The 8 billion chickens raised and processed annually in the US generate over one million metric tons of feathers per year. Although feathers contain > 90% crude protein, only a small fraction of feathers is processed as feed supplements. Feathers consist of almost pure p-keratin. With extensive disulfide bonds and cross-linkages, [3-keratin is made of p-sheets and twisted in a parallel manner to form stable fibrils. This highly rigid structure of keratin renders it insoluble and resistant to commonly-known proteases such as pepsin or trypsin. Because animals do not secrete enzymes that can break down disulfide bonds or cross-linkages of keratin, they are unable to digest feather protein. Consequently, poultry feathers are dumped or land-filled, notonly wasting voluminous amounts of potentially valuable animal protein but also adding 150,000 metric tons of nitrogen as environmental pollutants into land and water per annum. Meanwhile, high-quality proteins such as soybean meal are supplemented in diets for animals to meet their nutrient requirements. This supplementation accounts for a substantial portion of animal feed cost, and directly competes with human consumption for a limited world supply of edible food protein.
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