| Coffee peel is the main by-product of wet processing.According to the Ministry of Agriculture and Rural Affairs,China’s total coffee output stood at 137,900 t in 2019,ranking 13th in the world.Coffee peel is rich in nutrients,and the content of Dietary fiber(DF)can reach 70%,which can be used as the source of natural DF.Among them,soluble Dietary fiber plays a more important physiological function in the human body.At present,most of the coffee peel is directly discarded,which not only causes the waste of resources but also pollutes the environment,so it is necessary to develop and utilize the coffee peel.The research group has already studied the insoluble dietary fiber modified by superfine grinding of coffee peel.Therefore,this paper researched the soluble dietary fiber(CPSDF)of coffee peel,a byproduct of wet processing,and selected the green physical modification method for ultrasonic modification to make it better play its physiological function and improve the value of coffee peel.It provides a theoretical basis for the development and utilization of coffee by-products.The specific research contents and results are as follows:(1)The CPSDF,soybean,oat,inulin,jujube,and celery extracted by the traditional water extraction method were used as raw materials to compare the differences in particle size,monosaccharide composition,structural characteristics,physical and chemical properties and functional properties of the six SDF.The results showed that the grain size distribution of soluble dietary fiber in coffee peel was wider than that of other five kinds of SDF,but the monosaccharide composition of CPSDF was richer than that of the other five kinds of SDF.The structural characteristics of SDF from different sources showed similar spectral distribution,but the intensity of the characteristic band was slightly different,and the morphological structure was also slightly different.The oil retention of CPSDF was 2.18±0.03 mg/g,which had no significant difference from the other five SDF.The solubility was 90.9%,lower than the other five SDF.In terms of functional characteristics,CPSDF showed excellent NIAC,the adsorption capacity was 7.93 mg/g,significantly higher than the other five samples.(2)The CPSDF extracted from coffee peel by traditional methods was used as raw material,and was modified by ultrasonic power of 100 W,200 W,300 W,400 W,and 500 W,respectively,to compare the differences in the structural characteristics,physical and chemical properties,functional properties and antioxidant capacity of CPSDF before and after modification.The results showed that ultrasound could significantly reduce the particle size range of CPSDF,and the particle size distribution tended to be uniform.In terms of structure,different power had no significant effect on the monosaccharide types,and all showed characteristic absorption peaks of polysaccharides.However,the amorphous region of CPSDF was damaged by ultrasound,resulting in changes in crystallinity and decreased thermal stability.Ultrasound can significantly improve the WHC,OHC,and WS of CPSDF,and the highest water retention power is 300 W(8.92±1.57 g/g).The functional performance of CPSDF also had a positive effect,and the adsorption capacity of glucose and nitrite was significantly improved.The stuff effect of ultrasonic on antioxidation activity is complicated and needs further study.(3)CPSDF modified with different ultrasonic powers was used as raw materials to extract two forms of polyphenols in binding state and free state,respectively.The effects of ultrasonic modification on the composition and content of phenolic compounds in CPSDF were determined,and the differences in antioxidant capacity,α-amylase inhibitory effect,and α-glucosidase inhibitory effect were compared.It was found that the content of combined phenols in CPSDF was significantly higher than that in a free state,and the combined polyphenols accounted for 79%of the total phenolic content.After ultrasonic modification,the content of total phenols and flavonoids in CPSDF increased first and then decreased.The contents of total phenols and flavonoids were the highest(296.8 mg GAE/100 g and 1681.5 mg GAE/100 g).There were also significant differences in the composition and content of phenolic compounds between free and conjugated polyphenols,and correlation analysis showed at the contribution degree of phenolic compounds to antioxidation and α-glucosidase inhibition was different,and the interaction between phenolic compounds could better play the functional characteristics.(4)The dynamic changes of bound polyphenol metabolites in CPSDF treated with different ultrasonic powers were studied by LC-MS/MS metabolomics method.A total of 398 metabolites were detected,among which phenolic acids and flavonoids accounted for 92.97%.Cluster analysis and OPLS-DA showed that the differences between U1 vs U2,U3 vs U4 were small,and the differences between the other groups were large.Through the screening of metabolites with significant difference,the total number of metabolites with significant difference expression of U4 vs U5 was the largest,indicating that ultrasound can promote the release of content in the plant matrix by destroying the cell wall,but also destroy part of the released substances,resulting in varying degrees of changes in phenolic acids and flavonoids in polyphenols.It is the first time to use metabolomics to analyze the changes of polyphenols in ultrasonic modification.Therefore,how to analyze the factors regulating polyphenols and their formation pathways from the results of a large number of metabolites is the work that needs to be further strengthened. |
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