Investigaiton On The Preparation Of Covalent Complexes Of Milk Protein-caffeic Acid-glucose And Its Emulsion Stability With Astaxanthin Loaded

Food protein stabilized emulsions are excellent carriers for encapsulating,protecting and targeting delivery of nutrients.However,the conformation and aggregation characteristics of natural protein molecules are prone to change during processing,resulting in emulsion phase separation.Covalent modification of polyphenols or carbohydrates has been proved to improve the structural stability and emulsifying activity of proteins,enabling targeted delivery of nutrients.Astaxanthin is a powerful antioxidant with antioxidant,anti-inflammatory,anti-tumor and other biological activities.However,it is found that its thermal and light stability are weak,and its bioaccessibility is not high,which greatly limits its application in food processing.In the present work,milk protein was modified with caffeic acid and glucose to construct ternary covalent complex of milk protein-caffeic acid-glucose,and the contribution of covalent modification to improving protein stability and emulsification was evaluated.Then,the delivery efficiency of the emulsion system was evaluated with astaxanthin.Finally,the potential mechanisms of astaxanthin bioaccessibility in this system were analyzed by gastrointestinal digestion model.The specific results are as follows:（1）Two typical milk proteins（whey protein isolate（WPI）/casein（CS））were covalently modified by free radical grafting method and glycosylation to construct two milk protein-caffeic acid-glucose covalent complexes.The structure and functional characteristics of the covalent proteins were analyzed.It was found that under the same treatment,the grafting degree of WPI（51.67%）was higher than that of CS（39.53%）.After modification,the secondary and tertiary structures of the two proteins changed significantly,which was manifested as α-helix unfolding,β-sheet and increased random coil content.The α-helix content of WPI decreased from 20.6% to 15.5%,and β-sheet content increased from 33.5% to 39.8%.The random crimping content in casein increased from 21% to 25.6%.The hydrophobic groups of modified protein were more exposed,showing strong fluorescence quenching effect.In addition,the antioxidant capacity and emulsification ability of both proteins were significantly improved after modification.（2）The ternary covalent complex of casein,caffeic acid and glucose was used as stabilizer to prepare emulsion by high pressure homogenization,and the protective effect of the emulsion system on astaxanthin was evaluated.It was found that the emulsion prepared with 3% wt casein-caffeic acid-glucose ternary covalent complex as stabilizer had lower particle size（237.17 nm）and potential（-36.6 mv）,and better dispersion（PDI=0.14）.The emulsion showed good stability under the experimental conditions such as heating,freezing and thawing,salt ion stress and storage period,and could protect astaxanthin.The stability is better than that of sodium caseinate emulsion under acidic conditions（P < 0.05）.The rheological data showed that the emulsion system had higher elasticity than that of the sodium caseinate emulsion system,which can effectively avoid phase separation of the emulsion.（3）A simulated digestion model was established to analyze the digestive properties of astaxanthin emulsion stabilized by the ternary covalent complex of casein,caffeic acid and glucose.It was found that the emulsion stabilized by the ternary covalent complex group showed lower droplets size and higher antioxidant activity than sodium caseinate emulsion during gastrointestinal process.During the gastric digestion stage,the average particle size of emulsion droplets in ternary covalent complex group increased from245.96 nm to 4972.33 nm,which was significantly lower than that in sodium caseinate group（7361.00 nm）.The results of emulsion potential and laser confocal focalization showed that the emulsion particles of ternary covalent complex aggregated less and had stronger interaction force during gastric digestion.In addition,ternary covalent complex systems have been shown to reduce bile salt replacement rate and achieve better oil droplet digestion efficiency,thereby improving astaxanthin bioaccessibility.