Quality and Safety of Fresh-Cut Vegetables and Fruits

Non-Technical Summary:<br/>
Consumer demand for fresh food has stimulated the production of fresh produce and made it one of the fastest growing market sectors in the United States. Unfortunately, the increase in production and consumption has also been accompanied by a simultaneous increase in the reported number of foodborne disease outbreaks. Between 1996 and 2005, leafy green consumption increased 9.0% and leafy green associated outbreaks increased 38.6%. Foodborne illness outbreaks erode consumer confidence in the safety of fresh produce, and have adverse economic, social, and health consequences. Currently, commercial operations rely on a wash treatment with antimicrobials as the only step to reduce microbial populations on fresh produce. However, an industrial-scale operation that uses chlorinated water containing 50-200 mg/L free chlorine can only achieve a 1 to 2-log CFU/g reduction in bacterial population. As many published studies have demonstrated the health benefits of eating fresh produce, strategies to improve the microbial safety of fresh produce are clearly needed. In fresh produce antimicrobial treatments, or in any industrial and medical sanitation operation, an outstanding issue that has puzzled the industry and academia is the significant difference in killing efficacy of a sanitizer when applied to planktonic bacterial cells compared to cells adhering to a surface. With commonly used sanitizers, such as chlorine, peroxyacetic acid, and acidified sodium chlorite, it is easy to achieve an over 5-log reduction in pathogenic bacteria count or even a complete elimination in a time scale of less than one minute when the cells are suspended in a liquid. However, when the bacterial cells are attached to a surface, the inactivation efficacy with the same sanitizer is greatly reduced. Although many sanitizer-related studies have attempted to increase the inactivation efficacy of a wash operation, the root cause for such a discrepancy in inactivation has not been systematically investigated. There is a lack of fundamental understanding of the interactions among bacteria, produce, sanitizer, and washing solution hydrodynamics. Such knowledge is indispensable for the development of effective produce microbial safety. In this proposed study, we will conduct experiments to 1) evaluate methods of sampling and measuring flavor and nutrition of fresh-cut products to facilitate comparison to traditional shelf life factors; 2) develop new strategies to improve and better maintain inherent fresh-cut product quality and nutrition; 3) improve understanding of physiological mechanisms that affect fresh-cut product quality; 4) determine critical factors in controlled inoculation studies with human pathogens and surrogates that influence the outcome of quantitative microbial risk assessments; and 5) evaluate and control unintentional and intentional microbial contamination of intact and fresh-cut produce. This will provide insights for scientists, the produce industry, and consumers to develop improved understanding of issues on microbial safety of fresh produce.
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Approach:<br/>
The microfabrication of artificial produce surfaces will be conducted at the Micro-Nano-Mechanical Systems Cleanroom at University of Illinois. Deep reactive ion etching (DRIE) and photolithography techniques will be used. Confocal laser scanning microscopy (WITec alpha) in the Laser and Spectroscopy Facility at Material Research Laboratory of University of Illinois will be used to take a series of 2-D images (100 X 100 micro mater) by optically slicing the produce sample surfaces. Artificial surfaces will be used to examine the effect of temperature and air humidity on attachment of E. coli O157:H7. One feature will be selected to represent cantaloupe and another for spinach surface, with surface roughness and hydrophobicity measured using the above-mentioned methods. A nalidixic acid-resistant derivative of E. coli O157:H7, strain 87-23 (nonpathogenic) will be used. For the total population of E. coli cells on produce samples, 25 g of inoculated produce will be rinsed with sterile distilled water for 1 min, stomached in 225 mL of 0.1% (w/v) sterile peptone water for 2 min at 230 rpm with a stomacher blender, and E. coli cells in the homogenate will be determined using standard plating method with trypticase soy agar containing 50 mg/L nalidixic acid and the MPN method. The artificial produce surfaces will first be used to examine the attachment of E. coli O157:H7. The temperature and air humidity will be fixed and the focus will be on examining the effects of surface roughness, hydrophobicity, and cell size, geometry, and opening on the attachment of E. coli cells. The artificial surfaces with coatings for 3 hydrophobicity levels will be used. GFP-labeled E. coli O157:H7 will be visualized with a confocal laser scanning microscope. To examine the cell internalization, the self-contained X-ray microtomography system will include MicroXCT-400 and NanoXCT-100. New sanitizer compositions will be developed by introducing new compound(s) or utilizing currently available sanitizers in combination with new chemicals or surfactants. The inactivation tests will first be conducted in buffer and the inactivation kinetics will be obtained. Inactivation tests with selected produce will also be conducted with the most effective sanitizer composition(s). Physical methods, including ultrasound will be evaluated and developed in conjunction with the chemical sanitation methods. The design of ultrasonic cleaning chamber will pay attention to the acoustic field distribution which can be simulated with computer software.
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Progress:<br/>
2012/01 TO 2012/12<br/>
OUTPUTS: A simple pattern involving raised circular areas was used to test the procedure for making artificial plant surface. The design AutoCAD file was printed onto a soda lime chrome mask plate, which was then used to make a photoresist-coated silicon wafer. Polydimethylsiloxane (PDMS) molded on the wafer and allowed to solidify produced PDMS films with the desired pattern. E. coli K12 cells were inoculated on both the PDMS film and lettuce surface to observe the survival of E. coli. Two-hour survival of E. coli on PDMS was similar to that on spinach. Cells inoculated on PDMS film can be removed with a stomacher without damaging the film, which is also undamaged by autoclaving. The contact angle of the PDMS film was 108-110 degrees, so that the film is hydrophobic. Future work is needed to make the pattern mimic the surface topology of real produce, and to modify surface hydrophobicity and chemistry to match those of produce.
<br/>PARTICIPANTS: Nothing significant to report during this reporting period.
<br/>TARGET AUDIENCES: Nothing significant to report during this reporting period.
<br/>PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
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IMPACT: The polydimethylsiloxane (PDMS) file having a designed pattern can be used as a useful tool to conduct microbial attachment and removal tests. The PDMS artificial leaf surfaces will provide a platform free of the natural sample-to-sample variation due to produce surface topology and surface chemistry, and will thus allow us correlate produce surface characteristics, sanitizer solution chemistry, and bacterial population reduction.