Food Safety Update – Microbiological Risks, Controlled Environment Agriculture & Listeria Contamination in Produce

28th July 2023

Food Safety Update – Microbiological Risks, Controlled Environment Agriculture & Listeria Contamination in Produce


Welcome to our latest Food Safety Update. We aim to provide you with a snapshot of information on topical and relevant food safety issues and, where, applicable, the links to allow you to take your knowledge further.

We welcome your feedback on this service as well as any questions and comments on the topics included.

This edition discusses activities in the food safety and microbiology arena over the last year. We examine the increasing attention directed to minimally processed and ready to eat fresh produce.

Microbiological Risks Associated with Frozen Raw Produce Used in Uncooked Food Preparations

Recently New Zealand Food Safety (MPI) released a technical paper examining the microbiological risks of frozen raw produce that is consumed in uncooked preparations. This is important because it presents an updated view on a previous assumption that frozen produce did not represent a significant food safety risk.

Changes in consumption habits that mean more people are consuming frozen, raw produce in an uncooked form more often, have also led to an increased food safety risk from these products by increasing volumes consumed in this way.

A household survey of New Zealand consumers of uncooked preparations made from frozen raw produce, found berries were the most frequent type of produce consumed in this manner for the likes of smoothies or similar preparations on a weekly basis.

Frozen berries are consumed uncooked by 90% of users while other frozen fruit, such as banana and mango, are consumed uncooked by 60% of users. Raw frozen vegetables are consumed uncooked by only 25% of users, with the most common used without further cooking being carrots and spinach.

Food service businesses that prepare uncooked blended beverages and desserts were also surveyed and had similar results. These were most commonly sourced from wholesalers, with all brands of frozen fruit purchased appearing to come from imports.

Outbreaks traced back to frozen raw produce were predominantly associated with berries. The report chose to focus only on frozen berries for the microbial hazard investigation. Primary microbial pathogens associated with imported frozen berries were identified as HAV (Hepatitis A Virus) and Norovirus and so were the focus of their investigation.

Due to the characteristics of viruses, they are not able to reproduce without a host and so cannot be sampled and grown in a lab in the same way a bacterial pathogen could be. This lack of effective and economically viable options for detecting viral contamination on food samples means that it was not practical to measure the prevalence of viral pathogen contamination on imported frozen berry samples.

Instead, previous outbreaks, both in New Zealand and overseas, were examined to draw conclusions about the level of health risk frozen raw vegetables pose to the New Zealand public. Their findings strongly support a move to strengthen food safety risk management measures for frozen berries.

Perchec, A., Esguerra, C. (2023). Microbiological risks associated with frozen raw produce used in uncooked food preparations (New Zealand Food Safety Technical Paper No: 2023/14). Ministry for Primary Industries.

Controlled Environment Agriculture (CEA) Food Safety Considerations for Ready-to-Eat (RTE) Vegetables

CEA is a term commonly used to refer to farming operations enclosed within a structure, e.g., greenhouses or hydroponics. Many of these operations produce RTE products, such as leafy greens, in a way that is often assumed to be ‘safe’ from a food safety perspective. A recent article in Food Safety Magazine explores this concept of how CEA products may not be inherently ‘safe’ even if they have potential to be ‘safer’ than traditional farming methods.

CEA operations can separate production from the outdoor environment, e.g., the weather, and use water delivered nutrients to eliminate the use of soil. However, no CEA operation is able to operate in an entirely closed system. People, growing medium, water, air, insects, agrichemicals and seeds brought into the CEA environment all pose potential vectors for contamination to enter the CEA system. This means the assumption made by some growers and consumers, that all CEA products are inherently ‘safe’, is not true. The industry is vulnerable to neglecting risk mitigation efforts needed to reduce the chance of a foodborne illness outbreak occurring.

One large challenge for many CEA operations that produce RTE products, is combining what was traditionally two facilities into one. Farming and processing operations occurring in close proximity to one another increases the risk of cross-contamination, if adequate care is not taken to create physical separation between these processes. One potential separation method is using a wash system when transitioning produce between the farming and processing operations. Foot traffic between operations should be highly restricted, and the processing area subject to higher standards of sanitation.

Continuous production systems used by many operations also pose food safety risks. Care must be taken to ensure thorough cleaning of the production equipment occurs, such as nightly sanitation.

To mitigate the risks currently presented by CEA RTE operations, more complete risk analysis and preventative efforts must occur. Current risk assessment requirements are lacking, and third-party audits can bring prescriptive, focusing on minor details, rather than the fundamentals of food safety.

Ensuring optimal food safety and preventing outbreaks of foodborne illness will go a long way towards ensuring consumers continue to be willing to pay premium prices for the perceived benefits of RTE products.

Wilhelmsen, E. (2023, April). Is CEA a Safer Way to Grow and Process RTE Vegetables? Food Safety Magazine, 29(2), 14.

Listeria Contamination in Produce Facilities More Likely in Non-Contact Areas

Listeria monocytogenes is a problematic pathogen occurring naturally in the production environment. The issue with Listeria is that it can be fatal for immunocompromised people, and unborn babies. Elderly people are more likely to be hospitalised if exposed to high levels of Listeria and it can be fatal in many cases.

Recent research on Listeria is helping plug the knowledge gap on Listeria monocytogenes in the produce industry and how it needs to be managed.

A recent study examined contamination patterns, and the associated cleaning practices in three fresh produce processing facilities (cut iceberg lettuce, cut fruit and salad bowl). The study gathered practical data about the environments in produce facilities and the effectiveness of their cleaning programmes.

The objective was to understand how different physical zones in the facility, the process of sanitation, and the zones interconnectivity, affected the likelihood of contamination in fresh produce processing facilities.

The three zones were:

  1. Areas directly contacting fresh produce e.g. knives and conveyor belts (Zone 1).
  2. Surfaces that did not directly contact the produce, but are close by (Zone 2).
  3. More remote non-contact areas e.g. drains, floors, and ceilings (Zone 3).

The testing was undertaken before and after cleaning protocols were completed. This helped identify potential Listeria entry points. Testing highlighted that, regardless of the facility, Zone 3 returned the highest number of positive results for Listeria. Testing also included genetically fingerprinting some samples to determine if the samples were related. This highlighted that contamination present had moved from Zone 3 to Zone 1.

Subsequent testing looked at the effectiveness of cleaning actions and sanitising solutions against Listeria. This highlighted that Listeria was still sensitive to cleaning actions and sanitising solutions, and therefore the cleaning activities were likely not performed well, allowing Listeria to remain in the environment.

This research has broad implications for the fresh produce industry, when examining how environmental monitoring & cleaning should be conducted, to reduce the presence of pathogens such as Listeria.

The take home message: do not assume that all contamination comes from raw materials, and that drains, floors and ceilings can contribute to contamination. To reduce the risk of contamination, cleaning practices in Zone 3 should be reviewed regularly.

United Fresh will communicate more on the challenges of Listeria in future newsletters.

While the research was undertaken in high care processing facilities, the take home message for packhouses, is to look closely at cleaning methods, in the receival areas, as well as along the pack lines.

Fresh Produce Safety Centre Australia & New Zealand – Study Looks at Listeria Contamination Patterns in Processors (21 April 2023).

If you would like to know more on this information, please contact our Technical Advisory Group by emailing or calling 0800 507 555.