The starch of the bran or flour is first transformed into glucoses and dextrin by the action of an unorganized ferment called cerealin. The glucoses are then fermented by certain species of bacteria, Bacillus furfuris being perhaps the chief, with the formation of lactic, acetic, formic and butyric acids, and the evolution of carbon dioxide, hydrogen, nitrogen, and a small quantity of sulphuretted hydrogen.
The principal acid produced is lactic; the acetic acid is produced directly from the glucoses by the above-mentioned bacteria without any preliminary alcoholic fermentation by yeasts.
The mode of action of the drench on the skins may be summed up as follows:—
1. The solution of the last traces of lime which has not been removed by the bate, by the organic acids produced by the fermentation, and the subsequent swelling action of these on the skin fibres. The acids also dissolve a small amount of skin substance.
2. Simultaneously with (1) the distension and floating of the skins by gases produced by the fermentation, so enabling them better to take up the acids.
3. The mechanical absorption of dirt by the particles of bran or flour in the drench.
It is not proposed to enter into further detail, since the next chapter, consisting of the original papers of Dr. W. H. Willcox and the author, contains a pretty complete account of the analysis of the drench and theory of the drenching process.