117 | How Do Pathogens Get Into Fruit and Veg? | Chemical Hazards | Drugs in Supplements |
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How do pathogens get into fruit and veg (and what’s the story with melons?);
New and emerging chemical hazards;
Food Safety News and Resources;
The world’s most accurate pie chart (yum);
Food fraud news, emerging issues and recent incidents
Bit of a quiet one in food safety this week, although (sadly) more children have been found to have lead poisoning from cinnamon apple pouches and more adults are getting sick in the North American canteloupe outbreak.
But quiet is good in food safety land, right?
Welcome to Issue 117 of The Rotten Apple. Thank you for joining me.
In this week’s issue, I nerd out on the mechanics of pathogen internalisation in fresh produce and explore why melons seem so much more likely to cause food-borne illness outbreaks than other fruits. Plus I share four quick info bites about new chemical hazards from around the globe.
The scariest common food fraud has got to be food supplement adulteration. New insights have just been published, and you’ll find those in this week’s food fraud news.
Warning: This week’s Just for Fun section comes with a high risk of sugar-cravings.
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How Do Pathogens Get Into Fruit and Veg?
And why are melons so risky?
We don’t usually think of fruit as risky from a food safety perspective. But cantaloupes and peaches have both been involved in deadly recalls this year. There have been a least two deaths from Salmonella infections linked to whole cantaloupes and at least one death from listeriosis linked to whole peaches in the past few months.
It’s not just fruit. Raw sprouts and leafy greens are famously risky foods. But other veg can also cause outbreaks of illness. Like onions, which sickened 2546 people in 13 Salmonella outbreaks in the USA (CDC National Outbreak Reporting System) in the ten years to 2021.
In the US, 16% of outbreaks between 2010 and 2013 were attributed to fruits and vegetables (Bennett, et. Al 2018).
So how do bacterial pathogens like Salmonella, Listeria and E. coli end up in whole fruits and vegetables like cantaloupes, peaches and onions? Turns out there are a lot of contamination routes.
Perhaps the most famous whole fruit outbreak is one which was ultimately traced back to a contaminated fruit washing machine in a cantaloupe packing house owned by Jensen Farms (USA) in 2011.
A total of 33 deaths and one miscarriage from listeriosis were linked to cantaloupes processed on the farm. Another ten people who were infected with Listeria subtypes linked to the outbreak also died, but their deaths were not attributed to listeriosis (CDC, 2012).
In the Jensen Farms case, investigators concluded that the cantaloupes became contaminated with Listeria while being handled after harvesting. Food microbiologists call this ‘post-harvest’ contamination. Fruit and veg can also be contaminated before harvest (‘pre-harvest’).
Pre-harvest contamination can occur on the surface of fruits and vegetables, from soil, irrigation water, and animal faeces. But contaminants don’t always remain on the surface of produce. Bacterial pathogens can also get inside food plants, in a process called ‘colonisation’.
Seeds, water, animal faeces, birds and insects can all become sources of contamination that ultimately leads to colonisation with pathogens.
Soil contamination can take the form of dust and dirt getting onto plant surfaces. A dust storm in Australia is said to have contributed to the contamination of rock melons (cantaloupe) with Listeria in 2018, while a rare dust storm in the United Kingdom contributed to salad leaves being contaminated with pathogenic E. coli from dust carried from a nearby animal farm in 2022*.
Human pathogens can survive in soils for long periods. For example, Salmonella typhimurium persists for more than 7 months in soil, and Listeria for almost a year (Islam et al., 2004 and Piveteau et al., 2011).
Contaminated seeds are another potential source of pre-harvest pathogens in fruit and veg. In the lab, seeds of lettuce, cress, alfalfa and tomato which have been artificially contaminated (‘inoculated’) with bacteria can grow into plants that have pathogens inside their roots, stems and leaves (Liu et al 2018).
Irrigation water and flood water can also deliver pathogens into and onto food plants. Water sources near cattle farms seem particularly risky, and US authorities recommend buffer zones between leafy green growing areas and concentrated animal feeding operations (‘feedlots’) to prevent surface water and dust contaminated with animal faeces from getting onto leafy greens.
In 2005, a pond contaminated with Salmonella and used to irrigate tomatoes caused an outbreak in Romaine lettuce in the US and watercress in the UK have also been linked to outbreaks caused by water sources contaminated by nearby cattle farms*.
Wild animal faeces are also a source of contamination of fruit and veg. Outbreaks have been caused by pathogenic E. coli from strawberries contaminated with deer faeces and spinach contaminated with feral swine faeces*. Improperly composted manure of domestic animals has caused food-borne illness outbreaks in lettuce and spinach*.
Bacteria get inside plants by first attaching to the surfaces of the plants, where they may multiply to form ‘aggregates’, sometimes making use of biofilms, from where they grow through open pores on the plant’s surface to get inside the plant.
Once inside the food plant, pathogens cannot be easily removed by washing. Notably, washing with plain water does not remove all bacterial pathogens from leafy greens (Holvoet et al 2014).
Preventing pre-harvest contamination is a challenge. Using clean seeds, irrigating with clean water and keeping large animals off fruit and veg crops are key controls which can be relatively simple to implement for some farms. However other controls are extremely difficult, and these include controls to prevent dust, flood waters and small wild animals like rodents from coming into contact with produce while it is growing.
Probably the most important preventive measure for pre-harvest contamination of fruit and veg crops is to physically separate them from areas where there are animal farming operations, especially concentrated feeding operations which are a source of concentrated bacterial pathogens, contaminated water and dust.
Post-harvest contamination of fruit and vegetables is more preventable than pre-harvest contamination.
Post-harvest operations include the packing, washing, storage and preparation of fruit and veg. Good manufacturing processes (GMP) and HACCP-based food safety management systems can be used to identify hazards and reduce food safety risks. Key controls include monitoring of wash water, cleaning of process equipment, control of storage temperatures and personnel hygiene protocols.
Interestingly, researchers have shown that areas of damage on the leaves of leafy greens which have been bruised during harvest can support higher numbers of pathogens and other bacteria, and that mechanically damaged leaves allow the invasion of pathogens into the tissue of the leaves. Therefore, reducing damage to edible leaves could help reduce pathogen loads.
Post-harvest wash water contains a sanitising agent to remove some pathogens, however, the concentration of sanitiser must be carefully controlled. Post-harvest wash water can spread pathogens from contaminated leaves to non-contaminated leaves, and contaminated wash water has led to an outbreak of Salmonella from melons*.
Post-harvest contact surfaces can become a source of contamination for fruit and veg. For example, ready-to-eat salad leaves became contaminated with Listeria monocytogenes during contact with a conveyor belt which could not be properly cleaned due to its design, leading to an outbreak which sickened 32 people in Switzerland in 2013 and 2014.
Similarly, in 2021 Dole’s food safety team traced the source of Listeria contamination in a lettuce-linked outbreak to a harvester which had uncleanable areas. Read the fascinating story of Dole’s investigation in Issue 91.
When fruit is cut, bacteria on the surface can be transferred into the flesh, carried by the knife. Cut melons seem to be particularly at risk, being implicated in foodborne illness outbreaks more often than other types of fruit.
One reason given to explain this phenomenon is that the flesh of melons and watermelons has relatively low acidity compared to other fruits, with a pH of about 6 (Thomas et. al. 2024).
Other commentators report that the rough, porous surface of cantaloupe skin (rind) makes it difficult to sanitise during post-harvest washing, which could increase the likelihood of pathogens getting into the flesh when the fruit is cut (anecdotal). Chlorinated washing of whole cantaloupes does not completely remove enterococci bacteria (Gagliardi et al 2003).
A final factor which makes melons risky is that they are often grown in contact with the ground. Researchers found melon production soils had high levels of coliforms and enterococci bacteria in the furrows of fields which were flood-irrigated (Gagliardi et al 2003).
Takeaways for food professionals and consumers
Raw fruit and vegetables can be contaminated with human pathogens during pre-harvest and post-harvest operations. For fruit and veg that will be eaten raw, the contamination can lead to deadly outbreaks.
The prevention of contamination relies on the actions of growers, packing houses and post-harvest operators.
Washing fruit and vegetables does not remove all pathogens.
Fruit and vegetables grown in high-risk areas, including areas near animal feedlots, or where wastewater or wild animals could contaminate the crops should be cooked prior to eating.
In short: 🍏 Pathogens such as Salmonella, E. coli and Listeria can be present on the surface of fruit and vegetables, and can also get inside some plant foods 🍏 Pre-harvest contamination occurs while the food plant is growing 🍏 Bacteria can get inside plant foods by first attaching to plant surfaces, then multiplying, sometimes in biofilms, then growing through the pores of the plant to its interior 🍏 Post harvest contamination occurs during washing, packing, storage and handling 🍏 Melons are grown close to the ground, have low pH flesh and rough, porous skin (rind) which could contribute to the relatively high number of fruit outbreaks linked to melons 🍏
*Sources for these outbreak examples can be found in Thomas, et. al. 2024
Bardsley, R.R. Boyer, S.L. Rideout, L.K. Strawn (2019) Survival of Listeria monocytogenes on the surface of basil, cilantro, dill, and parsley plants Food Control, 95 (2019), pp. 90-94, 10.1016/j.foodcont.2018.07.047
Bennett, S., Sodha, S., Ayers, T., Lynch, M., Gould, L., & Tauxe, R. (2018). Produce-associated foodborne disease outbreaks, USA, 1998–2013. Epidemiology & Infection, 146(11), 1397-1406. doi:10.1017/S0950268818001620
Gagliardi, J.V., Millner, P.D., Lester, G. and Ingram, D. (2003). On-Farm and Postharvest Processing Sources of Bacterial Contamination to Melon Rinds. Journal of Food Protection, 66(1), pp.82–87. https://doi.org/10.4315/0362-028X-66.1.82.
Holvoet, A. De Keuckelaere, I. Sampers, S. Van Haute, A. Stals, M. Uyttendaele (2014) Quantitative study of cross-contamination with Escherichia coli, E. coli O157, MS2 phage and murine norovirus in a simulated fresh-cut lettuce wash process Food Control, 37 (2014), pp. 218-227, 10.1016/j.foodcont.2013.09.051
Islam, M.P. Doyle, S.C. Phatak, P. Millner, X. Jiang (2004) Persistence of enterohemorrhagic Escherichia coli O157:H7 in soil and on leaf lettuce and parsley grown in fields treated with contaminated manure composts or irrigation water J. Food Protect., 67 (2004), pp. 1365-1370, 10.4315/0362-028X-67.7.1365
Islam, J. Morgan, M.P. Doyle, S.C. Phatak, P. Millner, X. Jiang (2004) Persistence of Salmonella enterica serovar Typhimurium on lettuce and parsley and in soils on which they were grown in fields treated with contaminated manure composts or irrigation water Foodb. Pathog. Dis., 1 (2004), pp. 27-35, 10.1089/153531404772914437
Liu, Y. Cui, R. Walcott, J. Chen (2018) Fate of Salmonella enterica and enterohemorrhagic Escherichia coli cells artificially Internalized into vegetable seeds during germination Appl. Environ. Microbiol., 84 (2018), pp. 1-10, 10.1128/AEM.01888-17
Piveteau, G. Depret, B. Pivato, D. Garmyn, A. Hartmann (2011) Changes in gene expression during adaptation of Listeria monocytogenes to the soil environment PLoS One, 6 (2011), Article e24881, 10.1371/journal.pone.0024881
Thomas, G., Gil T.P., Müller, C.T., Rogers, H.J. and Berger, C.N. (2024). From field to plate: How do bacterial enteric pathogens interact with ready-to-eat fruit and vegetables, causing disease outbreaks? Food Microbiology, 117, pp.104389–104389.
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Chemical Hazards Quick Bites
European cranberrybush juice: poisonous?
Scientists who did a safety assessment on the juice of European cranberrybush (Viburnum opulus L.) using rats and mice discovered that a single dose did not show signs of acute toxicity but prolonged exposure could cause changes to the liver, kidney and adipose tissue.
BPA at high levels in Europeans
Bisphenol A (BPA) was detected in the bodies of European adults from 11 countries, with the chemical found in the bodies of 92% of adults. Swiss study participants exceeded safety thresholds for BPA in urine for 71 out of 100 people.
Food plants grown indoors could contain too much mercury
Lights used to grow plants in indoor agriculture operations, including light-emitting diodes (LEDs) could contaminate the plants with mercury if they are made with improperly chosen materials.
Curly kale from an indoor hydroponic vegetable farm contained 10 ppm of mercury, which is 200 times higher than the Singaporean food safety limit of 0.05 ppm of mercury. LEDs from the farm were discovered to be encapsulated in polyurethane which contained high concentrations of mercury.
Honey heavy metal contamination
A study of soil and honey samples from Romania found that soils contained high levels of heavy metals including copper, zinc, lead and cadmium and honey production was occurring near areas with levels of lead and cadmium above legal limits. Heavy metals were also found in some honey samples, showing that bioaccumulation and transfer of heavy metals from soils to plants to honey can occur.
Food Safety News and Resources
Our news and resources section includes not-boring food safety news plus links to free training sessions, webinars and guidance documents: no ads, no sponsored content, only resources that I believe will be genuinely helpful for you.
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The World’s Most Accurate Pie Chart (Just for Fun)
Warning: This week’s just-for-fun might give you sugar cravings!
What you missed in last week’s email
Special Offer: Huge Savings on Group Subscriptions (ends soon);
Olive crisis, a food fraud warning;
Dodgy Influencers Muddying Food Safety Messages;
The Potato Paradox;
Food fraud news, emerging issues and recent incidents
Below for paying subscribers: Food fraud news, horizon scanning and incident reports
📌 Food Fraud News 📌
Supplement fraud: new insights
Probably the scariest food fraud that happens relatively regularly is supplement adulteration with undeclared pharmaceutical drugs, including Viagra-like drugs and stimulants for weight loss.
People who take these supplements think they are getting safe, and often ‘natural’, supplements to support their health. Instead, they are getting unregulated, undeclared and unauthorised drugs… terrifying. There have been multiple deaths from adulterated weight loss teas and body-building supplements.
Scientists have analysed the incidence of unauthorised pharmaceutical adulterants and unauthorised supplements in the European Union and published new insights, as follows: