Untargeted volatile metabolomics using comprehensive two-dimensional gas chromatography-mass spectrometry – A solution for orange juice authentication

Orange juice is one of the most consumed fruit juices worldwide and its adulteration has been a long-lasting concern. In this study, an untargeted volatile metabolomics using a comprehensive two-dimensional gas chromatography-quadrupole mass spectrometry (GC × GC-qMS) was developed to systematically authenticate orange juice. At least 405 citrus whole fruits were collected, belongs to 58 types of orange samples and 23 types of non-orange citrus. The fruit juices were prepared in the laboratory and analyzed using the comprehensive GC × GC-qMS instrument. After optimizing the instrumental settings, this novel method was able to identified ~250 volatiles in each juice sample, covering a variety types of hydrocarbons, esters, alcohols, aldehydes, ketones and others. Combining with unsupervised principal component analysis and supervised partial least squares-discriminant analysis , this novel analytical tool was able to authenticate orange juice from a broad perspectives with a high accuracy in the cross-validation model: 1) differentiating orange juice from non-orange citrus juice (99% accuracy), 2) recognizing orange harvesting years (100% accuracy) and geographical origins (96% accuracy), and 3) distinguishing original pure orange juice from the reconstituted juice (94% accuracy). Key volatile metabolites associated with different categories of samples were also identified after thorough investigation of the loading values of the classification models. These metabolites have high potential to be used as food-markers to design targeted analytical methods for orange juice authentication. This novel comprehensive GC × GC-qMS-based method is ideal for governmental laboratories and the food industry to routinely authenticate orange juice.