A generous dusting of grated Parmesan perfects a plate of spaghetti. But how can a culinary connoisseur be sure that it really is the king of cheeses, Parmigiano-Reggiano? Augusta Caligiani of the University of Parma and colleagues have developed a method that can help determine whether Parmesan has been bulked out with less noble cheeses—or whether it is an outright imposter (J. Agric. Food Chem. 2016, DOI: 10.1021/acs.jafc.6b00913). The team had previously found that milk and cheese from cows fed a fermented fodder called silage contain traces of two unusual cyclopropane fatty acids (CPFAs), possibly made by lactic acid-producing bacteria in the silage. Now, the team has used gas chromatography-mass spectrometry to look for these molecules in 304 cheese samples. Cheeses with European Union regulations that forbid silage feeding—including Parmigiano-Reggiano, Fontina, and Gruyère—had no CPFAs, confirming their authenticity. But Grana Padano, which does use milk from silage-fed cows, contained 300–830 mg of telltale CPFAs per kg of fat. Grated Parmesan is particularly vulnerable to being adulterated with other cheeses, and this method could help detect counterfeits.