Protein precipitating capacity of crude canola tannins: Effect of pH, tannin, and protein concentrations

The protein precipitating capacity of canola tannins was evaluated using the protein precipitation assay of Hagerman and Butler (J. Agric. Food Chem. 1978, 26, 809-812) and the dye-labeled bovine serum albumin (BSA) assay of Asquith and Butler (J. Chem. Ecol. 1985, 11, 1535-1543). Condensed tannins were isolated from hulls of Cyclone, Excel, and Westar canola cultivars. The tannin content in the hulls ranged from 98 to 1973 mg of catechin equivalents/100 g of hulls, as determined by the vanillin assay. The effect of pH on the affinities of dye-labeled and unlabeled BSA, fetuin, gelatin, lysozyme, and pepsin was monitored. The optimum pH for the precipitation of dye-labeled and unlabeled BSA was found to be 3.5 and 4.0, respectively. The optimum pH for the precipitation of proteins was found to be 0.3-3.1 pH units below the isoelectric points of the proteins. The crude tannin extracts contained about 20% proanthocyanidins, which were soluble in ethyl acetate as determined by the vanillin assay. Canola tannins showed definitive thresholds prior to the formation of insoluble tannin-protein complexes as determined by the protein precipitation assay. There was also a linear correlation (r(2) = 0.975) between the amount of tannin-protein complex formed and the amount of tannin added to the system. Ethyl acetate soluble proanthocyanidins contributed to the protein-precipitating capacity of crude canola tannins isolated from low-tannin Cyclone canola hulls.