Short-Chain Fatty Acid Starch Stabilized Pickering Emulsions : Design, Properties and Applications
Sammanfattning: Pickering emulsions are emulsions stabilized by solid particles. Particles with optimal dual wettability toward both of the oil and water phases, can be adsorbed onto the interface, thereby providing the stability of the emulsions. Starch granules have attracted attention due to their positive characteristics, such as being widely available, inexpensive, biodegradable, and non-allergenic. Due to a relatively low degree of hydrophobicity, chemical modification of starch can improve starch hydrophobicity by esterification with a short-chain fatty acid (SCFA) group. The aim of this thesis was to perform SCFA starch modification by means of esterification of rice and quinoa starches with different SCFA groups and levels of modification. The physicochemical and functional properties of SCFA starches were investigated. As one of the future applications, the emulsifying capacity of SCFA starches was evaluated and in vitro digestion was carried out. Until recently, there have been no studies evaluating the effect of different types of SCFA starches and the levels of modification to the physicochemical properties, emulsification and digestibility. The rationale behind the selection of different types of SCFA starches at different levels of modification and the application of these to stabilize Pickering emulsions were discussed. The esterification of starch with short-chain fatty acids group was successfully quantified by direct stoichiometry, FTIR and 1H-NMR. SCFA starches have shown a different properties compared to their native forms. Native and SCFA-rice starches have a larger particle size compared to native and SCFA-quinoa starches. Both types of starches displayed a polyhedral shape. Upon modification, no changes in particle size were observed. SCFA starches exhibited a reduction in protein and amylose content. SCFA starches demonstrated low gelatinization and pasting temperature. The highest level of resistant starch was observed in the starches with the highest level of modification. Principle component analysis revealed that the physicochemical and functional properties of SCFA starches are highly influenced by the level of modification. SCFA starches were able to perform as a stabilizer in Pickering-type emulsions. The emulsifying capacity was improved by increasing SCFA chain length and levels of modifications. SCFA-quinoa starch Pickering emulsions were observed to have smaller droplet sizes, higher emulsification index, better Turbiscan stability index, and more stable droplet sizes that remained below 50 µm during 50 days of storage. This indicated that Pickering emulsions stabilized by SCFA-quinoa starches were more stable than SCFA-rice starches. In vitro digestion of SCFA starch Pickering emulsions showed that increasing SCFA chain length and modification level reduced the extent of starch hydrolysis. The results of this PhD project implied that increasing the chain length and modification level improved the overall hydrophobicity of the granules and hence improves the emulsification capacity and stability. Improved hydrophobicity resulted in a higher adsorption degree (less free starch) and a denser layer of particles at the interface. Hence, this dense layer protects the oil droplets and prevents the enzyme from getting access to the oil droplets. However, particle coverage was not complete due to the large sizes of the particles. There were therefore still small gaps between starch particles, resulting in lipolysis not being completely arrested. In future research, formulation of SCFA starch Pickering emulsions can be used to investigate the capacity of these emulsions to serve as a carrier for controlled release and targeted delivery of bioactive compounds to a specific location of the gastrointestinal tract, such as the distal locations of the small intestine or the colon.
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