The burning of fossil fuels such as coal and oil releases carbon dioxide that alters the balance of carbon isotopes naturally found in the environment — an effect that is now being found in food, reveals a US study.

Modern methods for tracking the origins of processed foods use isotopes — atoms of the same element that have different numbers of neutrons. Of the most common naturally occurring isotopes of carbon — carbon-12, with six neutrons, and carbon-13, with seven — the heavier carbon-13 isotope is rarer. In many plants, 108 out of 10,000 carbon atoms are carbon-13. However, in plants such as sugar cane and maize (corn), which use a different type of photosynthesis, 110 out of 10,000 atoms are carbon-13.

Tracking these ratios is a key part of how food regulatory bodies determine if low-cost sweeteners, such as corn syrup, have been added to foods. Because sweeteners from sugar cane and maize have a higher proportion of carbon-13, the carbon isotope ratio of the final product will be skewed.

As part of an undergraduate project intended to show how isotope analysis works, geochemist William Peck at Colgate University in Hamilton, New York, got his students to analyse maple syrup from different parts of the northeastern United States. "Our intent was really just to see if isotope values varied by geography or if anyone was putting in sweeteners," says Peck.

All of the isotope values that the class collected were much the same, but when the group compared their values to isotope values of maple syrup in papers from the late 1970s and early 1980s, they noted that there were significant differences. Their analysis revealed that the relative amount of carbon-13 in maple syrup seemed to have gone down since the 1970s. This got Peck wondering if it was possible that baseline isotope ratios might be shifting because of environmental changes.

Sticky problem

To work this out, Peck and his student co-author, Stephanie Tubman, obtained maple syrup samples from producers in the states of New York and Vermont, covering the period 1970–2006. Some of these samples were not in a good condition: "When we opened one old can of syrup, it smelled like freshly mown grass. It was disgusting. My student had to scrape inch-thick layers of mould off the top for analysis to work out if the mould might change the isotope ratio of the syrup," recalls Peck. Fortunately, it did not.

The researchers analysed 246 samples from this 36-year time period. They report in the Journal of Agriculture and Food Chemistry¹ that maple syrup isotope ratios have shifted over the years. Samples of 1970s syrup had 108.7 carbon-13 isotopes per 10,000 carbon atoms, whereas the 2006 average was 108.5 carbon-13 isotopes per 10,000 carbon atoms. So syrup carbon-13 values are approaching the average 108 value that maple trees and most plants should have, explains Peck.

The reason, he suggests, is that carbon released from the burning of oil or coal, which has very little carbon-13 compared to that found naturally in the atmosphere, is shifting environmental carbon isotope ratios accordingly. Atmospheric data show that isotope ratio changes correlate directly with the changes in the maple syrup isotopes over the course of the 36 years studied, Peck says.

"We've known that atmospheric carbon isotope values were changing, but nobody was applying this to food science," says geochemist John Valley at the University of Wisconsin-Madison. "Clearly, food-monitoring studies need to start taking atmospheric isotope data into account."

Sweet cheat

The findings raise the possibility that producers of foods that are monitored for carbon isotope ratios might be able to add cheap sweeteners without being caught. Yet Peck doubts this is the case. "The producers that could cheat have not had the necessary information do so effectively," he says.

And the findings apply to more than just food. Isotope analysis of human tissue is being considered in some countries to help determine where immigrants have come from. "I think this maple syrup study demonstrates the danger of tissue testing. If we are making serious decisions about peoples' lives with isotope analysis, we must remember that there are numerous effects that determine the final values," says Valley.

As for whether isotope ratios change the taste of maple syrup, for the moment, that remains a mystery. "We had a pancake party in class at the end to celebrate the findings," says Peck. "Nobody was brave enough to try syrups from the 1970s." 

• References

  1. Peck, W. H. & Tubman, S. C. J. Agric. Food Chem. advance online publication doi: 10.1021/jf100104s (2010).

http://www.nature.com/news/2010/100205/full/news.2010.56.html