| By adopting compounding mixture of different proteins and maltodextrin as oil microcapsules’ wall material, this thesis studied the proteins’ secondary structure change before and after the formation of microcapsule through infrared spectroscopy. It also probed into the emulsibility and emulsion stability of whey protein, isolated soybean protein, peanut protein as well as sodium caseinate. With all the components of the microcapsule wall material being marked with fluorescent probes, microcapsules’ external feature and internal structure has been analyzed with Scanning Electron Microscope and Laser Scanning Confocal Microscope (LSCM) respectively. Thus, the micro-structural and the formation mechanism of the oil microcapsules being used in this thesis have been researched, and the qualities of the oil microcapsule products have been examined at the same time.The interaction between protein molecules and maltodextrin microcapsule products during processing have been studied by Fourier transform infrared spectra, and the result shows that the amide I band of sodium caseinate, isolated soybean protein, peanut protein and whey protein all shift to the low wave number direction, indicating that hydrogen bond has been formed between protein molecules and its surrounding medium maltodextrin hydroxyl. By performing secondary derivative, deconvolution and Gaussian curve fitting on the amide I bond, the author of this thesis found that the secondary structure of protein has changed before and after the formation of the microcapsule. The random coil rate of sodium caseinate is the highest, which increased from25.57%to28.90%; while peanut protein’s random coil rate is13.22%, which is the lowest among all of them. The rate of a-helix in sodium caseinate and whey protein has increased after the microcapsule wall structure being formed. The rate of β-sheet in isolated soybean protein and peanut protein has also increased after the microcapsule wall structure being formed.In the present study, Rhodamine B and Fluorescein Isothiocyanate (FITC) were chosen as fluorescent probes to label maltodextrin and isolated soybean protein, which are the main materials of the microcapsule wall. Combined with Laser Scanning Confocal Microscope (LSCM) technique, the image of the microcapsule’s internal structure could be obtained directly. The results showed that the maltodextrin has a homogeneous distribution throughout the capsule wall while isolated soybean protein assembled in the membrane surface and the outer interface of the soybean oil microcapsules. During the process of the formation of the emulsion, protein tended to absorb to the surface of oil spontaneously and immediately for its emulsifying property, thus, an O/W interfacial membrane was formed. The stability of the emulsion depends on the structure and density of the protein adsorption layer on the surface of the emulsion droplet. The protein adsorption layer can prevent the coalescence between the emulsion droplets and thus emulsion is stabilized. The maltodextrin molecules distributed in the aqueous phase homogeneously. In the process of spray drying, as the water evaporated, maltodextrin molecules wrapped on the surface of membrane proteins, as a result, the thickness, intensity and density of protein membrane has increased, which enhanced microcapsule wall material’s mechanical strength. The other part of the proteins conglomerated on the outer wall of microcapsule membrane to reduce the surface tension of the droplet.By placing the fluorescence microscope image upside down, it is easy to discover that the O/W droplets have formed in the emulsion after it is homogenized through high pressure. The formed droplets are all in regular shape-basically in spherical shape. The powdered oil spreads out rapidly in good shape when it is put into the lukewarm water.The particle sizes of the soybean oil microcapsules made of four different kinds of proteins are vary. The average grain diameter of the microcapsules made of sodium caseinate is363.7nm, which is the smallest particle type among the fours kinds of soybean oil microcapsules, but the particle size is relatively more centralized. The average grain diameter of the microcapsules made of whey protein, isolated soybean protein and peanut protein are relatively larger than the microcapsules made of sodium caseinate, they are540.9nm,606.0nm,552.1nm respectively.During initial period, the four kinds of protein are used as wall material to embed soybean oil, and the result shows that the oil content on the surface is very low, which indicates that the emulsion system is stable and the combination of the wall material is dense. After12months’ probationary period, the oil contents on the surface of the four kinds of microcapsules all have increased, and the increase rate on the surface of microcapsules made of peanut protein is7.34%, which is the highest among the four. By measuring and examining the total content of oil in the microcapsules, the author of this thesis found that the newly prepared oil microcapsules’ soybean oil retention rates are all above90%. But as time goes on, all the retention rates have declined. The drop rate of sodium caseinate is the lowest while the drop rate of peanut protein is the highest, only67.89%after12month.By examining the changes of peroxide value and acid value of the soybean oil microcapsule products during the period of storage, the author found that in the270days’ probationary period, the soybean oil microcapsule products’ peroxide value took a "saddle pattern" as time goes on and the acid value increased from1.289to11.564. Meanwhile, the peroxide value and acid value of the soybean oil being wrapped by microcapsule remained more or less flat, which indicates that the wall material made of microcapsule with the above mentioned four kinds of protein and maltodextrin as its main components has dense structure, which can effectively protect the core material from oxygen and water. |
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