Compositional aspect and mechanism of surface formation of spray-dried microparticles with surface-active rice protein hydrolysates

Flavourzyme enzymatically hydrolyzed rice protein isolate to different degrees of hydrolysis (DH, 2, 6, and 10%) to stabilize formulated linseed oil emulsions with different protein concentrations (0.5, 1.0, and 1.5%). The emulsions were spray dried, and the microstructure, surface composition by X-ray photoelectron spectroscopy (XPS), and oxidative stability of the resultant microparticles were investigated. The physicochemical properties of powders were also evaluated. DH and protein concentration did not affect solubility, hygroscopicity, wetting time, and particle size. However, a strong and positive correlation was found between DH and oxidative stability. The lowest peroxide value (2.48–1.99 meq/kg oil) was found from the powder formulated with protein hydrolysate (DH 10%) and 1.5% of protein concentration during the entire storage study. Fluorescence optical microscopy and atomic force microscopies also allowed identifying the effects of these variables on oxidative stability. Through XPS analysis, it was possible to observe that higher DH favored the surface accumulation of proteins at the powder surface. The protein content on the particle surface (9.8%) was overrepresented compared with the bulk composition (1.5%). These results showed that the degree of hydrolysis exerted an influence on the distribution of components and the percentage of protein on the powder surface.