Tanning substances

Chevreul, M. E., Tanning substances formed by the action of nitric acid on... 

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. 

Of this second portion 50 c c. are evaporated in a tared dish on a water-bath, the residue being dried in a steam-oven to constant weight. It represents the non-tanning substances and is calculated per 100 grams of the original substance. 

Tanning Substances.—These maybe found by difference or determined directly by various methods. 

The value of tanning products depends primarily on their content of tanning substances (tannin), but their mode of employment depends also on the quantity and quality of the non-tannins they contain. 

The analysis of tannin, as far as the determination of the soluble, insoluble and tanning substances are concerned, is carried out, when required, by the methods already described for tanning products in general. It includes also the following tests and determinations. 

The principal determinations to be made on leather are those of moisture, ash, fatty substances, substances soluble in water (tannins and non-tannins) and nitrogenous substances (hide substances) from the results it is then possible to calculate the combined tanning substances and the true leather substances, also the leather yield and the degree of tannage. Other determinations which may be required are those of the sugars, free sulphuric add and lime (in the ash). 

Another quantity of 500 c.c. of the filtered aqueous solution (corresponding with 10 grams of the leather) is concentrated to 125 c.c., in which the soluble non-tannins are determined by means of hide powder, as described for tanning substances (see p. 340). Subtraction from the result of the amount of the soluble mineral matter gives the soluble organic non-tannins. 

Qualitative Investigation.—The aqueous solution of logwood extract is deep red if the extract is neutral, reddish-violet if alkaline, or yellowish if acid (extracts containing tanning substances are always acid) it is turned reddish-yellow and yellow by acids and reddish-violet and then brown by alkalies. 

Addition of extraneous tanning materials (see chapter on Tanning Substances) is indicated by the fact that a very dilute alcoholic solution, obtained in the cold and filtered, is coloured more or less intense black rather than green by ferric chloride. 

Biosynthesis The aromatic hydroxycarboxylic acid is first formed from polyacetates and then esterified enzymatically. Besides in lichens, D. also occur in tanning substances, e.g., in coffee or savory a well-known representative is digallic acid (see tannins). Lit. Culberson Karrer, No. 1023-1053 Zechmeister 41, 1-46 45, 103-234. 

Tannage process. Tannage consists in injection of tanning substances into the dermis structure and their interaction with functional groups of protein molecular chains, which results in formation of additional stable cross bonds. Tannage is one of the most important processes for leather manufacture. This stage in the leather industry radically changes dermis... 

In some instances crop plants were also selected for a higher content of certain types of secondary products. Cultivated apples, pears and other fruits have a better aroma, i.e., contain more volatile secondary substances, and in addition to a higher sugar content are much more attractively colored than their wild progenitors. The pigments (E 5.5.2) and scent volatiles are usually nontoxic for humans, in contrast, for example, to the tanning substances which many of the wild forms of our cultivated fruits contain. 

Fresh tea sprouts contain approximately 70% water. The other components present include catechims, chatechim tanning substance, proteins, minerals, wood fibre, lignin, pectin and between 2.0 and 3.5% caffeine [7]. 

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