Verifying food origins using stable isotopes

Hello. I’m Dr. Simon Kelly. This week, we’re going to take a look at the use of stable isotope analysis to detect food adulteration. This technique has been used since the early 1970s to detect the extension and substitution of food and drink with cheaper ingredients. Some examples are– the substitution of freshly squeezed fruit juice with juice from diluted concentrate, the addition of water to wine, the addition of sugar syrups to honey and maple syrup, and the substitution of natural flavours, such as vanilla, with synthetic alternatives made from wood tar. So how do we use stable isotope analysis to detect this kind of food fraud? First of all, let’s remind ourselves what we mean by stable isotopes.

Not all the atoms of an element are exactly the same. If we use carbon as an example, we can see that, in its most abundant form– carbon-12– it contains six protons and six neutrons. The presence of six protons means it is an atom of carbon with an atomic number of 6. However, the number of neutrons can vary between atoms, and these versions of carbon with different numbers of neutrons are called isotopes. Isotopes are atoms of the same element that differ only in the number of neutrons in their nucleus. Here is another example of a carbon isotope– carbon-13.

You can see that it has the same number of protons as carbon-12, but it has an additional neutron, taking the total to seven in the nucleus. It has a slightly higher mass, but it has the same number of electrons, and so behaves the same, chemically, as carbon-12. Carbon-13 is a stable isotope of carbon and does not undergo radioactive decay, unlike carbon-14. A good example of using carbon-stable isotope analysis to uncover food fraud is the detection of cane and corn syrup addition to honey as a cheap extender. A growing demand for honey, and the collapse of domestic bee colonies, means production of honey is falling. And rising prices have led to what industry calls “honey laundering.”

Products labelled as pure honey may, in fact, be a honey blend or honey syrup– that is, honey adulterated with cane sugar or corn syrup. The stabilised detection method relies on measuring differences in the ratios of carbon-13 to carbon-12 that are found in plant sugars present in nectar and plants that are grown for commercial sugar production. About 95% of all plant species, including the vast majority of flowering plants and food crops such as wheat, potatoes, and rice use the Calvin, or C3, pathway to metabolise carbon dioxide during photosynthesis. This process discriminates against carbon-13, giving rise to relatively low carbon-13 to carbon-12 ratios.

However, sugar cane and maise belong to a small variety of plants that use the Hatch and Slack, or C4, pathway, which discriminates less against carbon-13, giving rise to relatively high carbon-13 to carbon-12 ratios. These differences in the carbon-13 to carbon-12 ratios observed between honey and cane syrups can be precisely measured using a stable isotope ratio mass spectrometer, such as the one you see here, which is a Thermo Fisher Delta V connected to an elemental analyzer. First of all, a very small quantity of the honey test sample– usually about one milligramme– is placed in a small tin capsule and sealed.

The capsule is then placed inside the carousel and dropped into a high temperature furnace at 1,000 degrees Celsius in an oxygen atmosphere. Under these conditions, the carbon atoms in the sugars from the honey are completely converted into carbon dioxide. The carbon dioxide is carried by a stream of helium carrier gas through a drawing tube to remove the water of combustion and into a gas chromatography column to separate it from other gases. The carbon dioxide then enters the Thermo Fisher Delta V mass spectrometer, where the relative proportions of carbon-13 dioxide and carbon-12 dioxide are precisely measured after being separated by the magnetic field in the analyzer. Here is the ion chromatogram from the honey sample.

The software reports the corresponding ratio of carbon-13 to carbon-12. By comparing the measured ratio against a database of authentic honeys, the presence or absence of added cane and corn syrups can be confirmed. The sensitivity of the method is limited by the natural variation in honey and sugar syrup’s stable carbon isotope ratios. And this can be improved by extracting the protein from the honey and using it as an internal isotopic reference point. In other words, the carbon-13, carbon-12 isotope ratio of the sugar, and the protein in an authentic honey, should be very similar. This is the basis for the official test method, which can detect the addition of corn syrup or cane sugar at as little as 10% in most honeys.

As we’ve shown, stable isotope analysis is a very powerful technique. And it’s now being used far more widely throughout the food industry to check the provenance of their food products and to prevent food fraud from taking place.