PROTEIN PRECIPITATING CAPACITY OF CRUDE CANOLA TANNINS - EFFECT OF PH, TANNIN, AND PROTEIN CONCENTRATIONS

The protein precipitating capacity of canola tannins was evaluated usi ng the protein precipitation assay of Hagerman and Butler (J. Agric. F ood 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 We star canola cultivars. The tannin content in the hulls ranged from 98 to 1973 mg of catechin equivalents/100 g of hulls, as determined by th e vanillin assay. The effect of pH on the affinities of dye-labeled an d 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 prec ipitation of proteins was found to be 0.3-3.1 pH units below the isoel ectric points of the proteins. The crude tannin extracts contained abo ut 20% proanthocyanidins, which were soluble in ethyl acetate as deter mined by the vanillin assay. Canola tannins showed definitive threshol ds prior to the formation of insoluble tannin-protein complexes as det ermined by the protein precipitation assay. There was also a linear co rrelation (r(2) = 0.975) between the amount of tannin-protein complex formed and the amount of tannin added to the system. Ethyl acetate sol uble proanthocyanidins contributed to the protein-precipitating capaci ty of crude canola tannins isolated from low-tannin Cyclone canola hul ls.