When new research from the University of Leicester highlighted a potential risk of salmonella developing as a result of the presence of damaged leaves within bags of salad, Produce Business UK found out that it is research that is proving controversial
The study suggests that the juices released from damaged leaves can stimulate the presence of salmonella pathogen growth 2,400-fold, as well as increasing the pathogens adherence to the surface of the packets.
The research project was undertaken by Dr Primrose Freestone of the Department of Infection, Immunity and Inflammation and PhD student Giannis Koukkidis, from the University of Leicester, and published in the academic journal Applied & Environmental Microbiology. Previously very little research had been undertaken about the behaviour of salmonella once salad leaves have been bagged.
Salad leaves have been associated with outbreaks of salmonella in the past and the research by Freestone and Koukkidis suggests that the problem may be because leaves are contained within moist plastic containers that retain leached juice. The researchers used a variety of different leaves including spinach and lambs lettuce in their study.
Dr Freestone is careful to point out that her research did not look for evidence of salmonella in bagged salads. “Instead it examined how salmonella grows on salad leaves when they are damaged,” she explains. “Salad leaves are cut during harvesting and we found that even microlitres of the juices, which leach from the cut ends of the leaves, enabled salmonella to grow in water, even when it was refrigerated. These juices also helped the salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.
“This strongly emphasises the need for salad-leaf growers to maintain high food safety standards as even a few salmonella cells in a salad bag at the time of purchase could become many thousands by the time a bag of salad leaves reaches its use by date, even if kept refrigerated. Even small traces of juices released from damaged leaves can make the pathogen grow better and become more able to cause disease.”
The researchers say that they personally still eat salads but “have become more careful about buying salads, and using them quickly. We no longer store our salad bags beyond one day in the fridge”.
Efficacy of washing
The team also reports that washing did not displace pathogens that had already become attached to the leaves “as they internalized into the lettuce and spinach leaves via the surface stomata (leaf pores),” explains Freestone.
And anything that enhances adherence of foodborne pathogens to a leaf’s surface also increases their persistence and ability to resist removal such as during salad washing procedures. “Even more worrying for those who might eat a salmonella-contaminated salad, was the finding that proteins required for the virulence of the bacteria were increased when salmonella came into contact with the salad leaf juices,” Koukkidis adds.
Dr Freestone admits “the fact that salad garnish can cause food poisoning is a new one” but adds that even though it is rare, it must be considered by the industry. “The priority of salad growers and microbiologists now has to be working together to stop the pathogens entering the salad bag,” she stresses. “We have to work with salad growers to stop food pathogens such as salmonella and E-coli from getting into the salad bag.”
Mind the GAP
This is where the high standards of the UK fresh produce trade and commitment to good agricultural practice on the part of growers come to the fore. “Ready-to-eat prepared produce is grown according to good agricultural practice (GAP) and is washed prior to packing to remove soil and other debris, plant tissue exudates that occur during cutting and to reduce the number of associated micro-organisms,” says Nigel Jenney, chief executive of the Fresh Produce Consortium. “However, since washing will never eliminate microbiological contamination, minimising the potential for contamination in the field – by applying GAP – is key to assuring that the product is safe.”
Jenney also highlights the study’s methodology. “Let’s remember that this research has not evaluated findings from any tests on finished products available to the consumer,” he adds. “The UK fresh produce industry has a high standard of food hygiene, and food poisoning associated with the consumption of fresh produce is extremely rare in the UK. Good agricultural practices, hygiene preparations and packaging minimise the potential for contamination. The fresh produce industry has stringent standards in place to ensure that consumers can enjoy the considerable benefits of eating good quality fresh produce as part of a healthy diet.”
His stance is echoed by market leading prepared salad brand Florette which said in a statement: “We employ strict hygiene measures, including good farm practices, across all of our sites to remove any risk of contamination and gently wash our leaves to ensure they are safe to eat.”
However, others are wary as to whether there is anything new in the research and puzzled as to why it made national headlines. “I am surprised there has been so much furore about the research,” says Kaarin Goodburn, director of the Chilled Food Association. “This is school science, something you would do at GCSE; taking juice out of a leaf and adding germs. Producers select out damaged leaves so they are not put into bags. Pre-washing salads removes exudates and associated organisms. Growing crops using Good Agricultural Practice minimises any risk. This work provides no new knowledge and does not change established risk management strategies in the UK retail supply chain. [The University of] Leicester’s own press release acknowledges this, stating: ‘This strongly emphasises the need for salad leaf growers to maintain high food-safety standards.’
“All that has been done in this work is to break up cells to release their contents (nutrients), then add salmonella and watch it grow. Everyone in fresh produce knows that damaged produce spoils microbiologically more quickly than non-damaged. Because of this good quality leaves are used for leafy salads, often having gone through optical sorting systems to remove those with damage, which further reduces opportunities for microbial growth.”
The association stresses the same factors as the FPC when it comes to potential for growth of any organisms present, and points to the importance of the use-by date given on the products. “Because of these measures, the UK has an excellent safety record with these foods, which we would hope to see mirrored elsewhere in the world,” says Goodburn. “…We are happy to support research which is designed to deliver new knowledge given our focus on best practice.”
The researchers are now working with Campden BRI, a provider of science and technology services for the food and drink sectors, on collaboration with major salad and vegetable producers to discover ways of addressing the issues raised by their study.
Meanwhile, Dr Freestone stresses that the University of Leicester study is a long way from school science. “The paper was peer reviewed by experts and was accepted in a major American Society for Microbiology food microbiology journal, who thought it so important that its research contents should be released to the press,” she explains. “Furore implies public interest and concern, so this is certainly not school science and research that aims to reduce the risk of food poisoning is not trivial school science either.”
Regarding salad leaf damage, the Leicester academic points out aht cut salad leaves are damaged and because they are in a watery environment from the leaf washing, the juices leach out into the residual water. “Previous washing does not stop juice leaching while leaves are in the salad bag,” she says. “Salad leaves do get damaged during processing and transport and multi pack storage in supermarkets – you only have to look at a salad bag to see this bruising. And exudates clearly do leach into the water within bags, otherwise why in our publication did the salad bag water behave similarly to the leaf exudates in terms of pathogen growth, biofilm formation and other effects that might make salmonella more infective?”
In terms of advice, Dr Freestone suggests that growers and packers should introduce “more frequent controls for where salad vegetables are grown to check for high levels of enteric pathogens within the soil environment”. She also says that the water used for fresh produce washing should be free of any pathogens, and equipment checked for the presence of any pathogenic microorganisms.
“When packaging, make sure that the fresh produce is completely dry without any excess of water left in the salad bag environment, because we also showed that salad bag fluid containing cut salad leaf exudates can stimulate salmonella growth,” warns Dr Freestone. “We would also suggest reducing the sell by date of the plastic bag. Now it is currently six to seven days (bearing in mind the time needed to travel from European countries to the UK) but for the last one to two days before the salad bag expires and while it is still closed, we have noticed that leaves in the salad bag often look degenerate and mashed.”