High hydrostatic pressure (HHP) as a green technology opens up a new possibility for the fabrication of electrospun nanofibers: Part I-improvement of soy protein isolate properties by HHP

A common problem of electrospinning of plant proteins is that these kinds of proteins, including soy protein isolate (SPI), should be highly soluble and preferably in a random coil structure rather than the globular conformation. The current solution is to apply thermal treatment; however, it results in extensive organoleptic consequences (e.g. color degradation and off-flavor) and loss of nutrients. Thus, if a non-thermal technology such as high hydrostatic pressure (HHP) increases the disordered structure of the protein, it could facilitate the electrospinning of the protein and eliminate the barriers of thermal treatment, forming our hypothesis in this research. To this end, initially, extensive experiments including protein solubility, sulfhydryl content, turbidity, surface hydrophobicity, circular dichroism, and viscosity tests were carried out on HHP (200, 400 and 600 MPa) and thermally treated SPI samples (3.5% and 7% w/w). The results of sulfhydryl content revealed that disulfide bonds decreased at 400 MPa more intensely than other pressures while surface hydrophobic forces were pro-moted at this pressure. The circular dichroism test results showed that HHP treatment resulted in higher disordered structures and lower ordered ones, which is particularly helpful for the electrospinning process; besides, lower 8 -sheet content achieved by HHP treatment is especially useful for attaining defect-free fibers. For the first time, in this research, ideal electrospun fibers with average diameters of 300-400 nm were achieved through HHP processing, without using any thermal energy.