Pre-Treatment Of Defatted Soybean Flake And The Effect On The Extraction And Properties Of Soybean Protein

Soybean flake is a by-product of soybean oil, it can be classified into high-temperatureflake and low-temperature flake.Soybean protein isolated (SPI) is prepared fromlow-temperature flake through alkali extraction and acid precipitation,which has a quite highprotein content of over90%. The extraction process of the soybean oil by hexane cannotremove all the lipids, especially the polar lipids binding with the soybean protein.The residuelipids in the soybean flake will make effect on the product in the further process to a certainextent, and make some changes in the flavor and features of protein which is not expected.Therefore, the secondary treatment of defatted soybean flake through heat and solvent wasinvestigated in this paper; the changes of residual lipids content in the soybean flake werestudied. The effects of pre-treatment of low denatured soybean flake on the extraction andproperties of soybean protein were investigated.The content and composition of residual lipids in the soybean flake were investigated;the residual lipids were extracted by the classical solvent, chloroform:methano（l2:1，v:v）, andwater-saturated butanol whose polarity is very strong. Its content was2.10%.The residuallipids extracted from the soybean flake were separated by silica gel column chromatographyinto neutral lipids（25.24%）and polar lipids (74.76%).Then the neutral lipids and polar lipidswere seperated by Thin Layer Chromatography(TLC)for qualitative analysis respectively.Neutral lipids included triglyceride,diglyceride and free fatty acid,polar lipids mainlyconsisted of phosphatidyl choline (PC),phosphatidyl inositol (PI),phosphatidyl ethanolamine(PE) and phosphatidic acid (PA).Chloroform:methanol extracted about90%of the lipids fromthe flake, whereas water-saturated butanol resulted in an extract with more polar lipids thanthat from chloroform/methanol extraction. The fatty acid composition of residual lipid wasanalyzed, which mainly consisted of palmitic acid (C16:0) and linoleic acid (C18:2), oleicacid (C18:1), stearic acid (C18:0) and linolenic acid (C18:3), and the unsaturated fatty acidsaccounted for more than60%.Dry heating and solvent extraction were used to pre-treat the defatted soybean flake.Heattreatment conditions were80℃,30min,and the latter way included two solvents,85%ethanol(A) and hexane-ethanol azeotrope (82:18, v/v)(HA),the soybean flake was extracted at30℃for1h, and the residual lipids were determinated.Then SPI was prepared. The results showedthat heat treatment make no change of residual lipids in soybean flake. But the distribution ofprotein and lipid in the SPI and soybean residue were changed during the process of SPIextraction from soybean flake treated by heating，the lipids of H-SPI accounted for22.47%ofthe total lipids,that is26.10%lower than the control, the lipids of soybean residue accountedfor69.56%of the total lipids,that is30.99%more than the control, The loss of protein intosoybean residue was8.21%higher than the control. The residual lipids content of defattedsoybean flake extracted by A and HA decreased to1.18%and0.87%respectively.Thedecrease of residual lipids in soybean flakes treated by Aand HAreduced the lipids content ofthe corresponding SPI.But because the degree of denaturation of protein by solvent weredifferent, the extraction yield of SPI changed, hexane-ethanol azeotrope was better than85% ethanol, the protein extraction yield ofA-SPI decreased by6.18%。The defatted soybean flakes were fractionated into11S, lipoprotein (LP) and7S, and thecomposition of each sample was studied by SDS-PAGE and yield of protein.Results showedthat there are variety of proteins existed in LP, including7S,11S and the characteristic proteinof Oleosins.The ratio of LP in control flake was31.88%, while the proportion of LP in H, Aand HA-treated soybean flakes was reduced into22.51%,17.54%and16.77%respectively.Besides, the content of11S and7S was increased, it means that the removal of residual lipidsalso improve the purity of protein.The effects of pre-treatment of soybean flakes on the sensory and functional properties ofSPI were investigated.The theory of protein modification through heating and solvent weredifferent, the former reduced the residual lipids in SPI by the way of changing the bindingdegree between lipid and protein, the latter removed the residual lipids directly from soybeanflake to avoid the adverse effects of them on the protein.Nevertheless, whether heat treatmentor solvent treatment, the nature of SPI have been significantly improved,such as thesolubility,transparency and color became better, the composition of off-flavor reduced, therheological,texture and the microstructure properties of gel were improved significantly,additionally, the emulsion and foam stability were improved.