Saturday, July 13, 2024

Salads under scrutiny: Study reveals contamination risks and solutions in ready-to-eat salads

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In a research paper published in the journal Trends in Food Science & Technology, the authors have highlighted the critical points in packaged salad production processes that might be associated with the origin of microbial contamination.

They have also discussed the solutions available to ensure adequate microbiological quality control and health and safety.

Study: From farm to fork: Fungal and bacterial contaminants and their diagnostics in the production steps of ready-to-eat salads. Image Credit: monticello / Shutterstock


Ready-to-eat green leafy vegetables or ready-to-eat salads represent a very fast-growing market. These products are marketed already washed in sealed bags and are meant to be consumed raw, without cooking.

The ready-to-eat nature of these products highlights the need for stringent and comprehensive microbiological quality control to ensure health and safety. Various methods and conditions associated with vegetable cultivation, washing, bagging, storage, transport, and distribution are the major determinants of product quality.

Ready-to-eat salads may contain various pathogens, including bacteria, yeast, and filamentous fungi, which can reduce the shelf-life of products and impose human health risks. Dangerous pathogens (parasites and viruses) associated with foodborne disease outbreaks or contaminations have also been identified in these food products.      

Risks of microbial contamination

Staff training and hygiene are mandatory factors for food safety in ready-to-eat food production systems. Multiple sources, including soil, amendments, manure-based fertilizers, water, neighboring poultry or livestock farms, and wildlife residues, can potentially contribute to microbial contamination at pre- and post-harvest stages.

Water quality is considered to be the most critical factor for microbiological safety. The recent European Commission Regulation has advised using recycled water for irrigation. Water-based nutrient solutions for growing plants are expected to provide better microbiological safety than conventional soil-based systems as they operate in a controlled environment, avoid animal manure and soil-borne diseases, and minimize plant contamination with human pathogens.

However, recycled water and nutrient solutions, seeds, seedlings, and substrates have been identified as further sources of microbial contamination in soilless plantation technologies. This indicates that water in recirculating systems can catch contamination more rapidly than conventional soil-based cultivation.  

According to the Food Code of the U.S. Food and Drug Administration (FDA), ready-to-eat salads are potentially hazardous food products, as they were associated with several outbreaks between 1998 and 2008.

The microbiological criteria established by the European Commission Regulation for accepting ready-to-eat products are based on the presence, absence, or number of microbes and the concentration of toxins or harmful metabolites. The food safety criteria are the absence of L. monocytogenes and Salmonella spp. in 25 grams of non-expired products.

The main risk factors in ready-to-eat food production are transport, processing, and storage temperature, water quality and sanitation, equipment design, hygiene conditions, and worker personal hygiene and training.

Potential hazards may also arise at the critical control points along the production process, including reception of raw materials (collection and storage), selection (manual cleaning and cutting), disinfection (washing and drying), packaging (bags, boxes, wrapping), and distribution (local storage, transport to market).

Maintaining a low ambient temperature during the production and storage of food products is vital for reducing pathogen growth rates. However, it is not sufficient to completely prevent pathogen growth.

The use of disinfectants to maintain water quality is essential, as water and product contamination and cross-contamination with enteric pathogens can occur during washing. Chlorine is the most widely used disinfectant because of its high effectiveness and low price. Other disinfectants are ozone, electrolyzed water, acidified sodium chlorite, hydrogen peroxide, organic acids, bromine, and iodine.   

Equipment, cutting machines and tools, biofilm formation, and conveyor belts are the most significant contamination hotspots. During product packaging, it is vital to determine the correct combination of films, product weight, and gas composition to ensure product quality, safety, and shelf life.   

Proper labeling of ready-to-eat food products with information on ingredients, nutritional value, and origin is important for ensuring transparency for consumers. Labels should also include information on storage temperature, expiry date, and product source.

Microbiological methods for screening ready-to-eat salads

Several microbiological methods have been developed for detecting pathogenic microbes in ready-to-eat food products, including salads. Several primary agar media are available that can be used to detect microbes in these products.

Microbial colonies isolated from agar plates are characterized by biochemical and serological tests to identify primary human pathogens, including Escherichia coli, Staphylococcus aureusSalmonella enterica, and Listeria monocytogenes.

Molecular methods, such as polymerase chain reaction (PCR)-based assays, are also used for colony identity confirmation or for targeted identification of specific pathogens in enrichment media.

The application of high-throughput screening methods is increasing in food safety, targeting culturable and non-culturable pathogens and spoiling agents in ready-to-eat salads. Some novel techniques, including shotgun metagenomics and metatranscriptomics, are gaining popularity because they can directly sequence the target nucleic acids to provide necessary microbial information.   

The current focus of food industries is on sustainable, low-cost, type-specific new technologies that can benefit the production chain of ready-to-eat salads. Some novel methods have been developed to qualitatively and quantitatively assess microbial risks by monitoring critical production points in real-time.

Nanopore-based DNA sequencing technologies coupled with bioinformatics analysis pipelines work by feeding specially trained prediction models and multivariate analyses for the early detection of microbiological contamination.


While zeroing microbiological risk in ready-to-eat salads is statistically unrealistic due to the possibility of contamination throughout the production chain, advancements in sustainable technologies and rigorous microbiological quality control can significantly mitigate these risks and enhance food safety before salads reach consumers.

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