Preparation of emulsifiers from pea and corn starches using octenyl succinic anhydride modification
The objective of this thesis research was to modify pea starch – a representative pulse starch – using octenyl succinic anhydride (OSA) to prepare emulsifiers for enhanced industrial utilization. Pea starch (PS), normal corn starch (NCS), and waxy corn starch (WCS) were modified with 1%, 3% and 5% OSA (w/w, on a dry basis of starch, db). Native PS consisted of more amylose (41.5%) than NCS (32.9%) and WCS (1.7%). The degrees of substitution (DS), thermal properties, and pasting properties of the derived OS starches were determined. Incorporation of OS groups significantly alter the thermal and pasting properties of the three starches. Gelatinization temperatures and enthalpy changes of the starches were gradually decreased with an increasing level of added OSA. OS starches showed less tendency to retrograde compared to the control starches. OSA modification progressively reduced the pasting temperatures and increased the peak viscosities of the three starches as the DS increased. The OS starches displayed good capability of lowering canola oil-water interfacial tension (IFT). Canola oil-in-water (O/W) emulsions were prepared with the OS starches to evaluate their emulsifying properties, with gum Arabic (GA) being included as the industry standard for comparison. Droplet-size distributions, zeta potentials, accelerated stability, and storage stability of the resultant emulsions were determined and compared. Fresh emulsions produced with the OS starches showed droplet sizes considerably smaller than that of GA emulsion. NCS and WCS modified with 3% and 5% of OSA and GA exhibited good ability to stabilize O/W emulsions during 28-day storage at 4°C due to steric hindrance and electrostatic repulsion provided by the emulsifier layer on the droplets. However, the droplet sizes of all the OS-PS emulsions increased to some extents during the storage, suggesting the least stability of these emulsions. Emulsion stability under various environmental conditions was also investigated, including pH 2-7, 0-1.0 M sodium chloride in aqueous medium, and incubation at 90°C for 30 min. The droplet-size distributions and the stability of the emulsions were confirmed by confocal laser scanning microscopy observation. Under the same test condition, the droplet sizes of the generated emulsions were largely in a descending order of OS PS > GA ≥ OS NCS > OS WCS. The results obtained from the thesis suggested that steric repulsion between emulsion droplets provided by the emulsifier layer was the main factor to prevent droplet aggregation and coalescence in OS-starch and GA emulsions and that electrostatic repulsion only partially contributed to stabilize the emulsion systems. Overall, the ability of the emulsifiers in stabilizing O/W emulsions followed an ascending order of OS PS < GA ≤ OS NCS < OS WCS. The poorer stability of OS-PS emulsions was partly attributed to the higher amylose content and longer amylopectin branch chains of the starch, which could cause stronger re-association and a higher rate of retrogradation between starch molecules to weaken the steric hindrance in the resultant emulsions. Emulsifying properties of the OS starches and stability of the resultant emulsions were generally enhanced with a higher DS, with the most noticeable improvement being observed in OS-PS emulsions. This study demonstrated the effects of OSA modification on the thermal, pasting and emulsifying properties of different starches. The interrelationships between the molecular structures (e.g., amylose content and degree of substitution), functional properties (e.g., retrogradation rate), and emulsifying capability of the OS starches were also clearly elucidated. The new findings from the thesis can be used to further tailor the structures of OS PS to improve the emulsifying properties for potential industrial uses.
pea starch emulsifier, emulsion stability, OSA modification
Master of Science (M.Sc.)
Food and Bioproduct Sciences