Comparison of gas chromatography-mass spectrometry and electronic tongue analysis for the classification of onions and shallots

Onions and Jersey shallots belong to the same species (Allium cepa L.), but are from two different groups: cepa and aggregatum. The grey shallot belongs to Allium oschaninii O. Fedtsch. Onions and shallots differ in taste but however both contain same sulfur volatile compounds making sensory evaluation difficult. There is a practical need to reliably discriminate onion from shallot. The aim of this study was to evaluate and compare the classification of several cultivars of onions and shallots by the "electronic tongue" and by the chemical analysis of their fresh aroma. The "c-tongue" is an analytical instrument comprising an array of cross-sensitive chemical sensors. The fresh aroma of onion and shallot due to sulfur compounds (thiosulfinatcs and zwiebelanes) was analysed by GC-MS. Data processing was performed by PCA. The "c-tongue" and GC-MS chemical analysis were able to separate onions from shallots. The grey shallot and the white onion were separated from all samples by the two techniques, a result which fits well with its botanical nature. The differentiation between seed-propagated cultivars (all onions and a few shallots) from the vegetative produced ones (all the classic shallots) was also done. The two methods appeared compatible and sometimes complementary.