Comparison of Methods for Measuring Protein in Beer

Mixtures containing various combinations of haze-active (HA) and non-HA proteins (water soluble hordein and lysozyme, respectively) and HA and non-HA polyphenols (tannic acid and epicatechin) were prepared in buffer model systems. A battery of protein methods (Bradford Coomassie brilliant blue dye binding method [CBB], HA [sensitive] protein, the Bicinchoninic acid [BCA] method, and absorbance at 280 nm) were applied to each sample and the results were compared. The various methods gave quite different responses to the different test compounds. With concentrations of the various substances roughly equivalent to those found in beer, absorbance at 280 nm and BCA suffered strong interference from the polyphenols. The CBB and HA protein methods, on the other hand, responded very little to polyphenols. CBB response was much greater to lysozyme than to hordein. Tannic acid haze induction responded only to HA protein, but exhibited some nonlinearity. Results obtained when these methods are applied to beer must be interpreted with caution.