Studies On The Structure Characterization And Products’ Modification Of Walnut Protein

China’s walnut cultrivation area and output rank first in the world. Except for exports asfresh food and domestic sales, walnuts are mainly processed into products, like walnut oil,walnut powder and walnut milk. Walnut oil production produces a lot of defatted walnut flour.Because of different walnut oil extraction methods, the component of defatted walnut flourwas complex and function was poor, leading walnut protein is hard to wide application infood processing. In order to make effective use of plant proteins in this resource and toexpand the fields of walnut protein products in food processing and other applications, theresearch on structure characterization and products’modification of walnut protein wasconducted in this article. Structure and properties of walnut protein, walnut protein extractioncondition and isoelectric point, development of walnut protein isolate and concentrate and onlimited enzymatic hydrolysis of walnut protein isolate to improvement functionality weredeeply investigated.Important physical chemistry parameters characterization, structure and properties ofwalnut protein and protein fractions were deeply investigated. Results showed that walnutprotein fractions are albumin, prolamin, glutelin and globulin with content of7.54%,4.73%,72.06%,15.67%, respectively. Glutelin was the main protein composition of walnut. Thecontent of glutamic acid, arginine, aspartic acid was the highest in the amino acidescomposition of walnut protein and protein fractions and dominated by acidic amino acids andhydrophobic amino acid. Lysine was the first limiting amino acids of defatted walnut flour,albumin, glutelin and globulin and Leucine was the first limiting amino acids of prolamin.Molecular weight distribution of defatted walnut flour was broad with the range of3.54kDa~81.76kDa and molecular weight distribution of globulin, glutelin, albumin andprolamin was between11.25kDa~78.60kDa,14.25kDa,3.63kDa~67.14kDa,3.63kDa~67.14kDa and13.17KD, respectively. Subunits composition of walnut protein wascomplex with molecular weight ranging between14kDa~120kDa, and was made up of11bands on the non-reducing condition. The peptides with molecular weight between31kDa~43kDa and20kDa~28kDa was acid peptides and molecular weight between14.4kDa~20.1kDa was basic peptide. Globulin was made up of9bands, with molecularweight ranging between20kDa~120kDa. Albumin was made up of8bands, with molecularweight ranging between14kDa~60kDa. Glutelin was made up of5bands, with molecularweight ranging between14kDa~60kDa. The subunit composition of walnut protein fractionsand walnut protein were basically. By two-dimensional gel electrophoresis study, walnutprotein isoelectric point is mainly concentrated at pI4.8~6.8with few isoelectric pointconcentrated at pI8.4~9.0. Walnut protein was dominated by acidic protein.The preparation technology of walnut protein isolate and concentrate was determined bystuding the extraction condition and isoelectric point of walnut protein. Peeling methods forwalnut kernel was systematically studied among the methods of roasted, blanching and alkalisolution soaking. Result showed that peeling method of alkali solution soaking was best, with2%NaOH for4min. Among the variety of factors, the extraction rate was influencedeffectively on extraction pH. Optimum extraction conditions results by Response SurfaceMethod for walnut protein was: solid-liquid ratio was1:26(w/v); extraction temperature was53℃; extraction time was1.5h; extraction rate was82.68%. It was determined that the isoelectric point of alkali-soluble protein of walnut protein was pH4.5. The purity of walnutprotein isolates and concentrates prepared by above preparation technology was90.50%,75.56%, respectively, which are higher than walnut protein products produced by othertechniques.The protein structure and chemical composition, surface hydrophobicity, functionalproperties of walnut protein isolate and concentrate was systematicly studied with view toexploring protein structure and functional relationships. The secondary structure content ofwalnut protein concentrate and isolate was similar. It was showed that the secondary structureof walnut protein concentrate and isolate were changed during preparateion, with destructionof α-helix structure, significant increased of β-turns, random coil structure and a small amountincrease of β-sheet. Results showed that the molecular weight distribution of walnut proteinisolate was relatively single with molecular weight at89.6kDa. However, the molecularweight distribution of walnut protein concentrate was more complicated about11kDa~79kDa.The amino acids were not damaged through preparation of walnut protein concentrate andisolate, especially for cysteine and methionine. Walnut protein isolate showed relatively weaklarge-layer structure. However, walnut protein concentrates showed layers of smaller andmore compact structure. It was suggeseted that large-layer structure helps to increase thesolubility of walnut protein. Solubility and emulsifying property, foaming property of walnutprotein isolate and concentrate was significantly influenced by pH and NaCl concentrations.Low concentrations of NaCl (0.1~1.0mol/L) can improve the solubility, emulsifyingproperties and foaming properties of walnut protein isolate and concentrate. Solubility,emulsifying properties and foaming properties of walnut protein isolate was significantlyhigher than that of walnut protein concentrate, which was due to the difference onmicro-structure, molecular weight distribution and components. Under neutral conditions, fatabsorption capacity of walnut protein isolate was significantly higher than protein concentrate,which all were higher than that of soy protein isolate. Water absorption capacity of walnutprotein isolate was less than protein concentrate.Walnut protein isolate was limitedly hydrolyzed by Alcalase alkaline protease. Thecomposition, structure and functional properties of hydrolysate were systematically studied.Optimize conditions of enzymatic hydrolysis by response surface methodology was: pH8.7,enzyme0.05%(mL/g),5%substrate concentration, temperature is55℃. The results showedthat moderate hydrolysis can increase surface hydrophobicity, change secondary structure,makes molecular weight decrease and disperse, increase hydrophobic amino acids, reducehydrophilic amino acids, makes acid peptide hydrolysis first and alkaline peptide does nothydrolysis, make large layer structure becomes small layer structure with small particles. Thehydrolysates changes above can improve the nitrogen solubility index, emulsifying stability,foaming properties, fat absorption capacity and water absorption capacity. Walnut proteinisolate through limited hydrolyzed by Alcalase showed better functional properties with DHof6%form the perspective of improving nitrogen solubility index and emulsifying properties.