Study On The Protein And Processing Characteristics Of Seed-watermelon Kernels

The seed-watermelon?Citrullus lanatus var?is a variant of watermelon?Citrullus lanatus?,which is a genus belonging to the Cucurbitaceae family and is widely cultivated in the global temperate zone.Its seed kernels are rich in nutrients,containing high contents of lipids,proteins,vitamins and minerals,especially with the protein content being 36%to 40%,and with reasonable essential amino acid composition,making it an excellent plant protein resource.In this study,Xinjiang seed-watermelon kernels were used as research object,and the electrolyzed water extracted seed-watermelon protein?EWP?and ultrapure water extracted seed-watermelon protein?UWP?were compared and analyzed for their structural and functional differences.After developing the new extraction method for seed-watermelon kernels protein isolate?WMP?and investigating its structure and functional properties,the effects of AAPH and irradiation treatment on its structure and digestion characteristics were also studied,focusing on the antioxidant properties of the polypeptide in the digesta of the pre-treated seed-watermelon kernels protein.The main experimental results are as the following:?1?The optimal extraction process was obtained by the response surface optimization design,to be exact,with the temperature at 36.95?,pH of 9.99,the ratio of material to liquid being 1:28.7,and extraction time of 3.28 h,the EWP extraction rate was as high as 91.58%;when compared with UWP,EWP not only has higher amino acid content,but also has better solubility,emulsifying property and foaming property.In addition,the free sulfhydryl content of EWP is significantly higher than that of UWP,while the disulfide bond content of it is slightly lower than UWP,and the surface hydrophobicity is relatively higher.Also,the range of the diameter distribution of EWP is smaller,and there are more ordered structures such as?-helix and?-sheet.The results of Scanning electron microscopy showed that the EWP structure was more orderly and the appearance was uniformed.?2?As the AAPH concentration increased,the free sulfhydryl content of WMP decreased from 18.24?mol/g to 11.25?mol/g,the solubility decreased from 9.59mg/mL to 6.76 mg/mL,and its disulfide bond was increased from 39.57?mol/g to43.06?mol/g,the enthalpy??H?and denaturation temperature?Td?increased?p<0.05?.Its structural analysis showed that the?-helix in WMP had a significant decrease and the?-sheet increased.The results of electrophoresis and particle size distribution showed that the protein in WMP had obvious aggregation.What's more,by simulating the degree of hydrolysis curve measured by gastric digestion,together with the mass spectrometry results of the digested,it was shown that the AAPH treatment had a negative impact on the digestion characteristics of WMP.?3?After being treated with different levels of radiation,the?-fold content of WMP decreased from 52.4%to 47.5%,and the irregular curl content increased from21.2%to 29.1%.The molecular structure of the protein was unfolded,the surface hydrophobicity was enhanced.The hydrophobic group buried in?-sheet structure was exposed.The average particle size of the protein increased to 179.5 nm,the surface morphology became concave and rough,the surface area in contact with the aqueous medium increased,and the digestion rate increased to 8.53%.Furthermore,the small molecular peptides increased in the digesta,and the molecular weight range increased.?4?Comparing the antioxidant activity of polypeptides in the digesta samples of AAPH treated and irradiation treated WMP,it was observed that AAPH oxidation treatment significantly inhibited the antioxidant properties of the peptides in the digesta,while moderate irradiation treatment significantly improved at the antioxidant properties of the polypeptide in the digesta,especially the impacts on the DDPH radical scavenging ability and the Fe³⁺chelation ability were more prominent.The MLDI-TOF-MS/MS identified 4 kinds of antioxidant peptides in the digesta of seed-watermelonkernelprotein,namelyDVANSVCGILCSKF,CDIPPNGLKMASTFIG,RGEQGPAGAPGFQGLP,GDKGPTGATGLRGGP;The physicochemical properties,structural properties and safety of these six antioxidant peptides were investigated by means of bioinformatics.The study found out that these 4natural antioxidant peptides have high antioxidant activity and good safety record.It is also found that the antioxidant capacity of RGEQGPAGAPGFQGLP is close to 0.5mg/mL of VC,which make it a natural active peptide resource worth utilizing.