Some chemicals are easier to monitor than others, explains Angelika Tritscher, World Health Organization (WHO) secretary to the Joint Expert Committee on Food Additives (JECFA).
Chemicals added to foods deliberately, and fraudulently, to inflate profits are almost impossible to monitor. Their entry into the food supply is not accidental. Consequently, experts know little or nothing about it until an event occurs.
In 2008, the world discovered that Chinese milk distributors were adding melamine to milk so that it could be watered down without reducing the apparent protein content, the primary test of quality. The protein content of milk is approximated via a simple nitrogen assay, so milk distributors added melamine, which is nitrogen-rich, to watered down milk to fool the testers. Suspicions were only raised when babies became ill in startlingly high numbers, and with problems that rarely affect the young, such as kidney stones. According to the WHO, six children in China died and over 50,000 became ill.
Melamine is used in a range of industrial products, including laminates, glues, moulding compounds, coatings and fire retardants. China produces more melamine than any other country. It now seems likely the deception was facilitated by consistent oversupply, leading to a glut of melamine on the market, which forced the price down.
So-called food fraud, the deliberate and illegal adulteration of food to inflate profits, is a problem well known to regulators. But it seems they have struggled to uncover its full extent. In a speech given in 2008 during the European Food Fraud Conference, Ian Reynolds, deputy chair of the UK Food Standards Agency, said that in Europe ‘the true extent and cost of food fraud is largely unknown’. In many instances, it presents no more than an expensive nuisance to consumers, tricked into thinking their battery farmed eggs are free range or their regular potatoes are organic. But, as the melamine event shows, it can sometimes lead to massive public health crises.
‘This melamine event is probably the biggest food safety event we have ever had, with the biggest public health impact,’ says Tritscher. ‘The emergency phase is over. But we advise countries that they should be aware products may still circulate in the marketplace. Some of the products affected, such as milk powder, have shelf lives of a couple of years potentially.’
The WHO is drawing up a report, which will contain an analytical methods section, with strategy recommendations, so that member states can tailor their monitoring approach to their needs. But, in the short term at least, melamine monitoring is likely to be a feature of many national and international strategies. The EU, for example, has established a law requiring that all imported Chinese foods be tested for melamine, and for results to be reported to the European Commission via the rapid alert system for food and feed (RASFF).
When the melamine event hit the press, Mengshi Lin, a food science professor at the University of Missouri, US, was already developing better ways of detecting melamine in food. He had started work on the problem following a 2007 event, in which thousands of dogs and cats in the US were killed or made ill by melamine-adulterated pet food from China. Lin has developed an approach using surface enhanced Raman spectroscopy (SERS) and HPLC that has proved effective with a variety of food samples, including milk, wheat gluten, chicken feed, cake and noodles. Most spectroscopic approaches, such as IR or UV spectroscopy, allow researchers to infer the molecules present from the photons absorbed. Laser light is passed through a sample and photons with energies that correspond to key molecular energy changes are consumed. Raman spectroscopy makes use of inelastic scattering, where photons change energy by amounts that correspond to key molecular energy changes. In SERS, the molecules are adsorbed onto rough metal substrates, which enhances the typically weak phenomenon.
According to Lin, using SERS is both ‘faster and simpler’ than the ‘time consuming, labour intensive and expensive’ method approved by the US Food and Drug Administration (FDA) following the 2007 pet food incident, which takes over 30 mins/sample. Lin’s group is now looking at substrates to make such tests more practical and reduce costs. ‘A major problem with SERS is the availability and reproducibility of SERS-active nanosubstrates,’ says Lin. ‘Right now there are only a few commercial substrates available on the market, and their reproducibility and shelf lives need to be improved. In addition, most of them are gold- or silver-based substrates.’
In contrast, it was the Chinese students in his group who encouraged Renato Zenobi, professor of analytical chemistry at the Federal Institute of Technology (ETH) in Zurich, Switzerland, to investigate a better test for melamine in milk. His group investigates novel analytical applications for mass spectrometry, and has developed an approach called extractive electrospray ionisation (EESI). To perform mass spectrometry, molecules have to be made into ions and forced into the gas phase, a brutal process that typically leads to partial breakdown. This means that for the analysis of larger molecules, like melamine, ‘soft’ approaches are needed so that at least some molecules remain whole long enough to be detected. There is also a trend towards mass spectrometry techniques that work at ambient pressure, as this removes the need for cumbersome vacuum equipment. The most common of these is electrospray ionisation (ESI), in which samples are ‘pre-treated’ so that they can be made into a fine mist. EESI, like ESI, works at ambient pressure but requires no sample pre-treatment. It uses a continuous mist of a ‘background’ solvent, such as methanol, into which samples can be immediately introduced. ‘The instrument is constantly running,’ Zenobi explains. ‘As soon as you put something in, you see a chemical signature.’ It was the work of two days to adapt his approach, devising a test that takes just 30s/sample to analyse the melamine content. ‘It was a small step to find a simple way to turn milk into a fine mist, and we did that with an ultrasonic transducer,’ he adds.
‘The situation created an immediate need for an analytical method that is highly sensitive, fast and accurate,’ says Graham Cooks, a chemistry professor at Purdue University, US. ‘We took it as a challenge to use simpler instrumentation.’ His group has reported another type of mass spectrometry that incorporates a low temperature plasma probe to ionise samples at ambient pressure. His approach, like Zenobi’s, takes about 30s/sample. ‘Researchers are working to make these devices faster, easier to use and more portable. Perhaps one day everyone will have a mass spectrometer to analyse whatever comes their way.’
Rapidly developing appropriate tests in the immediate wake of an event like this is important, and analytical chemists play a role. But how might dangerous food fraud events be prevented in the future? Tritscher says that it is vital that all those involved, not just the food producers, take some of the blame. Chemical companies, she says, could help by taking more responsibility for where their products end up. As Tritscher puts it, ‘all that melamine had to go somewhere’.
Systemic change is also needed. ‘At the source of all this is the increase in global food trade, and emerging-market countries are entering that trade very aggressively and on a very large scale,’ Tritscher says. ‘In countries like China, Brazil and India, the economies are developing so rapidly, and expanding so much in terms of exports, that internal food safety systems may not keep up with the growth. In order to avoid these fraudulent practices, you have to have well functioning food safety systems set up. It’s not just laws and control but education as well.’
But events like this recent melamine incident are, by their very nature, unexpected, and no amount of regulation could ever entirely eliminate the risk of them occurring entirely. As Tritscher points out, ‘you can only find what you are looking for and you can only look for what you know about. You cannot screen for all of the hundreds of thousands of chemicals that exist in our world.’