Advantages of the KDS/BCA Assay over the Bradford Assay for Protein Quantification in White Wine and Grape Juice

The present study compared the performance of two colorimetric protein assays, the Bradford and the potassium dodecyl sulfate/bicinchoninic acid (KDS/BCA) assays, for use in wine and grape juice analysis. The Bradford assay was affected by protein type, whereas the KDS/BCA assay had lower protein-to-protein variation. Bovine serum albumin and lysozyme yielded an absorbance (562 nm) vs. protein concentration slope (dose-response curve) similar to that of wine proteins. In the Bradford assay, the presence of 12% ethanol and 200 mg/L of wine polyphenols decreased the protein absorbance by 28 and 16%, respectively, whereas in the KDS/BCA assay such interference was not significant. Among 64 white wines, the correlation between protein haze potential, determined by a heat test, and protein content was better for the KDS/BCA assay. This study confirmed the superiority of the KDS/BCA assay over the Bradford assay for quantifying protein in white grape juice and wine, and it yielded better predictive value with respect to the risk of white wine protein instability.