By Dr Sukhvinder Pal (SP) Singh, Department of Primary Industries and Regional Development

Since April 2023, the Safe Leafy Veg project has been actively investigating potential sources and pathways of microbial contamination in leafy vegetable production and postharvest systems. This project encompasses all leafy vegetables subject to the new Primary Production and Processing (PPP) standards, which took effect on 12 February 2025 across all states and territories, impacting growers and processors of all scales.

The project’s scope includes engaging with growers and processors to understand their food safety practices and preventive control measures. The response from growers has been overwhelmingly positive, indicating a strong willingness to adopt improved or new practices as needed.

Over 80 growers and processors have been visited across major leafy vegetable production regions, including East Gippsland, the Mornington Peninsula, Bacchus Marsh, Lower Murray, Adelaide-Virginia, Lockyer Valley, Fassifern Valley, Granite Belt, Perth Metropolitan, Gingin-Peel-Ord River, Greater Sydney, Riverina, and Tasmania. Growers and processors have shared critical control measures employed at production, harvest, and postharvest stages.

As part of the microbial risk mapping process, microbiological samples were collected from production (soil, water, produce, machinery), harvest (equipment), postharvest water, produce, premises and processing machinery) and retail stores (produce).

So far more than 8,000 microbiological assays have been conducted for three target pathogens (Listeria monocytogenes, Salmonella species and pathogenic E. coli) for their presence/absence. The aim of microbial risk mapping is to define the points of interventions to improve practices and direct resources of growers’ levy funds to maximise the food safety improvement gains. All participating growers have been receiving food safety recommendations to implement in their businesses in short, medium, and long terms.

Allowing irrigation water sediment to settle in farm ponds is important

The key learnings so far are highlighted below:

• Pathogenic E. coli is the leading microbiological risk identified in the risk mapping process and its potential sources and routes are speculated based on production practices. Salmonella and Listeria monocytogenes detections are relatively much lower. In 2025, whole-genome sequencing of these pathogens will provider deeper insights into pathogen reservoirs, transmission and persistence in the production and postharvest systems.
• Produce-pathogen pairing showed that herbs and lettuce showed higher association with pathogenic E. coli. Ready-to-eat baby spinach and mixed salads ranked lower to these products. Infrequent detections were observed in other leafy vegetables such as Asian vegetables, kale, chards, cabbages and spring onions.
• Among sources and/or routes of contamination, the following are of special interest:
  • Irrigation water quality (surface water)
  • Organic soil amendments containing animal manures
  • Co-existence with livestock (proximity and waste utilisation)
  • Extreme weather events (flooding, heavy rainfall, bushfires and dust storms)
  • Postharvest cross-contamination risks during handling and processing.

Irrigation water microbial risks

Surface water sources (e.g. rivers, dams, creeks and farm ponds) are the most common sources of irrigation water for the field production of leafy vegetables and are highly prone to microbial contamination and seasonal variation compared to ground water sources.

Approximately 711 water samples have been collected from participating farms, that included monthly sampling from 12 water sources (rivers, creeks, and dams) directly feeding vegetable production system in different states.

• Surface water quality was observed to decline during summer (December–January) and improve during winter. While most sources met microbial quality criteria (<100 CFU E. coli/100 mL), some samples still showed the presence of pathogens like Salmonella.
• Though generic E. coli population proved to be a good indicator of water quality compared to coliforms count, there is still gap in risk assessment, especially when the water sources upstream have contamination risks.
• The data suggested that pumping water from rivers and creeks and allowing the sediment to settle in farm ponds before its application in irrigation improves water quality. Mixing of river water with ground water and then storing in farm ponds is a common practice to improve water quality.
• The presence of ducks and other water birds raises the risk of microbial contamination in farm ponds, depending upon the storage volume of water. Water birds’ droppings collected from multiple farm ponds in several regions occasionally showed the presence of pathogenic E. coli but no Salmonella detections were observed.
• Some farm ponds receive run-off from vegetable and other crop production fields. This run-off could contain high levels of nutrients such as phosphorus leading to the development and proliferation of blue-green algae (BGA) in farm ponds. The risk of BGA toxins in these production farms is speculated to be higher as research studies have shown the translocation and accumulation of toxins from irrigation water in edible parts of crops.
• Overhead irrigation is common for baby leaf production while drip irrigation is popular for head leafy crops such as iceberg lettuce, cabbage and romaine. However, some farms have overhead irrigation system for head leafy crops as well. Depending upon the soil type, the overhead irrigation continues until the day of harvest.

Water quality and quantity are going to be major limitation factors in safe production of leafy vegetables. Preliminary recommendations on managing irrigation water risk, based on this project, are as follows:

• Identify and assess microbial risks associated with irrigation water sources, considering upstream contamination risks. Tabulate historical microbiological data on water quality and look at trends based on the seasons, water sources and major weather events during those periods.
• Water from rivers and creeks should not be directly pumped and used in irrigation systems. Sediment should be allowed to settle before its application.
• The water pumped from ponds should be passed through coarse filtration systems with frequent back flushing to further improve water quality. The project data from a farm using the filtration system showed that water quality measured as E. coli population was improved by this process. This practice during peak summers could further help mitigate risks associated with overhead irrigation water.
• With some practical and cost limitations, application of chemical sanitisers including chlorine and ozone are other options to enhance water quality.
• Heavy rainfall near harvest increases organic load on leafy vegetables via splashing, especially in heavier soils. Some growers use preharvest rinsing of standing crop with water containing a sanitiser to mitigate food safety risks associated with weather events.

Organic soil amendments risks

A soil boot swab by a farm staff member

Untreated raw animal manures, partially composted manures, and aged manures could contain pathogens such as pathogenic E. coli, Salmonella, Listeria and Campylobacter. These pathogens survive in soils for extended durations (from days to more than a year) depending upon the pathogen type and population, soil type, soil temperature, soil moisture, cropping system and several other factors.

The safe use of raw untreated manures is a global debate as there is no scientific consensus on the fate of pathogen survival in real-world scenarios. As a result, leading food regulators (e.g. US FDA) have reserved their standing on the safe use of raw animal products in horticultural production systems. Global Food Safety Initiative (GFSI) certified schemes allow the use of raw manures with a range of exclusion periods for their application. However, the use of composts containing animal products, conforming to Australian Standard AS4454, is considered safe.

The use of animal products (manures and composts) is a common practice in both organic and conventional production systems. However, baby leaf production systems were predominantly avoiding the use of animal products in soils and have adopted green manures (cover crops) to improve soil health. There are several compelling reasons for the use of animal products as a reliable source of soil and crop nutrition, circularity of plant-animal industries, and cost-effectiveness.

As part of this project, it was observed that soil texture (e.g. light-textured sandy soils) in some regions was the driving factor for using animal products to maintain soil health, structure and optimise use of water and fertilisers. The microbiological samples collected mainly from the baby leaf production soils did not show a major concern of pathogen survival, but the sampling
from other cropping systems is continuing to generate a better understanding on this aspect.

At least 15 leafy vegetable growers have expressed interest in participating in a pilot study on the fate of pathogens in soils amended with different types of organic products. This study will generate the first dataset on the pathogen prevalence and distribution and their potential to contaminate produce in the real-world scenarios over multiple cropping seasons. Such longitudinal studies provide useful data and insights into the nature and magnitude of risks and help develop strategies to manage risks. A data-driven food safety management approach has proven powerful in several other industries, such as melons and citrus.

Soil health management is critical to supporting sustainable leafy vegetable production systems with safer amendment options such as certified composts or cover crops. However, the use of raw animal products requires proper risk assessment and management strategies to mitigate associated food safety risks. The data from the proposed pilot study will provide better understanding of risks and management strategies.

Coexistence of animals and horticultural industries and food safety management

Lettuce harvesting in progress

Horticultural and animal industries have coexisted for centuries, but the risk of microbial contamination has lately amplified due to intensification of production systems, degradation of natural resources (soil and water), lengthy supply chains, produce consumption patterns and increased frequency and severity of extreme weather events. The animal operations have potential to contaminate water sources, produce, equipment, and premises.

Management of proper buffer zones to separate these operations and install hurdles to prevent airborne (e.g. wind breaks) and waterborne contamination (e.g. run-off control) are some of the practical and cost-effective measures. Proximity of leafy vegetable production fields to animal operations requires diligent risk assessment and management. This includes postharvest cattle and sheep grazing in leafy vegetable fields as a practice to utilise crop biomass as animal feed.

The exclusion periods applicable to raw manures should also be considered for animal grazing in leafy production systems. The project has identified significant opportunities to improve these measures, especially in regions where livestock graze in vegetable fields postharvest.

Extreme weather events

Extreme weather events are intensifying in frequency and severity. These include floods, heavy rainfall, droughts, dust storms, bushfires and heatwaves, all of which disrupt horticultural production and postharvest systems. While the direct impacts of such events on crop losses are widely recognised, their implications for microbial food safety—particularly in the fresh horticultural produce sector — remain less understood.

In this project, it has been examined how extreme weather events exacerbate microbiological contamination risks in leafy vegetable production systems. There is more work required to develop actionable strategies for risk mitigation and adaptation measures. As part of the recommendations to growers, adoption of climate-smart practices has been proposed to building resilience in the supply chain and safeguarding consumer health. Developing and maintaining an extreme weather events register is one of the strategies to manage risks based on records and subsequent remedial measures.

Harvest and postharvest cross-contamination risks

A project team member collecting microbiological swabs from plastic crate

Harvesting machinery and containers (bins and crates) have been identified as the potential routes of microbial cross-contamination in leafy vegetables. Cleaning and sanitising machinery and harvest containers after each shift/use is the basic requirement to manage these risks.

Postharvest washing and sanitisation processes offer huge opportunities for cross-contamination of leafy vegetables. Wash water quality, a lack of adequate sanitiser concentrations and recirculation of wash water are major risks identified in this project.

Triple wash system

The sanitisation flow during processing of leafy salads in a triple washing system also requires reconsiderations. For example, in response to increased organic load on produce subjected to heavy rainfalls, rinsing leafy salads with run-to-waste is a missing step that can go a long way to maximise sanitisation efficacy of washing systems.

Maintenance of appropriate sanitiser concentrations and frequency of wash water change are critical postharvest operations. It has been observed that triple wash systems in some processing facilities are being operated on a gradient of sanitiser concentration with the lowest or zero concentration in the first wash water tank and then higher concentrations in the second and third tanks. It is recommended that processors should review their sanitiser concentration gradient and the highest sanitiser concentration in the first tank is required to prevent cross-contamination and deal with higher organic load.

Food safety helpdesk

As part of this project, a food safety helpdesk service is available to all growers and supply chain partners to address their technical enquiries on food safety. This service has been popular with the industry to confidentially discuss their food safety concerns and find solutions. To access the helpdesk, contact SP Singh (details below).

Get involved

Over 80 leafy vegetable growers are engaging with this project to seek solutions to food safety process improvement and compliance issues. The project team is calling on growers and processors to participate in the project by sharing their food safety practices, be a part of the irrigation water quality or soil amendments trials and learn more about data-based
food safety risk management. The project team maintains confidentiality and privacy of all participants, and presents and uses data in aggregated and anonymised form for industry-wide food safety practice improvement.

FOR MORE INFORMATION
Dr Sukhvinder Pal (SP) Singh, Program Leader- Fresh Produce Safety and Traceability, NSW Department of Primary Industries and Regional Development. Email: sp.singh@dpi.nsw.gov.au or 0420 593 129.