Was the pomegranate juice you drank really as pure as its label claimed? Researchers have now developed a method to conclusively test whether expensive fruit juices have been adulterated with cheaper ones (J. Agric. Food Chem., DOI: 10.1021/jf104527e).
To make sure juices are authentic, researchers usually test for signature organic acids such as grape juice's tartaric acid and apple juice's malic and quinic acids. But the routine analysis, liquid chromatography coupled to ultraviolet detection, often gives false readings, has trouble detecting low levels of the acids, and struggles with reproducibility. As a result, confusion reigns over whether pure pomegranate juice contains tartaric acid, making it difficult to determine if some brands of pomegranate juice are unscrupulously topped off with grape juice.
So analytical chemist Stefan Ehling and biochemist Shannon Cole at the Grocery Manufacturers Association, a trade group, developed a liquid chromatography-mass spectrometry approach that is highly sensitive and unequivocal in identifying the organic acids. The investigators use a special chromatography column that separates polar organic acids.
With their method, the researchers confirmed that pomegranate juice does contain tartaric acid but at low levels: between 1 and 5 mg/L. Pomegranate juice adulterated by grape juice, by contrast, contains more than 50 mg/L of tartaric acid. And so, Ehling says, high levels of tartaric acid in supposedly pure pomegranate juice would indicate that something's not quite right.
This article originally was published with incorrect units in the final paragraph. The correct units are mg/L.