Casein as the major protein of milk is a promising protein source for biopolymer fibers. Current casein-based fibers are fabricated by dissolving caseins in alkaline media and wet spinning in a coagulation bath containing harsh chemicals. In milk, casein is present in so-called casein micelles (CMs). Based on the rennet-induced aggregation, we developed a process that can be applied for the spinning of micellar casein fibers in a sustainable way without the use of harsh chemicals. Fabricated fibers show a surface with a characteristic microstructure, which can also be detected embedded in a network structure inside the fiber. The fibers are stable under acidic and neutral conditions and decompose in alkaline media down to aggregates with sizes comparable to the characteristic microstructure. The so far reached tensile properties of the micellar fiber are between low and mid double-digit percentage range compared to casein azlons.
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Regenerated fibers from rennet‐treated casein micelles can be produced with a defined width and homogeneous surface morphology using a two‐substance nozzle integrated in a spinning setup. Extrusion is carried out under standard conditions in a coagulation buffer with 100 × 10−3 m CaCl2 at 60 °C. Electron microscopic images of dried fibers show a fine structure consisting of µm‐sized strands running parallel to the fiber axis and radially outward. Confocal fluorescence microscopy lifetime images show that the strands are rich in free calcium, which is released upon swelling in deionized water and diffuses out of the fiber. Fibers made with only 50 × 10−3 m calcium chloride swell at a higher rate to larger equilibrium swelling values because of fewer stabilizing calcium bridges. In HCl, initial swelling of the fibers is followed by deswelling to a defined equilibrium state with a densely packed casein matrix, as indicated by fluorescence microscopy images. Because of the large number of dissolved calcium contacts and the residual charges on the casein, swelling in HCl occurs very quickly. Elastic contributions of the network and released ions lead to deswelling of the fiber, which occurs fastest for those with 100 × 10−3 m CaCl2 due to the high proportion of free ions.
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