Quantification of thaumatin-like proteins in white wine using MALDI-TOF mass spectrometry

The main soluble proteins in bottled wine are pathogenesis-related proteins. Their concentration is influenced by the harvesting technique used, the time of the juice exposition to grape skins and possible microbial infection of grapes. The most typical are namely chitinases and thaumatin-like proteins (TLPs), which accumulate in grapes on ripening. They show a low molecular weight of 20-35 kDa and are resistant to proteolysis as well as the acidic pH of wine. Chitinases are considered the primary cause of heat-induced haze formation because of their irreversible denaturation and aggregation. This process can additionally be affected by the non-protein wine components. We focused on the development of a fast quantification method for wine TLPs using matrix-assisted laser desorption/ionization mass spectrometry. White wine samples were analyzed directly, after dialysis or they were subjected to protein concentrating procedures before the measurements. MS-based quantification was achieved by comparing the peak areas of an internal standard (cytochrome c, thaumatin or myoglobin) and TLPs in the acquired mass spectra. The content of TLPs in commercial wines was determined at milligrams per liter. Chitinases could not be quantified in this way because of a low concentration in the analyzed bentonite-fined wines resulting in missing spectral signals. Anyway, this procedure becomes an alternative to common quantification methods based on liquid chromatography.