The Food Standards Agency has funded a study to come up with a test to confirm the authenticity of organic foods. Scientists from the University of East Anglia in Norwich report in the Journal of Agricultural and Food Chemistry that they have come up with a way to verify purity by using nitrogen content as a proxy.

Study author Dr Simon Kelly estimates that the test in its current form can reliably characterise tomatoes about 60 percent of the time and sort lettuce and carrots correctly 30 and 10 percent of time, respectively.

"At the moment, the test is not unequivocal," he admitted. "What you can say clearly is that the average values for organic tomatoes and organic lettuce and organic carrots are statistically, significantly different than the conventional ones."

Researchers explain that nitrogen makes up over 75 percent of the atmosphere, though the isotope nitrogen 15 accounts for less than 0.5 percent of that amount. The heavy nitrogen, however, is enriched in crops grown with natural fertiliser, like manure. Ammonia in the manure is a source of nitrogen-both the lighter nitrogen 14 and the isotope.

Ammonia that is composed of molecular nitrogen 14 is more volatile and typically escapes the manure over time, leaving easily detectable amounts of nitrogen 15 present in the fertiliser as well as in the crops grown in it. Conventional crops are typically grown with synthetic fertilisers, which do not lead to this isotopic enrichment.

The researchers used a standard technique, employed since the 1970s, involving gas chromatography and mass spectrometry to determine nitrogen 15 levels in different samples of organic and conventionally grown tomatoes, lettuce and carrots. The results show a fairly clear difference in nitrogen isotope values between organic and non-organic tomatoes - the average value was 8.2 percent higher for the organic version. The other two vegetables showed less distinct differences, with organic lettuce being only a few percentage points higher than regular lettuce and the two varieties of carrots being largely indistinguishable.

But the report concluded: "We strongly advocate that end product tests such as the nitrogen isotope approach cannot and should not be thought of as a replacement for organic certification and inspection schemes."

"However, it is our view that any analytical techniques that assist in protecting consumers from fraud and help to protect the interests of all honest growers should be viewed positively."

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