Salmonella contamination in vegetables: causes and prevention

Food safety is a critical concern for consumers and the food industry alike. One of the most significant issues in the food industry is the risk of pathogen contamination in fresh produce. While pathogens like Listeria can grow in field environments even at low temperatures, others like Salmonella are typically found in areas where animals or birds have defecated on the field. Therefore, raw fruits and vegetables, particularly those grown in soil, are at risk for contamination from Salmonella. This article will explore the key factors contributing to Salmonella contamination in fresh produce and discuss effective prevention steps.   

Salmonella causes in vegetables   

8% of 1,200 outbreaks caused by Salmonella bacteria in the United States between 1990-2015 were leafy vegetables due to their large surface area. In addition, such vegetables grow both in the field and under the roof. Lettuce, for example, is one of the most consumed leafy vegetables worldwide, a carrier of Salmonella spp. and Escherichia coli O157:H7 bacteria. A meta-analysis of 1,296 articles conducted by Brazilian researchers found that the average prevalence of Salmonella spp. on lettuce was 0.041, with considerable variation between regions and countries, ranging from 0.001 for Japan to 0.5 for Burkina Faso, West Africa. Another study conducted in Canada found Salmonella bacteria in 0.29% of leafy herb samples.   

Source: Information provided by the National Reference Centre for Salmonella (NRC), without confirmation that cause of death was attributable to Salmonella infection 

A study conducted in Spain found that Salmonella bacteria can survive on the surface of cucumbers even after artificial inoculation. The researchers tested the bacterial count several times over 24 hours. They found that within the first two hours, the bacterial count decreased 10-fold, increased over the next few hours, and returned to its initial count after 24 hours. The study also showed that Salmonella bacteria are more resistant to the low humidity prevalent on the cucumber fruit surface than E. coli bacteria, whose number decreased significantly after only eight hours of infection. These findings confirm that Salmonella bacteria can survive on the cucumbers' surface during room temperature storage. Furthermore, tomatoes were found to be colonised by Salmonella with bacterial cells by using contaminated water for irrigation or spraying.   

Research indicates that contaminated soil can also lead to the contamination of plant tissues, including fruit. For example, Salmonella cells can enter the plant through natural openings such as the stomata or hydathodes. In addition, trichomes and transport through the phloem from the leaves are potential routes for bacterial cells to reach the vegetable.   

Prevention of Salmonella   

Salmonella control measures must begin during fruit and vegetable production and continue throughout the farm-to-table process through effective surveillance, consumer education, and food handler training on preventing salmonellosis.  

People involved in plant care and harvesting must follow proper hygiene rules. For example, leafy vegetables harvested by hand require adherence to Good Agricultural Practices (GAP) and Good Handling Practices (GHP) during harvesting. It includes practising personal hygiene, using protective gloves, and disinfecting tools to avoid microbial contamination of raw horticultural products.  

Food handler  

Studies have shown that food handlers are susceptible to intestinal parasites and Salmonella, which could increase the risk of food contamination through physical contact. Cross-contamination may also occur during food preparation when raw food encounters cooked or ready-to-eat food or if a food handler fails to wash their hands or use separate utensils and equipment for raw and cooked food. The Food Safety Authority of Ireland (FSAI) highlights the importance of proper personal hygiene, avoiding using equipment and utensils meant for raw food for cooked food, and displaying cooked food in areas designated for raw food.  

Hygiene  

According to the USFDA, poor hygiene in food handling can spread Salmonella in food. Restaurants have been associated with many foodborne outbreaks, with 59% of such incidents in the United States occurring in the food service industry. A staggering 75% of such cases are due to poor food handling practices.  

Government regulations: EU 

The EU has taken an integrated approach to control Salmonella in the food chain, which involves the top government level of EU Member States, the European Commission, the European Parliament, EFSA and ECDC. This approach covers various routes of Salmonella exposure and includes legislation that requires an absence of Salmonella in ready-to-eat foods. The EU Regulation 2073/2005 mandates the absence of Salmonella in food samples, with an absence of the bacteria required in ready-to-eat foods and proof of absence as part of buying specifications for raw and finished products.  

Read more: Comparison Of Environmental Regulatory Requirements For Salmonella And L.Mono 

End product testing: rapid pathogen detection methods 

Rapid testing methods that comply with ISO 16140 are highly appealing to the industry due to their simplicity, ease of use, and ability to be performed anywhere, at any time. In addition, these tests often use immunological or molecular techniques such as lateral flow devices or polymerase chain reaction (PCR). However, some rapid tests have had high false negatives and false positives rates, leading to new and improved methods such as the N-Light^TM Salmonella Risk screening method developed by NEMIS Technologies AG. This method detects potential Salmonella contamination in a food producer's environment. Therefore, it is suitable for use in food processing areas and equipment as part of an environmental monitoring program. 

Using the N-Light^TM screening method, performing Salmonella pathogen screening directly in the factory takes only five easy steps. The technique involves using hand tools such as the Bench Top Luminometer, Dry Block Heater, N-Light^TM Test Salmonella Risk, and Sterile Dry Swab with separate PBS Buffer. Compared to other rapid test methods such as LAMP and LFIA, the N-Light^TM Salmonella Risk is more sensitive than any chromogenic method. However, it is also one of the fastest methods available in the market, with results obtainable within 24 hours (to be more precise, 24 hours + 3 minutes + 10 seconds). 

Read more: Rapid Testing Methods For Salmonella Monitoring In Food Production  

Summary 

Control efforts for Salmonella must involve systematic processes throughout the food chain, from poultry production to consumer education and training of food handlers. The WHO has recommended strengthening food safety standards, enhancing surveillance efforts, and improving national and regional laboratory effectiveness to prevent the spread of Salmonella.  

Only strict measures can guide domestic production and international imports, while periodic food handlers training is essential. Emphasis should be on production control rather than the testing of final products. Public education and increased awareness of salmonellosis and potential sources of Salmonella infection are also crucial. 

Additional sources: 

1. https://www.aaem.pl/Fresh-vegetables-and-fruit-as-a-source-of-Salmonella-bacteria,156765,0,2.html 
2. https://www.foodsafetynews.com/2019/01/researchers-uncover-how-salmonella-infects-plants-before-harvest/ 
3. https://www.sciencedirect.com/science/article/abs/pii/S2214799318300456?via%3Dihub  
4. https://www.mdpi.com/2304-8158/10/5/907