Milk fiber or milk casein is a manufactured, natural polymer, protein fiber. Developed in the 1930s, milk fiber is a regenerated animal protein derived from skimmed milk.

Milk fiber’s manufacturing process consists of the following; after milk is drained and skimmed, it is heated and mixed with additives. These additives can be either of chemical or natural origin. This results in a spinning solution that is then processed into fibers by means of a wet spinning process. As long as milk fibers are produced from “waste milk products” and not from drinkable milk, this fiber can be considered sustainable.

Untreated milk fibers are compostable. Things like dye, toxic chemicals, blended fibers and trims can hinder its compostability.