Hide-powder method

Methods aimed at the determination of tannin material content in the extract The classical method of this type still used is the hide-powder method. These methods were devised to determine which percentage of the extract would participate in leather tanning. Their main drawback for their use in adhesives is their incapacity of detecting and determining the approximate 3-6 per cent of monoflavonoids and biflavonoids, or phenolic nontannins , present in the extract, which do not contribute to tanning capacity, but which do definitely react with formaldehyde and contribute to adhesive preparation. 

A photoelectric colorimeter was used by Faure and Pallu (1936) for the color of the tannin-iron complex. Kretzdorn (1949) also tested an iron chloride procedure, but citric acid (1 % to 3%) interfered. The hide-powder method was applied to dealoohol-ized wines by Ponte and Gualdi (1931). Feigl and Feigl (1946) proposed use of ,a -ferrous dipyridyl sulfate or a,a -ferrous phenanthroline sulfate for the colorimetric identification of tannin. Synthetic tannins did not react and seven red and white wines gave positive tests. 

Protein binding has been used in various guises as a method for analysis of tannins and tannin-containing extracts. The hide powder method has been used for decades in the leather industry. More recently, methods based on the residual absorbance of unprecipitated hemoglobin (10), radiometric titration with labelled bovine serum albumin (BSA) (51), and the ferric chloride induced color of protein-tannin complexes resolubilized in alkali (50) have all been suggested as methods of tannin analysis. 

General protease, a-amylase, and exoglucanase activities were estimated using hide powder-, amylose-, and celliilose-azure substrates, respectively, as described earlier (49). Here, standard curves were developed for the hydrolysis of each azure-linked substrate by standard enzymes of known activity. By this method, one cellulose-azure hydrolysis unit corresponds to one filter paper unit, one unit of hide powder-azure activity corresponds to the hydrolysis of 1.0 nmole of iV-benzoyl-L-tyrosine ethyl ester (BTEE) per min, and one amylose-azure unit of activity corresponds to the hydrolysis of 1.0 nmole of maltose from starch per 30 min. 

As the quantity of water-soluble constituents in the various hido powders varies greatly, it is always necessary, before using a hide powder for tannin determinations, to determine the quantity of soluble constituents by a blank test according to the method described here. 

Non-tanning Matters.—For this determination, the tanning substances are absorbed from the solution by hide powder, the liquid being then filtered and the dry residue estimated in the filtrate. The detanning of the solution may be effected by one of the two following methods. 

Various qualities of hide powder have been recommended for this determination in general, a powder answering the requirements already indicated for the preceding method may be employed. Also in this case some prefer to use a chromed hide powder, which will give more concordant results. 

With tanning extracts adulterated with cellulosic extracts, the indirect method does not, however, give reliable results, since cellulosic extracts contain non-tannins which are fixed by the hide powder. In such a case the tannin should be determined by the direct method, since these non-tannins have no appreciable action on permanganate the difference between the results of the two methods will give an approximate indication of the extent of the adulteration, if this is at all marked. 

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