Fresh-cut Iceberg Research Articles

Inactivation of Murine Norovirus 1, Coliphage φX174, and Bacteroides fragilis Phage B40-8 on Surfaces and Fresh-Cut Iceberg Lettuce by Hydrogen Peroxide and UV Light

Inactivation of Murine Norovirus 1, Coliphage φX174, and <i>Bacteroides fragilis</i> Phage B40-8 on Surfaces and Fresh-Cut Iceberg Lettuce by Hydrogen Peroxide and UV Light

Effects of Gamma Irradiation, Modified Atmosphere Packaging, and Delay of Irradiation on Quality of Fresh-cut Iceberg Lettuce

This study was conducted to investigate the effect of modified atmosphere packaging (MAP) and delay of irradiation application on the quality of cut Iceberg lettuce. Overall visual quality and tissue browning of cut lettuce were evaluated using a scale of 9 to 1, whereas texture was analyzed instrumentally during 14 days of storage at 4 °C. 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 as a result of lower O2 levels in the packages of irradiated samples compared with the levels in control packages. In general, irradiation at doses of 0.5 and 1.0 kGy did not affect firmness of the lettuce. After 14 days of MAP storage, overall visual quality of non-irradiated samples had a score of ≈4, a score below the limit of sales appeal, whereas the two irradiated samples had scores of 6.5 to 7.9, indicating the irradiated samples had a fair to good quality. Delaying irradiation by 1 day after preparation of cut lettuce did not significantly (P &gt; 0.05) affect cut edge browning, surface browning, or overall visual quality compared with lettuce irradiated immediately after preparation. Our results suggest that MAP is essential to minimize quality deterioration caused by irradiation.

Inactivation of murine norovirus 1, coliphage phiX174, and Bacteroides [corrected] fragilis phage B40-8 on surfaces and fresh-cut iceberg lettuce by hydrogen peroxide and UV light.

In this study, the inactivating properties of liquid hydrogen peroxide (L-H(2)O(2)), vaporized hydrogen peroxide (V-H(2)O(2)), UV light, and a combination of V-H(2)O(2) and UV light were tested on murine norovirus 1 (MNV-1) and bacteriophages (φX174 and B40-8) as models for human noroviruses. Disinfection of surfaces was examined on stainless steel discs based on European Standard EN 13697 (2001). For fresh-produce decontamination, a mixture of the viruses was inoculated onto shredded iceberg lettuce and treated after overnight incubation at 2°C. According to our results, L-H(2)O(2) (2.1%) was able to inactivate MNV-1 and φX174 on stainless steel discs by approximately 4 log(10) units within 10 min of exposure, whereas for B40-8, 15% of L-H(2)O(2) was needed to obtain a similar reduction in 10 min. Only a marginal reduction (≤1 log(10) unit after 5 min of exposure) by V-H(2)O(2) (2.52%) was achieved for the tested model viruses, although in combination with UV light, a 4-log(10)-unit decrease within 5 min of treatment was observed on stainless steel discs. Similar trends were observed for the decontamination of shredded iceberg lettuce, but the viral decline was reduced. These results demonstrated that both L-H(2)O(2) and a combination of V-H(2)O(2) and UV light can be used for norovirus inactivation on surfaces; V-H(2)O(2) (2.52%) in combination with UV light is promising for decontamination of fresh produce with much less consumption of water and disinfectant.

From the Editors

From the Editors

Modeling logistic performance in quantitative microbial risk assessment.

In quantitative microbial risk assessment (QMRA), food safety in the food chain is modeled and simulated. In general, prevalences, concentrations, and numbers of microorganisms in media are investigated in the different steps from farm to fork. The underlying rates and conditions (such as storage times, temperatures, gas conditions, and their distributions) are determined. However, the logistic chain with its queues (storages, shelves) and mechanisms for ordering products is usually not taken into account. As a consequence, storage times-mutually dependent in successive steps in the chain-cannot be described adequately. This may have a great impact on the tails of risk distributions. Because food safety risks are generally very small, it is crucial to model the tails of (underlying) distributions as accurately as possible. Logistic performance can be modeled by describing the underlying planning and scheduling mechanisms in discrete-event modeling. This is common practice in operations research, specifically in supply chain management. In this article, we present the application of discrete-event modeling in the context of a QMRA for Listeria monocytogenes in fresh-cut iceberg lettuce. We show the potential value of discrete-event modeling in QMRA by calculating logistic interventions (modifications in the logistic chain) and determining their significance with respect to food safety.

Suitability of aqueous chlorine dioxide versus sodium hypochlorite as an effective sanitizer for preserving quality of fresh-cut lettuce while avoiding by-product formation

Aqueous chlorine dioxide (ClO 2) has been postulated as an alternative to sodium hypochlorite (NaClO) for fresh-cut produce sanitization with the advantage of avoiding the risks associated with chlorination by-products. However, little is known about its influence on preserving quality and the potential formation of trihalomethanes (THMs) under typical processing conditions. The suitability of aqueous chlorine dioxide (3 mg L −1) as an effective sanitizer of fresh-cut iceberg lettuce stored under active modified atmosphere packaging (MAP) at refrigerated conditions was determined and compared with sodium hypochlorite (100 mg L −1). Fresh-cut lettuce washed with tap water was used as a control. The epiphytic microbiota were characterized by the evaluation of the major relevant microbial groups such as mesophiles, psychrophiles, Pseudomonas spp., Enterobacteriaceae, lactic acid bacteria, yeasts and moulds. Additionally, gas composition, sensory quality, vitamin C and individual and total phenolics were monitored after washing and during storage for 3 d at 4 °C followed by 7 d at 8 °C. In general, the natural microbiota of fresh-cut lettuce after washing and storage was equally affected by the different washing solutions, with the exception of yeasts which showed the highest growth after 10 d storage in samples washed with chlorine dioxide. None of the tested washings negatively affected sensory quality, which was acceptable after 10 d storage. Additionally, the content of bioactive compounds was not significantly affected either by washing solution or by storage time. The potential formation of THMs was evaluated by the analysis of lettuce washed in water with a chemical oxygen demand (COD) of 700 mg L −1 treated for 30 min with sodium hypochlorite (100 mg L −1) or chlorine dioxide (3.7 mg L −1). Trihalomethane formation was only detected in the process water in which sodium hypochlorite was applied (217 ± 38 μg L −1). However, THMs formation in fresh-cut lettuce was negligible despite the sanitation procedure. The formation of THMs was only detected in fresh-cut lettuce when sodium hypochlorite was used under very extreme conditions where lettuce was washed in water with a high level of organic matter (COD = 1800 mg L −1), high sodium hypochlorite concentration (700 mg L −1) and long contact time (60 min). Our data suggest that aqueous chlorine dioxide is as suitable as sodium hypochlorite for fresh-cut lettuce sanitation with the advantage of preventing the formation of THMs.

Efficacy of oregano oil in the inactivation of Salmonella typhimurium on lettuce

This study was conducted to determine the efficacy of oregano oil in the inactivation of Salmonella typhimurium inoculated onto iceberg lettuce. The effect of washing with oregano oil ( Oreganum onites), typical of Turkey, at three different concentrations (25, 40 and 75 ppm) and four different treatment times (5, 10, 15 and 20 min) on survival of S. typhimurium inoculated to fresh cut iceberg lettuce were determined at 20 °C and compared with a 50 ppm chlorine wash at the same conditions. The spot and dip inoculation methods for applying inoculum to iceberg and romaine type lettuce were evaluated and the effects of selective and nonselective media on the recovery of S. typhimurium were also examined. Populations of S. typhimurium recovered from spot and dip inoculated lettuce were not significantly different ( p > 0.05). No significant differences were found between two different media, and two different lettuce types ( p > 0.05). Reductions of S. typhimurium by washing with oregano did not exceed 1.92 logarithmic units regardless of the washing times and concentrations. The effectiveness of washing lettuce with 75 ppm oregano oil on inactivation of S. typhimurium was comparable with that affected by 50 ppm chlorine. The results suggested that oregano oil might be a suitable decontamination alternative to chlorine for lettuce.

Microbial antagonists of Escherichia coli O157:H7 on fresh-cut lettuce and spinach.

Fresh-cut lettuce and spinach can become contaminated with pathogens at numerous points from the field to the retail market. Natural microflora present on fresh produce may help reduce the pathogen load. The objective of this study was to isolate natural microflora from fresh-cut iceberg lettuce and baby spinach and to determine whether these bacteria were antagonistic toward Escherichia coli O157:H7. Samples were collected under conditions that mimicked actual practices between production and retail sale. Evidence of naturally occurring microorganisms on fresh lettuce (295 isolates) and spinach (200 isolates) and of possible antagonistic activity toward E. coli O157:H7 was documented. Inhibitory activity by several isolates was due to either acid production or antimicrobial peptides. Bacteria with inhibitory activity were isolated from every step in the processing and handling of the fresh-cut iceberg lettuce and baby spinach.

Effectiveness of bacteriophages in reducing Escherichia coli O157:H7 on fresh-cut cantaloupes and lettucet.

Consumption of produce contaminated with Escherichia coli O157:H7 has resulted in cases of foodborne illness. We determined the efficacy of a mixture of three E. coli O157:H7-specific bacteriophages (ECP-100) in reducing the number of viable E. coli O157:H7 on contaminated fresh-cut iceberg lettuce and cantaloupe. E. coli O157:H7 was spot inoculated on lettuce pieces (9 cm2) with a population of 3.76 log CFU/cm2, allowed to dry, and then sprayed with a control (phosphate-buffered saline) or ECP-100 to deliver 7.98 log PFU/cm2 to lettuce stored for 2 days at 4 degrees C. Cut pieces of cantaloupe were spot inoculated with E. coli O157:H7 (4.55 log CFU/ml) and treated with the control or ECP-100 (6.69 log PFU/ml), and then stored at 4 or 20 degrees C for up to 7 days. On days 0, 2, 5, and 7, cantaloupe samples were homogenized, and populations of E. coli O157:H7 were enumerated. Populations of E. coli O157:H7 on lettuce treated with ECP-100 on 0, 1, and 2 days (0.72, <0.22, and 0.58 log CFU/cm2 of lettuce) and stored at 4 degrees C were significantly (P < 0.05) lower than those treated with the control (2.64, 1.79, and 2.22 log CFU/cm2), respectively. Populations on cut cantaloupes treated with ECP-100 on days 2, 5, and 7 (0.77, 1.28, and 0.96 log CFU/ml) and stored at 4 degrees C were significantly lower than those cut cantaloupes treated with the control (3.34, 3.23, and 4.09 log CFU/ml), respectively. This study is the first to show the effectiveness of bacteriophages to reduce E. coli O157:H7 on fresh-cut lettuce and cantaloupes.

Optimization and evaluation of a decontamination step with peroxyacetic acid for fresh-cut produce

Since several disadvantages are associated with the use of sodium hypochlorite as a decontamination agent, the attention for alternative agents such as peroxyacetic acid (PAA) is increasing. In this study the effectiveness of PAA to remove the native microflora was tested in four types of fresh-cut vegetables: grated carrots, fresh-cut white cabbage, iceberg lettuce and leek. Furthermore, the influence of varying PAA concentrations (0, 25, 80, 150 and 250 ppm) and varying contact times (1, 5 and 10 min) was described by means of a linear model. The efficiency of PAA to remove the native flora was highly dependent on the type of fresh-cut produce: the highest microbial reductions were obtained for carrots (0.5–3.5 log cfu/g) and white cabbage (0.5–3.5 log cfu/g) followed by iceberg lettuce (0.4–2.4 log cfu/g). The obtained efficiency was the lowest for fresh-cut leek (0.4–1.4 log cfu/g). Furthermore, all the treated samples, regardless of the type of vegetable and the contact time and concentration of the PAA treatment, were acceptable for consumption.

Moderate and high doses of sodium hypochlorite, neutral electrolyzed oxidizing water, peroxyacetic acid, and gaseous chlorine dioxide did not affect the nutritional and sensory qualities of fresh-cut Iceberg lettuce (Lactuca sativa Var. capitata L.) after washing.

Besides the traditionally used sodium hypochlorite (20 and 200 mg L(-1)), alternative sanitizers such as peroxyacetic acid (80 and 250 mg L(-1)) and neutral electrolyzed oxidizing water (4.5 and 30 mg L(-1) free chlorine) as well as chlorine dioxide gas (1.54 mg L(-1)) were evaluated for their efficiency in reducing the microbial load of fresh-cut iceberg lettuce. An additional rinsing step with tap water and cooling of the sanitizing solutions, which are obvious for the fresh-cut industry, were not performed within the current study. The high doses of sodium hypochlorite and peroxyacetic acid tested within this study do not conform to the normally used concentrations within the fresh-cut industry. Neutral electrolyzed oxidizing water (30 mg L(-1)), peroxyacetic acid (250 mg L(-1)), and gaseous chlorine dioxide significantly reduced the total aerobic plate count of cut lettuce in comparison with water wash treatments alone. None of the treatments significantly affected the sensory quality of the lettuce, although small color changes were observed after colorimetric measurements. From a nutritional point of view water rinsing significantly decreased the vitamin C (maximum 35%) and phenol (maximum 17%) contents, but did not affect the carotenoid and α-tocopherol contents. Additional effects caused by adding a sanitizer to the wash water were not observed for vitamin C and phenols. Conversely, washing with 250 mg L(-1) peroxyacetic acid reduced the β-carotene content by about 30%, whereas using 200 mg L(-1) sodium hypochlorite reduced both the lactucaxanthin and the lutein contents by about 60%. Use of gaseous chlorine dioxide also had an impact on the lutein content (-18%). Furthermore, the α-tocopherol content was reduced by 19.7 and 15.4% when the two concentrations of neutral electrolyzed oxidizing water were used, respectively. These data represent the situation on day 0. In a next phase, shelf-life studies considering microbial and sensory quality and nutrient content should be conducted.

Respiration rate and ethylene production of fresh cut lettuce as affected by cutting grade

For designing optimal polymeric films for modified atmosphere packaging of whole heads as well as for minimally fresh processed (fresh-cut) Iceberg lettuce ‘Coolguard’, the effect of several cutting grades on respiration rate (RR) and ethylene production at 5ºC was studied. According to common industrial practices cutting grades less than 0.5 cm, between 0.5 and 1 cm, and 2 cm length were selected. Results from four experiments were compared to those obtained for whole heads in which a homogenous range of 6 to 8 ml CO2 kg-1 h-1 in RR was found. Compared to whole heads, in fresh-cut lettuce the RR was 2-fold higher. The lowest cutting grade showed the highest respiration rate, and no significant differences in RR among lettuce pieces of intermediate and the highest grades were found. No ethylene production was detected in whole heads, while in minimally processed lettuce pieces only traces were found. For avoiding risks of anaerobic respiration and excessive CO2 levels within packages containing fresh-cut lettuce pieces lower than 0.5 cm length, films with relatively high O2 permeability like standard polypropylene or low-density polyethylene must be selected.;

Effectiveness of organic acid, ozonated water and chlorine dippings on microbial reduction and storage quality of fresh‐cut iceberg lettuce

The comparative effects of organic (citric and lactic) acids, ozone and chlorine on the microbiological population and quality parameters of fresh-cut lettuce during storage were evaluated. Dipping of lettuce in 100 mg L(-1) chlorine solution reduced the numbers of mesophilic and psychrotrophic bacteria and Enterobacteriaceae by 1.7, 2.0 and 1.6 log(10) colony-forming units (CFU) g(-1) respectively. Treatment of lettuce with citric (5 g L(-1)) and lactic (5 mL L(-1)) acid solutions and ozonated water (4 mg L(-1)) reduced the populations of mesophilic and psychrotrophic bacteria by 1.7 and 1.5 log(10) CFU g(-1) respectively. Organic acid dippings resulted in lower mesophilic and psychrotrophic counts than ozonated water and chlorine dippings during 12 days of storage. Lactic acid dipping effectively reduced (by 2.2 log(10) CFU g(-1)) and maintained low populations of Enterobacteriaceae on lettuce for the first 6 days of storage. No significant (P > 0.05) changes were observed in the texture and moisture content of lettuce samples dipped in chlorine, organic acids and ozonated water during storage. Colour, β-carotene and vitamin C values of fresh-cut iceberg lettuce did not change significantly (P > 0.05) until day 8. Lactic and citric acid and ozonated water dippings could be alternative treatments to chlorine dipping to prolong the shelf life of fresh-cut iceberg lettuce. Copyright © 2007 Society of Chemical Industry.

Growth and bacteriocin production by lactic acid bacteria in vegetable broth and their effectiveness at reducing Listeria monocytogenes in vitro and in fresh-cut lettuce

The fresh-cut fruit and vegetable industry is searching for alternatives to replace chemical treatments with biopreservative approaches that ensure the safety of the product and fulfil consumer preferences for minimally processed foods. In this study, the use of bacteriocins produced by lactic acid bacteria has been tested as a substitute for chemical disinfection of fresh-cut iceberg lettuce. First, the ability of several non-plant origin bacteriocinogenic strains (nisin Z +, plantaricin C +, lacticin 481 +, coagulin + or pediocin PA-1 +) to grow in a lettuce extract at 4 °C, 10 °C and 32 °C was tested. All strains were able to grow, but bacteriocin production was predominantly detected at 32 °C. Addition of bacteriocinogenic supernatants (nisin +, coagulin + and a nisin–coagulin + cocktail) to tryptic-soy agar plates inoculated with Listeria monocytogenes reduced Listeria counts by approximately 1–1.5 log units compared with the control plates without bacteriocin, after 48 h of storage at 4 °C. The effect of washing with bacteriocin-containing solutions on survival and proliferation of Listeria monocytogenes was also evaluated in fresh-cut lettuce packaged in macro-perforated polypropylene bags and stored for 7 days at 4 °C. Washing fresh-cut lettuce with these solutions decreased the viability of Listeria monocytogenes by 1.2–1.6 log units immediately after treatment, but, during storage at 4 °C, bacteriocin treatments only exerted minimal control over the growth of the pathogen. Natural microbiota were little affected by bacteriocins during storage.

Modeling changes of sensory attributes for individual and mixed fresh-cut leafy vegetables

Changes of the main sensory attributes of three fresh-cut leafy vegetables (Iceberg and Romaine lettuce and chicory) were investigated at selected temperatures (2–20 °C). The aim of this work was to develop and apply a kinetic approach to model sensory quality changes in order to establish the appropriate function that describes the time–temperature dependence of each attribute. The changes of the sensory characteristics followed first order reaction kinetics and the temperature dependence of rate constants followed the Arrhenius relationship. The limiting quality factor, which determined the sensory shelf life for the three fresh-cut vegetables assayed, at any temperature, was the general appearance of the products. The activation energies obtained for general appearance were 71.1 kJ mol −1 for fresh-cut Iceberg lettuce, 69.5 kJ mol −1 for fresh-cut Romaine lettuce and 65.7 kJ mol −1 for fresh-cut chicory. Additional experimental tests showed that the predicted and experimental sensory shelf life for individual fresh-cut vegetables at constant temperature were not different ( P > 0.05). Under dynamic temperature conditions (sequence of different temperatures), the predicted and experimental values of browning, wilting, and off-odour were also not different ( P > 0.05), but the general appearance loss model overestimated the quality loss from 10 to 30%. The models of quality change for individual vegetables were used to predict the sensory shelf life of fresh-cut mixed vegetables. The experimental validation tests proved that these models provide a good approach to evaluate the shelf life of the mixed product. Results showed that the general appearance of fresh-cut Iceberg and Romaine lettuce dominated the sensory perception of mixed product.

(50) Effect of Delayed MA Packaging and Packaging Film on Quality Maintenance of Fresh-cut Iceberg Lettuce

Modified atmosphere (MA) packaging has been used to maintain quality of fresh-cut lettuce, but O2 depletion and excessive CO2 accumulation can be injurious. This study was conducted to evaluate the effect of delayed MA packaging using three different films on quality maintenance of fresh-cut iceberg lettuce. Iceberg lettuce leaves were sliced, washed, dried, and placed for 0, 5, 10, and 15 hours under air atmosphere at 5 °C. Samples were packaged with 2.9 and 16.6 pmol·kg·Pa·s, oxygen transmission rate (OTR) films and microperforated film (P-plus), and stored for 12 days at 5 °C. Results indicated that delayed packaging affected gas composition (O2 and CO2), fermentative volatile production, off-odor, color, and tissue electrolyte leakage. With 2.9 OTR film, longer delayed time maintained quality by inhibiting off-odor development and the accumulation of ethanol. However, delayed packaging was not effective in maintaining quality of samples packaged with 16.6 OTR film and P-plus film. Regardless of delayed time, all 16.6 OTR-packaged samples had severe discoloration after 7 days of storage. All delayed treatments packaged with P-plus film attained the desired O2 (3–5 kPa) and CO2 levels (10–12 kPa) from day 9 to the end of storage. A 15-hour delayed packaging into packages prepared from 2.9 OTR film and samples packaged with P-plus film maintained quality by inhibiting off-odor development. The results indicate that delayed packaging could be an alternative method when fresh-cut lettuce is packaged with low OTR films.

Ozonated water extends the shelf life of fresh-cut lettuce.

The use of ozonated water as a sanitizer to extend the shelf life of fresh-cut lettuce and the effect on the antioxidant constituents (polyphenols and vitamin C) were investigated. Fresh-cut iceberg lettuce (Lactuca sativa L.) was washed at 4 degrees C using three different ozonated water dips [10, 20, and 10 activated by ultraviolet C (UV-C) light mg L(-1) min total ozone dose], and the dips were compared with water and chlorine rinses. Treated lettuce was packaged in air or active modified atmosphere packaging (MAP) (4 kPa of O2 + 12 kPa of CO2 balanced with N2) and stored for 13 days at 4 degrees C. Despite its strong oxidizing activity, ozonated water did not stimulate the respiratory activity of fresh-cut lettuce. Moreover, ozonated water maintained the initial visual appearance of fresh-cut lettuce and controlled browning during storage in air. Initially, ozonated water and chlorine reduced the total mesophilic population by 1.6 and 2.1 log, respectively, when compared with water. Active MAP was effective in controlling total microbial growth, achieving 2.0 log reduction in relation to samples stored in air at the end of storage. On the other hand, active MAP caused a 2.0-3.5 reduction of coliforms on sanitized samples compared with water-washed samples. The most efficient treatments were ozone 20 and ozone 10 activated by UV-C, which were as effective as chlorine. Changes in individual phenolic compounds were independent of the washing treatments. In air, chlorogenic and isochlorogenic acid contents increased noticeably after 13 days while monocaffeoyltartaric and dicaffeoyltartaric acids remained unchanged. MAP effectively suppressed accumulation of caffeoylquinic derivatives, whereas caffeoyltartaric derivatives decreased during MAP storage to reach similar levels. The content of vitamin C (ascorbic acid and dehydroascorbic acid) decreased during storage, particularly under MAP. Ozonated water could be an alternative sanitizer to chlorine for fresh-cut lettuce due to good retention of sensorial quality and browning control with no detrimental reduction in the antioxidant constituents.

Growth of Listeria monocytogenes on iceberg lettuce and solid media

The growth of pathogenic bacterium Listeria monocytogenes on fresh-cut iceberg lettuce under constant temperatures was modelled in order to investigate microbial safety during distribution of this vegetable. We examined the effects of several incubation temperatures, ranging from 5 to 25 °C, on bacterial growth. These data were fitted to the Baranyi model and the curves showed a high correlation coefficient at all temperature ( R 2>0.95). In addition, the native bacterial flora of the lettuce did not affect the growth rate of L. monocytogenes regardless of incubation temperature. However, the lag time was reduced at a ratio of native bacteria to inoculated L. monocytogenes (100:1) at low incubation temperatures (5 and 10 °C). Furthermore, the maximum population density (MPD) was increased at a low ratio of native to inoculated L. monocytogenes (1:1) at all incubation temperatures. These results were compared with the previous work published by Buchanan et al. (1989) [Buchanan, R.L., Stahl, H.G., Whiting, R.C., 1989. Effects and interactions of temperature, pH, atmosphere, sodium chloride, and sodium nitrite on the growth of Listeria monocytogenes. J. Food Prot. 52, 844–851] that is being developed at the US Department of Agriculture (USDA) Agricultural Research Service's Pathogen Modelling Program (PMP). The broth-based Buchanan model for L. monocytogenes was found to markedly deviate from the observed data. In order to investigate this discrepancy, we examined the effects of medium environment and nutrient content on L. monocytogenes growth using tryptic soy agar plates (TSAP) and agar plates (AP) containing 1.7% sucrose. The inoculated bacteria on both TSAP and AP showed slower growth rates than that predicted by the PMP. The MPD of bacteria grown on TSAP was similar to the PMP model (∼9 log 10 CFU/ml or plate (circle of diameter of 90 mm)) regardless of the incubation temperature. By contrast, the MPD observed on AP was ∼4 log 10 CFU lower than that observed on TSAP or predicted by the PMP. Both the growth rate and the MPD of L. monocytogenes on AP were similar to those on lettuce. These results suggest that the solid medium and poor nutrient content inhibited the growth of L. monocytogenes on lettuce. The growth rates of the inoculated L. monocytogenes on all media were described using Ratkowsky's simple square root model.

Effect of 1-methylcyclopropene on phenylpropanoid metabolism, the accumulation of phenolic compounds, and browning of whole and fresh-cut ‘iceberg’ lettuce

Exposing whole heads or leaves of iceberg lettuce (Lactuca sativa L., iceberg) to 1-methylcyclopropene (1-MCP) significantly decreased the accumulation of phenolic compounds (absorbance of methanol tissue extract at 320 nm) and subsequent tissue discoloration induced by exposure to 1.0 μL L−1 ethylene in air at 5 °C. The 0.5 μL L−1 concentration of 1-MCP was just as effective as 1.0 μL L−1, and a 3-h exposure was just as effective as a 24-h exposure at 5 °C. In contrast, exposure to 1-MCP either before or after excision of mid-rib tissue did not interfere with the wound-induced increase in phenolic content of the tissue. It appears that wounding and ethylene act independently in the induction of phenylpropanoid metabolism and the accumulation of those phenolic compounds that contribute to browning of mechanically injured (e.g., fresh-cut) iceberg lettuce.

Survival of Campylobacter jejuni on various fresh produce

The survival of seven human and two chicken Campylobacter jejuni strains, with known Penner heat-stable (HS) serotypes and pulsed-field gel electrophoresis (PFGE) genotypes, was investigated on fresh-cut iceberg lettuce. In addition, the survival of four selected C. jejuni strains was assessed on cantaloupe pieces, cucumber slices, grated carrot and strawberries. Fresh produce was inoculated with 10 5 to 10 7 colony-forming units (CFU) of C. jejuni per gram, and the bacterium was enumerated using standard procedures after sample storage at 7 and 21 °C for 24, 48 and 72 h. The absolute values of the slopes (death rates) of the survival curves (log 10 CFU/g versus time) were calculated and compared. At 7 °C, the mean death rates (day −1) were 0.44 on cantaloupe, 0.41 on cucumber slices, 0.43 on grated carrot, 0.59 on iceberg lettuce and 1.02 on strawberries. The corresponding death rates (day −1) at 21 °C were 1.52, 1.55, 2.61, 1.39 and 8.74. The death rate of C. jejuni on strawberries (pH 3.4) was significantly ( P<0.05) higher than on other produce. Moreover, the death rate at 21 °C as compared with 7 °C was significantly higher ( P<0.05). Minor differences were observed in the survival of different C. jejuni strains. Our results suggest that after contamination of fresh produce, including strawberries, C. jejuni may survive sufficiently long to pose a risk to the consumer.