In the manufacture of many kinds of light leathers, skins, after bating, are drenched. The process usually consists in placing the skins in a mixture of bran and water, half to one per cent. of bran being used (5 to 10 grm. per litre, or 1/2 to 1 lb. per 10 gall.) at a temperature of 29° to 35° C. This ferments vigorously for 18 to 24 hours, with evolution of a considerable quantity of gas, and the formation of weak organic acids. The acids neutralize any lime which has not been neutralized in the bate, and they do this in a very gradual and mild way, being produced by bacterial fermentation exactly as required, in a similar manner to fermentations taking place in presence of calcium carbonate. The gases evolved bring the skins to the surface of the drench, and distend the fibres; the workman attending to the drenches puts the skins down under the liquor with a pole, but a better way is to have the drench vat fitted with a wooden paddle, which may be pulled round by hand when necessary; in this way the skins are put under the liquor without danger of tearing or trapping them; in the case of grain splits the paddle is a great advantage over the pole. The skins are allowed to rise two or three times, according to the condition required. For many kinds of alum leather the bate is omitted and the drench only is used. In English practice the bran is mashed in the vat, in which the drenching is to take place, at a temperature of about 95° F. (35° C.); the skins are then put in and the drench well stirred or paddled until the whole is uniform, the temperature is then usually down to 85° to 83° F. (29·5° to 28·2° C.), at which temperature the fermentation begins. The temperature allowed varies with the state of the goods and the prevailing exterior temperature; in winter it may be higher than in summer, but never usually higher than 90° F. (32·2° C.). In some continental works the bran is mashed in boiling water, allowed to stand for some hours to cool, and only the bran-water is used; in this case the fermentation is different to that in which the bran is mashed at a lower temperature. In the first process the starch of the bran undergoes changes to dextrin and glucoses, which are then fermented by bacteria with the production of acids and gases (see Chapter X.), in the latter case the starch, which has been gelatinized is fermented by other organisms, which probably secrete diastatic enzymes. Less gas is given off in this fermentation than where the bran is mashed at 95° to 100° F. The temperature of fermentation is kept low, usually 25° C. (77° F.). Bran contains a sufficient amount of nitrogenous matter,[151] to furnish the bacteria with nutriment, and until this is exhausted there is no fear of the skins being attacked. [151] Bran examined by Kjeldahl’s process contained 2·2 per cent. N, equivalent to about 13–14 per cent. proteids.