Lemon peel and Limoncello liqueur: a proteomic duet

Combinatorial peptide ligand libraries (CPLLs) have been adopted for investigating the proteomes of lemon peels and pulp, of a home-made alcoholic infusion of peels and of a very popular Italian liqueur called "Limoncello", stated to be an infusion of the flavedo (the outer, yellow skin of lemons). The aim of this study was not only to perform the deepest investigation so far of the lemon peel proteome but also to assess the genuineness of the commercial liqueur via a three-pronged attack. First, different extraction techniques have been used for the characterization of the peel (and additionally of the pulp) proteome, secondly a home-made infusion has been analysed and finally the proteome of the commercial drink was checked. The peel (the flavedo, not the underlying layer called albedo) proteome has been evaluated via prior capture with CPLLs at different pH values (2.2 and 7.2). Via mass spectrometry analysis of the recovered fractions, after elution of the captured populations in 4% boiling SDS, we could identify a total of 1011 unique gene products in the peel extracts and 674 in the pulp, 264 proteins in the home-made infusion and just 8 proteins (and protein fragments), together with 12 peptides, in one Italian Limoncello produced in the Sorrento Region, thus proving the genuineness of this product. On the contrary, cheaper Limoncellos were devoid of any protein/peptide, casting doubts on their production from vegetable extracts. This could be the starting point for investigating the genuineness and natural origin of commercial drinks in order to protect consumers from adulterated products.