Quantification of Interferences in the Direct Measurement of Proteins in Wines From the Champagne Region Using the Bradford Method

Abstract

At this time, the Bradford method is the most commonly used method in enology for direct protein quantification because of its rapidity and its reproducibility. This paper considers interferences which may falsify the estimation of protein in champagne Pinot noir and Chardonnay wines. Ultrafiltration with 10 kDa MWCO membrane shows that nonprotein compounds largely react with the Coomassie Brilliant Blue (CBB) and can produce an A_{595 nm} equal to 30% to 90% of the initial value. Ethanol and exogenous and endogenous phenolic compounds are principally responsible for these interferences. They act indirectly favoring the neutral form of the dye. However, interferences from ethanol and phenolics are not additive. Furthermore, the interaction is negative. Gelatins used in enology react 50 times less than BSA with the dye reagent. Both bentonite and vegetable charcoal treatments partially eliminate proteins and phenolics. The ratios A_{520 nm}/A_{595 nm} (eq. BSA estimated by the direct Bradford method) increases on increasing bentonite fining and decreases on charcoal treatment. For these reasons, the deproteinization effects of enological treatments are, at present, very difficult to estimate with direct measurement. A modified Bradford method involving of A_{595 nm} before and after ultrafiltration (3 and 10 kDa MWCO) is proposed as a more accurate measure of wine protein content.