Properties of Pseudomonads Causing Spoilage of Vegetables Stored at Low Temperature

Use of a Novel Sanitizer To Inactivate Salmonella Typhimurium and Spoilage Microorganisms during Flume Washing of Diced Tomatoes

ABSTRACT

As demand for fresh-cut produce increases, minimizing the risk of salmonellosis becomes critical for the produce industry. Sanitizers are routinely used during commercial flume washing of fresh-cut produce to minimize cross-contamination from the wash water. This study assessed the efficacy of a novel sanitizer blend consisting of peracetic acid (PAA; OxypHresh 15) with a sulfuric acid-surfactant (SS) antimicrobial (PAA-SS; ProduceShield Plus) against Salmonella during simulated commercial washing of diced tomatoes. Triplicate 9.1-kg batches of Roma tomatoes were dip inoculated in a two-strain avirulent Salmonella cocktail (Salmonella Typhimurium LT2 and MHM112) to achieve 5 to 6 log CFU per tomato and air dried for 2 h. After mechanical dicing, the tomatoes were washed in a pilot-scale processing line for 60 s with or without an added organic load in 90 ppm of PAA-SS (pH 1.8), SS at pH 1.8, 90 ppm of PAA, 5 or 10 ppm of free chlorine or sanitizer-free water as the control. Overall, PAA-SS (1.75 ± 0.75 log CFU/g) was significantly (P ≤ 0.05) more effective than water (0.69 ± 0.42 log CFU/g), chlorine (0.35 ± 0.36 log CFU/g), or SS (0.36 ± 0.19 log CFU/g) in reducing Salmonella. After washing for 20 s, PAA-SS was the only sanitizer to show a significant (P ≤ 0.05) reduction (1.93 ±0.59 log CFU/g) in Salmonella. All wash water samples were negative for Salmonella, except for 5 and 10 ppm of chlorine and the water control. Using PAA-SS with an organic load, yeast and mold populations were below the limit of detection (1.40 log CFU/g) and significantly (P ≤ 0.05) lower on diced tomatoes after 14 days of refrigerated storage compared with the other treatments (8.37 ± 0.08 log CFU/g), with SS at pH 1.8 (3.91 ± 0.93 log CFU/g) most effective against yeast and mold in the absence of an organic load. On the basis of these findings, the safety and shelf life of commercially washed diced tomatoes can be improved with PAA-SS.

HIGHLIGHTS

Control of spoiler Pseudomonas spp. on fresh cut vegetables by neutral electrolyzed water

In the present study, we evaluated the antimicrobial activity of neutral electrolyzed water (NEW) against 14 strains of spoilage Pseudomonas of fresh cut vegetables under cold storage. The NEW, produced from solutions of potassium and sodium chloride, and sodium bicarbonate developed up to 4000 mg/L of free chlorine, depending on the salt and relative concentration used. The antimicrobial effect of the NEW was evaluated against different bacterial strains at 10(5) cells/ml, with different combinations of free chlorine concentration/contact time; all concentrations above 100 mg/L, regardless of the salt used, were found to be bactericidal already after 2 min. When catalogna chicory and lettuce leaves were dipped for 5 min in diluted NEW, microbial loads of mesophilic bacteria and Enterobacteriaceae were reduced on average of 1.7 log cfu/g. In addition, when lettuce leaves were dipped in a cellular suspension of the spoiler Pseudomonas chicorii I3C strain, diluted NEW was able to reduce Pseudomonas population of about 1.0 log cfu/g. Thanks to its high antimicrobial activity against spoilage microorganisms, and low cost of operation, the application of cycles of electrolysis to the washing water looks as an effective tool in controlling fresh cut vegetable microbial spoilage contamination occurring during washing steps.

Microbiological quality and shelf life modeling of ready-to-eat cicorino

Growth of the microbial population resident in ready-to-use fresh cut cicorino, a variety of Chichorium intybus, was determined in the various preparation steps with the aim of defining the hazard critical control points. The investigation concerned cut cicorino from two producers. During the process a 1- to 1.5-fold increase of microbial counts was observed, and the retail product showed values of 10(5) to 10(6) CFU/g. The shelf life of the product was kinetically modeled in order to check the effects of storage temperature and assess the microbial indexes most relevant for hygiene and quality of the distributed product. A modified Gompertz function described the kinetics of microbial growth and allowed definition of a stability time and its dependence on temperature. Stability times were of 0.3, 3.7, and 4.7 days at storage temperatures of 20, 10, and 5 degrees C, respectively. Q10 (the fold decrease of stability time for an increase of 10 degrees C) was 3.85. The results from this study may be used to predict the effects of temperatures experienced in the distribution chain on bacterial levels in cicorino.

Effect of temperature on the inhibition kinetics of phenol biodegradation by Pseudomonas putida Q5

The temperature-dependent performance of mixed-culture wastewater treatment processes may be strongly influenced by their content of psychrotrophic bacteria. In this work, the effect of temperature on cell growth and phenol biodegradation kinetics of the psychrotrophic bacterium Pseudomonas putida Q5 were determined using both batch and continuous cultures in the range of 10-25 degrees C. The Haldane equation was found to be the most suitable substrate-inhibition model for the specific growth rate. The Haldane parameters mu(max) and K(I) were best modeled by a square-root dependency on temperature. However, the Arrhenius model provided a better prediction of the temperature dependence of K(S). The variation of the yield constant with temperature also was studied experimentally. Comparisons with results of previous workers are presented.

Production of pectate lyases and cellulases by Chryseomonas luteola strain MFCL0 depends on the growth temperature and the nature of the culture medium: evidence for two critical temperatures

Several extracellular enzymes that are responsible for plant tissue maceration were detected in culture supernatant of the psychrotrophic bacterium Chryseomonas luteola MFCL0. Isoelectrofocusing experiments showed that pectate lyase (PL) activity resulted from the cumulative action of three major isoenzymes, designated PLI, PLII, and PLIII. Cellulolytic activity was also detected in culture supernatants. These enzymes exhibited different behaviors with respect to growth temperature. PLII was not regulated by temperature, whereas PLI and PLIII were regulated similarly by growth temperature. Maximal levels of PLI and PLIII were produced at 14 degrees C when cells were grown in polygalacturonate-containing synthetic medium and at around 20 to 24 degrees C in nutrient broth. In contrast, thermoregulation of cellulolytic activity production differed from thermoregulation of PL. The level of cellulolytic activity was low in all media at temperatures up to 20 degrees C, and then it increased dramatically until the temperature was 28 degrees C, which is the optimal temperature for growth of C. luteola. Previously, we defined the critical temperature by using the modified Arrhenius equation to characterize bacterial behavior. This approach consists of monitoring changes in the maximal specific growth rate as a function of temperature. Our most striking result was the finding that the temperature at which maximum levels of PLI and PLIII were produced in two different media was the same as the critical temperature for growth observed in these two media.

Effect of hexanal on the shelf life of fresh apple slices

In this work the effects of hexanal, as a component of packaging atmosphere, on the shelf life of and evolution of naturally occurring microbial populations in fresh apple slices during storage at 4 and 15 degrees C were evaluated. Although hexanal had no bactericidal effects, in all conditions considered, this volatile molecule significantly extended the shelf life. In fact, the presence of hexanal in the storage atmosphere (at 4 degrees C) totally inhibited mesophilic bacteria and considerably prolonged the lag phase of psychrotrophic bacteria. Also, at 15 degrees C, hexanal strongly inhibited molds, yeasts, and mesophilic and psychrotrophic bacteria. Moreover, hexanal led to a yeast selection favoring species having a reduced spoilage potential due to their prevalent respiratory activity. When added to a modified atmosphere (70% N(2) and 30% CO(2)), this molecule was also very effective in preventing browning reactions for at least 16 days at 15 degrees C. No changes in hue angle values were observed in samples packaged in modified atmosphere with hexanal, even after 16 days of storage at 4 degrees C.

Degradation of pectic compounds during pasteurised vegetable juice spoilage by Chryseomonas luteola: a predictive microbiology approach

Predictive modelling consists in describing effects of environmental factors on microbial growth parameters. With food spoilage bacteria, this approach must be extended to both growth and food damage characterisation. In order to study the incidence of storage temperature on vegetable damage, using predictive microbiology tools, kinetics of pectic compound degradation were studied. Chryseomonas luteola has been chosen because of its ability to grow on post-harvested vegetables. Experiments were performed at refrigerated temperatures (0-10 degrees C) with low initial bacterial charges (10(1)-10(3.5) cfu/ml). Microbial specific growth rate (mu), stability phase before pectic degradation (Sp) and alteration percentage (Ap) were chosen as reference parameters. Then, sub-optimal temperature effects on these three parameters were estimated using modified Ratkowsky model. Results obtained in synthetic medium were compared with data observed in endive juice to appreciate the alteration of vegetable during post-harvest storage.

Changes in microbial populations on fresh cut spinach

The microbial populations found on fresh-cut spinach leaves that were stored in gas permeable bags at 10 degrees C for 12 days were examined and identified. The microorganisms consisted of mesophilic aerobic bacteria, psychrotrophic bacteria, Pseudomonadaceae, Enterobacteriaceae, Micrococcaceae, lactic acid bacteria and yeasts. Populations of mesophiles, psychrotrophs, Pseudomonadaceae and Enterobacteriaceae increased sharply during the storage period. The initial populations were 10(7), 10(6), 10(6) and 10(4) CFU.g-1 respectively. Populations reached 10(10) for the mesophiles, psychrotrophs and Pseudomonadaceae and 10(7) CFU.g-1 for Enterobacteriaceae after 12 days of storage. Micrococcaceae, lactic acid bacteria and yeasts remained constant (10(3)-10(4) CFU.g-1. The majority of the bacterial isolates were identified as Pseudomonas fluorescens, Aeromonas caviae and Staphylococcus xylosus. The yeasts, which were most frequently isolated, were classified in the genus Cryptococcus. No pathogens such as Listeria monocytogenes and Salmonella were detected. Observations with low temperature scanning electron microscopy (LTSEM) indicated that the microorganisms were not present on the surface of healthy unbroken leaves. Alternatively, they were found in areas where the cuticle was broken and could be seen infecting the internal palisade parenchyma.

The microbiology of minimally processed fresh fruits and vegetables

Minimally processed fresh (MPF) fruits and vegetables are good media for growth of microorganisms. They have been involved in outbreaks because of the consumption of products contaminated by pathogens. They are also sensitive to various spoilage microorganisms such as pectinolytic bacteria, saprophytic Gram-negative bacteria, lactic acid bacteria, and yeasts. Contamination of MPF fruits and vegetables occurs at every stage of the food chain, from cultivation to processing. Polluted environments during cultivation or poor hygienic conditions in processing increase the risk of contamination with foodborne pathogens. Although MPF fruits and vegetables may harbor psychrotrophic microorganisms such as fluorescent pseudomonads or Listeria monocytogenes, good control of refrigeration temperature limits growth of spoilage and pathogenic microorganisms. Modified atmospheres are often efficient to maintain or improve visual organoleptic quality of MPF fruits and vegetables, but their effects on microorganisms are inconsistent. Chemical disinfection can partially reduce the initial bacterial contamination; irradiation seems to be more efficient. The applications of legislations and quality assurance systems to control contamination, survival, and growth of foodborne pathogens in MPF fruits and vegetables are discussed.

Effects of Temperature, pH, and NaCl on Growth and Pectinolytic Activity of Pseudomonas marginalis

The interaction of temperature (4, 10, 18, and 30°C), pH (6, 7, and 8), and NaCl (0, 2.5, and 5%) and their effects on specific growth rate, lag phase, and pectinolytic enzymes of Pseudomonas marginalis were evaluated. Response surface methodology was adapted to describe the response of growth parameters to environmental changes. To obtain good conditions of storage, the combined action of salt and temperature is necessary. At 4°C with an NaCl concentration of 5% and a pH of 7, the lag time was 8 days and no growth was observed at 4°C with 5% NaCl and a pH of 6. In the absence of salt, P. marginalis could grow regardless of temperature and pH. Pectate lyase and pectin lyase were produced by P. marginalis , while pectin methyl esterase activity was not observed in our culture conditions. The enzyme production depended on temperature, pH, and salt concentration but also on the age of the culture. Pectinolytic enzymes were abundantly excreted during the stationary phase, and even at 4°C, after 2 weeks of storage, enzyme activities in supernatant culture were sufficient to damage vegetables. Both bacterial growth and enzymatic production have to be taken into account in order to estimate correctly the shelf life of vegetables.

A note on the microbiology of retail packs of prepared salad vegetables

Retail packs of mixed, prepared salad vegetables from two different manufacturers were stored at 7 degrees C until the end of storage-life (sell-by date plus 1 d), when the microbial flora was examined. The quality of the salads was acceptable at the end of storage life. The oxygen concentrations in packs were lower, and the carbon dioxide concentrations were higher, than those in air. High numbers of bacteria were present, with Pseudomonas spp. and Enterobacter agglomerans predominating in packs of both salads, together with lactic acid bacteria in one of the salads. Significant numbers of pectolytic bacteria including Pseudomonas spp. and Erwinia carotovora were detected. Despite the presence of high numbers of coliform bacteria, Escherichia coli was not detected in batches of one salad, and was detected in relatively low numbers in batches of the other. Yersinia spp., predominantly environmental strains of Yersinia enterocolitica, were isolated by enrichment from all samples tested; Staphylococcus aureus and enterococci were not detected.