Since the early twentieth century, a lot of different approaches have been followed to produce fibers from regenerated proteins such as silk fibroin, collagen, gelatin, or milk protein. These fibers, known as “azlons,” were developed to compete with other natural protein fibers such as wool or silk in order to improve material properties, provide new functionalities, or reduce the ecological production footprint. According to Brooks [
1], three different generations of regenerated protein fibers can be identified, each developed in response to contemporaneous economic and social factors. They are produced by dissolving the proteins in alkaline media and forcing the solutions through a spinneret into an acidic coagulation bath followed by an additional crosslinking step [
2‐
4]. Commonly crosslinking is performed with formaldehyde. Recently, other more biocompatible crosslinking agents such as citric acid or enzymatic treatment are used for post-treatment. [
5,
6] In the bioinspired spinning process, regenerated silk fibers are formed from a silk protein solution, which is aligned along the spinning jet axis by elongation forces [
7]. In the natural spinning process, silk proteins form micelle-like configurations with anisotropic liquid crystalline properties which, after shear stress and dehydration, become fibers when pulled out [
7,
8].
Casein, the main protein in milk (80%), can be described as a group of natively unfolded or intrinsically disordered molecules [
9]. Until now, casein could only be used in combination with other additives for the production of regenerated protein fibers. In milk, the caseins form the so-called casein micelles (CMs) due to their poor solubility properties. CMs are roughly spherical, colloidal structures with diameters ranging from 50 to 600 nm [
10]. Their broad size distribution can be described by a log-normal distribution [
11]. CMs are complex association colloids composed of four natively unfolded phosphoproteins α
S1-, α
S2-, β-, and κ-casein and colloidal calcium phosphate. [
10,
12,
13] Their primary structure shows distinct hydrophilic and hydrophobic regions, which led to their consideration as diblock copolymers within the dual binding model of hydrophilic and hydrophobic interactions [
14,
15]. According to the model, the hydrophobic regions of the caseins associate with each other, while phosphoserine-rich spots in the hydrophilic blocks associated with colloidal calcium phosphate particles distributed in the micelle. A comprehensive experimental proof of the dual-binding hypothesis was provided by in-situ light scattering studies under high hydrostatic pressure [
16]. The contribution of hydrophobic interactions to overall stability was singled out from the electrostatic interactions by temperature-dependent measurements, which were sensitive to variations in pH and calcium concentration. On the outside of the CM, κ-casein builds up a so-called “hairy” layer with a thickness of about 5–10 nm. The hydrophilic C-terminal part (casein macropeptide, CMP) expands into the surrounding solution, while the hydrophobic N-terminal part of the molecule is anchored in the micelle. The κ-casein surface layer thus formed stabilizes the CM sterically and prevents unlimited growth by shielding further hydrophobic contacts [
11,
12]. After enzymatic cutting of the κ-casein brush, CMs can be converted into so-called para-casein micelles (para-CM), which agglomerate at ambient temperature and form a gel. During this process, chymosin, the main enzyme of the rennet extract from the calf’s stomach, specifically cleaves κ-casein at the Phe
105-Met
106 bond [
17]. The hydrophilic C-terminal part of the κ-casein is released into the serum while the N-terminal part remains attached to the micelle. If the process is carried out at temperatures above 18 °C, after hydrolysis of about 80% of the κ-casein, the micelles lose their colloidal stability and begin to aggregate [
11]. The subsequent sol-gel transition takes place via the formation of large primary clusters, which then combine in a second step to form a space-filling cluster gel [
17,
18]. The aggregation strongly depends on environmental conditions such as temperature, pH, and Ca
2+ concentration [
19]. The forces stabilizing the aggregates are not yet fully known but hydrophobic interactions are probably major contributors, because coagulation hardly takes place at low temperatures [
20,
21]. However, when the enzymatic cleavage is carried out in the cold, the para-CMs keep their colloidal stability, so that they can be used specifically for controlled structure formation [
22] and for studying the rennet-induced coagulation process [
23].
So far, only non-micellar sodium or calcium caseinate has been used as raw material to produce regenerated casein fibers. Sudha et al. used casein/soy blends to fabricate fibers of 100–250 μm diameter in a wet-spinning process [
24]. SEM images showed that pure casein fibers had a very smooth surface, while hybrid fibers showed increasing surface roughness with increasing soy content. Yang and Reddy employed a wet-spinning process using sodium sulfate and acetic acid as coagulation agents [
6]. Subsequent crosslinking of the fibers with citric acid and sodium hypophosphite followed by a thermal treatment led to the enhanced mechanical stability of the fibers. Tomasula et al. used casein/pullulan blends to produce sub-micrometer-sized fibers in an electrospinning process [
25].
Here we show a simple process how fibers can be produced from rennet-treated casein micelles in a wet-spinning process without further additives. We report on typical microstructural features inside and on the surface of the fiber as well as on its solubility behavior and mechanical properties.