| Recently, driven by people’s health consciousness constantly enhancing, the personal attention of functional foods rised. Whereas, the nutritional food components which are closely related to people’s health, such as ω-3 fatty acids, β- carotene, lutein et al. are really sensitive to environment. These nutrients are not stable and easily degraded, when they encounter environmental risk factors( light, heat, oxygen and p H et al.). Microencapsulation can protect them from the outside world, and prevent the formation of harmful substances. However, the wall materials of microcapsule is high cost and some of them are not conform to the standards for use of food additives. Additionally, the commonly used wall materials like plant proteins are sensitive to processing environment. Because of these deficiencies, it greatly limits the technique of micro-encapsulation in the field of food industry.Using widely distributed and cheap soy protein isolate(SPI) and maltodextrin, we developed two multi-functional food emulsifying stabilizer with good solubility, emulsifying ability and antioxidant capacity, respectively, through combined modification technique of enzymatic hydrolysis and glycosylation, which were used to prepared fish oil microcapsules as coating materials. The preparation processes, physico-chemical properties and the application effect in fish oil microencapsulation were analyzed and evaluated. The main contens are as follows:(1) According to the emulsifying properties, emulsifying stability and Inhibiting activity of lipid oxidation, suitable conditions for enzymatic hydrolysis-glycosylation system were chosen: concentration of SPI 5%(w/v), enzyme concentration(E/S) 0.3%(w/w), ratio of SPI hydrolysates and maltodextrin 6/1, reaction temperature 80 oC, concentration of SPI hydrolysates 7%. The functional properties of conjugates of SPI hydrolysates-maltodextrin were studied. Result showed that the functional properties of SPI, such as heat stability, emulsifying capacity, foaming property and DPPH free radical scavenging capacity et al. were greatly improved after the combined modification. Especially, compared with native SPI and hydrolysates of SPI(HSPI), the emulsifying property of the combined modified SPI products improved by 27% and 19%, respectively.(2) The molecular structure of the conjugates of HSPI and maltodextrin was characterized by various analytical techniques. Results showed that the molecular weight of SPI decreased after hydrolysis, whereas, increased after the conjugation of hydrolysates and maltodextrin. With a PAS stain, red bands appeared, indicating the maltodextrin was successfully grafted. Results of fluorescence spectrum showed that the fluorescence intensity(FI) was enhanced after hydrolysis procedure, indicating the exposure of the hydrophobic region inside protein molecule, whereas, FI decreased with the subsequent conjugation. According to the circular dichroism(CD) spectra, the content of ordered structure decreased but that of random structure increased after SPI was hydrolyzed. It is speculated that, the folded structure of SPI was unfolded, which helps the conjugation of maltodextrin and also would be significant for steric hindrance of polysaccaride.(3) Fish oil microcapsules were prepared with conjugates of SPI hydrolysates and maltodextrin, followed by the investigation of the particle size distribution of fish oil emulsion through dynamic light scattering(DLS), atomic force microscopy(AFM), and confocal laser scanning microscopy(CLSM). Compared with control samples, emulsions prepared by HSPI-Maltodextrin showed a narrow size distribution, lower polydispersity index(PDI) and smaller particle size. Especially, the particle size and PDI of HSPI-Maltodextrin(reaction time, 270 min) based emulsions was 194 nm and 0.094, respecially, much lower than those of SPI/maltodextrin mixture based emulsions(719 nm and 0.537), which demonstrating the emulsifying capacity of SPI was greatly improved after combined modification. Evalution of emulsion storage stability exhibited, all emulsions prepared by combined modified SPI products exhibited good stability with no visual instability such as creaming during storage.(4) Investigation of fish oil microencapsulation efficiency(MEE) and oxidative stability during storage showed that, the MEE of HSPI-Maltodextrin coated microcapsules(HSPI-Md 300 min) was 87%, which was much higher than that of SPI/Maltodextrin mixture and conjugates based microcapsules(66% and 57%, respecially). And oxidation rate of microcapsules prepared with HSPI-Maltodextrin was relatively lower than that of control samples. After 4 weeks storage, the peroxide value and the content of propanal was only 15.64 mmol/kg and 4195.40, respecially, whereas, those of control microcapsules(SPI/Maltodextrin mixture) reached up to 28.58 mmol/kg and 10057.87. Microcapsules coated by HSPI-Maltodextrin exhibited uniform and porous structures, as evidenced by Scanning Electron Microscope(SEM).(5) Another novel microcapsule coating material was developed through glycosylation of SPI and maltodextrin followed by Neutrase hydrolysis, which is a new combined modified technique. According to the degree of graft(DG) and browning, suitable conditions of glycosylation between SPI and maltodextrin were chosen: ratio of SPI hydrolysates and maltodextrin 2/1, concentration of SPI hydrolysates 4%, reaction temperature 80 oC, p H 7.0, reaction time 80 min.(6) The functional properties of of SPI-Maltodextrin conjugates hydrolysates were studied. Results showed that the functional properties of SPI, such as solubility, emulsifying capacity, heat stability and antioxidative capacity et al. were greatly improved after the combined modification. Especially, compared with native SPI and SPI-Maltodextrin conjugates, solubility of SPI-Maltodextrin conjugates hydrolysates at p I was greatly improved by 22.6-fold and 7.6-fold, respectively. The emulsifying activity index(EAI) increased from 105.75 m2/g(SPI) to 147.75 m2/g, emulsifying stability was improved by 2.2-fold, and heat stability was enhanced from 97 oC to 108 oC(SPI-Maltodextrin conjugates hydrolysates, DH 1.8%) after combined modification of SPI.(7) The molecular structure of HSPI-maltodextrin conjugates hydrolysates was characterized by various analytical techniques. Results showed that the molecular weight of SPI increased after the conjugation between SPI and maltodextrin, whereas, decreased to various degrees after subsequent hydrolysis. With a PAS stain, red bands appeared, indicating the maltodextrin was successfully grafted. As evidenced by infrared spectroscopy(IR),-OH stretching vibration frequency increased after conjugation, indicating the polyhydroxy polysaccharide was successfully grafted. After the subsequent hydrolysis, no abatement was observed for-OH stretching vibration frequency and C=O stretching vibration, indicating the amido bonds formed through glycosylation were not destroyed. Analysis of fluorescence spectrum showed that, the graft of hydrophilic polysaccharide decreased the FI, however, the FI was enhanced after hydrolysis procedure, demonstrating the exposure of the hydrophobic region inside protein molecule. According to the CD spectra, the content of ordered structure decreased but that of random structure increased after hydrolysis procedure. It is speculated that, the folded structure of SPI was unfolded, which helps the enhancement of protein molecule flexibility, which is critical for its functional properties.(8) Fish oil microcapsules were prepared with SPI-Maltodextrin conjugates hydrolysates, followed by the investigation of the particle size distribution of fish oil emulsion through DLS, AFM and CLSM. Compared with control samples, emulsions prepared by SPI-Maltodextrin conjugates hydrolysates showed a narrow size distribution, lower PDI and smaller particle size. Especially, the particle size and PDI of SPI-Maltodextrin conjugates hydrolysates(DH, 2.9%) based emulsions was 232 nm and 0.091, respecially, much lower than those of SPI/maltodextrin mixture based emulsions(401 nm and 0.293), which demonstrating the emulsifying capacity of SPI was greatly improved after combined modification. Evalution of emulsion storage stability exhibited, all emulsions prepared by combined modified SPI products exhibited good stability with no visual instability such as creaming during storage.(9) Investigation of fish oil microencapsulation efficiency(MEE) and oxidative stability during storage showed that, the MEE of SPI-Maltodextrin conjugates hydrolysates coated microcapsules(DH, 1.8%) was 97%, which was much higher than that of SPI/Maltodextrin mixture and conjugates based microcapsules(66% and 27%, respecially). And oxidation rate of microcapsules prepared with SPI-Maltodextrin conjugates hydrolysates was relatively lower than that of control samples. After 4 weeks storage, the peroxide value and the content of propanal was only 4.72 mmol/kg and 2688, respecially, whereas, those of control microcapsules(SPI/Maltodextrin mixture) reached up to 24.06 mmol/kg and 10848.87, respecially. Microcapsules coated by SPI-Maltodextrin conjugates hydrolysates exhibited uniform and porous structures, as evidenced by Scanning Electron Microscope(SEM). |
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