How safe is the food on your plate?

University of Pretoria researchers follow fresh produce from farm to table to ensure that consumers receive safe and nutritious fruits, vegetables and mushrooms.

In an ideal world, a farmer might sell a harvest of fresh produce to families nearby, who would eat that produce within the week.

The more common reality in South Africa and the world however, is that it can take up to three months or even more for food to travel from the farm to table.

“The longer this supply chain, the less control you have over the product and the harder it is to maintain quality and safety,” says Prof Lise Korsten, who leads the food safety research programme at the DST/ NRF Centre of Excellence in Food Security, which is co-hosted by UP and the University of the Western Cape.

Fresh fruit and vegetables are an important part of a healthy diet. But how do we know whether the food is safe from harmful pathogens?

“Our research helps ensure that fresh produce stays safe along that journey for as long as possible.” Korsten explains that many complex factors must be studied so that farmers, sellers, consumers, government and everyone in between have access to the best information available to make sure that food is safe to eat, wholesome and nutritious.

Broad questions UP researchers are investigating include: how regulators ensure that fruit and vegetables are free of potentially harmful pesticides, other agricultural chemicals and human and plant pathogens; how farmers can prevent contamination and reduce losses due to polluted water or human error; and, whether fresh produce from formal markets like retail stores are safer than fresh produce from informal traders.

Considerations like human health, plant diseases, legal frameworks and governance, says Korsten, mean that UP researchers approach food safety in a holistic, multi-disciplinary way.

Pathogen hunters fight mushroom spoilage

As an example, she highlights the hunt for pathogens on mushroom farms currently being undertaken by two postdoctoral students, Dr Noncy Gomba and Dr Nazareth Siyoum.

Although pathogens that affect mushrooms are not harmful to humans, growing mushrooms hygienically - in clean air, the most suitable growing medium, and free from disease-carrying flies, for instance - ensures that mushroom growers suffer minimal losses during production and that the mushrooms are free of human pathogens as well.

Korsten says that most mushrooms available in South Africa are safe, nutritious and delicious - they can in fact be eaten raw, even without washing.

Managing food losses from farm to table

Other research in Prof Korsten’s programmes considers postharvest spoilage or contamination of fresh produce that might make food unfit for human consumption, and how this could be prevented by best practices and more effectively regulated by authorities.

“We’ve been trained to recognise spoilage and it can look disgusting,” says Korsten. She warns, however that unsafe food is not always easy to recognise - it could for example be contaminated by pathogens that are not visible, like Escherichia coli or Salmonella species.

Complying with international safety standards for pesticides will make our fresh produce safer

Pesticide residues above levels allowed by the Department of Health are also a potential threat to human health. In his recent PhD thesis, one of Korsten’s postgraduate students, Dr Mbulaheni Mutengwe, found a critical shortage of local laboratories and inspectors and recommended that regulatory capacity needs to be expanded to more effectively control pesticide residues on local and imported food.

This kind of increased government regulation would boost public trust in food safety, but Korsten notes that consumers also trust big retailers to make sure that food are safe, which are in some ways secured through self-regulation. She adds however that food safety is everyone’s responsibility, and not just that of retailers and regulators.

Korsten gives the example of contamination on the farm by irrigation water that is polluted by mining-related chemicals or by sewage that introduces E. coli and other pathogens. “Water resources are scarce and farmers can’t be solely responsible for cleaning it - the burden must be shared by all, including the municipalities and even the mines that pollute our environment.

Fresh produce from informal traders: Is it safe to eat?

Another important focus of food safety research at UP is fresh produce sold informally, for instance from the back of a bakkie directly after harvest, or by informal traders who may have acquired produce not acceptable for formal markets.

Dr Stacey Duvenage, another postdoctoral researcher working with Korsten, says preliminary research shows that at the point of sale, there’s not much difference in safety between fresh produce sold formally and informally - although more research along the supply chain is needed, this is good news for the millions of South Africans who rely on either.

Click on the pages below to learn more about pathogens on mushroom farms, managing food losses, whether it's safe to eat fresh produce from informal traders, and what international safety standards require in terms of pesticides.

Watch the video or view the infographic or gallery in the sidebar to learn more.


Pathogen hunters fight mushroom spoilage

Leaving no door handle un-swabbed, mushroom detectives sift through potential crime scenes looking for clues that might cause some of our most nutritious food to spoil before it reaches your table.

Leaving no door handle un-swabbed, mushroom detectives sift through potential crime scenes looking for clues that might cause some of our most nutritious food to spoil before it reaches your table.

Pathogen detectives Dr Noncy Gomba and Dr Nazareth Siyoum however, use microscopes in place of a magnifying glass, and wear lab coats instead of trench coats.

They are post-doctoral researchers at UP’s Department of Plant and Soil Sciences, and are always hot on the trail of pathogens and practices that can lead to mushroom production losses and spoilage. They strive to make sure the mushrooms that land on your plate are fresh and suitable to eat.

According to Cancer SA, and the Heart and Stroke Foundation, South Africans would benefit from eating more mushrooms as they are highly nutritious. South Africa lags behind in mushroom consumption compared to countries such as China, where mushrooms rank among the top five most-consumed types fresh produce.

Many consumers might be surprised by how safe mushrooms are: when produced correctly, they can even be eaten fresh without cooking or washing. This is according to Prof Lise Korsten, who leads the DST/NRF Centre of Excellence in Food Security.

However, mushrooms do spoil relatively quickly; the time between harvest and consumption should ideally be within a week or two. Refrigeration may be an option to extend shelf-life and washing can remove dirt but this causes a change in the microbiome of the mushroom over time. These natural microorganisms protect the mushroom from external mushroom diseases and pathogens.

Such losses are a serious issue: consumers do not realise that for every packet of fresh mushrooms they find on the shelves, many others were spoiled, leaving a trail of dead mushroom bodies along the supply chain. Mushroom pathogen hunters like Gomba and Siyoum see this carnage as a trail of breadcrumbs they can follow to the cause.

“We hunt for pathogens that lurk in the air conditioning systems that blow fresh air into the mushroom growing rooms,” says Gomba. Contamination does not end with the air; it can also come from the raw materials used such as straw and chicken manure.

“Preventing contamination is akin to playing a musical instrument,” says Gomba. “You have to fine tune this balance.”

The suspects always on these detectives’ radar are pathogens known to affect fungi. Although not harmful to humans, they can still negatively affect mushroom production - things like Psuedomonas bacteria or yeasts that cause spoilage. These organisms may be responsible for severe post-harvest losses, or simply compromise the quality of your mushrooms at home.

Gomba visits farms, tests for pathogens and gives reports to farmers. She detects where pathogens harmful to fungi may be hiding in important areas such as growing rooms, door handles, and the equipment used throughout the production process.

Based on her findings, she then makes recommendations on how a farmer can prevent contamination.

“The pathogens I work with do not affect humans but they are spread around the farm by flies, and these flies can also carry human pathogens,” says Gomba. “So when we control flies, we control fungal pathogens that affect mushrooms and pathogens that affect humans.”

Occasionally, some human pathogens can also be introduced in the production system and make their way onto mushrooms from contaminated water or unhygienic handwash practices from pickers and packers.

Siyoum studies the microbes that live on mushrooms and how to control those that cause disease. She says the microbiome of mushrooms is made up of a delicate balance of microorganisms, which when disturbed, can introduce pathogens that are harmful to the mushrooms.

Siyoum’s work directly benefits farmers who are keen to keep their mushrooms healthy and their productivity high. This research also impacts us as consumers - human gut microbiomes are positively affected by the balanced microbiome of fresh mushrooms, according to these researchers. South Africans’ health and nutrition would benefit from eating more fresh mushrooms.

Click on the pages below to learn more about managing food losseswhether it's safe to eat fresh produce from informal traders, and what international safety standards require in terms of pesticides.

Click on the gallery for more.

Managing food losses from farm to table

When consumers see fresh produce on the market shelf or on their plate, they often have very little idea of the losses incurred on the journey from the farm.

Understandably, a consumer may feel upset when a fruit or vegetable spoils soon after purchase, or may not even buy fresh produce with imperfections. But losses at the point of sale dwarf losses that occur during the months that go by after harvesting, during which packing, storage and transport activities occur.

Losses are however often preventable or manageable if best practices are followed. Part of the mission of food safety researchers at the Department of Plant and Soil Sciences at the University of Pretoria, led by Professor Lise Korsten, is to provide farmers, consumers and everyone in between with information to minimise waste and losses.

To determine where and how severe food losses are, researchers consider all the links along the supply chain, starting from when the farmer plants a crop to when a consumer plants a fork into the food product. The first step is to ensure that seeds and seedlings are of a good quality and not diseased.

“We test seeds for viability in South Africa because they must reach at least 80% germination,” says Korsten. “The farmer needs information that can help them cultivate properly, and knowledge about plant material and the environment is therefore important to prevent losses.”

Major losses can happen during production due to pests and diseases, which is why it is so important to protect crops during production. Losses can also occur up to and at the point of sale. Unfortunately, at this stage spoilage of fresh fruits and vegetables often means they are not fit for human consumption - thus unsafe to eat. Fresh produce that has imperfections such as bruises, which might indicate spoilage, is sometimes sold to the informal markets, and ultimately to unsuspecting consumers.

“The longer the supply chain is, the more you lose and the harder it is to maintain quality and safety,” she says.

One way UP researchers have helped prevent losses is through the introduction of essential oil sachets that can be put in boxes to reduce avocado diseases. The product relies on cutting-edge, but affordable, nano-technology, edible oils and essential oils to help manage post-harvest losses of these fleshy fruits. This research was completed by PhD student Dr Malick Bill in collaboration with Prof Dharini Sivakumar from the Tshwane University of Technology.

Prof Korsten emphasises that the responsibility of food safety falls on everyone at every link of the supply chain, including farmers, researchers and inspectors. What connects all these links is a cycle of knowledge generation and implementation.

She adds that losses, wastage and food safety go hand in hand, and that consumers should be informed. Ensuring food safety can be as simple as looking at food to determine whether it is spoilt, but human pathogens and contaminants can hide in plain sight. This means that practices like washing, cooking and existing indigenous knowledge systems related to the preparation and preservation of food are important to secure safe food for all.

Click on the pages below to learn more about pathogens on mushroom farmswhether it's safe to eat fresh produce from informal traders, and what international safety standards require in terms of pesticides.


Fresh produce from informal traders: Is it safe to eat?

Unlike restaurants or conference venues that must be formally licensed to prepare and serve food, catering for local church weddings or funerals does not need to comply with specific health and safety standards.

The question is, how safe are such “occasional” foods really compared to foods from the formal sector?

Salads prepared the day before may or may not be kept in a fridge, and the raw produce used to make the salads may have been purchased from informal markets that don’t take food safety regulations into account. On the other hand, the produce may be prepared entirely hygienically and cooked adequately to ensure the safety of guests.

Dr Stacey Duvenage, a post-doctoral researcher in Prof Lise Korsten’s food safety team based at UP’s Department of Plant and Soil Sciences, says although millions of South Africans consume fresh produce from informal markets and settings, it is uncertain if it is as safe as produce bought from retailers and other formal food outlets such as restaurants.

“Previous research focussed mainly on ‘on farm’ food safety, but now the focus has shifted to the market and food plate. We no longer look at the production only but rather at the whole chain up to the point of consumption. Therefore we can ask, ‘how safe is the food on my plate?’

“We don’t yet know if food from formal or informal markets are more risky for consumers or not,” she says. “So now we are focussing on post-production at the consumer level.”

To get a clearer picture of the situation in Gauteng Province, the research group is investigating the safety of fresh produce prepared in churches, in people’s homes and in school feeding programmes.

Part of the challenge, says Duvenage, is that food safety guidelines are lacking for informal growers, vendors and school gardens. In addition, the people who rely most on the informal sector are often also the most food insecure.

Duvenage, who is part of the UP food safety research team working with the Institute for Food, Nutrition and Well-being (funded through the DST/ NRF Centre of Excellence Food Security), hopes that these studies will help establish such guidelines, and provide authorities with recommendations for better regulation of the informal sectors.

Fortunately, Korsten’s work ties in with the food safety and regulatory control programmes, which has a direct link with government to inform policy.

One of the key questions is, ‘how much microbial contamination is too much?’. Duvenage explains that microorganisms are naturally found on fresh produce, and that only some are harmful to humans, and only in certain quantities.

“In South Africa we don’t have a standard yet for fresh produce on farm or at the point of sale, only for ready to eat food,” she says. “Internationally standards may be unrealistic for South Africa and other countries, so if we can identify a healthy microbial load it will have international impact as well.”

“The goal of Prof Korsten’s life’s work has been to identify an organism that can be used as a quality indicator for a healthy microbial load on fresh produce,” adds Duvenage.

Duvenage’s own work is bringing them ever closer to that goal: she analyses disease-causing bacteria on crops. Her analysis paints a clear picture of the microbes found in and on the fresh produce, and it can also pinpoint potential sources of contamination.

“Irrigation water, soil, people who handle the fresh produce, contact surfaces - all of these are possible sources of contamination,” she explains. In a laboratory, she compares bacteria from these sources to those on the fresh produce to identify possible matches.

“We also look at characteristics of the bacteria, like whether they are harmful to humans and resistant to antibiotics.” She says this is important because it is estimated that by 2050, a large number of people will die due to the spread of antibiotic-resistant bacteria.

“After our analyses we give feedback to growers, sellers and handlers. We would like them to put strategies in place such as disinfection practices, better irrigation methods and improved personal hygiene practices.”

She explains that most often, when farmers follow correct practices, produce will not be contaminated with disease-causing bacteria.

In the formal sector, this is usually the case. “It is highly controlled with certified processing facilities and top notch technologies for washing, disinfection, packaging and refrigerated transport,” she says.

In the informal sector however, the supply chain is much shorter and uncontrolled. “Hawkers may enter a farm and pick fresh produce without any specific hygienic practices, and then sell the produce from the back of a ‘bakkie’ to a local community or other sellers.”

Duvenage hopes that more studies into the differences between formal and informal markets will reveal lessons both can learn from each other to ensure food safety.

Click on the pages below to learn more about pathogens on mushroom farmsmanaging food losses, and what international safety standards require in terms of pesticides.

Click on the gallery, video or infographic for more.

Complying with international safety standards for pesticides will make our fresh produce safer

The food safety research team at the DST/NRF Centre of Excellence in Food Security has proposed changes to current pesticide control that would help bring the standard of local fresh produce to the same level of compliance as exported fresh produce.

Dr Mbulaheni Mutengwe and the food safety research team at the DST/NRF Centre of Excellence in Food Security has proposed changes to current pesticide control that would help bring the standard of local fresh produce to the same level of compliance as exported fresh produce.

In terms of minimising pesticide residues, produce that is imported or exported must by law undergo routine inspections, but this is not always the case for locally-traded fresh produce.

As part of his doctoral work completed under Prof Lise Korsten at UP’s Department of Plant and Soil Sciences, Mutengwe has made policy proposals to address various challenges in how pesticide residues are regulated.

South Africa is a major exporter of fresh produce, trading internationally with countries around the world. This important market demands stringent regulation and quality control measures for SA produce: the Perishable Products Export Control Board has been mandated by the Department of Agriculture, Forestry and Fisheries to inspect all agricultural products earmarked for export.

Locally-traded products, however, are not regularly monitored for maximum residue limits in the same way as export commodities. Similarly, imported products are also not well monitored, leaving the local consumer with an important concern: is local fresh produce and imported food as safe as the food we export?

The European Union and the United States have dedicated food safety bodies such as the European Food Safety Authority, and the Environmental Protection Agency, which report annually on pesticide level residues in fresh produce. SA doesn’t currently have the same data for fresh produce traded on the local markets.

Although SA has been testing pesticide residues on export crops, there is no coordination between various role players in terms of monitoring pesticide residues on local and imported products.

Mutengwe analysed pesticides found in locally traded fresh produce and compared the data to that obtained from exported produce and that which is imported into the country. For both local and imported products he compared the levels of compliance with the internationally set and locally adopted Maximum Residue Levels (MRLs).

For the comparison, he first had to describe how well produce destined for export complies with the set MRLs – in other words, what percentage of samples (from available data) exceeded the maximum residue levels allowed by the Department of Health.

Dr Mutengwe analysed a total of 43 756 data sets of fresh produce destined for the export market from 2009 to 2014. He found that 98.9% of the samples complied, and of those that did not, 0.8% had traces of unregistered pesticides (mostly grapes) and 0.3% exceeded the allowed levels (mostly oranges).

With these numbers as a reference, he then collected 199 samples of fresh produce from local fresh produce markets in the City of Tshwane and Johannesburg, which he then sent to the South African Bureau of Standards’ (SABS) laboratories for analysis.

Mutengwe found that 91% of the locally-traded fresh produce complied with the South African set MRLs, while 8% had traces of unregistered pesticides and 1% exceeded the allowed levels. The results show that local fresh produce has 7.9% more non-compliance when compared to the exported fresh produce.

Mutengwe also looked at produce imported to South Africa from other countries (European countries, other African countries and South American countries). He found that 95.5% complied, 3.5% had traces of unregistered pesticides, and 1.19% exceeded allowed pesticide levels.

With this evidence, Mutengwe recommended that government should expand its inspection capacity and that it should include testing for food safety assurance on all locally sold fresh produce. Mutengwe further saw a need to strengthen the local food control systems by engaging all relevant stakeholders and to support the expansion of the South African laboratory framework for pesticide residue analysis.

Following from this work, the Minister of Agriculture, Forestry and Fisheries, Mr Senzeni Zokwana, has since appointed two new roles namely Product Control for Agriculture (Prokon) and Leaf Services, for local and import inspection related to fresh produce, grain and its related products.

Overall, Mutengwe’s research has shown the importance of linking the scientific community with industry and government to achieve more effective food safety standards.

Prof Lise Korsten

June 26, 2018

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Table of contents

  • Professor Lise Korsten

    As Co-director of the Department of Science and Innovation-National Research Foundation Centre of Excellence (CoE) in Food Security (FS), Professor Lise Korsten coordinates and integrates food security studies at the University of Pretoria (UP)....

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