Insoluble fibrous protein---Collagen

Collagen, a family of insoluble fibrous proteins found in all multicellular organisms, is one of the most abundant proteins in nature. It is the most abundant protein in mammals, comprising about 25–30% of total body protein (Bailey & Light, 1989) , 1–2% of bovine skeletal muscle, and 4–6% of high-connective tissue muscles (Whiting, 1989) . It is a major component of skin, bone, cartilage, tendon, blood vessels, basement membrane (endomysium), and teeth (Bandman, 1987 ; Stryer, 1988) . 

Collagen is a rod-shaped molecule approximately 300 nm in length and 1.5 nm in diameter. Its basic subunit is tropocollagen (mol wt 300 kDa), which consists of three helical polypeptide α -chains ( α 1, α 2, and α 3) coiled around one another into a triple-stranded superhelix stabilized by hydrogen bonds. Variations in the composition of the α -chains of tropocollagen give rise to various collagen phenotypes, 19 of which have been identified (Bailey & Paul, 1998 ; McCormick & Phillips, 1999) . 

Properties and Applications 

Collagen sources for use in processed meats include skin, hide, bone, offal, and skeletal muscle (Bailey & Light, 1989) . Collagen can be added to meat products either (1) as a constituent of high-collagen meat raw materials (typically high-collagen tissues, such as skeletal muscle connective tissue, beef hides, pork skins, and tripe) or (2) in concentrated form as a direct additive. It has been concentrated from bone (as bone collagen extract), beef hides, pork skins, and skeletal meat connective tissue (Gillett, 1987) . 

Skeletal meat collagen can be concentrated by mechanical desinewing or extracted by low-temperature rendering followed by extrusion, dehydration, grinding, flaking, or milling (Gillett, 1987 ; Jobling, 1994 ; Prabhu & Doerscher, 2000 ; Prabhu & Hull, 2001) . In either form, collagen can significantly affect the processability and organoleptic attributes of meat products.