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theses

With an increasing trend on the development of "clean-label" food products, environmental friendly materials have attracted more and more attentions during the recent years. As a natural biodegradable protein from maize, zein has a good potential to be used to develop new carrier systems due to its Generally Recognized as Safe (GRAS) status, abundant source, commercial availability, low cost, and low digestion properties. The water-insoluble and amphiphilic characteristic of zein makes it a perfect candidate to form colloidal particles to stabilize Pickering emulsions. The successful assembly of particles at emulsion interface largely relies on their surface activities, which can be tuned through the modification of surface charges and hydrophobicities.

In this study, zein was hydrophilically modified with sodium caseinate (SC) to form zein/SC complexes and hydrophobically modified with saturated fatty acids (FA) to form zein/FA complexes. The obtained zein/SC complexes displayed good emulsification capacities, which were suitable to stabilize a wide range of oils with middle or low polarities including toluene, n-octane, hexane, and general cooking oils extracted from vegetables. The fluorescence image of fluorescein isothiocyanate (FITC)-labeled zein particles indicated that the emulsions were stabilized mainly by zein-based colloidal particles. With the presence of polyglycerol polyricinoleate, zein/SC complexes were able to stabilize W/O/W double emulsions either through magnetic stirring method or through high speed homogenization method. The Pickering emulsions stabilized by zein/SC complexes were further utilized to encapsulate resveratrol, a plant phytoalexin extracted from grape, peanut and other few plants.

Resveratrol has been widely reported for its health beneficial potentials, such as anti-oxidation, anti-inflammation, anti-obesity, anti-tumor activities, etc. However, the applications of resveratrol in the food industries are quite limited because of its low chemical stability, low oral bioavailability, and poor water solubility. In this study, a series of delivery systems were developed to compare their delivery efficacy and bioaccessibility of resveratrol using in vitro models. Zein/SC complexes were utilized to fabricate Pickering emulsions through high speed homogenization, while conventional emulsions were stabilized by lecithin using high pressure homogenization. The Simulator of the Human Intestinal Microbial Ecosystem (SHIME) and TNO gastro-Intestinal Model (TIM-1) were utilized to mimic the digestion in the upper GI tract. The results from TIM-1 indicated that conventional emulsions displayed the highest bioaccessibility compared with Pickering emulsions and bulk oil solutions. In SHIME experiment, the structure of Pickering emulsions was still observable after 2-hour digestion in the stomach, which explained why Pickering emulsion showed slower release effect of RES compared with conventional emusions. After 180 min digestion in the jejunum by the TIM-1 model, the digested samples were loaded on Franz Cell Diffusion apparatus to see the permeability of various formulations through porcine small intestine membrane. It was found that Pickering emulsions had significantly lower permeability as compared with conventional emulsions and bulk oil solutions (P < 0.05). The permeability of resveratrol was driven by concentration gradients between donor compartment and receptor compartment. Through anaerobic fermentation study, it was found that lecithin stabilized emulsions after SHIME digestion retarded the growth of probiotics, Lactobacillus rhamnosus GG, while zein/SC complexes stabilized Pickering emulsions even promoted its growth, since zein and sodium caseinate could serve as a protein source for bacteria. Conclusively, Pickering emulsions are more suitable for target delivery of RES into colon, while conventional emulsions can be used to improve the bioaccessibility in small intestine.

Besides Pickering emulsion, zein/SC complexes were further utilized to fabricate colloidosomes that are characterized by their hollow shell structures through layer-by-layer (LbL) self-assembling technique. Quartz crystal microbalance equipped with dissipation monitoring (QCM-D) was applied to simulate the deposition process of polyelectrolytes (chitosan, sodium alginate, and zein) on the zein/SC surface. The result from QCM-D analysis showed that the adlayer on zein/SC surface had a soft and viscoelastic property. This presented method successfully produced robust colloidosomes, which not only survived from multiple washing steps, but also did not show flocculation and coalescence phenomenon during the LbL coating process. And the polysaccharide coatings strengthened the structure of shells, and offered them a viscoelastic property to stand harsh dehydration conditions.

In the following section, fatty acid (FA)/zein (Z) complexes were prepared by using FAs with different chain lengths (i.e. lauric acid (LA), myristic acid (MA), palmitic acid (PA), stearic acid (SA) and oleic acid (OA)), different FA/Z ratios (i.e. 0.1:1, 0.2:1, 0.4:1, and 1:1), and different pH values (i.e. 3.5, 5.0, 8.0, 10.5) in order to precisely tune the surface activities of zein. Results showed that stable FA/Z complexes were able to be formed under alkaline condition, rather than acidic or neutral pH, which was due to the film forming property of zein in acidic conditions. According to the contact angle (CA) measurement, the CA increased gradually from 64.88 ± 4.09˚ (LA/Z, 0.5:1) to 85.25 ± 2.00˚ (SA/Z, 0.5:1) with an increase of the carbon chain length. However, oleic acid, a monounsaturated omega-9 fatty acid, reduced the hydrophobicity, giving the lowest CA of 61.08 ± 6.14˚. Scanning electron microscopic (SEM) images indicated that FAs could lead to the partial fusion of the particles, which explained why FAs could reduce the brittleness and increase the plasticity of zein. The SA/Z complexes showed good emulsification capacity to stabilize O/W/O double emulsions through handshaking. The LA/Z complexes were further applied to improve the bioavailability of lipophilic ingredients, i.e. hesperidin in this study. Based on in vitro study conducted through the TIM-1 model, the bioavailability of hesperidin was successfully enhanced from 53% to around 70% as compared with bulk oil solutions. This study indicated the good potentials of using zein as an environmental friendly particle emulsifier to develop novel delivery systems for various nutraceuticals.

ETD_10862

Ph.D.

Includes bibliographical references

doi:10.7282/t3-crmm-v612

ETD doctoral

The author owns the copyright to this work.