Metabolite variation in three edible Italian Allium cepa L. by NMR-based metabolomics: a comparative study in fresh and stored bulbs

Introduction: In fruits and vegetables, comparative analysis of metabolic plant profiles has a high potential for quality control of active components. Onion (Allium cepa L.) is used fresh or stored as food, spice, and in traditional medicine. Its metabolic content, often with nutraceutical value, makes its level an important factor in agronomic production. Objective: To describe for the first time the metabolome of "San Pietro" white onion (WP), and compare its chemical profile with the red onion var. Tropea (RT) and the yellow onion var. Montoro (CM). Furthermore, we also aim to obtain a multivariate model based on NMR fingerprints to discriminate the three Italian A. cepa L. cultivars. Methods: For the chemical fingerprinting we used NMR-based metabolomics. We investigated the aqueous and chloroform extracts of fresh onion at harvesting time, and after 9-month storage. Principal component analysis (PCA), Partial least squares discriminant analysis (PLS-DA) and Orthogonal partial least squares (OPLS-DA) were used to build reliable models. Results: We obtained a clear discrimination of A. cepa L. varieties for the fresh and stored batches. The statistical model highlighted higher levels of fructo-oligosaccharides (FOS) in the fresh WP; RT showed a high content of glucose, citrate and amino acids, while CM had many sulfur components. In the stored samples (CMS, RTS), carbohydrates and sulfur components decreased, while in WPS the free monosaccharides concentration increased. Linoleic acid was overexpressed in the apolar extracts of CMF and WPF cultivars. Conclusion: Metabolomics allows a reliable differentiation among onion varieties, and highlights the potential of fingerprinting for food authentication purposes.