Minimizing pathogen transference during lettuce harvesting by optimizing the design of the harvesting device and operation practices.

Laboratory studies have shown that a field harvesting-coring knife can transfer pathogenic bacteria like E. coli O157:H7 to lettuce where it can grow when exposed to ambient temperatures for short periods. However, specific scientific data that reflect realistic contamination conditions during in-field lettuce harvesting-coring and which are suitable for use in risk assessments are not available, but are urgently needed. Specific information also is unavailable on how to prevent/reduce pathogen contamination from in-field coring and reduce food safety risks. This research project proposal describes a series of studies designed to provide scientific data to address these issues and provide answers to key questions identified by the produce industry. By working closely with industry cooperators, we will provide scientific data obtained under realistic conditions that will assist industry in making informed decisions about food safety improvement practices and provide practical solutions to manage this potential problem. We anticipate that development of prototypes of modified coring knives with improved food safety features and optimization of sanitation treatments will provide the needed tools for industry to minimize pathogen contamination cost-effectively. We will also employ creative engineering approaches to develop a new automated coring device that will not only eliminate the source of contamination via soil contact and human handling thus significantly improving food safety, but also reducing labor costs. Specific scientific information and practical solutions developed from this research will support immediate and near-term improvements in lettuce field-coring/harvesting operations relative to reducing potential risks from pathogen contamination and transfer to product.

This project addresses CPS priority research area on 'Transference of human pathogens to leafy greens during harvest,' and plans to provide scientific information and practical approaches to mitigate the potential risk of Escherichia coli O157:H7 transference during lettuce core-in-field (CIF) stage. Specifically, we will 1) determine the required pathogen contamination levels, and harvesting practices that facilitate pathogen transference to lettuce via contaminated coring knives, 2) develop approaches to reduce the risk of pathogen transference by improving coring knife design and sanitation, 3) identify improved harvesting practices as well as technical innovations to effectively eliminate soil contact as a source of coring knife contamination; and 4) identify post-harvest handling practices that can be implemented in the field to mitigate food safety risks. Advanced molecular and microbiological techniques will be applied to quantify pathogen concentrations spanning a range of realistic contaminant loads and field conditions; creative engineering approaches will be utilized to optimize coring knife design and develop enhanced sanitation practices, and to develop a hands-free automated coring device that effectively eliminates the contamination source of coring knives via soil contact. The following CPS RFP questions will be addressed: 'What level of pathogen must be present to facilitate transfer by trimming and coring?, What in-field sanitation practices could be implemented to mitigate coring knife pathogen transference?, What post-harvest treatments could be implemented in?field to reduce or mitigate this risk'. The near term outcome will result in scientific information needed for risk assessment of the pathogen transference potentials during lettuce field coring process for a realistically low to increased contaminant range, as well as the development of several improved practices and new technologies to reduce the risk. This information is crucial for the industry to develop science-based guidance and practical interventions for establishing food safety policies, and managing potential risks. Development of several prototypes of a coring knife with improved food safety features, and an evaluation of sanitation procedures for lettuce coring devices will provide industry with tools and procedures to directly improve the CIF process safety.

Objective 1: Determine the pathogen levels required for pathogen transference to the edible portions of lettuce via contaminated coring knives. 

Objective 2: Reduce the risk of coring knife pathogen transference by developing improved coring knife design and sanitation procedures. 

Objective 3: Eliminate the potential for coring knife contamination via soil contact by separating the cutting and coring process. 

Objective 4: Identify post-harvest handling practices that can be used to effectively manage the potential food safety risks during CIF.

1) Pathways and critical factors in relation to pathogen transference from soil to lettuce were identified. Pathogens transferred from soil to harvested lettuce are a function of pathogen concentration in soil, and the amount of soil present on the harvested lettuce, which are then impacted by a number of factors including lettuce growing and harvesting conditions, CIF lettuce harvesting methods, and knife design and disinfection. Optimizing harvesting practice, and improving harvest knife design and disinfection can significantly minimize this potential risk. 

2) The cutting blade and coring ring of a CIF harvest knife play significantly different roles in pathogen contamination and transference from soil to lettuce. Analysis of the current CIF harvesting practice observed in California reveals that the cutting blade has higher potential to be contaminated by the soil, but less opportunity to transfer pathogens onto to harvested lettuce. On the contrary, the coring ring has less potential to be contaminated by soil; but much higher potential to transfer pathogens onto the harvested lettuce.

3) The current CA Leafy Green Marketing Agreement calls for attention to CIF harvesting to minimize pathogen transfer; yet, detailed information is needed as to how to minimize pathogen contamination from soil during harvesting/coring operations. Since the cutting blade is used to cut lettuce off the stem that touches the ground, it is important to minimize the potential for the cutting blade to contact soil whenever possible. However, since the blade to soil contact may be inevitable at some point during harvesting under the current CIF practice, avoiding contact between the blade and the edible portions of the lettuce plays a vital role in minimizing pathogen transfer. Field observations of the current California CIF harvesting practices, and random samples received from California indicate that most harvesters in California (at least those harvesting for leading fresh-cut processors) are being careful to minimize contact between blade and soil, and between blade and harvested lettuce. However, evaluation of random samples received from Florida and Mexico suggests that CIF harvesting in these regions need more improvement. A cutting blade that touches soils should NOT be used to touch the edible portions of the lettuce. 

4) Harvesting CIF lettuce in a wet field (due to rain, irrigation etc.) represents a great challenge to minimize transfer of soil to harvested lettuce. Extreme care must be exercised to avoid harvest knife contact between soil, and lettuce, if CIF lettuce has to be harvested under this condition. 

5) The harvest knives currently widely used in the industry need improvement. The rough weld between the cylindrical ring and its shaft harbors pathogens and is difficult to disinfect. Both of our newly designed knife prototypes harbored fewer pathogens and were easier to disinfect. 

6) The ability of ultrasound and surfactant to enhance chlorine disinfection efficacy was tested. While surfactant did not show any improvement on pathogen reduction, ultrasound improved chlorine efficacy for pathogen reduction on the roughly welded surface.