Quantification of Proteins in White and Rosé Wines

The Bradford method is used worldwide in biochemistry to quantify proteins. Proteins might react with a dye, the Coomassie Brilliant Blue (CBB), to form blue complexes measured at 595 nM. The Bradford method is the most commonly used method in oenology for must and wine protein quantification because of its rapidity, reproducibility, accuracy, and ease of implementation. Nevertheless, different interferences may falsify the quantification of protein because of interactions between nonprotein compounds and the dye reagent. Thus, ultrafiltration with 3–10 kDa molecular weight cut-off (MWCO) membrane showed that nonprotein compounds largely react with the Coomassie Brilliant Blue (CBB) and can produce an A595 nM equal to 30–90% of the value estimated by direct quantification. Ethanol and phenolic compounds are the main responsible for these interferences. Moreover, both bentonite and vegetable charcoal treatments partially eliminate proteins and phenolics. In this chapter a modified Bradford method involving the measurement of A595 nM before and after ultrafiltration (3–10 kDa MWCO) and taken in consideration the global interference with the CBB, will be presented, allowing us for a more accurate measure of the wine protein content.