Surface Of Lettuce Research Articles

Lactobacillus-isolated exopolysaccharide as emulsifier ensure extended stability of eugenol encapsulation, potent anti-microbial activity, and application on fresh produce

In this study, we employed a novel encapsulation approach to develop a stable eugenol oil-in-water (O/W) emulsion by using Lactobacillus plantarum ATCC 8014 isolated exopolysaccharide (EPS) as a natural emulsifier. The emulsion was optimized and analyzed for physical stability, microstructure, rheology, and chemical behavior using fourier-transform infrared analysis (FTIR). The eugenol emulsion consisting of a 40:60 O/W ratio with 3 % w/v EPS exhibited a desired droplet size of 192 ± 1.89 nm with multimodal size distribution, polydispersity index of 0.362 ± 0.01 and zeta-potential of −32 ± 1.90 mV. Scanning electron microscopy (SEM) revealed porous and granulated structures with profound consistency. EPS maintained its emulsifying activity across processing conditions (−20 to 70 °C, pH 3–9, salinity 1 %–30 %), with stability over 90 days. Both EPS and eugenol synergistically enhanced the antimicrobial activity of emulsion against Listeria monocytogenes B600 and Salmonella Poona 36. The practical application of optimized emulsion was assessed by demonstrating a reduction in microbial load on the lettuce surface. Microbial load was determined by immersion and sprayed treatment methods which resulted in more than 3.0 log reduction. This potential application of lactobacillus–derived EPS as a natural emulsifier for encapsulating bioactive essential oils will enable its future applications for food safety.

UV-C light-activated gallic acid and ultrasound: A novel approach for reducing enteropathogens inoculated in lettuce (Lactuca sativa L.)

The effect of UV-C light-activated gallic acid (AGA) alone or combined with ultrasound (US) on the inactivation of E. coli and S. Typhimurium inoculated in aqueous solution and lettuce was studied. The contact time (0–30 min) and AGA concentration (0.05–0.25 M) were evaluated against microorganisms inoculated in aqueous solutions. At selected conditions, the combined effect of AGA and US process was evaluated against microorganisms inoculated in lettuce leaves; samples were taken from washing peptone water and the surface of lettuce using a cotton swab. The microbial inactivation of AGA in lettuce was described using the Monod model, which adequately reproduces the inactivation kinetics (R > 0.96). In aqueous solutions, 10 min of contact time and 0.15 M and 0.25 M of AGA were required to reduce more than 5 log reduction of S. Typhimurium and E. coli, respectively. In lettuce, the E. coli showed higher resistance than S. Typhimurium to the AGA treatment, obtaining a maximum microbial reduction (4.33 ± 0.17 log reduction) using washing peptone water as a sampling method and AGA plus US process. Moreover, the AGA treatment combined with the US process presented a higher microbial inactivation compared to a common disinfectant (colloidal silver). Therefore, it is possible to indicate that AGA combined with the US may be an adequate alternative to reduce enteropathogens in lettuce.

Causal inference and mechanism for unraveling the removal of four pesticides from lettuce (Lactuca sativa L.) via ultrasonic processing and various immersion solutions

This study explores the reduction of carbamates (CAs) and pyrethroids (PYs) − commonly used pesticides − in lettuce using various immersion solutions and ultrasonic processing. It also examines the role of machine learning and molecular docking in understanding the mechanisms of pesticide reduction. The results revealed that the highest reduction of both CAs and PYs exceeded 80 % on lettuce leaves. In most samples, the reduction increased with the power of ultrasonic processing and processing time. The results of machine learning models (XGBoost and SHAP) showed that during the immersion cleaning of CAs and PYs, as well as during both immersion cleaning and ultrasonic processing of CAs + PYs, the reduction was most influenced by the initial pesticide levels and immersion time. Gas Chromatography-Mass Spectrometry (GC–MS) analysis of lettuce’s wax layer identified 24 compounds, including fatty alcohols, fatty acids, fatty acid esters, and triterpenoids. Despite the absence of active sites, the lipophilic nature of long-chain aliphatic compounds aids in pesticide binding, while triterpenoids form strong hydrogen bonds with pesticides, indicating a robust adsorption on the lettuce surface. This study aims to offer insights into the efficient removal of chemical pesticide residues from fruits and vegetables, addressing critical concerns for food safety and human health.

Effects of ultrasound on disrupting metabolite profiles of Pseudomonas fluorescens biofilms cultured on the surface of lettuce

Effects of ultrasound on disrupting metabolite profiles of Pseudomonas fluorescens biofilms cultured on the surface of lettuce

Fates of attached E. coli o157:h7 on intact leaf surfaces revealed leafy green susceptibility

Leafy greens, especially lettuce, are repeatedly linked to foodborne outbreaks. This paper studied the susceptibility of different leafy greens to human pathogens. Five commonly consumed leafy greens, including romaine lettuce, green-leaf lettuce, baby spinach, kale, and collard, were selected by their outbreak frequencies. The behavior of E. coli O157:H7 87-23 on intact leaf surfaces and in their lysates was investigated. Bacterial attachment was positively correlated with leaf surface roughness and affected by the epicuticular wax composition. At room temperature, E. coli O157:H7 had the best growth potentials on romaine and green-leaf lettuce surfaces. The bacterial growth was positively correlated with stomata size and affected by epicuticular wax compositions. At 37 °C, E. coli O157:H7 87-23 was largely inhibited by spinach and collard lysates, and it became undetectable in kale lysate after 24 h of incubation. Kale and collard lysates also delayed or partially inhibited the bacterial growth in TSB and lettuce lysate at 37 °C, and they sharply reduced the E. coli O157:H7 population on green leaf lettuce at 4 °C. In summary, the susceptibility of leafy greens to E. coli O157:H7 is determined by a produce-specific combination of physiochemical properties and temperature.

Enhancing lettuce yield via Cu/Fe-layered double hydroxide nanoparticles spraying

Layered double hydroxides (LDHs) have been widely used in the field of plant engineering, such as DNA/RNA transformation and enhancing plant disease resistance. However, few studies have examined the direct effects of LDHs on plants and their potential utility as nanofertilizers. In this study, the retention capacity of Cu/Fe-layered double hydroxide nanoparticles (CuFe-LDHs) was assessed by comparative experiments on vegetables. The results showed that the retention of CuFe-LDHs in leafy vegetables was high, such as lettuce. Phenotypic analysis revealed that the fresh and dry weights of lettuce leaves were both increased by spraying 10–100 μg/mL CuFe-LDHs. Using the optimal concentration of 10 μg/mL, we conducted further experiments to elucidate the mechanism of CuFe-LDHs promoting lettuce growth. It was found that the application of CuFe-LDHs had a significant effect on growth and induced physiological, transcriptomic, and metabolomic changes, including an increase in the chlorophyll b content, net photosynthetic rate, and intercellular carbon dioxide concentration, as well as modifications in gene expression patterns and metabolite profiles. This work provides compelling evidence that CuFe-LDHs can efficiently adsorb on the surface of lettuce leaves through hydrogen bonding, promote lettuce growth, mitigate the toxicity of heavy metal ions compared to their raw materials at the same concentration and offer a molecular-scale insight into the response of leafy vegetables to CuFe-LDHs.Graphical

Impact of irrigation water quality on human norovirus surrogate survival during leafy green production.

The impact of water quality on the survival of human norovirus (NoV) was determined in irrigation water field run-off (tail water) and well water from a representative Central Coast vegetable production site in the Salinas Valley, California. Tail water, well water, and ultrapure water samples were inoculated separately with two surrogate viruses for human NoV-Tulane virus (TV) and murine norovirus (MNV)-to achieve a titer of 1×105 plaque forming units (PFU)/ml. Samples were stored at 11, 19, and 24°C for 28 days. Additionally, inoculated water was applied to soil collected from a vegetable production site in the Salinas Valley or to the surface of growing romaine lettuce leaves, and virus infectivity was evaluated for 28 days in a growth chamber. Virus survival was similar for water stored at 11, 19, and 24°C and there was no difference in infectivity based on water quality. After 28 days, a maximum 1.5 log reduction was observed for both TV and MNV. TV decreased by 1.97-2.26 log and MNV decreased by 1.28- 1.48 logs after 28 days in soil; infectivity was not influenced by water type. Infectious TV and MNV were recovered from lettuce surfaces for up to 7 and 10 days after inoculation, respectively. Across the experiments there was no significant impact of water quality on the stability of the human NoV surrogates. Overall, the human NoV surrogates were highly stable in water with a less than 1.5 log reduction over 28 days and no difference observed based on the water quality. In soil, the titer of TV declined by approximately 2 logs over 28 days, while MNV declined by 1 log during the same time interval, suggesting surrogate-specific inactivation dynamics in the soil tested in this study. A 5-log reduction in MNV (day 10 post inoculation) and TV (day 14 post inoculation) was observed on lettuce leaves, and the inactivation kinetics were not significantly impacted by the quality of water used. These results suggest that human NoV would be highly stable in water, and the quality of the water (e.g., nutrient content, salinity, and turbidity) does not significantly impact viral infectivity.

Melatonin reduces pink rib discoloration in wounded lettuce midribs

Pink rib discoloration is a physiological disorder resulting in loss of quality and marketability of minimally processed and whole head lettuce (Lactuca sativa L.). Although the biochemistry of pink rib is complex and a comprehensive understanding is lacking, multiple research efforts have focused on inhibiting or reducing pink rib through exogenous application of small molecules such as L-cysteine. In this study, the effects of naturally occurring phytochemicals [serotonin (0.5 or 1 g L−1), naringenin (0.5 g L−1), L-cysteine (0.5 g L−1), and melatonin (1 g L−1)] on pink rib expression were examined in iceberg and Romaine lettuce. The exogenous application of these natural compounds was investigated for reduced pink rib discoloration in lettuce during simulated commercial storage and examined for metabolite and enzymatic activity changes. Immersing wounded midribs for 30 s in serotonin, naringenin, or L-cysteine solutions had no or only a slight inhibitory effect to inhibit pink rib discoloration while those immersed in a melatonin solution had reduced development of pink rib in both iceberg and Romaine lettuces after 5 d at 5 °C. Phenylalanine ammonia lyase (PAL) activity and chlorogenic acid concentration increased upon pink rib discoloration, consistent with previous reports. However, melatonin-treated samples did not result in reduced PAL activity, chlorogenic acid content, or polyphenol oxidase (PPO) activity. Untargeted metabolite analysis showed that melatonin treatment resulted in a global effect on metabolite changes in the wounded lettuce midrib tissue. To compare the severity of pink rib among treatments, pink rib was quantified throughout storage visually and by converting digital images of the lettuce midribs to the L*a*b* color space. The a* component of L*a*b* represents redness versus greenness, thus its values were used to objectively measure pink rib severity. Melatonin was shown to be a promising compound to minimize development of pink rib discoloration at cut surfaces of iceberg and Romaine lettuce during commercial handling and storage.

Automatic detection of pesticide residues on the surface of lettuce leaves using images of feature wavelengths spectrum.

The inappropriate application of pesticides to vegetable crops often results in environmental pollution, which seriously impacts the environment and human health. Given that current methods of pesticide residue detection are associated with issues such as low accuracy, high equipment cost, and complex flow, this study puts forward a new method for detecting pesticide residues on lettuce leaves. To establish this method, spectral analysis was used to determine the characteristic wavelength of pesticide residues (709 nm), machine vision equipment was improved, and a bandpass filter and light source of characteristic wavelength were installed to acquire leaf image information. Next, image preprocessing and feature information extraction were automatically implemented through programming. Several links were established for the training model so that the required feature information could be automatically extracted after the batch input of images. The pesticide residue detected using the chemical method was taken as the output and modeled, together with the input image information, using the convolutional neural network (CNN) algorithm. Furthermore, a prediction program was rewritten to generalize the input images during the prediction process and directly obtain the output pesticide residue. The experimental results revealed that when the detection device and method designed in this study were used to detect pesticide residues on lettuce leaves in a key state laboratory, the coefficient of determination of the equation reached 0.883, and the root mean square error (RMSE) was 0.134 mg/L, indicating high accuracy and that the proposed method integrated the advantages of spectrum detection and deep learning. According to comparison testing, the proposed method can meet Chinese national standards in terms of accuracy. Moreover, the improved machine vision equipment was less expensive, thus providing powerful support for the application and popularization of the proposed method.

Optimize lettuce washing methods to reduce the risk of microplastics ingestion: The evidence from microplastics residues on the surface of lettuce leaves and in the lettuce washing wastewater

Airborne microplastics have been identified as an emerging contaminant that can adhere to the surface of leafy vegetables, and if not completely removed by washing, there is a high risk that human consumption of these plastics may cause harm to humans. In this study, we simulated atmospheric pollution by spraying microplastic particles (MPs) with particle sizes of 100 nm and 500 nm to determine whether MPs particles would adhere to the lettuce surface and whether different common cleaning methods (water rinsing, ultrasonic vibration cleaning, and edible detergent cleaning) would be effective in removing MPs from the leaf surfaces. We scanned the leaf surface with a laser confocal microscope and examined the wash water with a flow cytometer and found that simple rinsing was not effective in removing plastic particles from lettuce leaves. In comparison, ultrasonic vibratory cleaning showed a better efficiency, 4 times higher than more MPs being washed from the leaves. The most effective method was detergent washing, with the washing efficiency increased by 6.9 times. Ultrasonic vibrations can partially break the chemical bonds between MPs and plant surfaces, and detergents' surfactants can enhance MPs' hydrophilicity. MPs with a particle size of 100 nm were more difficult to clean than those with a particle size of 500 nm. This finding has important implications for the interaction of MPs contamination with vegetables and the cleaning of vegetables.

Curli Production Influences Cross-contamination by Escherichia coli O157:H7 When Washing Fresh-cut Romaine Lettuce

Escherichia coli O157:H7 expresses extracellular proteins called curli that are essential for surface colonization. Transfer rates of E. coli O157:H7 0018+ (curli+), and 0018- (curli−) from inoculated to noninoculated lettuce pieces during washing were quantified in this study. Romaine lettuce pieces were inoculated with ∼6 log CFU on just the surface, just the cut edges, or both surface and cut edges. Samples were dried for 2 h in a biosafety cabinet and then washed with ten (10) noninoculated lettuce pieces in 500 mL of water for 30 s. The curli− strain was more readily removed (3 log reduction) compared to the curli+ (1 log reduction) when only the lettuce surface was inoculated (p > 0.05). The same was true when only the lettuce piece edge was inoculated (p > 0.05), although the magnitude of the reduction was less. There was no significant difference in reduction of curli+ strain between any of the surfaces. There was a significant difference (p < 0.05) in reduction of the curli− strains when comparing the leaf surface (more removal) to the cut leaf edge (less removal). The curli+ strain always showed significantly (p < 0.05) more transfer to noninoculated leaves regardless of the inoculation location. The curli+ strain transferred about −1 log percent (∼0.1%) to noninoculated pieces, while the curli− strain transferred about −2 log percent (∼0.01%) CFU to the noninoculated pieces. Mean log percent transfer was not significantly different within the curli+ or curli− experiments (p > 0.05). When the leaf surface was inoculated, there was about 2 log percent (i.e., close to 100% transfer) into the wash water for both the curli+ and curli− strains. When only the cut edges or surface and edge were inoculated, observed mean transfer rates were lower but not significantly different (p > 0.05). Further research is needed to more fully understand the factors that influence bacterial cross-contamination during the washing of fresh produce.

Antibacterial Activity and Mechanism of Linalool against Shigella sonnei and Its Application in Lettuce

Shigella sonnei (S. sonnei) infection accounted for approximately 75% of annual outbreaks of shigellosis, with the vast majority of outbreaks due to the consumption of contaminated foods (e.g., fresh vegetables, potato salad, fish, beef, etc.). Thus, we investigated the antibacterial effect and mechanism of linalool on S. sonnei and evaluated the effect of linalool on the sensory quality of lettuce. The minimum inhibitory concentration (MIC) of linalool against S. sonnei ATCC 25931 was 1.5 mg/mL. S. sonnei was treated with linalool at 1× MIC for 30 min and the amount of bacteria was decreased below the detection limit (1 CFU/mL) in phosphate-buffered saline (PBS) and Luria-Bertani (LB) medium. The bacterial content of the lettuce surface was reduced by 4.33 log CFU/cm2 after soaking with linalool at 2× MIC. Treatment with linalool led to increased intracellular reactive oxygen species (ROS) levels, decreased intracellular adenosine-triphosphate (ATP) content, increased membrane lipid oxidation, damaged cell membrane integrity, and hyperpolarized cell membrane potential in S. sonnei. The application of linalool to lettuce had no effect on the color of lettuce compared to the control. The sensory evaluation results showed that linalool had an acceptable effect on the sensory quality of lettuce. These findings indicate that linalool played an antibacterial effect against S. sonnei and had potential as a natural antimicrobial for the inhibition of this foodborne pathogen.

Effect of encapsulation and natural polyphenolic compounds on bacteriophage stability and activity on Escherichia coli in Lactuca sativa L. var. longifolia

AbstractThe goals of the present study were to characterize the phages isolated by magnetic nanocomplex, to determine the effect of natural polyphenols on phage activity against E. coli and the stability of free and encapsulated bacteriophages under UV irradiation, and their efficacy against E. coli on romaine lettuce (Lactuca sativa L. var. longifolia) as a fresh food model. The morphology of the two magnetically isolated phages were characterized using the transmission electron microscope (TEM). All isolated phages were classified in a morphological family of tailed phages of the order Caudovirales, Myoviridae family. The PCR results showed that the stx1 and cI genes were not on any phage. Assays were carried out to determine the effect of four polyphenolic natural extracts on phages under UV radiation. The highest activity and stability of phages expressed in phage titer (PFU/ml) were observed in the presence of polyphenols from tarbush leaves (Flourensia cernua DC). Coliphages were encapsulated in alginate/chitosan and alginate/PVA carriers with and without polyphenols from tarbush leaves. Encapsulation led to a higher phage stability under UV radiation. All encapsulated formulations and free coliphages were applied on the surfaces of romaine lettuce containing E. coli cells. An effective reduction of 2 log of E. coli titers from lettuce was observed. Encapsulated phage formulations with and without polyphenols from tarbush leaves could be considered for future application in fresh food preservation.

Inactivation of Pectobacterium carotovorum subsp. Carotovorum and Dickeya chrysanthemi on the surface of fresh produce using a 222 nm krypton–chlorine excimer lamp and 280 nm UVC light-emitting diodes

Pectobacterium carotovorum subsp. carotovorum and Dickeya chrysanthemi have been reported to cause soft-rot diseases to fresh produces. The aim of this research was to investigate the inactivation efficacy of a 222 nm KrCl excimer lamp and 280 nm UVC-LED against P. carotovorum subsp. carotovorum and D. chrysanthemi on fresh produces. When samples were subjected to UVC treatment by a excimer lamp or UVC-LED at 450 mJ/cm2, it was observed that soft-rot bacteria on the surface of iceberg lettuce, strawberry, and tomato were reduced by a maximum of 5, 3, and 5 log units, respectively. The surface properties of fresh products, such as hydrophobicity, roughness, and topographic image, were analyzed under the hypothesis that the characteristics of the sample surface affected the inactivation efficacy of the excimer lamp and UVC-LED. It was determined that there was an inverse correlation between the surface roughness and inactivation efficacy of UVC treatment. The texture properties of samples were not significantly different (P > 0.05) from those of untreated samples. The results of this research substantiate that the excimer lamp and UVC-LED can be applied to decontamination processes for inactivating soft-rot bacteria, thereby prolonging the shelf life of fresh produce and reducing economic losses.

Leaf fitness and stress response after the application of contaminated soil dust particulate matter

In this study, we observed the effect of the application of soil dust enriched with risk elements (Cd, Pb, As and Zn) to leaf surfaces of lettuce (Lactuca sativa var. capitata) while it was grown under hydroponic conditions. This study aimed to determine how low soil dust particulate matter (PM) doses affected the activity of or damaged the photosynthetic apparatus and how the uptake of risk elements was associated with both epigenetic changes (5-methylcytosine content, i.e., 5mC) and stress metabolism. During the study, we obtained many results pertaining to risk element contents and biochemical (total phenolic content (TPC), malondialdehyde (MDA) content and the amount of free amino acids (AAs)) and physiological (photosynthesis parameters: net photosynthetic rate, transpiration rate, intercellular CO2 concentration, stomatal conductance, instantaneous water-use efficiency, maximum quantum yield of PSII, chlorophyll and carotenoid contents, and leaf water potential (WP)) plant features. The results showed an increase in MDA and 5mC. However, the transpiration rate, WP and free AAs decreased. In conclusion, contamination by very low doses of soil dust PM had no direct or significant effect on plant fitness, as shown by the TPC and 5mC content, which indicates that plants can overcome the oxidative stress caused by the accumulation of risk elements. From the above, we propose the use of epigenetic changes as biomarkers of potential changes in the activation of plant metabolism under stress caused by environmental pollution.

Understanding inactivation of Listeria monocytogenes and Escherichia coli O157:H7 inoculated on romaine lettuce by emulsified thyme essential oil

Effects of thyme essential oil (TEO) emulsion (TEE) with cationic charge formulated using cetylpyridinium chloride (CPC) on attachment strength and inactivation of Listeria monocytogenes and Escherichia coli O157:H7 on romaine lettuce surface were examined in this study. Regardless of the inoculation time (2 h and 24 h), pathogen attachment was stronger on the adaxial surface of the romaine lettuce than on the abaxial surface because of the lower roughness of the former. Moreover, attachment strength increased with increasing inoculation time. TEE washing had the strongest inhibitory effect on pathogen attachment at 2 h when compared with that of TEO, CPC, and sodium hypochlorite (SH), demonstrating a 3.32 and 2.53 log-reduction in the size of the L. monocytogenes and E. coli O157:H7 populations, respectively, compared to the control samples. Additionally, the TEE washing effects were maintained even after inoculation for 24 h, and it decreased attachment to adaxial surface of the samples. These results indicate that TEE could be a good alternative to SH in improving the microbiological safety of romaine lettuce.

Investigation of synergistic antibacterial effect of organic acids and ultrasound against pathogen biofilms on lettuce

The aim of this study was to investigate the inactivation effect of organic acids and ultrasound against Escherichia coli and Listeria monocytogenes biofilms on the surface of lettuce leaves and to determine the effect of these inactivation treatments on the quality characteristics of the lettuce. Lettuce leaves with biofilms formed following inoculation with E. coli and L. monocytogenes were treated individually or simultaneously with ultrasound and different organic acids (acetic acid, malic acid, citric acid and lactic acid) for periods of 10, 30 and 60 min. Combined treatments caused synergistic and antagonistic inactivation of pathogen biofilms. In addition, the anti-biofilm effect of combined treatments was found to be higher compared with a single treatment. With an increase in treatment time from 10 min to 60 min, biofilm detachment rate was increased on the surface of the lettuce. The reduction in E. coli and L. monocytogenes biofilms varied from 0.43 to 5.54 log CFU/cm2 and from 0.40 to 4.23 log CFU/cm2, respectively. Lactic acid and malic acid showed stronger anti-biofilm activities than the other organic acids. All these results indicate that biofilm inactivation on lettuce samples depends on target microbial species, the type of organic acid, the treatment time, and method. Additionally, decontamination treatments for 60 min did not adversely affect the colour, texture or sensory quality of the lettuce. In conclusion, the combined treatment of ultrasound and organic acid is suggested as a promising hurdle technology in order to remove pathogen biofilms from fresh produce.

Inactivation of mixed Escherichia coli O157:H7 biofilms on lettuce by bacteriophage in combination with slightly acidic hypochlorous water (SAHW) and mild heat treatment

Escherichia coli O157:H7 is one of the most important foodborne pathogens that can persist in leafy green vegetables and subsequently produce biofilms. Biofilm formation is an ongoing concern in the food industry as biofilms are relatively resistant to a variety of antimicrobial treatments. In the present study, we evaluated the combined effects of phage FP43 and mild-heated slightly acidic hypochlorous water (SAHW) in reducing established biofilms on lettuce. Prior to the sequential treatments involving phage-SAHW and SAHW-phage for long-term storage, equal inoculum densities of E. coli O157:H7 and E. coli O91:H- were added on iceberg lettuce surfaces and the lettuce samples were stored at 10 °C for 48 h to allow biofilm formation. The sequential treatment with phage FP43 and SAHW significantly decreased the number of adhered cells, especially the combination of phage FP43 at 25 °C for 2 h and mild-heated SAHW, which considerably eliminated E. coli viable biofilm cells to undetectable levels (>3 log CFU/piece). However, the biofilms were not completely removed, as evidenced via SEM observation. Additionally, sequential treatment with SAHW and phage caused continuous reductions in viable counts, decreasing the viability of E. coli O157:H7 and total E. coli to the lower limit of detection after incubation for 5 d. Meanwhile, bacterial regrowth was observed after treatment with SAHW alone. These results indicated that the combination of phage and SAHW could be considered as a promising strategy to control the formation of E. coli O157:H7 biofilms on lettuce.

Comparing the effectiveness of Cinnamomum zeylanicum essential oil and two common household sanitizers to reduce lettuce microbiota and prevent Salmonella enterica recontamination

AbstractThe washing step is necessary to remove biological and physical hazards from minimally processed vegetables. Nevertheless, the risk of foodborne diseases could persist even after washing due to postsanitizing contamination, and little is known about the antimicrobial effect of residual sanitizers. This study was conducted to compare the effectiveness of sodium hypochlorite (SH), sodium bicarbonate, and Cinnamomum zeylanicum essential oil (CEO) as sanitizers on lettuce (8°C, 48 h). First, the effect of sanitizers in reducing total aerobic mesophilic and psychrotrophic bacteria, yeast and molds, lactic acid bacteria, and Enterobacteriaceae on lettuce was evaluated with some insights on lettuce quality attributes (pH, color, and sensory analysis). Then, the capability of the treatments in preventing postwashing Salmonella adhesion on lettuce surface was investigated. Commercial SH disinfectant (solution at 2%) and CEO (0.5%) reduced microbial contamination in lettuce, without affecting the overall acceptability after 48 h at 8°C. SH reduced postsanitizing Salmonella adhesion of about 2.7 Log colony forming unit (CFU)/g. The microbial reduction was confirmed by confocal laser scanning microscopy, which also evidenced Salmonella internalization within stomata. Interestingly, CEO as well reduced Salmonella adhesion but with lower efficacy (0.44–1.00 Log CFU/g reduction), while sodium bicarbonate (15 mg/ml) was not effective. In conclusion, SH and CEO seem to be effective sanitizing agents, capable of improving the microbiological profile of fresh produce. In addition, the residual sanitizers, that remain on lettuce after washing, play a role in reducing Salmonella adhesion.

Use of aqueous ozone rinsing to improve the disinfection efficacy and shorten the processing time of ultrasound-assisted washing of fresh produce

In minimal processing industry, chlorine is widely used in the disinfection process and ultrasound (US) increases the disinfection efficacy of chlorine and reduces the cross-contamination incidence during washing. Tap water (TW), which has no disinfection effect, is generally used to rinse off sanitizer residues on the surface of disinfected fresh-cut vegetables. In this study, aqueous ozone (AO), a low-cost and residue-free sanitizer, was used to replace TW rinsing in combination with US (28 kHz)–chlorine (free chlorine [FC] at 10 ppm, a concentration recommended for industrial use) for the disinfection of fresh-cut lettuce as a model. US–chlorine (40 s) + 2.0 ppm AO (60 s) treatment resulted in browning spots on lettuce surface at the end of storage. In contrast, US–chlorine (40 s) + 1.0 ppm AO (60 s) did not lead to deterioration of the sensory quality (sensory crispness, color, and flavor) and a change in total color difference, and the activities of browning-related enzymes were significantly lower. Moreover, US–chlorine (40 s) + 1.0 ppm of AO (60 s) treatment led to significantly lower counts of Escherichia coli O157:H7, Salmonella Typhimurium, aerobic mesophilic (AMC), and molds and yeasts (M&Y) on days 0–7 than US–chlorine (60 s) + TW (60 s) and single 1.0 ppm AO (120 s) treatments, suggesting that AO provided an additional disinfection effect over TW, while reducing the overall processing time by 20 s. Cell membrane permeability analysis (alkaline phosphatase, protein, nucleotide, and adenosine triphosphate leakage) showed that the combination with 1.0 ppm AO caused more severe cell membrane damage in E. coli O157:H7 and S. Typhimurium, explaining the higher disinfection efficacy. 16S rRNA sequencing revealed that following US–chlorine (40 s) + 1.0 ppm of AO (60 s) treatment, Massilia and Acinetobacter had higher relative abundances (RAs) on day 7 than after US–chlorine (60 s) + TW (60 s) treatment, whereas the RAs of Escherichia–Shigella was significantly lower, indicating that the former treatment has a superior capacity in maintaining a stable microbial composition. This explains from an ecological point of view why US–chlorine (40 s) + 1.0 ppm of AO (60 s) led to the lowest AMC and M&Y counts during storage. The study results provide evidence that AO has potential as an alternative to TW rinsing to increase the disinfection efficacy of US–chlorine.