Modulation of extruded collagen films by the addition of co-gelling proteins

• Co-gelling proteins are more prone to be incorporated in native collagen.
• The substitution of collagen by co-gelling proteins decreases the tensile strength.
• The addition of gluten impairs the tensile strength of telopeptide-poor collagen films.
• Precipitation dominates the tensile strength, rather than the degree of native crosslinks.
• Collagen entanglement decreases extrudate swell and increases tensile strength.

Collagen can be modified by the addition of co-gelling proteins. The extrusion of these gels might lead to collagen films with new functionalities, e.g. microstructure and texture. An amount of 4% (w/w), 2.75% (w/w) telopeptide-poor or native collagen and 2.75% (w/w) of both collagen types containing 1.25% (w/w) soy protein isolate, blood plasma or gluten were extruded utilizing a laboratory nozzle extruder system to form films. Gels and films were analyzed using rheology, tensile tests and microscopy. Results indicated that co-gelling proteins are more prone to incorporation in highly crosslinked native collagen gels, as indicated by a maximal consistency index k* of 2.00 10−3 Pa s−n*, rather than cluster-like telopeptide-poor collagen gels, as indicated by a maximal consistency index value of 0.50 10−3 Pa s−n*. However, the film forming ability of collagen could not be matched by any other protein, as shown by decreased complex viscosities when co-gelling proteins were added. The addition of gluten to telopeptide-poor collagen impaired the film strength due to phase separation leading to lumps. Both collagen types featured comparable tensile strengths, ranging from 0.42 to 1.70 kPa, suggesting that the ionic bonds caused by precipitation determine the film strength, rather than initial covalent crosslinks. The 4% (w/w) pure collagen gels of either type yielded the thinnest films, however, with the highest tensile strength and complex viscosity. Results thus suggest that addition of co-gelling proteins presents a suitable approach to modify the gel strength in order to create collagen films with altered elasticity or tensile strength, leading to sausages with modified sensory attributes, e.g. bite or snap.