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Casein, as the main component of milk protein, has good hydrophilicity, biocompatibility, and functionality, and is widely used in fields such as food packaging, tissue regeneration, drug delivery, and cosmetics. Electrospinning is a common method for preparing nanofibers. The physical and mechanical properties of nanofibers can be customized by adjusting process parameters, solution characteristics, and environmental conditions. However, the low molecular weight and self - aggregation properties of casein limit its electrospinning performance. κ-carrageenan is a polysaccharide derived from seaweed and is often used as a thickener and stabilizer in food. Research has found that adding κ- carrageenan is expected to improve the performance of casein nanofibers. Therefore, it is of great significance to explore the interaction between them and its impact on the performance of nanofibers.
1.1 Preparation of Electrospinning Dopes
Firstly, the activation conditions of κ-carrageenan in the ethanol/water system were explored. Preliminary experiments showed that it exhibited strong thickening/gelation properties when heated to 60 - 80 °C in a 40 - 50% ethanol - water solution. Subsequently, by changing the dispersion and mixing temperatures (20 °C or 60 °C), different combinations of κ-carrageenan/casein electrospinning dopes were prepared. At the same time, the content of κ-carrageenan in the electrospinning dope was adjusted (0 - 2 wt%), and its influence on the performance of nanofibers was studied. The electrospinning device was used to fabricate nanofibers from these dopes.
1.2 Characterization Methods
A variety of instruments were used to comprehensively characterize the electrospinning dopes and nanofibers. The apparent viscosity was measured using an HR - 3 hybrid rheometer, the pH and conductivity were determined using a pH and conductivity meter, and the surface tension was measured using a du Nouy ring precision tensiometer. The surface morphology of nanofibers was observed by scanning electron microscopy (SEM), and parameters such as fiber diameter, bead number, and bead area were analyzed using Image J software. The specific surface area of nanofibers was measured by a surface area analyzer, the thermal properties of solid dispersions were studied by differential scanning calorimetry (DSC), and the quasi - static mechanical properties of nanofibers were tested by a dynamic mechanical analyzer (DMA). These characterizations helped to understand the properties of the nanofibers prepared by the electrospinning machine more comprehensively.
The activation conditions of κ-carrageenan in the water system are clear, but there is no conclusion in the dispersion system containing ethanol. The study found that the physical appearance and apparent viscosity of κ-carrageenan in a 50% v/v ethanol/water mixture were significantly affected by concentration and temperature. For example, a 0.3 - 0.4 % w/v κ-carrageenan dispersion would gel when heated to 80 °C and cooled to 20 °C. Changes in ethanol content and temperature can alter the physical properties of κ-carrageenan dispersions, which is attributed to changes in solvation and hydrogen - bonding interactions.
The temperature for preparing and mixing casein and κ-carrageenan dispersions has a significant impact on the quality of electrospun fibers. When both were prepared and mixed at 60 °C, electrospun fibers with the maximum deposition and the fewest bead defects could be obtained (Figure 1). However, when prepared and mixed under other temperature conditions, problems such as thicker fibers, more beads, and increased inter - fiber porosity would occur. This indicates that temperature is crucial for the partial dispersion of casein aggregates and the conformational transition of κ-carrageenan, thereby affecting the interaction between the two and the spinning performance.
2.2 Morphological Attributes of κ-carrageenan/Casein EMs
As the content of κ-carrageenan increased, the apparent viscosity and conductivity of the electrospinning dope increased. The change in κ-carrageenan content had a significant impact on the morphology of nanofibers (Figure 2). EMs without κ-carrageenan (K - 0) had the most bead defects and the smallest average fiber diameter. When the κ-carrageenan content was 1 wt% (K - 1), the bead number and area reached the minimum, the average fiber diameter increased, and the specific surface area increased. When the κ-carrageenan content was further increased to 2 wt%, the bead number and area increased again, and the surface area decreased. This is because changes in κ-carrageenan content can alter surface tension and phase separation, thus affecting the morphology of nanofibers.
2.3 Quasi - Static Mechanical Properties of κ-carrageenan/Casein EMs
Casein EMs without κ-carrageenan were brittle and difficult to peel from the aluminum foil, so their mechanical properties were not evaluated. As the κ-carrageenan content increased from 0.5 wt% to 1 wt%, the elastic modulus increased by about 3 times, and the tensile strength increased by 2 times. The significant reduction in bead defects in K - 1 EMs contributed to the improvement of its mechanical properties. However, when the κ-carrageenan content was further increased to 2 wt%, the elastic modulus and tensile strength decreased, and the elongation at break also decreased significantly (Figure 3). This indicates that an appropriate amount of κ-carrageenan can enhance the mechanical properties of nanofibers, but excessive addition will lead to a decrease in performance.
2.4 Water Sorption of κ-carrageenan/Casein EMs
Casein EMs without κ-carrageenan (K - 0) rapidly adsorbed about 35 wt% of moisture within the first 5 h, and the final moisture content reached about 56% after 48 h. Electrospun mats containing κ-carrageenan continuously adsorbed moisture rapidly within 48 h. Among them, EMs with 1 wt% κ-carrageenan had the strongest water absorption, and the water absorption rate reached 140 wt% at 48 h (Figure 4). This is related to the fact that EMs with 1 wt% κ-carrageenan have the largest specific surface area. After water absorption, the fibers would fuse and expand, and EMs with different κ-carrageenan contents showed different morphological changes.
2.5 Thermal Behavior of Solid κ-carrageenan/Casein Dispersions
The DSC curve showed that the freeze - dried κ-carrageenan/casein solid dispersion had a broad endothermic peak in the range of 80 - 120 °C. This peak was attributed to the evaporation of moisture in the solid, and the peak temperature (Tendo) was related to the energy of the polymer - water interaction. At all κ-carrageenan concentrations, the transition temperature of the solid dispersion prepared at 60 °C was higher than that prepared at 20 °C, and the enthalpy change (ΔHendo) was lower (Figure 5). This indicates that when mixed at 60 °C, κ-carrageenan binds to casein in a disordered single - chain form, while when mixed at 20 °C, κ-carrageenan may exist in a double - helix conformation.
Through this study, it is known that by using a 50% (v/v) ethanol/water mixture, preparing a κ-carrageenan/casein dispersion containing 1 wt% κ-carrageenan (relative to the total polymer) at pH 10 and a mixing temperature of 60 °C, κ-carrageenan/casein EMs with the fewest bead defects (2×10−3/μm2) and an inter - fiber porosity of 52% can be formed. The 1 wt% κ-carrageenan/casein EMs had a maximum strength of 0.2 MPa, a modulus of 12.4 MPa, and good structural stability.
The research results show that the mechanism for the formation of casein/carrageenan electrospun fibers includes: using environmental stimuli to promote the interaction between casein and κ-carrageenan, dissociating casein protein aggregates into smaller aggregates or monomers, and simultaneously promoting the transition of κ-carrageenan from its helical native conformation; using a low relative content of κ-carrageenan can maximize the interaction between casein and κ-carrageenan, while a higher relative content of polysaccharide will promote polysaccharide - polysaccharide interactions, resulting in thick fibers with poor mechanical properties. This study successfully prepared independent EMs containing more than 98 wt% casein, providing a theoretical basis and technical support for its applications in fields such as food, cosmetics, packaging, and biomedicine. Future research can further expand the performance optimization and functional expansion of these nanofibers in practical applications and promote the development of related fields.
Article Source: https://doi.org/10.1016/j.foodhyd.2024.110855
