A study of the proline isomerisation in typical Italian wines

In this paper the presence of the D isomer of proline in wine was investigated. The analyses were carried out on samples of two different types of Italian red wines, ‘Merlot’ and ‘Cabernet’, at different stages of aging during the period 1987–1996. The quantitative determination was performed using an RP–HPLC apparatus equipped with a diode array detector. Marfey's reagent, (1-fluoro-2,4-dinitrophenyl)-5-L-alanine amide (FDAA), was used for the derivatisation, and the obtained diastereomers were separated on a Supelcosil LC-18 analytical column (25 cm × 0.4 cm id), adopting a gradient procedure and making use of water/acetonitrile buffer as the mobile phase. L-Norluecine was used as the internal standard for quantitative determination, and the detection of the derivatised components was made at a fixed wavelength of 340 nm. The data obtained show an interesting correlation between the age of the sample and the presence of the D-proline isomer, due to a racemisation process that can be described as a first-order kinetic reaction. The values of the kinetic constants obtained for the two wines are very similar but differ significantly from those observed for wines aged for more than 10 years. This is probably due to enzymatic degradation processes, which can occur in wine after longer periods of time.