| The coffee beverage containing milk is popular among consumers because of its rich coffee polyphenols and protein,and its unique taste and nutritional health.The health benefits of coffee polyphenols are not only closely related to the biological accessibility of polyphenols,but also affected by the processing conditions and ingredients in the production process.Heattreat Thermal processing is an important processing step in the production of coffee beverages.However,what effect does the heat treatment condition have on the interaction between milk protein and coffee polyphenol? How does the bioaccessibility of coffee polyphenols change after thermal processing? It is not clear about such issues at present.This paper focused on the two beverage systems of milk coffee and soy milk coffee to study the effect of heat treatment on the bioaccessibility of coffee polyphenols.β-lactoglobulin,β-casein,soybean 7S protein and coffee polyphenols(chlorogenic acid,caffeic acid and ferulic acid)were studied in this study and the solution system was simulated.The effect of heat treatment on the bioaccessibility of coffee polyphenols in pure protein systems was researched.Furthermore,the interaction between the protein and the polyphenol was analyzed by the binding constant,the number of binding sites,the thermodynamic constant and the conformational change of the protein.This could provide important theoretical basis and guidance for improving the health function quality of coffee beverage.Major contents and results were as follows.The effects of heat treatment on the bioaccessibility of coffee polyphenols in milk and soymilk coffee systems were studied by simulated digestion experiments in vitro.The results were as follows.The addition of milk and soymilk could improve the bioaccessibility of total phenol and chlorogenic acid,caffeic acid and ferulic acid in different heat treatments.Also,the bioaccessibility of coffee polyphenols was better with the increase of heat treatment temperature.The effect of milk on the bioaccessibility of coffee polyphenols was better than that of soymilk.There was no significant change in the bioaccessibility of total phenols and the bioaccessibility of chlorogenic acid and caffeic acid in milk coffee system after 90℃/30 min and 121℃/15 min heat treatment.The bioaccessibility of ferulic acid in milk and soymilk coffee decreased by 5%~12%,while that of chlorogenic acid in soymilk coffee increased by8%~10%.The effect of milk and soymilk on the bioaccessibility of coffee polyphenols in different heat treatments might be related to the interaction between the protein of milk and soymilk and coffee polyphenols.The effect of heat treatment on the bioaccessibility of coffee polyphenol monomers(chlorogenic acid,caffeic acid and ferulic acid)in pure protein(β-lactoglobulin,β-casein and soybean 7S protein)system was studied by simulated digestion experiment in vitro.The results were as follows.Heat treatment reduced the bioaccessibility of chlorogenic acid,caffeic acid and ferulic acid in the pure protein simulation system.Also,the degree of the reduction in the bioaccessibility of coffee polyphenols increased as the enhancement of heat treatment temperature.The bioaccessibility of chlorogenic acid decreased significantly more than caffeic acid and ferulic acid.After 90°C/30 min and 121°C/15 min heat treatment,thebioaccessibility of chlorogenic acid,caffeic acid and ferulic acid decreased by 60%~89%,13%~53% and 12%~22%,respectively.The three proteins totally had no significant effect on the bioaccessibility of coffee polyphenols subjected to different heat treatment.However,the bioaccessibility of caffeic acid under 121°C/15 min heat treatment was significantly increased by 12%~18%.The interaction of chlorogenic acid,caffeic acid and ferulic acid with pure protein was studied by fluorescence,circular dichroism,liquid chromatography-mass spectrometry and particle size analysis.The results were as follows.Three kinds of coffee polyphenols had fluorescence quenching ability to β-lactoglobulin,β-casein and soybean 7S protein,and all of them were static quenching mechanism.Their interaction with coffee polyphenols,which were based on non-covalent bonds,altered the secondary structure and conformation of the protein,and the number of binding sites n was 1.The interaction intensity of three coffee polyphenols with β-lactoglobulin was greater than that of β-casein and 7S protein,and the binding of ferulic acid and protein was generally weaker than chlorogenic acid and caffeic acid.The interaction between protein and coffee polyphenol could be affected by heat treatment to varying extent.For the β-lactoglobulin system,the interactions between chlorogenic acid,caffeic acid and protein were enhanced by about 173% and 314%,12% and175%,respectively,after heat treatment at 90°C/30 min,121°C/15 min.The interaction between ferulic acid and protein were weakened by 76%,45%.Covalent bond binding was observed and the system protein aggregates particle size increased.For the β-casein system,the interactions of chlorogenic acid and protein were reduced by 28% and 59% after heat treatment,respectively,as well as the interaction of caffeic acid and protein was enhanced by68% and 283%.The ferulic acid and protein were enhanced by heat treatment at 90 °C by178%,while it was reduced by 53% after heat treatment at 121°C.No significant effect on the distribution of protein particle size was observed by heat treatment.For 7S protein system,the interaction of caffeic acid and ferulic acid with protein increased by 124%,90%,150% and23% respectively after heat treatment,while the interaction of chlorogenic acid with protein decreased by 72%,68%,respectively.Also,the particle size of protein in the system increased.Due to the interaction of coffee polyphenol and protein,protein had a protective effect on polyphenol in heat treatment,which improved the bioaccessibility of coffee polyphenol after heat treatment to a different extent. |
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