Small granular starches from rice, quinoa and amaranth were modified with octenyl succinic anhydride (OSA) at 5 defined intervals (0 to 3.0%) and investigated with respect to emulsifying capacity and stability. Starch granules’ surface were characterized by Brunauer–Emmett–Teller and contact angle measurements. Emulsifying capacity was characterized by multiple light scattering (MLS) and particle size analysis. Stability towards environmental stress was characterized by centrifugation and MLS.
Surface hydrophobicity and emulsifying capacity correlated with starch type and modification level. Quinoa stabilized emulsions had the smallest droplet size (e.g. 59.2 µm at 3.0% OSA) and superior stability, both before and after centrifugation, especially at the lowest modification levels. Rice and amaranth had larger droplets (99.8 and 84.1 µm at 3.0% OSA respectively). Amaranth, despite its small size showed poorer performance than quinoa, especially at lower modification levels. The higher emulsifying efficiency of quinoa starch granules attributed to the higher protein content.
The surface area measurements performed by physisoption showed that the surface area calculated from light scattering was underestimated for the quinoa starch granules, and confirmed by scanning electron microscope images.
Based on measurements of a sessile drop of Miglyol on a starch pellet, the surface hydrophobicity of starch granules was affected by the starch type and the level of modification. Starches from rice and quinoa presented higher contact angles in the native state, and contact angle decreased with increasing modification level. Although amaranth starch had lower contact angle in both the native and corresponding modified levels, there was no decreasing trend in the contact angle for amaranth as the OSA modification level increased.
The emulsifying capacity of starch granules differed with starch type, and has a positive correlation the level of OSA modification, where the starch granules from quinoa, displayed higher emulsifying capacity in native and modified levels followed by rice. Amaranth presented a significantly lower emulsifying capacity at all levels of modification. The accelerated stability test showed that the stability of emulsions from quinoa starch granules in native and various modification levels was higher, even after exposure to acceleration as high as 5250 g. The higher initial and accelerated stability of quinoa starch stabilized emulsions is attributed to its’ higher protein content, in addition to having a suitable granule size. The high protein content increases the hydrophobicity and makes quinoa granules adsorb stronger to the oil-water interfaces. This role of the protein should be considered when designing starch isolation processes as excessive washing in alkaline solutions which may reduce protein content to such a degree that it may be detrimental to the starches’ performance as a Pickering type emulsifier.