| Jackfruit is widely loved by consumers due to its unique flavor,taste and rich nutrients.Polyphenols have attracted much attention among many nutrients because of their physiological effects such as lowering cholesterol,reducing cardiovascular and cerebrovascular diseases,and inhibiting tumors.Bound polyphenols are mainly combined with DF in the form of covalent bonds and released by alkaline hydrolysis,acid hydrolysis,enzymatic hydrolysis,etc.DF cannot be digested and utilized in the gastrointestinal tract,the residue of digested DF reaches the colon,the covalent bonds are broken by microorganisms and microbial enzymes,then the bound polyphenols are released in the colon.Free polyphenols in the system are absorbed by the human body or catabolized by microorganisms for their own growth.At present,there were few research reports on the bound polyphenols in JDF.In this study,DF rich in bound polyphenols was prepared from jackfruit pulp freeze-dried powder,and optimize the extraction of bound polyphenols by three methods:alkaline hydrolysis,acid hydrolysis and enzymatic hydrolysis,then bound polyphenols were identified by UPLC-QTOF-MS/MS technology,the antioxidant properties of bound polyphenols were explored by in vitro antioxidant experiments,the total polyphenol content,composition and antioxidant activity of the three hydrolyzed extracts were compared to obtain the best extraction process of bound polyphenols from JDF;The total polyphenol content,composition and antioxidant activity and its effects on the microecology of extracts from three stages of digestion(oral cavity,stomach,and small intestine)and fermentation at different time points(1-48h)were studied by in vitro gastrointestinal digestion and colonic model.The results of this study provided an effective method for releasing antioxidant bound polyphenols from JDF,and provide experimental basis for further study of the digestive and fermentation characteristics.The main findings are as follows:(1)The total polyphenol contents were used as an indicator in this experiment,DF was prepared from jackfruit pulp freeze-dried powder as raw material.Three factors including concentration,liquid-solid ratio and treatment time were selected to optimize the extraction of bound polyphenols from JDF by three methods:alkaline hydrolysis,acid hydrolysis and enzyme hydrolysis.The optimal extraction process based on regression model optimization is as follows: In alkaline hydrolysis,the alkaline concentration was 8 M,the liquid-to-solid ratio was 20 m L/g,and the treatment time was 4 h,at this time,the total polyphenols content was 64.9 mg GAE/10 g DM;In acid hydrolysis,the acid concentration was 9%,the liquid-to-solid ratio was 20 m L/g,and the treatment time was 4h,at this time,the total polyphenols content was 21.8 mg GAE/10 g DM;In enzymatic hydrolysis,the enzyme concentration was 2.4 mg/m L,the liquid-solid ratio was 20 m L/g,and the treatment time was 90 min,at this time,the total polyphenols content was 25.0 mg GAE/10 g DM.Therefore,alkaline hydrolysis is the best technological condition for the effective extraction of bound polyphenols from JDF.(2)A total of 17 polyphenols(4 phenolic acids and 13 flavonoids)were identified in the alkaline extracts,and only two polyphenols were identified in both the acid and enzyme extracts by UPLC-QTOF-MS/MS.In addition,the antioxidant activities of the three hydrolyzed extracts were determined by ABTS free radical,oxygen free radical and peroxygen free radical scavenging ability assay.It was found that the free radical scavenging ability of alkaline hydrolyzed extracts was significantly higher than that of acid hydrolyzed and enzyme hydrolyzed extracts,no matter which antioxidant method was used(p<0.05).In conclusion,alkaline hydrolyzed extracts have more species and higher antioxidant activity than acid hydrolyzed and enzyme hydrolyzed extracts,further indicating that alkaline hydrolysis is a more effective method to extract bound polyphenols from JDF.(3)The total polyphenol content,polyphenols species and antioxidant activity of the three stages of digestion(oral,stomach,and small intestine)extracts of JDF were significantly lower than those of alkaline,acid and enzyme extracts(p<0.05).Small intestine stage had the highest total polyphenols content(3.53 mg GAE/10 g DM),followed by oral stage(2.48 mg GAE/10 g DM),and stomach stage had the lowest total phenol content(1.70 mg GAE/10 g DM).Only two kinds of bound polyphenols were identified in oral,gastric and small intestine stages,antioxidant activity and total polyphenol content showed a similar trend at different stages of digestion.(4)The total polyphenol content of the extracts at different fermentation time points(1-48h)decreased first and then increased significantly and finally stabilized(p<0.05).After the microorganisms adapted to the new environment,the covalent bond between the bound polyphenols and DF was destroyed by microorganisms and microbial enzymes,the bound polyphenols were released into the system in large quantities and the total polyphenol content increased,but at the same time,the released polyphenols will be degraded by microorganisms for their own growth,resulting in the reduction of total polyphenols content.A total of about 20 bound polyphenols and metabolites were identified and the main polyphenols compounds were quantified at different fermentation time points,the possible metabolic pathways of gallic acid,protocatechuic acid and naringin were proposed.The antioxidant activity and CAA value of the fermented extracts showed a similar trend to the total phenol content.The study of microbial community structure showed that the bound polyphenols and their metabolites could promote the growth of beneficial bacteria such as bifidobacterium,which playde an important role in maintaining the balance between beneficial bacteria and pathogenic bacteria in the human colon. |
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