UJ team finds faster means to detect food toxins

Johannesburg – Researchers at the University Johannesburg (UJ) have developed a more sensitive method to test for dangerous levels of heavy metals, like arsenic, cadmium and chromium in vegetables and water. 

The method can also be used to test other foods, and the instrumentation used is readily available in laboratories in developing countries.

  The method is faster and more economical to directly identify toxic and carcinogenic heavy metals in vegetables and drinking water. Trace metals such as lead (Pb), arsenic (As), cadmium (Cd) and thallium (Tl) are toxic even at very low concentration levels. Arsenic, cadmium and chromium hexavalent compounds are also recognised as carcinogens by International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP).

  Combining accurate established techniques, the method makes it possible to test for several trace metals simultaneously.

  SARChI chairperson: Nanotechnology for Water at UJ, Professor Philiswa Nomngongo says the study provides a simple, fast and sensitive method for laboratories with limited resources. 

  “The research results can also improve food quality for consumers. This study contributes data that can be used as a reference when setting up or revising guidelines for the maximum allowable levels in common vegetables and palatable water. The method is environmentally friendly and conforms to green analytical chemistry principles. It does not introduce secondary pollution,” she says.

  The method uses ultra-sound assisted cloud-point extraction and dispersive micro-solid phase extraction to preconcentrate samples from vegetables and water. The samples are then directly analysed with inductively coupled plasma optical emission spectrometry.

  All over the world, unwanted trace metals show up in vegetables and drinking water. These metals are among many pollutants that seriously affect human health. Monitoring which metals occur in foods and beverages can really challenge a laboratory.

  Often labs cannot directly measure some trace metals in foods, because they occur in ‘too low doses’: the equipment is not designed to detect such low concentrations. This means that sophisticated methods and expensive equipment are needed to detect the presence of some trace metals.

  Secondly, vegetables are inherently very complex to accurately analyse for chemical elements. This creates long, time-consuming procedures to prepare samples to be ready for metals testing. Combined, this means that monitoring for trace metals in agricultural produce is generally slow, expensive, and can only be done by highly qualified scientists. For developing countries, monitoring can be inaccessible because of that.

  To stay healthy, humans need to eat vegetables and drink water. High quality vegetables contain many micronutrients, including trace metals the body needs to function well.

But too high doses of some metals can make people sick, though health conditions such as cancer may take years to manifest. Others are so toxic that very low amounts can make people extremely ill within days or weeks.

  “We need to eat foods containing some trace metals, such as copper, zinc, and iron. But others are toxic; lead, arsenic cadmium, mercury, among others. On the one hand, vegetables form a vital part of human nutrition. On the other, they are good accumulators of heavy metals. Knowing the level of metal contamination in the vegetables we eat and water we drink can make a difference to health and quality of life,” says Nomngongo.

    Co-author of the study, Dr Luthando Nyaba says previous studies used similar methods, but for analysis of organic pollutants.

  “This is the first time where a clay-based adsorbent is combined with a cloud point extraction method for simultaneous analysis of trace metals in vegetables and palatable water. In this method, we convert solid vegetable samples into liquid form, making it possible to directly analyse trace metals with a suitable analytical instrument. Direct, simultaneous analysis means that more vegetable samples can be analysed at the same time, and quicker than was possible before,” adds Nyaba.

  Unwanted metals affect food crops and drinking water world-wide. Heavy metals pollution from urbanisation, factories, mines, and other industries filter into sources for drinking water and irrigation in agriculture. Some agricultural fertilizers, including re-purposed sewage sludge, also affect food crops.

Many of these metals bio-accumulate in the human body, in animals, plants and the environment. This means that the metals cannot be removed and that the resulting problems can only be managed, not cured.

  The World Health Organisation (WHO) and national governments publish drinking water guidelines and other standards, which show what levels of metals in food and water are likely to affect human health. And the WHO’s International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP) publish lists of known and probable human carcinogens.

  “Locally, the research results can help improve the quality of life for communities that depend on the studied water sources.

The results can also assist government and environmental protection agencies to set, review and enforce water quality regulations,” says Nomngongo.