Identification of Factors Affecting the Deterioration Rate of Fresh-Cut Lettuce in Modified Atmosphere Packaging

Optimization of the Use of a Commercial Phage-Based Product as a Control Strategy of Listeria monocytogenes in the Fresh-Cut Industry

A commercial phage biocontrol for reducing Listeria monocytogenes has been described as an effective tool for improving fresh produce safety. Critical challenges in the phage application must be overcome for the industrial application. The validation studies were performed in two processing lines of two industry collaborators in Spain and Denmark, using shredded iceberg lettuce as the ready-to-eat (RTE), high process volume product. The biocontrol treatment optimized in lab-scale trials for the application of PhageGuard ListexTM was confirmed in industrial settings by four tests, two in Spain and two in Denmark. Results showed that the method of application that included the device and the processing operation step was appropriate for the proper application. The proper dose of Phage Guard ListexTM was reached in shredded iceberg lettuce and the surface was adequately covered for the successful application of phages. There was no impact on the headspace gas composition (CO2 and O2 levels), nor on the color when untreated and treated samples were compared. The post-process treatment with PhageGuard ListexTM did not cause any detrimental impact on the sensory quality, including flavor, texture, browning, spoilage, and visual appearance over the shelf-life as the phage solution was applied as a fine, mist solution.

Weather factors, soil microbiome, and bacteria-fungi interactions as drivers of the epiphytic phyllosphere communities of romaine lettuce

Lettuce is associated with seasonal outbreaks of Shiga toxin-producing Escherichia coli (STEC) infections. Little is known about how various biotic and abiotic factors affect the lettuce microbiome, which in turn impacts STEC colonization. We characterized the lettuce phyllosphere and surface soil bacterial, fungal, and oomycete communities at harvest in late-spring and -fall in California using metagenomics. Harvest season and field type, but not cultivar, significantly influenced the microbiome composition of leaves and surface soil near plants. Phyllosphere and soil microbiome compositions were correlated with specific weather factors. The relative abundance of Enterobacteriaceae, but not E. coli, was enriched on leaves (5.2%) compared to soil (0.4%) and correlated positively with minimum air temperature and wind speed. Co-occurrence networks revealed seasonal trends in fungi-bacteria interactions on leaves. These associations represented 39%-44% of the correlations between species. All significant E. coli co-occurrences with fungi were positive, while all negative associations were with bacteria. A large proportion of the leaf bacterial species was shared with those in soil, indicating microbiome transmission from the soil surface to the canopy. Our findings provide new insight into factors that shape lettuce microbial communities and the microbial context of foodborne pathogen immigration events in the lettuce phyllosphere.

Extending lettuce shelf life through integrated technologies

Lettuce, a leafy vegetable used in cuisines worldwide, is a highly perishable product sensitive to postharvest losses caused by biotic and abiotic factors. The existing technologies and approaches used during plant cultivation, harvest, processing, transportation, and storage can limit the postharvest issues, but further improvements are needed to meet a growing demand for excellent product appearance, combined with superb quality, biosafety, and low economic and environmental cost. This review summarizes our current understanding of lettuce postharvest physiology and genetics with focus on enzymatic discoloration of wounded surfaces and rapid tissue deterioration. Discussed are existing and emerging integrated technologies and approaches that can facilitate achieving outstanding postharvest quality of lettuce products.

Seasonality, shelf life and storage atmosphere are main drivers of the microbiome and E. coli O157:H7 colonization of post-harvest lettuce cultivated in a major production area in California

BACKGROUND

Lettuce is linked to recurrent outbreaks of Shiga toxin-producing Escherichia coli (STEC) infections, the seasonality of which remains unresolved. Infections have occurred largely from processed lettuce, which undergoes substantial physiological changes during storage. We investigated the microbiome and STEC O157:H7 (EcO157) colonization of fresh-cut lettuce of two cultivars with long and short shelf life harvested in the spring and fall in California and stored in modified atmosphere packaging (MAP) at cold and warm temperatures.

RESULTS

Inoculated EcO157 declined significantly less on the cold-stored cultivar with short shelf life, while multiplying rapidly at 24 °C independently of cultivar. Metagenomic sequencing of the lettuce microbiome revealed that the pre-storage bacterial community was variable but dominated by species in the Erwiniaceae and Pseudomonadaceae. After cold storage, the microbiome composition differed between cultivars, with a greater relative abundance (RA) of Erwiniaceae and Yersiniaceae on the cultivar with short shelf life. Storage at 24 °C shifted the microbiome to higher RAs of Erwiniaceae and Enterobacteriaceae and lower RA of Pseudomonadaceae compared with 6 °C. Fall harvest followed by lettuce deterioration were identified by recursive partitioning as important factors associated with high EcO157 survival at 6 °C, whereas elevated package CO₂ levels correlated with high EcO157 multiplication at 24 °C. EcO157 population change correlated with the lettuce microbiome during 6 °C storage, with fall microbiomes supporting the greatest EcO157 survival on both cultivars. Fall and spring microbiomes differed before and during storage at both temperatures. High representation of Pantoea agglomerans was a predictor of fall microbiomes, lettuce deterioration, and enhanced EcO157 survival at 6 °C. In contrast, higher RAs of Erwinia persicina, Rahnella aquatilis, and Serratia liquefaciens were biomarkers of spring microbiomes and lower EcO157 survival.

CONCLUSIONS

The microbiome of processed MAP lettuce evolves extensively during storage. Under temperature abuse, high CO₂ promotes a lettuce microbiome enriched in taxa with anaerobic capability and EcO157 multiplication. In cold storage, our results strongly support a role for season and lettuce deterioration in EcO157 survival and microbiome composition, suggesting that the physiology and microbiomes of fall- and spring-harvested lettuce may contribute to the seasonality of STEC outbreaks associated with lettuce grown in coastal California.