Study On The Milk Protein-tea Polyphenol Interactions And Tea Polyphenol Bioaccessibility

Tea polyphenol is a type of nature polyphenols,which has a rich flavor and many pharmacological effects,so it is popular with consumers in recent years.However,tea polyphenols are easy to interact with proteins,and under different temperature and pH conditions,there is a significant difference with different structural proteins,which may have an impact on the bioaccessibility of tea polyphenols and its effects on human body.Currently researches on the protein-polyphenol interactions are mostly concentrated on the changes of the content of milk protein and polyphenol in vitro digestion,but there are few researches on the effect of heat treatments on the interaction and polyphenol bioaccessibility.The effect of milk on the polyphenol bioaccessibility is still also controversial.Therefore,this paper chose two proteins(whey protein and casein)and four tea polyphenols [epigallocatechin gallate(EGCG),epigallocatechin(EGC),epicatechin gallate(ECG)and epicatechin(EC)] as raw materials,to explore the effects of the two heating conditions(80°C/20 min,100°C/5 min)and two heating methods(heating the mixture,protein alone heating)on the protein-polyphenol interactions.Moreover,this paper explored the effects of milk protein and heat treatments on the tea polyphenol bioaccessibility in vitro simulation,and analysed the link between interaction and bioaccessibility.Firstly,this paper selected ?-lactoglobulin and ?-casein as typical proteins of whey protein and casein.Based on the results of fluorescence and circular dichroism,four kinds of tea polyphenols had fluorescence quenching ability on ?-lactoglobulin and ?-casein,among which EGCG and ECG were the strongest,EC was the weaker and EGC was the weakest.It was static quenching and the binding site was 1.The combination was spontaneous.Heat treatment and interaction with tea polyphenols changed the protein secondary structure and conformation.The binding strength of polyphenols with ?-lactobulin was greater than that with ?-casein.The effects of different heat treatment temperature and heating methods on the interaction of different milk proteins and polyphenols were different.The protein preheat treatment could increase the interaction of milk protein and polyphenols in different degrees.The mixed heat treatment significantly enhanced the ?-lactoglobulin-tea polyphenols interaction(1.24~3.21 times)and ?-casein-EGC/EC interaction,but it decreased the intensity of ?-casein with EGCG and ECG.Then,commercial whey protein,casein and tea polyphenols were selected to prepare simulated system respectively.To explore the effects of milk protein and heat treatment on the tea polyphenols bioaccessibility,the content of tea polyphenols was determined in vitro digestion.The results showed that under the same heat treatment,adding milk protein could improve the bioaccessibility of tea polyphenols in generally,and adding whey protein was better than adding casein.In non-protein system,heat treatment could reduce the bioaccessibility of tea polyphenols in different degrees by 5.30~39.73%.In whey protein system,protein 80°C pre-heating generally increased the bioaccessibility of tea polyphenols by 0.94~9.84%.The rest of heat treatments were generally shown to reduce the bioaccessibility.In casein system,protein pre-heating could increase the bioaccessibility of tea polyphenols generally by 0.65~15.69%,but mixed heating reduced the bioaccessibility of tea polyphenols by 8.00~29.73%.Overall,protein 80°C preheat treatment improved the bioaccessibility of tea polyphenols in the system.Finally,analyzed the relationship between the milk protein-tea polyphenol interactions and tea polyphenols bioaccessibility,it found that after protein pre-heating,the interaction was enhanced and tea polyphenol bioaccessibility was also generally increased due to the increased of milk protein protection;however after whey protein 100°C pre-heating,due to the high degree of protein degeneration and rapid hydrolysis,the bioaccessibility of tea polyphenols was reduced.The bioaccessibility of tea polyphenols under mixed heat treatment was related to binding strength and heating conditions.