|Abstract:||Oil structuring techniques in food science field have been focused on finding alternative oil-thickening methods for substitution of partial hydrogenation and interesterification. Oleogelation without chemically modification of liquid oils becomes the solution to exclude trans fat in the final products. One of the common oleogelation approaches to fabricate solid-like oil is using oleogelators to form stable network which can immobilize the oil phase and further construct into an oleogel system. Among various type of gelators, it is important and challenging to find an oleogelator that meet the needs of safe, biocompatible, biodegradable and cost-effective before and after digestion. Which natural proteins originated from animals or plants are suitable candidates for their ability to self-assembly into supramolecular structures and eventually lead to continuous networks that involve in the gel formation with non-covalent interactions. In this research, oleogelation mechanism of zein in 70% ethanol and oleic acid mixture was investigated.
Zein, a corn prolamin contains around 50% hydrophobic amino acid residues was used as an oleogelator in a 70% ethanol/zien/oleic acid emulsion system stored for 7, 14 and 21 days. Ternary phase diagrams were constructed and mapped out the numbers and extents of the phases. Within 21 days of storage time, up to five phases were identified, and self-assembled oleogels with different transparency were observed. The flow behavior of emulsions and viscoelastic properties of zein-based oleogels were determined by rheological measurements. The emulsions showed shear thinning behaviors with viscosity positively correlated with oleic acid content. It was assumed to be the high volume fraction of the dispersed oil phase that affected the interfacial tension of the emulsion. Both viscosity and storage modulus increased with increasing of protein concentration, which indicated the rigidity of the network was contributed mainly by the zein structures. After storage, all of the gel elasticity increased with time. And accordingly, the highest gel strength was found at the solvent composition where 70% ethanol and oleic acid ratio closed to 1. Although higher amount of oleic acid could act as fillers in the emulsion system, but excess amount might affect the solvent polarity and cause the zein molecules to aggregate instead of forming into elongated entangled network structures. Which at 15% zein concentration, the optimum composition for most rigid gel was 45-15-40 and 40-15-45 (70% ethanol/zein/oleic acid, (%, w/w/w) for 14 and 21 storage days, respectively.
The microstructural changes during zein molecules self-assembly process were monitored by ultra-small x-ray scattering (USAXS). The results showed one to two levels of structures were formed inside zein-based oleogels. The primary level of zein building blocks with radius of gyration Rg1 and shape factor P1 revealed between high q and intermediate q region (0.01 < q < 0.5 Å-1) would self-assemble from 1D rod-like structure to 2D plate of sheet-like structures. At this level, the size and shape were mainly affected by solvent composition, which Rg1 enlarged with the increasing of 70% ethanol, but not by storage time. The secondary level structures located at low q (q < 10-3 Å-1) showed the network were composed of similar 3D fractal structures with Rg2 ranged from 251 to more than 1200 nm. The Rg2 values increased with higher 70% ethanol content and longer storage time indicated the dynamic gelation process of zein-based oleogel was resulted from the rearrangement of zein molecules in different solvent environment. It was suspected that with less oleic acid and lower protein concentration, zein molecules had higher mobility to attach onto the surface of oleic acid oil droplets and further developed into larger scale of structures.
Lutein was successfully encapsulated in the zein-based oleogel with modified preparation method which sonication was applied. Based on tube inversion method and the rheological measurements, the gelation process was significantly facilitated by high shear which the formation of up-side-down gel with G’ > G” was observed at 3rd day instead of 21st day of storage (30-15-55). And with the determination of conductivity and microscopy, the oleogel system was confirmed as an oil-in-water emulsion with micro-sized oil droplets distributed within the hydrophilic continuous phase. And the ribbon-like strands observed by AFM could be the network structures that formed by zein with the interactions between oleic acid.
The results of this study provided insights about the self-assembly gelation mechanism of the zein-based oleogel. As a model system, it demonstrated similar behavior to spontaneous protein aggregation inside the cell. By understanding the effects of protein concentration, solvent composition, storage time, shear force applied and addition of antioxidant on the gelation process of the oil-in-water emulsion gel system, this research not only help to design the desirable texture and structural properties of the controlled-release system for chemically or heat sensitive compounds, but also a different perspective to approach the aging phenomenon.