The Preparation And Functional Properties Of Rapeseed Protein And Its Protein Hydrolysates' Antioxidant Activity

Rapeseed is one of the important oilseed crops in China.After oil extraction,this protein rich meal is usually only used for animal feed or fertilizer.The protein rich meal as a good source of protein has a well-balanced amino acid composition.In this study,the nature of rapeseed composition,preparation of protein,the in vitro digestibility,functional properties of rapeseed protein and antioxidant activity of hydrolysates were investigated in order to provide a theoretical basis for rapeseed protein processing and comprehensive utilization. The experimentation results were as follows:1.The influence of pH on solubility of protein,glucosinolate,phytic acid and tannin were invesgated.The results indicated that protein had the highest solubility(59.24%) at pH 12.0.The glucosinolate and phytic acid solubility of meal tested were high in acidic conditions.The highest solubility of glucosinolate and phytic acid were 74.42%and 56.46%at pH 2.0,respectively.The solubility of tannin was high(＞50%) and the maximum solubility of tannin was 84.8%at pH 11.0.2.Four proteins were isolated from rapeseed protein.The contents of albumin in rapeseed meal were 35.60%,globulin 24.51%,prolamin 0.82%and glutelin 29.70%.And the relative quantity of major subunits and molecular weight distribution of the proteins were investigated by SDS-PAGE.Albumin was composed of 9 polypeptide chains,which comprising 53.2%12S(cruciferin) and 44.9%2S(napin).Globulin was composed of 5 polypeptide chains,comprising 45.6%12S and 54.0%2S.Glutelin had the characteristic of aggregation and degradation.3.The pH,extraction temperatures,liquid-solid ratio during extraction of rapeseed proteins were optimized by Response Surface Methodology(RSM).The results indicated the highest extraction yield(78.85%) of rapeseed protein was obtained,when the defatted rapeseed meal was extracted twice at 55.0℃for 30min with 0.1%NaHSO₃(pH 12.0) and liquid-solid ratio 19:1(mL/g).The effect of pH values between 3.0 and 7.0 on the precipitation of extracted rapeseed proteins was investigated to recover the protein dissolved in NaOH aqueous solution.The maximum yield of precipitated protein was observed at pH 5.0.4.Under the sequential pepsin and trypsin digestion model,nitrogen releases of rapeseed protein isolates(PPI),12S fraction(RP-12S) and 2S fraction(RP-2S) were 84.51%,78.33% and 80.39%,respectively.And the nitrogen releases of PPI,RP-12S and RP-2S were all higher than that of SPI.During the pepsin digestion,the nitrogen release order was RP-2S＞SPI＞RP-12S＞PPI.Then during the trypsin digestion,the order was PPI＞RP-12S＞RP-28＞SPI.The nitrogen release of PPI increased rapidly and reached 27.77%in trypsin digestion.12S fraction could be quickly digested;while the nitrogen release of 2S fraction tended to digested slowly during the whole digest process.Thermal pretreatment decreased the final rapeseed protein digestion.The 12S digestion during pepsin digestion was significantly increased after thermal pretreatment,while 2S digestion was significantly decreased during this period.The digestion of PPI,RP-12S and RP-2S were decreased during trypsin digestion.5.Physicochemical and functional properties of SPI,PPI,RP-12S and RP-2S were investigated.Physicochemical properties:Surface hydrophobicity of PPI was at high pH(＞7.0). RP-12S had the minimum surface hydrophobicity at pH 7.0 and the maximum at pH 5.0; RP-2S had the maximum surface hydrophobicity at low pH(＜7.0).The SH_F of PPI,RP-12S and RP-2S were lower than SPI,but the disculfide bond were higher than SPI.Functional properties:Compared with SPI,RP-2S had higher solubility,oil holding capacity,foaming and emulsifying activity.Foaming stability was poor at pH 5.0-6.0. Emulsifying activity had a significantly positive correlation with solubility(r=0.7905^*). The emulsion activity declined at high pH(＞8.0).For RP-12S,its water holding capacity and oil holding capacity were lower than those of SPI,while its foaming capacity and stability were higher in alkaline condition.The RP-12S emulsifying activity in alkaline condition was higher and had a significantly positive correlation with solubility (r=0.9094^**).For PPI,its solubility,foaming capacity and foaming stability were lower than those of SPI.While its water and oil holding capacities were higher.The PPI emulsifying activity which had a significantly positive correlation with solubility (r=0.8281^*) was better at pH＞7.0.Consequently,the surface properties and the interaction between 2S fraction and water were prominent.The interaction between PPI and water was prominent,but the surface properties of PPI were poor.The 12S gel texture properties were better in alkaline condition.6.Adding xanthan appropriately could improve 12S gel texture properties.The best texture properties were obtained by adding 0.75%xanthan to 12S.Hardness,cohesiveness, gumminess,chewiness and resilience were improved with the increase of pH.The highest adhesiveness and springiness were obtained at pH 7.0.7.The hydrolysis results indicated the hydrolysis capacity of pepsin was poor(DH＜10%); Trypsin and papain hydrolysis capacities were better than pepsin(DH 15%～25%); Hydrolysis capacity of protease from Aspergillus melleus and protease from Bacillus sp. were the highest ones(DH＞30%) in five enzymes.After hydrolyzation with pepsin,trypsin and papain,the color of hydrolysates was increased.Antioxidant activity experiment showed that the hydrolysate obtained with trypsin had the highest antioxidative activities. The trypsin hydrolysate was composed of four different molecular weight fractions.