Poly(ethylene oxide) (PEO) is known for facilitating the electrospinning of biopolymer solutions, which are otherwise not electrospinnable. The objective of this study was to improve the understanding of the positive effects of PEO on the electrospinning of whey protein isolate (WPI) solutions under different pH conditions. Alterations in protein secondary structure and polymer solution properties (viscosity, conductivity, and dynamic surface tension), as induced by pH changes, significantly affected the electrospinning behavior of WPI/PEO (10% w/w: 0.4% w/w PEO) solutions. Acidic solutions resulted in smooth fibers (707 +/- 105 nm) while neutral solutions produced spheres (2.0 +/- 1.0 mu m) linked with ultrafine fibers (138 +/- 32 nm). In comparison, alkaline solutions produced fibers (191 +/- 36 nm) that were embedded with spindle-like beads (1.0 +/- 0.5 mu m). 13C NMR and FTIR spectroscopies showed that the increase in random coil and a-helix secondary structures in WPI were the main contributors to the formation of bead-less electrospun fibers. The electrospinning-enabling properties of PEO on aqueous WPI solutions were attributed to physical chain entanglement between the two polymers, rather than specific polymerpolymer interactions. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012