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Far-red (FR) light influences plant development significantly through shade avoidance response and photosynthetic modulation, but there is limited knowledge on how FR treatments influence the growth and nutrition of vegetables at different maturity stages in controlled environment agriculture (CEA). Here, we comprehensively investigated the impacts of FR on the yield, morphology, and phytonutrients of ruby streaks mustard (RS) at microgreen, baby leaf, and flowering stages. Treatments including white control, white with supplementary FR, white followed by singularly applied FR, and enhanced white (WE) matching the extended daily light integral (eDLI) of FR were designed for separating the effects of light intensity and quality. Results showed that singular and supplemental FR affected plant development and nutrition similarly throughout the growth cycle, with light intensity and quality playing varying roles at different stages. Specifically, FR did not affect the fresh and dry weight of microgreens but increased those values for baby leaves, although not as effectively as WE. Meanwhile, FR caused significant morphological change and accelerated the development of leaves, flowers, and seedpods more dramatically than WE. With regard to phytonutrients, light treatments affected the metabolomic profiles for baby leaves more dramatically than microgreens and flowers. FR decreased the glucosinolate and anthocyanin contents in microgreens and baby leaves, while WE increased the contents of those compounds in baby leaves. This study illustrates the complex impacts of FR on RS and provides valuable information for selecting optimal lighting conditions in CEA.

The process of washing tomatoes in dump (flume) tanks has been identified as a potential source of cross-contamination. This study's objective was to assess the potential for Salmonella entericacross-contamination at various inoculation levels in the presence of free chlorine (HOCl) and organic matter. Uninoculated tomatoes were introduced into a laboratory-based model flume containing tomatoes inoculated with a cocktail of five rifampin-resistant S. entericaserovars at 104, 106, or 108CFU per tomato in water containing 0 or 25 mg/L HOCl and 0 or 300 mg/L chemical oxygen demand (COD). Uninoculated tomatoes exposed to the inoculated tomatoes were removed from the water after 5, 30, 60, and 120 s and placed in bags containing tryptic soy broth supplemented with rifampin and 0.1% sodium thiosulfate. Following incubation, enrichment cultures were plated on tryptic soy agar supplemented with rifampin and xylose lysine deoxycholate agar to determine the presence of Salmonella. HOCl and pH were measured before and after each trial. The HOCl in water containing 300 mg/L COD significantly declined (P≤ 0.05) by the end of each 120-s trial, most likely due to the increased demand for the oxidant. Higher inoculum levels and lower HOCl concentrations were significant factors (P≤ 0.05) that contributed to increased cross-contamination. At 25 mg/L HOCl, no Salmonellawas recovered under all conditions from uninoculated tomatoes exposed to tomatoes inoculated at 104CFU per tomato. When the inoculum was increased to 106and 108CFU per tomato, cross-contamination was observed, independent of COD levels. The results from this study indicate that the currently required sanitizer concentration (e.g., 100 or 150 mg/L) for flume water may be higher than necessary and warrants reevaluation.

Fresh produce continues to be the main source of foodborne illness outbreaks in the United States, implicating bacterial pathogens such as Escherichia coliO157:H7 (EHEC). The efficacy of nanoemulsified carvacrol (NCR) as a washing treatment in reducing EHEC on fresh produce was investigated. Fresh baby spinach, romaine lettuce, and iceberg lettuce leaves (2.5-cm-diameter cores) were spot inoculated with a five-strain cocktail of nalidixic acid–resistant EHEC at ∼6 log CFU/cm2. After air drying for 1 h, 20 pieces of each inoculated produce leaf were immersed in water-based treatment solutions (200 mL per group), including water alone, 25 or 50 ppm of free chlorine, and 0.25 or 0.75% NCR for 2 min. Inoculated produce leaves without any treatment served as baseline. Produce leaves were stored at 10°C, and surviving EHEC populations were enumerated on days 0, 2, 7, and 14. The viability of EHEC following NCR treatments on the fresh produce was visualized under a fluorescence microscope. NCR treatment at 0.75% immediately reduced EHEC populations on iceberg lettuce by 1.3 log CFU/cm2as compared with the produce treated with water alone (P< 0.05). Antimicrobial activity of NCR against EHEC was comparable to chlorine treatments on day 0 for all produce (P> 0.05). After 14 days of storage at 10°C, populations of EHEC on 0.75% NCR-treated romaine lettuce were reduced by 2.3 log CFU/cm2compared with the recovery from 50 ppm of chlorine-treated samples (P< 0.05). Microscopic images revealed that EHEC cells were observed to be clustered on the baseline samples, indicating the development of cell aggregation, compared with the scattered cells seen on NCR-treated leaf surfaces. Treatments with NCR did not significantly affect the color of the fresh produce leaves during 14 days of storage at 10°C. Results of this study support the potential use of NCR as a water-soluble natural antimicrobial wash treatment for controlling EHEC on fresh produce.

Monitoring and maintenance of water quality in dump tanks or flume systems is crucial to maintaining proper sanitizer levels to prevent pathogen cross-contamination during postharvest washing of tomatoes, but there is limited information on how organic matter influences sanitizer efficacy in the water. The main objective of this study was to monitor water quality in flume tanks and evaluate the efficacy of postharvest washing of tomatoes in commercial packinghouses. Flume tank water samples (n= 3) were collected on an hourly basis from three packinghouses in Florida and analyzed for pH, total dissolved solids (TDS), free chlorine, chemical oxygen demand (COD), oxidation-reduction potential, and turbidity. Additionally, three flume-water samples were collected and tested for total aerobic plate count (APC), total coliforms (TC), and Escherichia coli. Fresh tomatoes (n= 3), both before and after washing, were collected and analyzed for the same bacterial counts. Turbidity, COD, and TDS levels in flume water increased over time in all packinghouses. Correlations observed include COD and turbidity (r= 0.631), turbidity and TDS (r= 0.810), and oxidation-reduction potential and chlorine (r= 0.660). APC for water samples had an average range of 0.0 to 4.7 log CFU/mL and TC average range of 0.0 to 4.7 log CFU/mL. All water samples were negative for E. coli. The average APC for pre- and postflume tomatoes from the three packinghouses was 6.0 log CFU per tomato and ranged from 2.2 to 7.4 log CFU per tomato. The average TC count was <1.5 and 7.0 log CFU per tomato for pre- and postwash tomatoes, respectively. There was no significant effect (P> 0.05) of postharvest washing on the microbiological qualities of tomatoes. Water quality in flume tanks deteriorated over time in all packinghouses during a typical operational day of 4 to 8 h.

Graphical abstract:

Fresh and fresh-cut tomatoes are high in phytonutrients. However, illness outbreaks associated with contaminated tomatoes have significantly impacted public health and the economic well-being of the tomato industry. Scientific information is needed to develop an effective, practical food safety standard to reduce pathogen contamination. The aim of this study was to assess factors impacting the deterioration of the quality of tomato wash water and the proliferation of indigenous microorganisms during a simulated dump tank washing process. Freshly harvested grape tomatoes were sorted into four groups: prime, defective, underripe, and nontomato debris. Tomatoes with leaf or stem harvest debris, combined or separate, were washed in tap water with or without free chlorine. Water samples were analyzed for total dissolved solids, turbidity, chemical oxygen demand, and chlorine demand. Microbial populations in water and on tomatoes as impacted by chlorine concentration and water filtration (300 μm) were also quantified. Field debris and defective tomatoes were the major contributors to microbial populations in wash water. Field debris, although accounting for <1% of the total weight of harvested material, contributed 37.84% of total dissolved solids, 46.15% of turbidity, 48.77% of chemical oxygen demand, and 50.55% of chlorine demand in the wash water. Water quality deterioration was proportional to the cumulative quantity of tomatoes and debris washed, and free chlorine at ≥5 mg/L significantly reduced the Enterobacteriaceae,aerobic mesophilic bacteria, and yeast and mold populations. These results highlight the importance of minimizing field debris and defective fruits in harvested grape tomatoes to reduce the microbial load and prevent deterioration of wash water quality. This information will be useful for the development of data-driven harvesting and packinghouse food safety practices for grape tomatoes.

Here, we report a novel, “dry”, nano-aerosol-based, antimicrobial technology using engineered water nanostructures (EWNSs) for leafy vegetable disinfection. These EWNS based nano-sanitizers are synthesized using a combined process of electrospray and ionization of aqueous solutions of generally recognized as safe active ingredients (AIs). A novel application of the EWNS to vegetable surfaces was carried out in this study. The EWNS-based nano-sanitizers were synthesized using an AI cocktail (10% hydrogen peroxide, 1% citric acid, 0.1% lysozyme and 0.0025% nisin), and their efficacy against foodborne pathogen surrogates and plant pathogens was assessed using a leafy vegetable model, namely spinach (Spinacia oleracea). The EWNSs were directed onto a coupon cut from spinach leaves, which were inoculated with microorganisms relevant to food safety, such as E. coli. It was shown that a 2 min exposure to EWNS-based nano-sanitizers significantly (P< 0.05) reduced the cell populations of Escherichia coli, Listeria innocua,and Pseudomonas fluorescenson spinach by 3.5, 1.2 and 2.6 log, respectively. The EWNS also reduced Penicillium italicumspores by 1 log in 15 minutes and bacteriophage MS2 by 1.4 log in 5 minutes. More importantly, the estimated delivered dose of AIs using the EWNS approach was minuscule (258 pg cm−2for a 15-minute exposure). No significant difference (P> 0.05) in color or surface pH of spinach was found after 15 minutes of exposure. In summary, EWNS based nano-sanitizers are highly effective against foodborne pathogen surrogates and plant pathogens using only nanograms of nature-derived active ingredients and without an impact on produce quality.

Lettuce (Lactuca sativaL.) is one of the most consumed vegetables in the world and different management practice can result in considerable variability of the secondary metabolites. Flow injection mass spectrometry (FIMS) combined with analysis of variance–principle component analysis (ANOVA–PCA) was used to study differences in the secondary metabolites originating from different lighting conditions (Sunlight, white light, and florescent light) and lettuce varieties (Romaine and Lollo Rossa). Ultra-high-performance liquid chromatography–high-resolution accurate mass spectrometry was used for putative marker compound identification. Quinic acid, caffeic acid, chlorogenic acid, L-chicoric acid, and quercetin malonyl glucoside varied significantly for Romaine lettuce grown under different light conditions. The study showed that the combination of FIMS fingerprinting and ANOVA–PCA can be a useful tool for the characterization of the sources of variance in plant materials regarding to genetic, environmental, and management factors.

Highly efficient antimicrobial agents with low toxicity and resistance have been enthusiastically pursued to address public concerns on microbial contamination in food. Silver nanoclusters (AgNCs) are known for their ultrasmall sizes and unique optical and chemical properties. Despite extensive studies of AgNCs for biomedical applications, previous research on their application as antimicrobials for food applications is very limited. Here, for the first time, by incorporating AgNCs (∼2 nm in diameter) into zein films that are widely used as food packaging materials, we developed a novel coating material with potent antimicrobial activity, low toxicity to human cells, and low potential to harm the environment. In addition, we systematically evaluated the antimicrobial activities and cytotoxicity of AgNCs-embedded zein films and compared them to zein films embedded with AgNO3or Ag nanoparticles with diameters of 10 and 60 nm (AgNP10 and AgNP60, respectively). At equivalent silver concentrations, AgNCs and AgNO3solutions exhibited considerably higher antimicrobial activities than those of AgNP10 and AgNP60 solutions. Moreover, AgNCs exhibited less cytotoxicity to human cells than AgNO3, with a half maximal inhibitory concentration (IC50) of 34.68 μg/mL for AgNCs, compared to 9.14 μg/mL for AgNO3. Overall, the novel AgNCs coating developed in this research has great potential for antimicrobial applications in food packaging materials due to its high antimicrobial efficacy, ultrasmall size, and low cytotoxicity.

AbstractCantaloupes, marketed as “Rocky Ford,” were implicated in the U.S. multistate outbreak of listeriosis in 2011, which caused multiple fatalities. Listeria monocytogenescan survive on whole cantaloupes and can be transferred to the flesh of melons. The growth of L. monocytogeneson fresh-cut “Athena” and “Rocky Ford” cantaloupe cultivars during refrigerated storage was evaluated. Fresh-cut cubes (16.4 cm3) from field-grown cantaloupes were each inoculated with 5 log10CFU/mL of a multi-strain mixture of L. monocytogenesand stored at 4°C or 10°C. Inoculated fresh-cut cubes were also: (1) continuously stored at 4°C for 3 days; (2) temperature-abused (TA: 25°C for 4 h) on day 0; or (3) stored at 4°C for 24 h, exposed to TA on day 1, and subsequently stored at 4°C until day 3. L. monocytogenespopulations on fresh-cut melons continuously stored at 4°C or 10°C were enumerated on selected days for up to 15 days and after each TA event. Brix values for each cantaloupe variety were determined. L. monocytogenespopulations on fresh-cut cantaloupe cubes stored at 4°C increased by 1.0 and 3.0 log10CFU/cube by day 7 and 15, respectively, whereas those stored at 10°C increased by 3.0 log10CFU/cube by day 7. Populations of L. monocytogeneson fresh-cut cantaloupes stored at 10°C were significantly (p< 0.05) greater than those stored at 4°C during the study. L. monocytogenesshowed similar growth on fresh-cut “Athena” and “Rocky Ford” cubes, even though “Athena” cubes had significantly higher Brix values than the “Rocky Ford” fruit. L. monocytogenespopulations on fresh-cut cantaloupes exposed to TA on day 1 and then refrigerated were significantly greater (0.74 log10CFU) than those stored continuously at 4°C for 3 days. Storage at 10°C or exposure to TA events promoted growth of L. monocytogeneson fresh-cut cantaloupe during refrigerated storage.

Radish (Raphanus sativusvar. longipinnatus)microgreens were produced from seeds inoculated with Escherichia coliO157:H7 by using peat moss-based soil-substitute and hydroponic production systems. E. colipopulations on the edible and inedible parts of harvested microgreen plants (7 days postseeding) and in growth medium were examined. E. coliO157:H7 was shown to survive and proliferate significantly during microgreen growth in both production systems, with a higher level in the hydroponic production system. At the initial seed inoculation level of 3.7 log CFU/g, E. coliO157:H7 populations on the edible part of microgreen plants reached 2.3 and 2.1 log CFU/g (overhead irrigation and bottom irrigation, respectively) for microgreens from the soil-substitute production system and reached 5.7 log CFU/g for those hydroponically grown. At a higher initial inoculation of 5.6 log CFU/g seeds, the corresponding E. coliO157:H7 populations on the edible parts of microgreens grown in these production systems were 3.4, 3.6, and 5.3 log CFU/g, respectively. Examination of the spatial distribution of bacterial cells on different parts of microgreen plants showed that contaminated seeds led to systematic contamination of whole plants, including both edible and inedible parts, and seed coats remained the focal point of E. coliO157:H7 survival and growth throughout the period of microgreen production.

Effective cold chain management is a critical component of food safety practice. In this study, we examined the impact of commonly encountered temperature abuse scenarios on the proliferation of Salmonella entericaand Listeria monocytogeneson fresh-cut cantaloupe. Inoculated fresh-cut cantaloupe cubes were subjected to various temperature abuse conditions, and the growth of S. entericaand L. monocytogeneswas determined. During 1 week of storage, Salmonellacell counts on fresh-cut cantaloupe increased by –0.26, 1.39, and 2.23 log units at 4°C (control), 8°C, and 12°C (chronic temperature abuse), respectively, whereas that of L. monocytogenesincreased by 0.75, 2.86, and 4.17 log units. Under intermittent temperature abuse conditions, where storage temperature fluctuated twice daily to room temperature for 30 min, Salmonellacell count increased by 2.18 log units, whereas that of L. monocytogenesincreased by 1.86 log units. In contrast, terminal acute temperature abuses for 2 to 4 h resulted in upwards to 0.6 log unit for Salmonella,whereas the effect on L. monocytogeneswas less significant compared with L. monocytogeneson cut cantaloupe stored at 4°C. Significant deterioration of produce visual quality and tissue integrity, as reflected by electrolyte leakage, was also observed under various temperature abuse conditions.

We have developed a two-step replica molding method for rapid fabrication of biomimetically patterned plant surfaces (BPS) using polydimethylsiloxane (PDMS-BPS) and agarose (AGAR-BPS). Beyond providing multiple identical specimens that faithfully reproduce leaf surface microstructure, this approach also offers unique chemical, physical, and biological features. PDMS-BPS provide good structural durability for SEM examination, have surface wettability comparable to plant surfaces for coating development, and allow for real-time monitoring of biosynthesis through incorporation into microfluidic devices. AGAR-BPS are compatible with bacterial growth, recovery, and quantification, and enable investigation of the effects of surface topography on spatially varying survival and inactivation of Escherichia colicells during biocide treatment. Further development and application of these biomimetically patterned surfaces to study (and possibly modify) other aspects of plant–bacteria interactions can provide insight into controlling pathogen contamination in a wide range of applications.

The dynamic interactions of chlorine and organic matter during a simulated fresh-cut produce wash process and the consequences for Escherichia coliO157:H7 inactivation were investigated. An algorithm for a chlorine feed-forward dosing scheme to maintain a stable chlorine level was further developed and validated. Organic loads with chemical oxygen demand of 300 to 800 mg/liter were modeled using iceberg lettuce. Sodium hypochlorite (NaOCl) was added to the simulated wash solution incrementally. The solution pH, free and total chlorine, and oxidation-reduction potential were monitored, and chlorination breakpoint and chloramine humps determined. The results indicated that the E. coliO157:H7 inactivation curve mirrored that of the free chlorine during the chlorine replenishment process: a slight reduction in E. coliO157:H7 was observed as the combined chlorine hump was approached, while the E. coliO157:H7 cell populations declined sharply after chlorination passed the chlorine hump and decreased to below the detection limit (<0.75 most probable number per ml) after the chlorination breakpoint was reached. While the amounts of NaOCl required for reaching the chloramine humps and chlorination breakpoints depended on the organic loads, there was a linear correlation between NaOCl input and free chlorine in the wash solution once NaOCl dosing passed the chlorination breakpoint, regardless of organic load. The data obtained were further exploited to develop a NaOCl dosing algorithm for maintaining a stable chlorine concentration in the presence of an increasing organic load. The validation tests results indicated that free chlorine could be maintained at target levels using such an algorithm, while the pH and oxidation-reduction potential were also stably maintained using this system.

Salmonellainternalization in tomatoes during postharvest handling is a major food safety concern. This study was conducted to determine the effect of immersion time, immersion depth, and temperature differential between bacterial suspension and tomato pulp on the internalization of Salmonella entericain tomato fruits. The effect of storage temperature and duration on the survival and growth of internalized Salmonellacells was also evaluated. Overall, immersion time significantly affected the incidence and extent of S. entericainternalization (P< 0.0001), with a linear correlation between immersion time and Salmonellainternalization. The depth of Salmonellainternalization in tomato tissues also increased with increasing immersion time. Immersion time also significantly influenced the degree to which the temperature differential affected Salmonellainternalization. With an immersion time of 2 min, the temperature differential had no significant effect on Salmonellainternalization (P= 0.2536). However, with an immersion time of 15 min, a significantly larger Salmonellapopulation became internalized in tomatoes immersed in solutions with a −30°F (−16.7°C) temperature differential. Internalized S. entericacells persisted in the core tissues during 14 days of storage. Strain type and storage duration significantly affected (P< 0.05) both the frequency detected and the population of internalized Salmonellarecovered, but storage temperatures of 55 to 70°F (12.8 to 21.1°C) did not (P>0.05). These findings indicate the importance of preventing pathogen internalization during postharvest handling.

This study evaluated the dynamic effects of free-chlorine (FC) concentration, contact time, and organic load on the inactivation of Salmonella, Escherichia coliO157:H7, and non-O157 Shiga toxin–producing E. coli(STEC) in suspension. Bacterial cells from four strains each of Salmonella, E. coliO157:H7, and non-O157 STEC were inoculated separately or as a multistrain cocktail into solutions with varying FC concentrations. Lettuce or tomato extract was used to simulate the organic matter present during commercial fresh and fresh-cut produce wash operations. After exposure to FC for various lengths of time, the bacterial survival and water-quality changes were determined. In the absence of organic matter in a wash solution, pathogen inactivation is primarily a function of initial FC concentration (P< 0.0001), exposure time (P< 0.0001), and pathogen strains (P< 0.0001). In general, an over 4.5-log CFU/ml pathogen reduction was found after exposure to >0.5 mg/liter FC for over 30 s, or to >1.0 mg/liter FC for over 5 s. When the combination of FC concentration and contact time were less than or equal to the above conditions, survival of pathogens was strain dependant and ranked as: Salmonella>E. coliO157:H7 > non-O157 STEC. When organic matter was present in the wash solution, pathogen inactivation efficacy was specifically dependent on the residual FC concentration, which directly relates to both the initial FC concentration and the organic load. Prevention of pathogen survival in chlorinated produce wash solutions can be achieved by maintaining sufficient FC concentration and reducing the accumulation of organic matter.

This study was undertaken to examine the effect of ultrasound in combination with chlorine on the reduction of Escherichia coliO157:H7 populations on lettuce coring knives. Two new coring devices designed to mitigate pathogen attachment were fabricated and evaluated. The coring rings of the knives were dip inoculated with soil slurry containing 106E. colicells and treated with chlorinated water with and without ultrasonication for 30, 60, and 120 s. The rough welding joints on currently used in-field lettuce coring knives provided a site conducive to bacterial attachment and resistant to cell removal during sanitation treatment. The two modified coring knives harbored significantly fewer E. colicells than did the currently used commercial model, and the efficacy of the disinfection treatment was high (P< 0.05). Ultrasound treatment reduced the E. coliO157:H7 counts to below the detection limit of 1.10 log CFU/cm2at both the coring ring blade and welding joint within 30 s in 1 ppm of chlorinated water. The redesigned coring knives and an ultrasound plus chlorine combination treatment may provide practical options for minimizing the microbial safety hazards of lettuce processed by core-in-field operations.

Tomatoes have been implicated in salmonellosis outbreaks due to possible contamination through bacterial internalization during postharvest handling. This study was conducted to determine the effects of tomato variety, temperature differential between tomato pulp and bacterial suspension, and the time delay between stem removal and immersion in bacterial suspension on internalization of Salmonella entericaserovar Thompson in tomato fruit. Mature green tomatoes at 32.2°C were immersed in water containing approximately 106CFU/ml S. entericabacteria. Different tomato varieties (Mountain Spring, Applause, and BHN961), temperature differentials (–10, 0, and 10°F, or –5.6, 0, and 5.6°C, respectively), and post-stem removal times (0, 2, and 16 h) were evaluated for their effects on S. entericainternalization. The incidence and density of internalized cells were determined by culture enrichment and most-probable-number methods, respectively. Overall, variety and post-stem removal time by variety interaction significantly affected the incidence of S. entericainternalization (P< 0.0001), while temperature differential had no significant effect (P= 0.36). Mountain Spring tomatoes were less susceptible to S. entericainternalization than were Applause and BHN961. Increasing the time interval between stem removal and immersion greatly reduced pathogen internalization in BHN961 and Applause, while it had no effect in Mountain Spring tomatoes. The variety and interactions between varieties and post-stem removal times (P= 0.0363) and between temperature differentials and post-stem removal times (P= 0.0257) had significant effects on the populations of internalized S. enterica.Furthermore, all internalized S. entericacells were found within the core tissue segments immediately underneath the stem scars.

Fresh-cut leafy greens contaminated with Escherichia coliO157:H7 have caused foodborne outbreaks. Packaging conditions, coupled with abusive storage temperatures of contaminated lettuce, were evaluated for their effect on the potential virulence of E. coliO157:H7. Shredded lettuce was inoculated with 5.58 and 3.98 log CFU E. coliO157:H7 per g and stored at 4 and 15°C, respectively, for up to 10 days. Lettuce was packaged under treatment A (modified atmosphere packaging conditions used for commercial fresh-cut produce, in gas-permeable film with N2), treatment B (near–ambient air atmospheric conditions in a gas-permeable film with microperforations), and treatment C (high-CO2and low-O2conditions in a gas-impermeable film). E. coliO157:H7 populations from each treatment were determined by enumeration of numbers on MacConkey agar containing nalidixic acid. RNA was extracted from packaged lettuce for analysis of expression of virulence factor genes stx2, eae, ehxA, iha,and rfbE. E. coliO157:H7 populations on lettuce at 4°C under all treatments decreased, but most considerably so under treatment B over 10 days. At 15°C, E. coliO157:H7 populations increased by at least 2.76 log CFU/g under all treatments. At 15°C, expression of eaeand ihawas significantly greater under treatment B than it was under treatments A and C on day 3. Similarly, treatment B promoted significantly higher expression of stx2, eae, ehxA,and rfbEgenes on day 10, compared with treatments A and C at 15°C. Results indicate that storage under near–ambient air atmospheric conditions can promote higher expression levels of O157 virulence factors on lettuce, and could affect the severity of E. coliO157:H7 infections associated with leafy greens.

This study was conducted to investigate the effect of free chlorine concentrations in wash water on Escherichia coliO157:H7 reduction, survival, and transference during washing of fresh-cut lettuce. The effectiveness of rewashing for inactivation of E. coliO157:H7 on newly cross-contaminated produce previously washed with solutions containing an insufficient amount of chlorine also was assessed. Results indicate that solutions containing a minimum of 0.5 mg/liter free chlorine were effective for inactivating E. coliO157:H7 in suspension to below the detection level. However, the presence of 1 mg/liter free chlorine in the wash solution before washing was insufficient to prevent E. coliO157:H7 survival and transfer during washing because the introduction of cut lettuce to the wash system quickly depleted the free chlorine. Although no E. coliO157:H7 was detected in the wash solution containing 5 mg/liter free chlorine before washing a mix of inoculated and uninoculated lettuce, low numbers of E. coliO157:H7 cells were detected on uninoculated lettuce in four of the seven experimental trials. When the prewash free chlorine concentration was increased to 10 mg/liter or greater, no E. coliO157:H7 transfer was detected. Furthermore, although rewashing newly cross-contaminated lettuce in 50 mg/liter free chlorine for 30 s significantly reduced (P= 0.002) the E. coliO157:H7 populations, it failed to eliminate E. coliO157:H7 on lettuce. This finding suggests that rewashing is not an effective way to correct for process failure, and maintaining a sufficient free chlorine concentration in the wash solution is critical for preventing pathogen cross-contamination.

This study investigated the effect of storage temperature and time on the survival and growth of Escherichia coliO157:H7, the growth of indigenous microorganisms, and the changes in product quality of packaged baby spinach. Commercial packages of spinach within 2 days of processing were cut open at one end, sprayed with fine mists of E. coliO157:H7 inoculum, resealed, and then stored at 1, 5, 8, and 12°C for 12 days until their labeled best-if-used-by dates. Microbial enumeration and product quality evaluation were conducted on day(s) 0, 3, 6, 9, and 12 postinoculation. Spinach held at 12°C supported significant (P< 0.001) E. coliO157:H7 growth, with a 1.0-log CFU/g increase within 3 days postinoculation, which was followed by additional growth during continued storage. E. coliO157:H7 grew slowly when held at 8°C, with a significant (P< 0.01) level of growth reached after 6 days of storage. However, on products held at 1 and 5°C, E. coliO157:H7 populations declined significantly (P< 0.01 and P< 0.001, respectively) within 3 days of storage. Aerobic mesophilic bacteria, psychrotrophic bacteria, and yeast and mold populations increased significantly at all storage temperatures, with more growth on products held at elevated temperatures. Product quality scores remained high within the first 6 days of storage, with a sharp decline noted on samples held at 12°C on day 9. Results suggest that E. coliO157:H7 can grow significantly on commercially packaged spinach held at 8°C or above before significant product quality deterioration occurs.

Washing produce with sanitizing solutions is an important step in reducing microbial populations during postharvest handling. Little information exists regarding the effects of washing solution flow conditions on the efficacy of pathogen reduction during washing. This study was undertaken to investigate the effects of washing conditions such as flow velocity, agitation rate, and contact time on the reduction of Escherichia coliO157:H7 populations from the surfaces of cantaloupe rind and cut apples. Top surfaces of cylindrical samples were spot inoculated with E. coliO157:H7 and treated with peroxyacetic acid (POAA; 80 mg/liter) solution under different flow velocities and agitation rates and with different washing modes. Test results indicate that the reduction rate of E. coliO157:H7 increased with the increase in flow velocity and agitation rate under the testing conditions. In a 3-min treatment in the flow-through chamber, the E. coliO157:H7 count reduction on cantaloupe rind and cup apples reached 2.5 and 2.3 log CFU/cm2, respectively, when the flow velocity increased from 0.0 to 0.8 m/min. Agitation conducted at the bottom of the treatment chamber reduced the E. coliO157:H7 population on cut apples by 1.2 log CFU/cm2in 3 min, whereas in the treatment with the agitation over the top of the chamber, the survival count of E. coliO157:H7 was reduced by only 0.8 log CFU/cm2. The experimental data were used to fit four microbial reduction kinetic models. It was found that E. coliO157:H7 reduction from the fruit surfaces was best described by the Weibull model. These findings may be useful in designing produce wash systems for achieving enhanced pathogen reduction and improved produce quality and safety.

Fresh-cut tissues are subjected to severe injury during preparation that leads to increased respiratory activity and quality deterioration. Modified atmosphere packaging (MAP) has been used to maintain quality of fresh-cut produce, but O2 depletion and excessive CO2 accumulation can be injurious. This study was conducted to evaluate the effect of delayed packaging and MAP using two different oxygen transmission rate (OTR) films on quality maintenance and shelf stability of fresh-cut romaine lettuce (Lactuca sativa L.). Romaine lettuce leaves were cut, washed, dried, and placed for 0,4,8, and 12 hours at 5 °C in ambient air before packaging. Fresh-cut samples were placed into packages prepared from films having OTRs of 8.0 and 16.6 pmol·s-1·m-2·Pa-1, flushed with N2 to reach an initial headspace O2 level of 1.5 kPa O2, and stored at 5 °C for up to 14 days. Delayed packaging affected gas composition, fermentative volatile production, off-odor development, color, CO2 injury, and tissue electrolyte leakage. With increasing delay before packaging, fermentative volatile production, off-odor development, and CO2 injury progressively decreased and discoloration increased. The modified atmospheres obtained with 16.6 OTR film increased discoloration when present, and generally had less off-odor development and CO2 injury compared to MAP with 8.0 OTR film. Delayed packaging affected overall quality of fresh-cut romaine lettuce packaged with both films. A 12-hour delayed packaging into packages prepared from 8.0 OTR film maintained quality by inhibiting CO2 injury, off-odor development, and tissue electrolyte leakage. However, an 8-hour delayed packaging into packages prepared from 16.6 OTR film was better at maintaining the quality of fresh-cut romaine lettuce at 5 °C for 14 days. The results indicated that delayed packaging could be an alternative method to optimize or balance package O2 during suboptimal OTR film packaging conditions. [ABSTRACT FROM AUTHOR]

Chlorine is widely used as a sanitizer to maintain the microbial quality and safety of fresh-cut produce; however, chlorine treatment lacks efficacy on pathogen reduction, especially when the fresh-cut processing water contains heavy organic loads. A more efficacious sanitizer that can tolerate the commercial processing conditions is needed to maintain microbial safety of fresh-cut produce. This study evaluated the efficacy of Escherichia coliO157:H7 reduction on fresh-cut carrots using new and traditional sanitizers with tap water and fresh-cut processing water scenarios. Fresh-cut carrot shreds inoculated with E. coliO157:H7 were washed in sanitizer solutions including 200 ppm chlorine, citric acid–based sanitizer (Pro-San), 80 ppm peroxyacetic acid-based sanitizer (Tsunami 100), and 1,000 ppm acidified sodium chlorite (SANOVA) prepared in fresh tap water or simulated processing water with a chemical oxygen demand level of approximately 3,500 mg/liter. Samples were packaged and stored at 5°C. Microbial analyses performed at days 0, 7, and 14 indicate that the organic load in the process water significantly affected the efficacy of chlorine on pathogen removal and was especially evident on samples tested during storage. Acidified sodium chlorite provided a strong pathogen reduction even under process water conditions with up to a 5.25-log reduction when compared with the no-wash control. E. coliO157:H7 was not recovered on acidified sodium chlorite–treated samples during the entire 14 days of storage, even following an enrichment step. These results suggest that acidified sodium chlorite holds considerable promise as an alternative sanitizer of fresh-cut produce.

A lettuce outbreak strain of E. coliO157:H7 was used to quantitate the pathogen's survival in ground beef and its transfer to hands, cutting board surfaces, and lettuce. Overnight storage of inoculated beef at 4°C resulted in no pathogen growth, while room-temperature storage allowed multiplication. Hamburger patty formation allowed the transfer of bacteria to hands. Contaminated fingers subsequently transferred the pathogen to lettuce during handling. E. coliwas transferred from hamburgers to cutting board surfaces; overnight storage of boards decreased the numbers of recoverable pathogens by ~1 log CFU. A 15-s water rinse failed to remove significant numbers of pathogens from cutting boards whether it was applied immediately after contamination or following overnight room-temperature storage. Three lettuce leaves were successively applied to a single contaminated cutting board area both immediately after contamination and after overnight room-temperature storage of contaminated boards. Another set of leaves was pressed onto boards immediately following contamination and was then stored overnight at 4°C before pathogen enumeration. The numbers of pathogens transferred to the first pressed leaves were larger than those transferred to the second or third leaves. There were no significant differences in the numbers of pathogens recovered from leaves pressed immediately after contamination whether pathogens were enumerated immediately or following overnight storage at 4°C. However, fewer pathogens were transferred to leaves pressed to boards stored overnight at room temperature prior to contact with lettuce. Twenty-five lettuce pieces were successively pressed onto one area on a board containing 1.25 × 102CFU of E. coli. Pathogens were transferred to 46% of the leaves, including the 25th exposed leaf.

Active food packaging materials that are sustainable, biodegradable, and capable of precise delivery of antimicrobial active ingredients (AIs) are in high demand. Here, we report the development of novel enzyme- and relative humidity (RH)-responsive antimicrobial fibers with an average diameter of 225 ± 50 nm, which can be deposited as a functional layer for packaging materials. Cellulose nanocrystals (CNCs), zein (protein), and starch were electrospun to form multistimuli-responsive fibers that incorporated a cocktail of both free nature-derived antimicrobials such as thyme oil, citric acid, and nisin and cyclodextrin-inclusion complexes (CD-ICs) of thyme oil, sorbic acid, and nisin. The multistimuli-responsive fibers were designed to release the free AIs and CD-ICs of AIs in response to enzyme and RH triggers, respectively. Enzyme-responsive release of free AIs is achieved due to the degradation of selected polymers, forming the backbone of the fibers. For instance, protease enzyme can degrade zein polymer, further accelerating the release of AIs from the fibers. Similarly, RH-responsive release is obtained due to the unique chemical nature of CD-ICs, enabling the release of AIs from the cavity at high RH. The successful synthesis of CD-ICs of AIs and incorporation of antimicrobials in the structure of the multistimuli-responsive fibers were confirmed by X-ray diffraction and Fourier transform infrared spectrometry. Fibers were capable of releasing free AIs when triggered by microorganism-exudated enzymes in a dose-dependent manner and releasing CD-IC form of AIs in response to high relative humidity (95% RH). With 24 h of exposure, stimuli-responsive fibers significantly reduced the populations of foodborne pathogenic bacterial surrogates Escherichia coli(by ∼5 log unit) and Listeria innocua(by ∼5 log unit), as well as fungi Aspergillus fumigatus(by >1 log unit). More importantly, the fibers released more AIs at 95% RH than at 50% RH, which resulted in a higher population reduction of E. coliat 95% RH. Such biodegradable, nontoxic, and multistimuli-responsive antimicrobial fibers have great potential for broad applications as active and smart packaging systems.