| As a typical carotenoid,?-carotene has drawn much attention in the related research field due to its abundant sources and strong pro-vitamin A activity as well as antioxidant ability.To overcome the defects of ?-carotene including poor water solubility,chemical instability and low bioavailability,food-grade emulsion has turned out to be one of the ideal delivery systems for encapsulation of ?-carotene and preparation of functionalized aqueous-based products(e.g.beverage,oral liquid,cosmetic lotion,etc.).Emulsions with smaller size,such as microemulsions and nanoemulsions,usually have relatively high physical stability,which appear more advantageous in preparation of the transparent aqueous-based products.However,the smaller the particle size of the emulsion droplets,the larger specific surface area and the higher surface energy of the emulsion,which is subsequently not beneficial for improving the storage stability of ?-carotene in emulsion system.The immobilization of?-carotene-loaded emulsion droplets in the hydrogel network could help to reduce the diffusion of oxgen molecules from external environment to the droplets inside,thus affording the improvement of ?-carotene stability.In this paper,different biocompatible surfactants and oil components were used to prepare emulsion systems for ?-carotene encapsulation,including microemulsion and double emulsion.Based on the established emulsion systems,emulsion-filled calcium alginate hydrogel systems were then constructed for further encapsulation and control release of?-carotene,through low-energy extrusion and double emulsion templating method.The effects of different delivery systems were analyzed on ?-carotene encapsulation,stability,in-vitro release behavior and antioxidant capacity,in order to investigate in depth the stabilization mechanisms of various delivery systems for ?-carotene.As a whole,the paper could be divided into the five parts as follows:(1)The fully diluted microemulsion system was constructed and the comprehensive characterization methods were employed to determine the structural transitions in microemulsion system during aqueous phase dilution,i.e.conductivity,viscosity,small angle X ray scattering,etc.Microemulsions of oil-in-water(O/W)and bicontinuous(B.C.)type were selected as carrier systems for ?-carotene,respectively.The effects of different factors(i.e.pH,ionic strength,storage temperature and time)on system structure and ?-carotene stability were studied and the antioxidant capacity of different microemulsion system was analyzed and compared by ABTS assay.The results showed that Tween60/isopropyl myristate/ethanol/glycerol/water system experienced a structural transition from water-in-oil(W/O)type to B.C.type,and then to the final O/W type during aqueous phase dilution.The solubilized location of ?-carotene varied synchronously with structural transition of the microemulsion system.?-Carotene was relatively stable in O/W and B.C.type systems which were placed within near-neutral environment(pH 6?7).Moreover,?-carotene in microemulsion of B.C.type was also manifested to have higher storage stability,thermal stability as well as dilutable stability of salt solution.The antioxidant capacity of microemulsion system increased with the increase of ?-carotene content.(2)Polymer introduced in microemulsion system could not only stabilize the system by steric or electrostatic repulsion,but also cause droplets flocculation and resultant instability due to bridging or depletion mechanism.On the basis of construction and structural determination of the ?-carotene-loaded microemulsion mentioned above,the influences of dilution of aqueous sodium alginate solution on microstructure of the system and stability of?-carotene were analyzed,through several approaches including rheology,dynamic light scattering,electron microscopy,to characterize the interaction between sodium alginate molecules and emulsion droplets.Moreover,the effects of different concentrations of sodium alginate and ?-carotene on the antioxidant capacity of the mixed system were investigated by ABTS assay.The results concluded that the stability of the mixed system containing microemulsion and sodium alginate was dependent on the alginate concentration,which was the consequence of droplets flocculation owing to polymer depletion effects.The addition of high concentration of sodium alginate(>1.0 wt%)into the mixed system could be helpful to improve system stability,but had no obvious effect on improvement of ?-carotene stability.The addition of high concentration of sodium alginate in the mixed system containing O/W microemulsion could stabilize the ?-carotene and thus improved the antioxidant capacity of the system to some extent.The increase of ?-carotene content in the microemulsion system was beneficial to enhance the antioxidant capacity of the mixed system including microemulsion and sodium alginate.(3)The mixed system containing microemulsion and sodium alginate was crosslinked with calcium ions by extrusion method,to prepare emulsion-filled calcium alginate hydrogel beads in millimeter scale.The influences of preparation parameters were studied on the size and pore-space volume of the beads,and the retention and release of ?-carotene and oil droplets in the beads during hardening processes.The differences in antioxidant performance and in-vitro release behavior of calcium alginate hydrogel beads were further discussed,which were fabricated from different types of microemulsions.The results showed that the release behavior of ?-carotene in hydrogel beads was mainly affected by alginate concentration in the mixed system,calcium ions concentration in the hardening solution and pH of the release medium.These were mainly attributed to the variations of the pore-space volume within the hydrogel beads and the electrostatic interaction between?-carotene-loaded oil droplets and polymer molecules in the hydrogel matrix.Reducing the concentration of sodium alginate in the microemulsion/sodium alginate mixed system could significantly lower the antioxidant capacity of the resulting calcium alginate beads.The antioxidant capacity of calcium alginate beads was enhanced by increasing ?-carotene content in the microemulsion system.(4)O1/W/O2 double emulsions were fabricated by a two-step emulsification method to encapsulate ?-carotene and the encapsulation efficiency(EE)of ?-carotene in the inner oil phase(O1)was improved by alginate addition into the intermediate aqueous phase(W)of the double emulsion.The factors correlated to the encapsulation efficiency of ?-carotene were discussed and the stabilization mechanism of the double emulsion system was established,according to the analysis of the relationship between emulsion microstructural properties and the encapsulation efficiency of ?-carotene.The results concluded that the number of the inner oil droplets encapsulated in an outer aqueous droplet could be adjusted by varying the shear rates corresponding to the primary and secondary emulsification,and in hence,the O1/W/O2 double emulsion could be formed with controllable droplet size.The EE of?50 wt%could be achieved in the freshly prepared O1/W/O2 emulsion when using aqueous alginate of 2 wt%as the intermediate phase and ?-caroteneloaded R-(+)-limonene of 0.1 wt%as the inner oil phase.In addition,the encapsulation efficiency of ?-carotene showed a positive correlation to the outer-to-inner droplet size ratio of the O1/W/O2 double emulsion.(5)?-Carotene-loaded calcium alginate hydrogel microparticles with micron scale were fabricated using O1/W/O2 double emulsion templating method,by virtue of two gelation mechanisms,i.e.,external and interfacial gelation,and revealed the mechanism how the gelation technology affected the encapsulation and storage stability of ?-carotene.The results showed that the prepared O1/W/O2 double emulsion system could be applied as templates for formation of calcium alginate hydrogels to encapsulate ?-carotene.The resulting hydrogel microparticles prepared by both gelation mechanisms were spherical and their average size in water dispersion system was slightly larger than that of the aqueous droplet templates in the double emulsion.?-Carotene content in the inner oil phase of the double emulsion system was responsible for the antioxidant capacity of the hydrogels prepared via double emulsion templating method.The hydrogels fabricated through interfacial gelation mechanism had relatively high stability and antioxidant capacity,owing to the formed shells of CaCO3 on the surface of the hydrogels. |
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