Microbial Safety, Quality and Shelf-Life of Fresh Produce and Other Ready-to-Eat Foods

<P>NON-TECHNICAL SUMMARY: The overall goal of this five year project is to enhance the microbial safety and quality of ready-to-eat, fresh-cut produce with three segments in the fresh produce chain being specifically targeted, namely processing, packaging, and retail distribution. In the first of these objectives, a series of laboratory as well as pilot-plant based studies will be conducted to quantify the extent of cross-contamination that occurs during simulated commercial and retail slicing/dicing of fresh fruits and vegetables as related to the various parameters of both the process (e.g., cutting force, friction, knife blade design/material) and the product (e.g., texture, bulk density, water, cellulose, lignin). Dr. Bradley Marks will then use these findings to develop and test a generalized mathematical model for pathogen attachment/transfer/internalization during slicing/dicing of fresh produce which we will then validate in a series of independent experiments using other types of fresh produce and slicing/dicing conditions. Since commercially diced produce is normally conveyed and washed before packaging, the impact of various single and/or combined treatments (e.g., chlorine, peroxyacetic acid, electrolyzed water, ozone sonication) on pathogen reduction during washing and conveying will also be assessed by several collaborators from other universities using our unique pilot-scale processing line for fresh-cut fruits and vegetables (e.g., lettuce, tomatoes, onions). Thereafter, optimal packaging systems will be developed to enhance microbial safety and quality of the types of fresh-cut produce being investigated with assistance from Dr. Eva Almenar from the School of Packaging. Based on a wealth of both in-transit and in-store time/temperature data that was previously collected across the country in conjunction with several other USDA grants led by Dr. Keith Vorst at Cal Poly, we will also will assess the growth of E. coli O157, Salmonella, L. monocytogenes, background bacteria, and yeasts/molds in the same packaged products from the previous objective under real-world conditions encountered during commercial transport, retail storage and display using a programmable incubator to mimic these same time/temperature fluctuations during periods of temperature abuse. Overall, these efforts will lead to the development of 1) improved practices for handling fresh-cut produce at both the processing and retail levels, and 2) improved risk assessments for fresh-cut produce that will help in guiding future government decisions in relation to the Food Safety Modernization Act. </P>
<P>APPROACH: Objective 1. Quantify the impact of different product, process and equipment parameters on the transfer of various foodborne pathogens during pilot plant-scale production of fresh-cut fruits and vegetables. Experimental design: A range of fruits and vegetables of varying composition and structure (density, cellulose and lignin content, surface texture), including those previously associated with foodborne outbreaks and/or recalls (e.g. leafy greens, tomatoes, onions, celery, cantaloupe among others) will be surface-inoculated with multi-strain cocktails of E. coli O157:H7, Salmonella or L. monocytogenes and then processed to mimic foodservice or large-scale commercial practices. Manual countertop slicers and dicers will be used to duplicate foodservice practices; whereas the MSU pilot-scale commercial processing line which includes an Urschel shredder and dicer, step conveyor, various types of roller conveyors, a 3.3-m long recirculating flume tank for washing, mechanical shaker table and centrifugal dryer will be used to duplicate large-scale commercial practices. Objective 2. Evaluate a range of chemical and physical strategies for reducing pathogenic and spoilage organisms on fresh-cut fruits and vegetables during simulated commercial production in a pilot-scale processing line. Experimental design. Various fruits and vegetables will again be surface-inoculated with different pathogens and subjected to pilot-plant-scale processing as described in Objective 1. While Objective 1 focused on quantifying the extent of bacterial transfer in the absence of chemical sanitizers, Objective 2 will assess the efficacy of a range of commonly used chemical sanitizers (e.g., chlorine, peroxyacetic acid, ozone, chlorine dioxide, electrolyzed oxidizing water) in combination with different physical treatments (e.g., sonication, pulsed UV light, and light activated photocatalytic nanocoatings) for inactivation of various foodborne pathogens (e.g., E. coli O157:H7, Salmonella, L. monocytogenes, Clostridium difficile, viruses) on fresh-cut fruits and vegetables during simulated commercial production using the MSU pilot-scale processing line. Objective 3. Evaluate a Portable Multi-use Automated Concentration System (PMACS) for concentrating pathogens from centrifugation water using a pilot-scale leafy green and tomato/citrus processing line containing wash water of various organic loads.Experimental design. Recovery of E. coli O157:H7, L. monocytogenes and Salmonella Typhimurium from flume tank and centrifugation water containing 0, 2.5, 5, 7.5 and 10% (w/v) homogenized lettuce solids will be assessed with the Portable Multi-use Automated Concentration System (PMACS) after processing 50 lbs. (22.7 kg) batches of iceberg lettuce simultaneously inoculated with all three pathogens at 104,102 and 100 CFU/g, using a chlorine-based sanitizer with sanitizer-free water serving as the control (n = 90). Objective 4. Assess the impact of the physiological state of pathogens on their survival on fresh-cut produce during processing and subsequent storage under simulated time/temperature profiles encountered during transport and retail storage/displayExperimental design. The physiological state of a bacterium (e.g., healthy, sublethally injured, viable-but-non-culturable, dead) influences its ability to adapt and survive new stresses and persist in foods. The goal of this project is to use gene expression data to identify the physiological state of EHEC and L. monocytogenes on pre-inoculated lettuce plants during harvest, processing, transport, and retail storage/display (Figure 1). Lettuce plants grown at North Dakota State University (NDSU) in a growth chamber under the same temperature and light conditions as those found in Salinas Valley will be inoculated with EHEC or L. monocytogenes. After 5 days of assimilation under these same conditions, the lettuce will be harvested, sent to MSU and subjected to flume washing, shaker table dewatering and centrifugal drying using the pilot-scale leafy green processing line. Four different wash water conditions will be used: water alone, 50 ppm chlorine, 50 ppm peroxyacetic acid, and 50 ppm with T-128. Objective 5. Develop optimal packaging systems for enhanced microbiological safety and quality of fresh-cut produce.Experimental design: Prototype packages will be developed for different types of fresh-cut produce based on modified atmosphere composition, package dimensions and material permeability. After inoculation with E. coli O157:H7, Salmonella, L. monocytogenes or C. difficile, the products will be packaged and analyzed for levels of pathogenic and spoilage bacteria, respiration rate, color, firmness, weight loss, and off-flavors at 3 to 5 day intervals during 3 weeks of storage at 4 and 10 °C The equilibrium gas concentration (steady-state condition) of the different packages will be determined by headspace analysis using gas chromatography. Up to three sensory analyses (uninoculated products) using consumer panels (n = 75) will be conducted in the MSU Sensory Evaluation Laboratory using uninoculated, pathogen-free control samples. Objective 6. Assess and model the growth of pathogens, mesophilic aerobic bacteria, and yeasts/molds in packaged fresh-cut produce during transport, retail storage and display using previous collected time/temperature histories. Experimental design. Different time/temperature histories collected from previous work which represents varying degrees of abuse during product transport, retail storage and display will be uploaded to a Blue M Environmental Chamber (FRS-13B; All-Temp Engineering, San Jose, CA) at MSU for product incubation. In triplicate experiments, inoculated and uninoculated packages of fresh-cut produce from the prior objectives will be evaluated at pre-determined intervals for growth of the target pathogen with mesophilic aerobic bacteria, psychrotrophic bacteria, lactic acid bacteria, yeast, and mold quantified according to standard methods while the environmental chamber cycles through temperature profiles matching the actual profiles collected from the retail study. </P>