Lime fleshings - Glue-stock
From www.leather-dictionary.com - The Leather Dictionary
During fleshing, the lime fleshings (also called glue-stock), which also contain water and chemicals from the liming stage, are removed from the skin. This material is approx. 10 - 30% of the total weight of the rawhide.
This stock was used to manufacture glues in the past and hence the term "glue stock". However, this is scarcely needed today. Nowadays, glue stock is sold for the production of gelatine. This is used in pharmaceutical products (gelatine capsules) and foodstuffs (gummy bears, sausages etc.).
Some tanneries are working to separate and sell the fats and proteins, or to use them for energy production (biogas).
Video about the leather production
The leather production in a modern tannery.