Carboxymethylated Modification And Antioxidant Activity Of Apple Pomace Polysaccharides

The apple planting and processing volume in Shaanxi province ranks first in the country,in recent years,with the increase in the production of concentrated apple juice,the discharge of apple slag has also increased dramatically.At present,the comprehensive utilization of apple pomace is low,and a small part is used for animal feed after rough processing,resulting in great waste of resources and environmental pollution.The apple pomace contains more dietary fiber,it can improve the the content of water-soluble dietary fiber and the physical and chemical properties after modification,which lays a foundation for the sustainable use of apple pomace.In this study,apple pomace dietary fiber was used as the base material.Firstly,the twin-screw extruder is used for extrusion modification treatment and extracting apple pomace polysaccharide to obtain carboxymethylated apple pomace polysaccharide.Then the structure characterization and morphological identification of apple pomace polysaccharide before and after carboxymethylation modification were carried out.Finally,the antioxidant activity of apple pomace polysaccharides before and after modification was determined to explore its structure-activity relationship.The main findings are as follows:?1?The optimal extrusion process of apple pomace dietary fiber:The material particle size is 20 mesh,the water addition amount is 30%,the screw rotation speed is 1500 r/min,and the rest time is 90 min,at this time,the APP yield was19.33%,which was 544.33%higher than that before the modification.Moreover,the WHC,WBC and SC of the dietary fiber after extrusion modification were increased by 46.2%,90.2%and 249.1%,respectively.?2?The best process for carboxymethylation modification of apple pomace polysaccharide:alkalization temperature is 50?,chloroacetic acid is 1.00 g,etherification temperature is 65°C,etherification time is 2.5 h,at this time,the degree of substitution of the carboxymethylated apple pomace polysaccharide was0.446.The results of scanning electron microscopy showed that the surface of the modified polysaccharide was uneven and attached with a large number of fragments.Infrared spectroscopy revealed a COO-vibration absorption peak in the carboxymethyl group,indicating that the polysaccharide was successfully modified by carboxymethylation.The solubility test showed that the solubility of polysaccharide after carboxymethylation modification was increased by 143.75%.?3?The carboxymethylated apple pomace polysaccharide was isolated and purified to obtain five components,namely CMAPP-1,CMAPP-2,CMAPP-3,CMAPP-4 and CMAPP-5 components.The degree of substitution is 0.449,0.198,0.432,0.315,0.265,respectively.Structural characterization of APP and CMAPP-1and CMAPP-3 with high degree of substitution were performed.Scanning electron microscopy showed that the surface of APP before carboxymethylation modification was smooth and flat,the structure was compact and there were no obvious particles on the surface.After modification,the surface of the polysaccharide was rough and uneven,and a large number of flaky substances with different shapes were attached.UV spectrum analysis shows that the modified polysaccharide does not contain protein,nucleic acid and pigment.Infrared spectroscopy revealed that all three polysaccharides had characteristic peaks of polysaccharides,and a new vibrational absorption peak COO-appeared in CMAPP-1 and CMAPP-3 after modification,indicating that the carboxymethyl substitution was successful.The molecular weights of APP,CMAPP-1 and CMAPP-3 were 42.11×10⁴Da,26.11×10⁴Da and32.98×10⁴Da,respectively.Analysis of monosaccharide components indicated that all three polysaccharides contained fucose,arabinose,galactose,glucose and xylose before and after carboxymethylation modification.According to the nuclear magnetic analysis,all three polysaccharides are pyran type,APP and CMAPP-1 have both?configuration and?configuration glycosidic bond,CMAPP-3 only has?configuration glycosidic bond,and the carboxymethyl modification occurs at the C-6 position.The congo red test showed that there were no triple helix structures in APP,CMAPP-1 and CMAPP-3.?4?The reduction of three polysaccharides showed that the three polysaccharides had certain reducing power before and after carboxymethyl modification,and the polysaccharide reduction ability was better than that before carboxymethylation.The results of DPPH free radical,?OH free radical,O₂^-?free radical and ABTS+radical scavenging test on three polysaccharides showed that the scavenging ability of apple pomace polysaccharide increased with the increase of polysaccharide concentration.The carboxymethylation modification can significantly increase its antioxidant activity.Moreover,the antioxidant activity of CMAPP-1 is better than that of CMAPP-3,indicating that the greater the degree of substitution,the higher the antioxidant activity.