Short communication: Electrospinning of casein/pullulan blends for food-grade applications

Electrospinning is a complex process that produces fibers with diameters on the micrometer or nano-scale from an electrified jet of a polymer solution. The objective of this study was to create electrospun fibers for food use from aqueous solutions of calcium (CaCAS) or sodium caseinate (NaCAS). Fibers were not formed from electrospinning of solutions of either caseinate (CAS) at 50 °C, but were formed from blends of either CAS solution with aqueous solutions of the food-grade polysaccharide, pullulan (PUL), when using mass ratios from 2:1 to 1:4 of PUL/CAS. The CAS in the spinning solutions ranged from 3 to 15% (wt/wt) and the PUL ranged from 5 to 15% (wt/wt). The PUL/CaCAS 1:2 fibers showed the lowest fiber diameter sizes (FDS) of 172 ± 43 nm, as determined by scanning electron microscopy, and were smaller in size than fibers electrospun from 15% (wt/wt) PUL solution. The PUL/NaCAS solutions were more viscous and formed fibers with occasional branching and less uniform FDS at higher NaCAS contents. Reductions in NaCAS in these solutions reduced viscosity and improved jet stabilities with consequent improvement in fiber morphology leading to more uniform FDS. Fibers with less defects and more homogeneous FDS were formed from PUL/CaCAS blends with more CaCAS, showing that each CAS interacted differently with PUL and formed the best fibers at different solution conditions. Calcium bridging may also underlie the anomalous behavior of the PUL/CaCAS blends by forming crosslinks with the phosphoserine residues, further enabling chain entanglements for fiber formation. The PUL/NaCAS fibers tended to be larger than the PUL/CaCAS fibers, which may also be due to other factors such as solution surface tension and conductivity, which also affect fiber quality and size. The shear viscosities at 100 s(-1) of the solutions producing fibers were within the range of 0.07 to 0.16 Pa/s, with the smallest standard deviations in FDS noted for solutions with viscosities within about 25% that of PUL. This is the first example of caseinate fibers prepared using a food-grade carrier rendering a product with potential use in food and packaging applications.