Differential microbiota shift on whole romaine lettuce subjected to source or forward processing and on fresh-cut products during cold storage

Authors: Gu, Ganyu; DING, QIAO - University Of Maryland; REDDING, MARINA - Orise Fellow; YANG, YISHAN - Orise Fellow; O'BRIEN, REGINA - Orise Fellow; GU, TINGTING - University Of Florida; ZHANG, BOCE - University Of Florida; Zhou, Bin; MICALLEF, SHIRLEY - University Of Maryland; Luo, Yaguang - Sunny; Fonseca, Jorge; Nou, Xiangwu

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2024
Publication Date: 3/6/2024
Citation: Gu, G., Ding, Q., Redding, M., Yang, Y., O'Brien, R., Gu, T., Zhang, B., Zhou, B., Micallef, S.A., Luo, Y., Fonseca, J.M., Nou, X. 2024. Differential microbiota shift on whole romaine lettuce subjected to source or forward processing and on fresh-cut products during cold storage. International Journal of Food Microbiology. https://doi.org/10.1016/j.ijfoodmicro.2024.110665.
DOI: https://doi.org/10.1016/j.ijfoodmicro.2024.110665

Interpretive Summary: Raw Romaine lettuce is typically transported from growing areas in California and Arizona to distant population centers in the US for processing and marketing, a practice termed forward processing. Recent repeated outbreaks of shigatoxin producing E. coli associated with romaine lettuce consumption enhanced concerns that romaine lettuce forward processing might impact lettuce microbiome and the survival of foodborne pathogens such as E. coli O157:H7. In collaboration with the fresh produce industry, ARS scientists systematically examined the impacts of forward processing practices on lettuce microbial dynamics and quality parameters. Our collaborative study showed that forward processing typically led to approximately 1 week delay in product processing, which was associated with high microbial counts and faster deterioration of products during storage. Significant microbiome shifts were observed with forward processing. The scientific information generated from the research can be used by the scientific research communities, government regulatory agencies and industry for developing best practice guidelines for improving food safety for forward processed fresh produce.

Technical Abstract: Romaine lettuce in the U.S. is primarily grown in California or Arizona and either processed near the growing regions (source processing, SP) or transported to other regions to be processed in facilities serving distant markets (forward processing, FP). The potential impact of FP on romaine lettuce food safety and quality has raised concerns from the industry, partially due to the multiple outbreaks of Escherichia coli O157:H7 in recent years that often exhibited patterns of case clustering in Northeast and Midwest. Therefore, a collaborative investigation with the romaine lettuce industry was conducted to assess the quality and microbiome dynamics of SP and FP romaine lettuce products from harvest to storage. In this study, freshly harvested romaine lettuce from a commercial field destined for both forward and source processing channels was tracked from farm to processing facility. In both the summer and fall seasons, whole romaine lettuce heads from the farm, the tracked bins delivered to forward and source facilities, and packaged fresh-cut lettuce after processing, were collected for microbiological and quality analyses. Cold-stored romaine lettuce was tested 1, 4, 7, and 14 days after processing. Amplicon (16S rRNA) high-throughput sequencing was performed using both Illumina MiSeq and Nanopore MinION to describe shifts in bacterial communities associated with romaine lettuce. A noticeable fluctuation (up to 20 kPa decrease) of air pressure during FP delivery was recorded. Microbial populations, especially total bacteria and coliform counts, of both whole-head lettuce before processing and fresh-cut lettuce after storage from FP were significantly (p<0.05) higher than that of SP. The levels of total bacteria estimated by qPCR followed the same trend. The compositions of microbial communities analyzed by MiSeq and MinION were comparable, which were significantly affected by romaine lettuce production season, postharvest delivery and processing conditions. Alpha diversity of lettuce microbiota was significantly higher in fall than summer, and declined during storage. The shifting patterns of certain dominant taxa were differentially impacted by production season and processing type. Considering product quality, the loss of tissue integrity of FP romaine lettuce, especially after 2-week of storage, was significantly higher than SP products (p<0.05). Besides, SP lettuce samples showed higher visual quality than FP products on days 7 and 14 after processing. Understanding the microbial ecology of romaine lettuce microbiota across the farm to storage continuum, under both SP and FP conditions, will provide insights on how to improve the microbiological safety and quality of lettuce products.