Carbon-dating wines produced since the first atomic bomb tests can identify fake vintages, say chemists.
Varying weather conditions mean that vintages can differ widely, and Graham Jones of the University of Adelaide says that some experts reckon that five percent of fine wine being sold is fake.
"The problem goes beyond ordinary consumers being overcharged for a bottle of expensive wine of a famous winery with a great year listed on the label," Jones said.
"Connoisseurs collect vintage wines and prices have soared with 'investment wines' selling for hundreds of thousands of dollars a case at auction."
But Jones' team has found that radioactive carbon dioxide produced from atomic bomb tests and absorbed by grapes can be used to accurately determine wine vintages.
The technique is similar to radio-carbon dating. It works by comparing the amount of carbon-14, a less common form of atmospheric carbon, to carbon-12, which is more stable and abundant. The ratio of these two carbon forms, or isotopes, was constant in the atmosphere for thousands of years.
"Until the late 1940s all carbon-14 in the Earth's biosphere was produced by the interaction between cosmic rays and nitrogen in the upper atmosphere," Jones says.
"This changed in the late 1940s up to 1963 when atmospheric atomic explosions significantly increased the amount of C-14 in the atmosphere. When the tests stopped in 1963 a clock was set ticking — that of the dilution of this 'bomb-pulse' C-14 by CO2 formed by the burning of fossil fuels."
The scientists used an accelerator mass spectrometer to determine the C-14 levels in the alcohol components of 20 Australian red wines with vintages from 1958 to 1997, and then compared these measurements to the radioactivity levels of known atmospheric samples. They found that they could accurately predict the vintage of wines to within the relevant year.