acoustic field in an ultrasonic washing tank: A baseline model will be built with grid generated with Abacus CAE. The symmetry of the tank will be utilized so only 1/4 of the tank will be used as the analysis domain. The analysis will be performed by Abaqus/Analysis which is the core module of Abaqus. The output database can be visualized with Abaqus/CAE dynamically. In data post-processing, a MATLAB program will be used to get the frames, node coordinates, and pressures to generate 3D figures of the pressure distribution. The acoustic field distribution produced by computer simulation will be validated with selected measurement methods.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Natural Toxins
Viruses and Prions
Bacterial Pathogens
Chemical Contaminants
Escherichia coli