【作者】 吴磊燕；

【导师】 温其标；

【作者基本信息】 华南理工大学 ， 食品科学， 2010， 博士

【摘要】 玉米醇溶蛋白(zein)疏水性好、易成膜,但纯玉米醇溶蛋白膜脆而易碎,延伸率差,工业化应用难,可用化学方法改变zein成膜性。本文利用玉米淀粉生产的副产物玉米蛋白粉(CGM)为原料提取玉米醇溶蛋白(zein),研究磷酸化改性对zein的物理性质及结构的变化;用界面吸附动力学和界面流变学研究SDS-磷酸化zein复合物的界面吸附和流变特性,模型分析磷酸化zein的吸附特点,推断磷酸化对zein结构的影响。研究塑化剂和磷酸化改性对zein蛋白膜表面和机械性质的影响,用静态和动态接触角分析zein蛋白膜表面亲水/疏水性质及动态润湿变化。从玉米蛋白粉中提取玉米醇溶蛋白(zein),用SDS-PAGE电泳、zata电位仪、差异扫描量热法(DSC)和圆二色谱仪(CD)研究zein的性质,结果表明该提取物主要成分为α-zein,等电点pH5.97,Tg为165.42℃;磷酸化后zein蛋白黏度升高,电负性增强,等电点向酸性偏移,二级结构发生变化,结构松散,α-螺旋降低,玻璃态转化温度Tg升高;O-磷酸键和N-磷酸键都存在于磷酸化zein蛋白中,pH5.0、7.0和9.0时磷酸化程度分别为3.31、6.68和5.85mol P/mol zein。磷酸化前后zein与不同浓度SDS混合溶液在空气-水界面、油(正己烷)-水界面上的表(界)面压力变化和吸附动力学不同;溶液的表(界)面压力都随吸附时间的增加而增大,各体系的溶质吸附到表(界)面,既有单溶质体系的吸附、渗透和重排,也有多溶质体系的竞争吸附、复合物吸附,甚至不相容吸附;根据Ward和Tordai吸附动力学模型公式得到各蛋白溶液界面上的扩散-控制吸附常数kdiff,判断各体系溶质吸附过程属于扩散-控制吸附,磷酸化提高zein蛋白的界面活性;用半经验公式得到蛋白溶液界面上的渗透速率常数和重排速率常数,分析得知,在研究的吸附时间内,各溶液体系在空气-水表面既有渗透吸附,又有重排现象,油-水界面上则只有强的渗透吸附。振动频率增加,空气-水界面或油(正己烷)-水界面上吸附的蛋白溶质(zein蛋白、磷酸化zein蛋白、SDS分子或蛋白与SDS混合)膜的弹性模量( E_d )增加,黏性模量( E_v )降低,且E_d大于E_v,各溶质形成弹性界面膜;随吸附时间延长,各溶液形成的界面膜的E_d和E_v都增加,界面膜的E_d、E_v与吸附在界面上的zein蛋白结构相关;磷酸化程度越高,在空气-水界面上形成的蛋白膜E_v越大,油-水界面上的油相溶剂化作用减弱了界面相内疏水力作用强的zein蛋白分子间的作用力,影响界面蛋白膜的E_d、E_v值。亲水/疏水性不同的甘油、油酸和聚乙二醇塑化剂按不同比例加入zein蛋白膜,改变蛋白膜的表面性质和机械性质,各蛋白膜中的水分含量、表面微结构、粗糙度Z值都不相同;甘油含量增加,甘油zein膜的表观接触角下降,而含油酸和聚乙二醇的zein膜的表观接触角却随着塑化剂含量的增加而增大;塑化剂膜表面动态接触角变化速率可用指数方程y = A1×exp (-x/t1) + y0拟合;随着塑化剂含量增加,zein蛋白膜的抗拉强度TS减小,延伸率EB增加。磷酸化也改变zein蛋白膜的表面和机械性能,改变效果跟磷酸化程度、添加塑化剂的种类有关;磷酸化增加zein蛋白膜的临界表面张力γc,膜表面疏水性降低;动态接触角变化速率增大,变化幅度与表面微结构和塑化剂种类有关,也可用指数方程y = A1×exp(-x/t1) + y0拟合;磷酸化使zein蛋白膜表面粗糙度Z值增大,但结构紧密,膜的机械性质如延伸率与塑化剂改善的蛋白膜相比提高3～4倍。磷酸化后,zein蛋白引入了磷酸根基团,蛋白质的物理性质、界面性质及所成膜的性质都发生改变。磷酸化提高zein蛋白的界面活性,磷酸化程度越高,界面蛋白膜的E_v越大,zein膜的延伸率也显著提高,膜柔韧性效果越好。因此,用磷酸化法改善zein膜的柔韧性有效、可行,适合工业化应用。更多还原

【Abstract】 Zein, a hydrophobic protein from corn, is often used as food and non-food packaging materials for its good film forming. But pure zein film has poor elongation, and too brittle and fragile to industrial application. This problem may be solved by chemical methods. In this paper, zein was extracted from corn gluten meal （CGM）, a by-product of corn starch production, and was phosphorylated by POCl₃. The physical properties of modified zein were determined. In order to evaluate the effective of phosphorylation to zein, the air-water or hexane/water interface properties of modified or un-modified zein with SDS complex solutions were studied by the methods of adsorption kinetics and interfacial rheology. For estimation the structure of phosphorylated zein, the adsorption characteristics were analyzed by a model. Variations in surface and mechanical properties of zein films with plasticizer or phosphorylated zein were also studied. Surface hydrophilic/hydrophilic of zein films and dynamic wetting changes on its surface were evaluated by static and dynamic contact angle. The more details are as follows:Properties of zein were determinated by SDS-PAGE electrophoresis, zeta potential instrument, differential scanning calorimetry （DSC）, circular dichroism instrument （CD）. The data showed thatα-zein was the major components in the extraction, and its physical properties are as follows: Isoelectric point pH5.97, Glass transition temperature （Tg） 165.42℃. After phosphorylation, zein solution possessed higher viscosity and negativity, loosed structure and lower isoelectric point. The far-UV CD data indicated that secondary structures of zein have changed and theα-helix reduced. DSC data showed that the glass transition temperature Tg was increased. Both O-phosphate bond and the N - phosphate bond are present in phosphorylated zein and the degree of phosphorylation at pH5.0, 7.0 and 9.0 was 3.31, 6.68 and 5.85 mol P/mol zein respectively.There is variation in surface pressure and absorption kinetics of air-water or oil （hexane）–water interface between zein and modified zein with SDS complex solutions at different concentrations. Surface pressure of all solutions increased with adsorption time. Not only adsorption, penetration and rearrangement in single system, but also competitive, complexation or even incompatibility adsorption in mixed system would be appear during solute adsorbed to interface. The process of adsorption belong to diffusion-controlled adsorption according to the diffusion rate constant kdiff calculated by a formula proposed by Ward and Tordai, and the result of diffusion-determining during adsorption was attained. Interface active of zein was improved after phosphorylation. The rate of penetration and rearrangement of adsorbed zein molecules have also been analyzed by a semi-empirical equation: The data showed that within adsorption time, adsorption at air-water interface has not only penetration but also rearrangement, while only strong penetrations at oil-water interface.Another important feature of protein-based films is rheologiy at interface. With frequency increases, more solute of zein, phosphorylated zein, SDS molecules or zein-SDS complex were adsorbed onto interface of air-water or oil-water, resulting in the increase of elastic modulus （E_d） and the decrease of viscosity modulus （E_v）. The flexible interfacial protein film was attained and E_d is greater than E_v. E_d and E_v at the two type interfeaces should be very dependant on the different structure of protein in interfacial films and increased with time. The E_v at air-water interface increased with the degree of phosphorylation increased. Effect of oil salvation at oil-water interface reduced the force between the hydrophobic protein molecules, affecting E_d, E_v value.Surface properties and mechanical properties of zein protein films, including water content, micro-structure and roughness Z of film surface, were changed by adding different proportions plasticizers, which properties of hydrophilic/hydrophobic are different each other. The apparent contact angle of zein film containing glycerol decreased with plasticizer increased, while the film containing oleic acid or polyethylene glycol plasticizer has the opposite result. A exponential equation y = A1×exp（-x/t1） + y0 can be fitted with the rate changes of dynamic contact angle on zein films. Tensil strength （TS） decreased and elongation （EB） increased by adding more plasticizers. Changes on surface and mechanical properties also happen to the phosphorylated zein films, and be affect by the degree of phosphorylation and type of plasticizer. Phosphorylated zein films possessed higher critical surface tensionγc, lower hydrophobicity and higher dynamic contact angle rate. The magnitude of changes was depended on micro-structure and type of plasticizer. Exponential equation y = A1×exp （-x/t1） + y0 also suit to dynamic contact angle of phosphorylated zein films. Elongation of phosphorylated zein films, possessed compact structure, has 3⁴ times than the films with plasticizer only, although its roughness Z is larger than plasticizer films.After phosphorylation, phosphoryl group was introduced into zein molecules, resulting in changes on physical properties, interfacial properties and the film forming properties. The elongation of phosphorylated zein films has improved dramatically and the E_v of interfacial films also increased with the degree of phosphorylation increased. Thus, phosphorylation is an effective, feasible method to improve the flexibility of zein films, suitable for industrial applications.更多还原