GO INTO a trendy pub and the beer list will be accompanied by tasting notes as purple as in any upmarket wine bar. The “grassy aromas” and “citrus notes” come from the flowers of Humulus lupulus, or the hop plant. These vary in flavour from region to region and between different varieties of the plant. Brewers therefore tend to be rather particular about obtaining specific types of hops from specific plants in specific places, to ensure the flavour of their beer does not change unpredictably.

But they cannot always be sure of what they are buying. Unscrupulous growers can adulterate high-quality hops with cheaper varieties, which can affect a beer’s taste. Detecting doctored shipments can be difficult. Existing tests focus on measuring levels of chemical telltales such as essential oils. But they are not very sensitive, typically requiring adulteration of 10% or more before triggering an alert. Now, as they report in the Journal of Agricultural and Food Chemistry, Miha Ocvirk, a PhD student at the Institute of Hop Research and Brewing in Slovenia, and Iztok Kosir, his supervisor, think they have a better idea.

The plan is to borrow a fraud-detection technique used for other types of valuable food, such as honey and olive oil. It relies on the fact that not all atoms of a given element are created equal. All carbon atoms, for instance, have six protons in their nucleus (that is what makes them atoms of carbon, instead of, say, nitrogen, which has seven protons). But the number of neutrons varies. Most carbon atoms have six neutrons, making 12 particles in total. But about 1% sport an extra neutron, and are thus dubbed “carbon-13”. Atoms of a given element with different numbers of neutrons are known as “isotopes” of that element.

Mr Ocvirk wants to track carbon-13, nitrogen-15 and sulphur-34 in hops. Levels of those isotopes vary between soils, and different varieties of plant absorb them in different quantities. Measuring them in a shipment could reveal whether it has been tampered with.

To test their idea the researchers collected hop flowers from nine different regions, from New Zealand to Slovenia, and put them into a mass spectrometer. Nitrogen-15 values varied substantially, from three parts per thousand in New Zealand to eight in the Czech Republic. Sulphur-34 values differed too, with the east coast of America averaging two parts per thousand and South Africa averaging 11. That allowed the team to tell which plants had grown where. The researchers report that isotopic analysis is about twice as sensitive as the chemical sort, able to spot adulterated shipments in which just 5% of the contents are from elsewhere.

Subtler variations in carbon isotopes also hinted at which variety of hops had produced a given flower, but that effect was weaker. Mr Ocvirk and Dr Kosir reckon those results could be firmed up by including isotopes of oxygen, hydrogen and lead in the analysis. Expect the first bottles of isotopically certified beer in your local soon.