| Lipophilic bioactive substances,such as carotenoid and polyunsaturated fatty acid,possess various physiological functions.However,most of lipophilic bioactive substances have poor solubility in water,are easily deteriorated due to oxidiation,and have low bioaccessibility,which limit their application in the field of functional food.To solve this problem,the microcapsule technology can be used to encapsulate bioactive substance,and the anti-solvent preparation method is not only simple and fast to operate,but also zein can be used as wall materials of microcapsule,which accords with the development direction of the food industry,so it has great development prospects.At present,there are only 3 articles that have reported the microencapsulation of functional oils using the anti-solvent method,and our previous study found that the structure of microcapsule prepared according to the existing method was loose and porous,resulting in the retention ratio of encapsulated bioactive substance of only 40% after 28 days storage at room temperature.Therefore,it is of great significance to further improve the stability of the microcapsule for promoting the industrial application of this method.In view of this,in this study,firstly,?-carotene was used as a lipophilic bioactive substance model,and then the existing preparation method was improved,the ultrasonic treatment was used to stabilize the system and the effects of various factors on the structure and properties of microcapsule during the preparation process were investigated.On this basis,the structure and properties of microcapsules were further optimized through cross-linking technology.The main research contents and results of this paper are as follows:1.Preparation of ?-carotene-loaded MCT/Zein core-shell microcapsules and its propertiesThe zein solution was used as the shell layer,and the ?-carotene dissolved in medium-chain triglyceride(MCT)was served as core layer.The ?-carotene-loaded MCT/ Zein core-shell structure microcapsule was prepared by the anti-solvent method.Ultrasound was used during the preparation process to stabilize the system,and the optimal preparation conditions were determined.The effects of various factors on the particle characteristics of microcapsule(i.e.,Z-average diameter,PDI and Zeta-potential)and ?-carotene encapsulation efficiency were studied,and the optimal mass ratio of oil phase to zein solution was 5:90,and the adding speed of water was 40 mg/s.Under the optimized conditions,the obtained microcapsules had Z-average diameter of 252 nm(PDI=0.42),Zeta-potential of 54.60 m V,and ?-carotene encapsulation efficiency of 91.4%.The observation of laser confocal microscope(CLSM)revealed that the microcapsules had a complete core-shell structure and the average shell thickness was 140 nm.The experimental results of DPPH scavenging rate showed that the preparation process of microcapsule had no significant effect on the antioxidant activity of ?-carotene.Under different environmental pressures(i.e.,salt solution treatment,heat treatment,freeze-thaw treatment and storage treatment),microcapsule can improve the stability of ?-carotene to a certain extent.But it was worth noting that microcapsule tended to aggregate together during processing,resulting in decrease of Zeta-potential,and increase of Z-average diameter and PDI,and thus had poor physical stability.The results of in vitro digestion experiment showed that ?-carotene retention ratio in microcapsule was only 55.3% after simulated gastric fluid(SGF)digestion for 2 h,and ?-carotene was completely released after simulated intestinal fluid(SIF)digestion for 6 h,and ?-carotene bioaccessibility was 48.5%.2.Preparation of ?-carotene-loaded cross-linked MCT/Zein core-shell microcapsules and its propertiesIn order to further improve the stability of ?-carotene-loaded MCT/Zein core-shell microcapsule and ?-carotenebioaccessibility,zein was modified with citric acid(CA)and tannic acid(TA)as cross linkers respectively,and then CA cross-linked microcapsule(CCM)and TA cross-linked microcapsule(TCM)were prepared.The effects of the mass ratio of crosslinkers to zein and the time of cross-linking on the particle characteristics of microcapsules and the?-carotene encapsulation efficiency were studied,and the optimal cross-linking conditions were determined: the mass ratio of CA and TA to zein were both 1: 100,the cross-linking time was 48 h.Under this optimized condition,the Z-average diameter of CCM was 472 nm(PDI=0.43),Zeta-potential was 47.17 m V,and the?-carotene encapsulation efficiency was 94.4%;and the corresponding values of TCM were 285 nm(PDI=0.41),55.63 m V and 93.9%.The CLSM results showed that both CCM and TCM had a complete core-shell structure and the average shell thicknesswas 161 nm and 143 nm,respectively.The surface structure of the microcapsules were observed by scanning electron microscopy,and it was found that compared with uncross-linked microcapsules(UCM),the surface of the microcapsules after cross-linking was smooth and dense,with only a few holes and cracks;in addition,cross-linking was significantly improved the stability of CCM and TCM under different environmental pressure.The results of differential scanning calorimeter and X-ray diffraction analysis showed that the thermal stability and crystallinity intensity of the shell complex of cross-linked microcapsules were significantly improved,and fourier infrared transform spectroscopy and circular dichroism spectroscopy confirmed that there existedinteractions between cross linkers and zein.In addition,the cross-linking effect of CA and TA also significantly improved the stability of microcapsules in SGF and the?-carotene bioaccessibility in SIF.The ?-carotene retention ratio in CCM and TCM were 72.8% and 79.7% after SGF digestion for 2 h,respectively;while ?-carotene bioaccessibility were 72.4% and 77.2% after SIF digestion for 6 h,respectively.This research provides theoretical basis and experimental data for the development of a new type of lipophilic bioactive substance-loaded microcapsule system,and will also promote the application of microcapsules prepared by anti-solvent method in the field of functional food. |
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