Emulsifying properties and structure changes of spray and freeze-dried peanut protein isolate

The emulsifying properties of peanut protein isolate (PPI) prepared by spray- and freeze-drying methods were investigated together with the change in protein structure due to drying. Oil binding, water holding capacities and solubility of freeze-dried PPI were significantly higher (p ＜ 0.05) than those of the spray-dried one. The spray-dried PPI had higher emulsifying activity index (EAI), whereas the freeze-dried PPI had higher emulsion stability index (ESI). The freeze-dried PPI had significantly higher surface hydrophobicity, disulfide bonds and β-sheets than the spray-dried one (p < 0.05). While the latter contained more hydrogen bonds than the former (p < 0.05), as shown by the Fourier transform infrared spectroscopy, which suggested that the spray-dried PPI had relatively higher unfolded or flexible structure than the freeze-dried PPI. Folded and wrinkled morphology of spray-dried PPI but plate-shaped structure in the freeze-dried PPI suggested that droplet shrinkage and solute concentration led to the distinct morphology, respectively. These two different drying processes greatly brought about different structure and properties thereof. Thus, the freeze-dried PPI produced more stable emulsions (higher ESI), while the spray-dried PPI occupied the oil water interface faster (higher EAI).