Study On Preparation And Solubility Of Walnut Protein Flour

China leads the world cultivated area and annual output of walnuts. Except for eating directly,walnuts are mainly used for oil extraction. Defatted walnut flour is the by-product of walnut oil process.The protein solubility of defatted walnut flour is very poor and the color is brown. Defatted walnut flourcannot be used in food product and can only be used as feed with low added value, because there is nopeering process before oil extraction. The composition, amino acid and protein fraction wereinvestigated. For the purpose of co-production of walnut protein and oil, peeling method and coldpressing technology were used to produce defatted walnut flour. The effects different proteinmodification methods on the physicochemical and functional properties of walnut protein werecompared, and limited enzymatic hydrolysis was used to improve the solubility of walnut protein.Walnut protein flour with high solubility was prepared, which broadened the applied range in food andincreased the added value of products.Elementary composition, amino acids, protein fractions of walnut and protein solubility andthermal properties of walnut protein were investigated.The content of oil, protein, ash and moisturewere61.56%,19.12%,2.03%and2.84%, respectively. Essential amino acids were rich in walnuts, andthe content of glutamic acid, arginine and aspartic acid were4.04%,3.01%and1.99%, which wasapparently higher than other amino acids, belonging to the characteristic amino acids of walnuts. Walnutproteins are composed of albumin, globulin, prolamin and glutelin. The content of glutelin was thehighest and was the main composition of protein fractions. Walnut glutelin was composed of7subunitswith the molecular weight about39.81kDa,37.21kDa,35.09kDa,22.17kDa,20.63kDa,18.60kDaand17.23kDa, respectively. Nitrogen solubility index (NSI) of walnut protein was10.63%, and it wasinsoluble at pH5.0-6.0. Walnut globulin had high solubility in acid environment, while walnut glutelinhad high solubility in alkaline environment. DSC results showed that initial denaturation temperature ofwalnut globulin was84.45°C, while walnut glutelin92.96°C, indicating that globulin was heat sensitiveand glutelin was heat stable.The peeling and oil extraction methods of walnut were compared, and the preparation technologyof walnut protein flour was determined. The effect of peeling methods were compared, and hot waterwith0.1%NaOH was used with little negative effect on walnut proteins. The color of peeled walnutwas white, and the NSI was improved from5.10%to10.63%. The effect of oil extraction methods onthe extent of denaturation and solubility of walnut proten were compared. The results showed that coldpressing had little negative effect on walnut protein, and its enthalpy value was declined from17.05J/gto14.53J/g with low level of extent of denaturation. The process of cold press was optimized anddefatted walnut flour was prepared with4.21%oil,55.87%protein, and7.32%NSI.Effects of some modification methods on the physicochemical and functional properties of walnutprotein were investigated, and the results showed that physical modification, such as heat treatment,ultrasonic wave and dynamic high pressure microfluidization, had little positive effects on protein solubility, while enzymatic hydrolysis had significant positive effects. Some different kinds of proteaseswere used to modify the walnut protein, and trypsin, neutrase and flavourzyme can apparently improvethe solubility of walnut protein with low level of degree of hydrolysis. SDS-PAGE profile indicated thatthe subunits of walnut protein changed a lot after hydrolysis, and35kDa subunits can easily be digestedby proteases, while20kDa subunits cannot be digested. SEC profile showed that most of walnutglutelin transferred into soluble protein with33.97%content of total soluble protein, while walnutglobulin was merely affected, indicating that the target of enzymatic hydrolysis was walnut glutelin.After the modification of protease, the protein solubility in neutral and weak acidic environment wasimproved significantly.The NSI walnut protein flour increased from4.5%to26.12%at pH4.0, and7.32%to40%at pH7.0. The emulsifying ability of walnut protein was increased enzymaticmodification, while emulsifying stability was decreased a little. The foaming ability and stability wereboth increased20times after modification.Trypsin was selected as the optimal enzyme among several commercially available proteasesaccording to the evaluation and analysis of solubilization of limited enzymatic hydrolysis. Responsesurface methodology was used to optimize the limited enzymatic hydrolysis conditions of walnutprotein flour.The optimum process parameters were defined as follows: the ratio of solid to liquid was10:1, the hydrolysis temperature was43°C, the hydrolysis time was52min, and the dosage of trypsinwas0.4%. Under the optimum conditions, the DH of modified walnut protein was as low as3.25%,while the NSI of walnut protein was significantly improved from7.32%to78.16%. Emulsifying abilityand foaming ability of walnut protein were also increased to56.59m2/g and96.3%significantly. Theapplication of modified walnut protein flour in solid drink was studied preliminary, the result showedthat the dosage of emulsifying agent were apparently reduced, and the stability of protein dispersionprepared by modified walnut protein flour with high solubility was significantly increased.