Study On The Changes Of Pumpkin Seed Quality With Roasting And Its Protein Properties

Plant-based milk,which have the advantages of being rich in dietary fiber,no cholesterol,low calories,are increasing in the dairy market.Pumpkin seeds are a good source of plantbased milk because of their high protein content(34%-38%),low allergenicity and health benefits such as relieving high blood pressure and lowering cholesterol.Studies have shown that heat treatment can improve the nutritional quality and physical and chemical properties of plant milk raw materials.In this paper,the effects of roasting on the quality and emulsion stability of pumpkin seeds(Cucurbita pepo L.)were studied,and the protein structure and properties were characterized,in order to provide theoretical basis for the processing of pumpkin seeds milk and promote the development and application of pumpkin seeds in plantbased diet.The main research contents are as follows:Firstly,in order to study the effect of heat treatment on flavor and nutrition of plant milk,pumpkin seeds were roasted and pretreated(120°C,160°C,200°C,10 min,200 g).The composition and content of volatile compounds and fatty acids in roasted pumpkin seeds were analyzed by SPEM-GC-MS and GC.The effects of roasting temperature on total phenolic content,flavonoid content and antioxidant activity of pumpkin seeds were investigated by colorimetric method,and phenolic acid composition and content were identified by high performance liquid chromatography.Furthermore,the average particle size and molecular weight distribution of roasted pumpkin seeds protein were determined.The results showed that with the increase of roasting temperature,the content of pyrazines in pumpkin seeds increased significantly,which made the seeds have strong burnt flavor.The total phenolic content,total flavonoids content and antioxidant activity of roasted pumpkin seeds were 2.44-3.82 mg-GAE/g,0.61-1.72 mg-RE/g and 3.21-4.92 μmol-TE/g,which increased with the roasting temperature.Roasting temperature had no significant effect on the composition and content of fatty acids in pumpkin seeds.The results of protein electrophoresis and particle size analysis showed that the optimal roasting temperature of pumpkin seeds was 160°C.Then the effect of heat treatment on the stability of pumpkin seeds milk was studied.The effects of roasting temperature on the microstructure,particle size and rheological properties were studied by confocal laser microscopy,particle size analysis and rheometer.At the same time,the storage stability,centrifugal stability and processing stability of pumpkin seeds milk were investigated.The results showed that the size of pumpkin seed milk decreased,viscosity increased,centrifugal stability and storage stability were significantly improved due to the roasting pretreatment.The average particle size(210.99 nm)of pumpkin seeds milk prepared from pumpkin seeds roasting at 200°C was the smallest,and no stratification was observed within 30 days.Ion concentration,freeze-thaw cycle and heat treatment had little significant effect on its stability.Finally,in order to study the effect of protein as a natural emulsifier on the stability of pumpkin seeds milk,the protein isolates,albumin,globulin and glutenin of pumpkin seeds were extracted by alkali solution and acid precipitation method and Osborne grading method respectively,and their physicochemical and functional properties were studied.The purity of protein isolate,albumin,globulin and glutenin were 84.59%,92.33%,81.53% and 84.32%,respectively,and the isoelectric points were 4.50,3.12,3.86 and 4.09.The protein group with the highest content was albumin.The total amino acid content of isolated protein was the highest(47.00 g/100g),while the essential amino acid content of globulin was the highest(36.34%),which was slightly higher than that of soybean protein(32.70%).The contents of β-fold and α-helix in isolate,albumin and globulin of pumpkin seeds were higher,while the random curl with more gluten content had the lowest denaturation temperature.The surface hydrophobicity of pumpkin seeds protein isolate was the highest,while the albumin was the lowest.The surface hydrophobicity index was 556.33 for isolate protein,167.61 for albumin,453.30 for globulin and 315.86 for gluten.There was no significant difference in water and oil retention of the four pumpkin seed proteins,and the solubility was the lowest around p I.At the same concentration of Na Cl,the solubility of protein isolate was the best while that albumin was the worst,and the concentration of salt solution had the greatest influence on albumin.At the same temperature,albumin had the worst solubility and globulin had the best solubility.The effect of p H on foamability and emulsification of pumpkin seed protein was the same as that of solubility.Under the same p H and Na Cl concentration,globulin showed better foaming and emulsification,while albumin had better foaming and emulsification stability.As heat treatment temperature increased,the functional properties of pumpkin seed proteins are reduced to varying degrees.