大豆蛋白自组装纤维聚集体的界面及乳化性质研究

大豆分离蛋白(SPI)经酸性热处理后可自组装形成具有交叉β-折叠结构的纤维聚集体,研究了SPI纤维化对其界面吸附动力学、界面膜扩张流变特征及乳化性质的影响。由于纤维化过程中伴随的蛋白水解,酸性热处理明显改善了SPI在等电点附近的溶解度,但中性p H处溶解度明显下降。SPI纤维化过程中,蛋白表面压增加速度和Ed值明显增强,说明其拥有较高的界面活性,且增强了蛋白分子在界面上的相互作用,易于在界面形成较厚的多层结构。纤维聚集体的形成使蛋白乳液粒径随加热时间延长而不断增加,但其p H稳定性明显增强,明显抑制了其在中性条件下的絮凝,这一改善受加热时间强烈影响。     

等电点附近大豆分离蛋白乳化稳定性的研究

等电点附近的大豆蛋白由于所带电荷减少、疏水相互作用增强而以聚集体的形式存在且其溶解性较差,故鲜有研究者关注该条件下大豆蛋白的乳化特性。本研究从颗粒稳定乳液的角度出发,分别以等电点附近(p H 5.0)和远离等电点(p H 7.0)两个条件制备了大豆分离蛋白(soy protein isolate,SPI)稳定的乳液,比较了两种条件下SPI的界面性质及所得乳液的储藏稳定性。结果发现,p H 5.0时SPI的溶解度仅为4.70±0.15%,远远低于p H 7.0时的93.28±1.89%;然而SPI浓度为0.50%时,p H 5.0的界面压却高于p H 7.0;以p H 5.0条件制备的SPI乳液,其界面蛋白吸附量高达87.03±1.28%,而p H 7.0制备的乳液仅为36.15±1.48%;p H 5.0的乳液两个月后液滴的平均粒径为63.15±0.30μm,与新鲜制备乳液(62.36±0.41μm)相比基本不变;p H 7.0的乳液经过两个月储藏后其液滴平均粒径从45.78±0.38μm增加至55.19±1.86μm。可见,以等电点附近条件制备的SPI乳液依然具有良好的储藏稳定性。     

一步法构建大豆蛋白双重乳液及其性质研究

本研究发现在中性条件下可以利用酸热处理的大豆分离蛋白（SPI）并通过简单的一步均质法制备得到W/O/W双重乳液,进一步研究了不同酸热处理时间对SPI在中性条件下的溶解度及其乳液性质（如乳析率、粒度）的变化。结果表明,不同时间的酸热处理会导致SPI在中性条件下的溶解度出现不同程度的下降（未加热SPI高达78.60%,酸热处理SPI为36.02%~53.29%）,可能是由于形成了不溶性的蛋白纤维聚集体,维系这些不溶聚集体的主要作用力是氢键和二硫键。酸热处理可以显著改善SPI稳定的乳液的乳析稳定性,且长时间的酸热处理效果更佳（如未加热SPI,38.70%;酸热处理2~6 h,34.90%~36.50%;酸热处理12~20 h,17.50%~19.70%）,但是酸热处理会导致乳滴粒度增大（如未加热SPI,24.63 μm;酸热处理SPI,28.12~33.97 μm）。增大乳液体系中的油含量也可以明显降低乳液的乳析率和粒度,随着油含量从20%增加到60%,乳析率从76.20%下降到3.30%,粒度从28.23 μm减小到19.06 μm。本研究结果为制备性质可控的双重乳液提供了新方法。     

碱溶热处理对大豆分离蛋白溶解度、热聚集和流变性质的影响

以大豆低温脱脂豆粕为原料提取大豆分离蛋白(SPI),在传统碱溶酸沉法制备SPI的碱溶阶段采用不同程度的热处理(25、50、60、70、80℃),研究了碱溶热处理对SPI溶解度、热聚集和流变性质的影响。其结果表明,碱溶热处理破坏了SPI的溶解度,可溶性部分含有大量由二硫键结合的大分子聚集体。碱溶热处理能提高蛋白弹性模量的同时降低凝胶点温度。适当的碱溶热处理(60℃)不仅能保留较好的蛋白溶解度,还能获得最佳的凝胶弹性。     

以大豆蛋白微粒构建高蛋白食品体系的稳定性研究

本研究以大豆分离蛋白(SPI)为原料,在p H 3.0、p H 3.4和p H 3.8等低酸性p H条件下对其进剪切热处理,制备获得粒度分布在2～100μm的大豆蛋白微粒。当微粒制备的p H条件向SPI等电点(p H 4.5)靠近时,形成蛋白微粒的结构越致密,稳定性越高。在p H 3.8条件下制得微粒的内部蛋白含量达到34.22%,在p H 2.0～8.0范围内其粒径不发生明显改变,蛋白溶出率不超过13%。经过二次热处理(95℃、30 min),该微粒的粒径和形貌没有发生明显变化,其分散液的粘度明显低于SPI分散液,在12%的浓度下仍然不形成凝胶。微粒化过程可以封闭大豆蛋白的疏水基团和氢键结合位点,使蛋白分子间的相互作用减弱,导致粘度降低。该大豆蛋白微粒可应用于饮料、酸奶等高蛋白食品体系中,在保持其口感的同时增加其蛋白质含量。     

琥珀酰化调控大豆分离蛋白电荷密度对其构象及乳化性的影响

通过加入琥珀酸酐对大豆分离蛋白（soybean protein isolate,SPI）进行改性,探究琥珀酰化调控SPI电荷密度引起的构象改变和乳化性改善之间的关系。通过荧光光谱、紫外光谱、傅里叶变换红外光谱分析不同酰化程度SPI的构象变化,采用扫描电子显微镜、电位、表面疏水性、分子柔性及乳化性等手段表征琥珀酰化对SPI理化性质的影响。结果表明：琥珀酰化反应发生,琥珀酰基基团成功接枝于SPI上,SPI等电点下降,电负性增强;SPI微观结构产生变化,光滑无规则片状结构的表面出现小孔穴,分子质量增加;电荷密度的增加导致SPI三级结构展开,色氨酸暴露,酪氨酸被包裹,荧光光谱和紫外光谱出现红移现象,证明酰化后的SPI处于更亲水的环境中;SPI游离氨基与琥珀酰基基团反应使得其中N—H键变为C—N键,导致酰胺III带改变;酰化后的SPI表面疏水性下降,分子柔性增加,乳化活性与乳化稳定性得到改善。该研究证实,琥珀酰化SPI电荷密度的增加可以影响SPI的空间构象从而使乳化性明显改善。     

不同离子强度大豆分离蛋白热诱导聚集体的研究

目的:研究离子强度对大豆分离蛋白热诱导聚集的影响.方法:采用可见分光光度计,在波长600 nm处测定不同离子强度下的大豆分离蛋白热处理溶液的吸光值,并将其作为溶液的浊度;采用体积排阻色谱(SECHPLC)、动态激光光散射(DLS)及zeta电位仪研究不同离子强度下大豆分离蛋白热诱导聚集体的分子质量分布、粒径分布及zeta电位.结果:随着离子强度的增加,大豆分离蛋白聚集体溶液的浊度增加,体系的聚集体部分及平均流体动力学半径(Rh)大幅增加,zeta电位逐渐降低.结论:电荷屏蔽作用使分子间斥力降低,促进了高离子强度下聚集的产生.     

大豆分离蛋白-葡聚糖糖基化产物作为乳化剂和活性物质载体的性能分析

研究大豆分离蛋白(soy protein isolate,SPI)-葡聚糖共价接枝物的制备及其乳化性质和作为姜黄素载体的性能。在95℃的"大分子拥挤"体系条件下,2种大分子通过美拉德反应进行共价接枝。根据糖基化产物在SPI等电点附近(p H 4.5)和中性(p H 6.5)条件下的溶解性将其分成2个组分,分别为MC45和MC65,并对其性能表征分析。十二烷基硫酸钠-聚丙烯酰胺凝胶电泳和分子质量测定分析显示,反应后生成大分子质量的接枝物。MC45制备的乳液在酸性环境中的粒度低于MC65,其平均粒径约为10μm,并且受离子浓度和温度的影响较小。与天然蛋白、SPI-葡聚糖混合物及MC45相比,荷载量、1,1-二苯基-2-三硝基苯肼(1,1-diphenyl-2-picrylhydrazyl,DPPH)自由基清除率分析表明MC65组分具有更好的姜黄素运载性能,MC65组分制备的姜黄素纳米颗粒荷载量为30.21μg/mg,DPPH自由基清除率为18.33%,其粒径最大。     

酸性热致大豆分离蛋白聚集体制备高冻融稳定性类蛋黄酱乳液

本文对酸性（pH 3.0）热致大豆分离蛋白聚集体（AHSPI）的平均粒径（Dh）、表面电位（ζ）、表面疏水性（H0）与溶解度进行了表征与测定,并用酸性（pH 3.0）天然大豆分离蛋白（ANSPI）、中性（pH 7.0）热致大豆分离蛋白聚集体（HSPI）作为对比分析,同时选用了AHSPI代替蛋黄制备高冻融稳定性的类蛋黄酱乳液,探讨了不同蛋白浓度对类蛋黄酱乳液的外观质地、微观结构、流变性质、DSC特性的影响。结果表明：AHSPI的平均粒径（204.67 nm）及H0（79418）均大于ANSPI与HSPI（pH 7.0）,但ζ电位绝对值（35.93 mV）小于ANSPI与HSPI（pH 7.0）,溶解度（89.47%）较HSPI（pH 7.0）更好。用AHSPI制备的类蛋黄酱乳液经过3次冻融循环保持稳定的最低浓度可达0.3 wt%,冻融前后乳液结构保持不变,只是乳液粒径的略微增大,流变性质表明类蛋黄酱乳液具有以弹性为主的凝胶网络结构,随着蛋白浓度的增大,油滴粒径逐渐减小,粘弹特性增强。DSC热分析表明：随着蛋白浓度的增加,类蛋黄酱乳液的结晶凝固点降低（-18→-28 ℃）,而蛋黄酱即使具有低于冷冻温度（-20 ℃）的结晶凝固点（-27.89 ℃）,仍发生了冻融破乳现象,类蛋黄酱乳液较好的界面稳定性赋予了其较好的冻融稳定性,为开发一类天然低胆固醇高冻融稳定性的酱类乳液提供了研究基础。     

不同处理对小米分离蛋白溶解性及亚基带分布的影响

以小米分离蛋白为研究对象,通过离子浓度、pH、温度对小米分离蛋白进行处理,研究其溶解性及亚基组分的变化。结果表明,低盐离子浓度下蛋白会产生电荷屏蔽作用,抵御外来盐离子且造成蛋白剧烈的聚集反应,中、高盐离子浓度会降低蛋白的溶解度;当溶液pH处于蛋白等电点附近会引起蛋白质不断碰撞,产生大量沉淀,pH远离等电点则溶解度增加;适当的温度会增强蛋白溶解,中、高温会使得蛋白变性,发生热聚集现象,而温度在沸点附近会破坏蛋白聚集体结构,蛋白聚集体变小,相应溶解度增加;部分亚基带经过β-巯基乙醇的还原后,亚基内的二硫键断裂,分解成低分子亚基带,导致电泳图中部分亚基带消失,产生新的亚基带。     

大豆球蛋白的纤维聚集体行为及其稳定性研究

为了明确蛋白质的纤维聚集行为,本研究以大豆球蛋白(soy globulin,11S)为原料,从亚基层面对酸性条件下热诱导的11S纤维聚集过程进行跟踪,监测蛋白及其亚基的水解过程、结构变化及其稳定性。结果表明,11S的纤维化是一个多步骤的过程,包括多肽链的水解、自组装成淀粉样纤维聚集结构及逐渐生长成宏观可见的具有扭曲螺旋结构的纤维聚集体。与11S纤维化过程的单指数增长相比,酸性亚基的纤维化过程存在迟滞期。酸性亚基在纤维化聚集的初期主要贡献于纤维聚集的成核过程,碱性亚基的加入改变其纤维聚集进程。蛋白质的纤维化过程会增加11S在等电点处的溶解度,降低中性和酸性pH下的溶解度。此外,碱性环境(pH值10.0)会导致11S纤维聚集体全部溶解、宏观纤维长度变小、结构发生改变。以上研究结果旨在为合理利用蛋白纤维化聚集体作为新的功能性食品配料提供理论依据。     

Effect of pH on the properties of soy protein–pectin complexes

The interactions of a commercial soy protein isolate (SPI) and a 2:1 SPI:high methoxy pectin (PEC) complex were evaluated over a range of pH values (3-7). The SPI formed very large (> 50 ??m) and largely insoluble aggregates (< 10%) close to its isoelectric point (IEP, pH 4 and 5) and smaller, more soluble (> 80%) particles at higher and lower pH values. The addition of PEC increased the solubility of SPI close to its IEP (pH 4 and 5) and prevented the formation of very large aggregates. However, PEC reduced the solubility of SPI at higher and lower pH values presumably via a depletion mechanism. The ??-potential of diluted SPI dispersions decreased from positive to negative with increasing pH, passing through zero at pH 4.6, the isoelectric point (IEP) of the protein. At pH < 6, the addition of PEC reduced the charge of the protein suggesting the formation of a complex while at pH 6 or 7 there was no evidence of complex formation. The increased SPI solubility in the IEP in the presence of PEC is probably due to the formation of charged complex which do not aggregate while the decreased solubility of protein in the presence at high and low PEC is probably due to the formation of insoluble complexes and a depletion interaction respectively. Thermal treatment (30 min, 90 °C) enhanced the solubility of the SPI:PEC complexes close to the IEP (pH 4 and 5), but reduces it at low pH (pH 3). The SPI:PEC complexes could be manufactured in the form of a beverage at pilot scale where their solubility was enhanced by homogenization.     

不同pH条件下米谷蛋白的理化及结构特性研究

为探索溶解大米蛋白的最适pH值,扩大其应用范围,表征了不同pH(pH 3.0,4.0,7.0)条件下大米蛋白中主要成分——米谷蛋白的理化及结构性质。结果表明,与中性条件下相比,酸性条件下米谷蛋白的溶解度和结构性质发生了明显改变,pH 7.0时米谷蛋白分子结合紧密,形成庞大的分子聚集体,溶解度仅(6.24±1.25)%;而在酸性条件下,米谷蛋白逐渐分散,分子间二硫键断裂,呈现分散疏松的小分子体状态,pH 3.0时其溶解度最高,达到(72.47±2.36)%。     

Influence of sugars on the characteristics of glucono-δ-lactone-induced soy protein isolate gels

The effects of the reducing sugars (glucose and lactose) and the non-reducing sugar (sucrose), heated in combination with soy protein isolate (SPI) at neutral pH, on the physicochemical and rheological properties of SPI were determined. After formation of gels induced by glucono-δ-lactone (GDL), the textural profile and physicochemical bonds of the non-heated and heated SPI gels were investigated. The gelation of SPI was induced in three stages of processing that is similar to some tofu-making procedures. First, SPI was heated in the presence of sugars at neutral pH above the denaturation temperature of SPI; then gelation was induced by GDL at iso-electric pH and finally the acidic gels were heat treated again. Heat treatment with glucose at neutral pH resulted in SPI with higher glycation degree than with lactose, whereas SPI heat treated in the presence of sucrose was not glycated. GDL-induced gels of SPI glycated with glucose was more soluble in water than gels of SPI reacted with lactose, which in turn was more soluble than the control and gels of SPI heated in the presence of sucrose. This indicates a change in the net charge of proteins caused by the glycation reaction. Glucose and lactose had a protective effect on protein denaturation at neutral pH, albeit less than sucrose, resulting in GDL-induced gels with increased water holding capacity and reduced gel hardness than sucrose. Chemical analysis indicated that disulphide bonds were involved in maintaining the structure of the gels, and solubility profiles of gels in different buffers indicate that other types of covalent bonds besides disulphide bonds were formed in gels of glycated SPI, resulting in reduced gel elasticity.     

Stabilization of acidic soy protein-based dispersions and emulsions by soy soluble polysaccharides

Soy soluble polysaccharides (SSPS) are shown to prevent destabilization of soy protein isolate (SPI) dispersions and SPI-based oil-in-water (O/W) emulsions under acidic conditions. Addition of SSPS above a critical concentration (0.25 wt%) increased the stability of 0.50 wt% SPI dispersions against aggregation and phase separation under conditions where SPI would normally precipitate (near its isoelectric point). Though SSPS neutralized SPI surface charge via electrostatic interaction, there was increased stability against aggregation due to steric repulsion. At acidic pH, addition of 1 wt% NaCl electrostatically screened protein–polysaccharide complexation which led to SPI precipitation and sedimentation. However, the order of salt addition had a significant impact on charge screening, with salt added before pH adjustment reducing SPI–SSPS complexation whereas it had less effect when added afterwards. Salt penetration efficacy diminished with decreasing pH. O/W emulsions (5 wt% oil) prepared with 0.50 wt% SPI destabilized at pH 4–5 due to protein aggregation, but addition of ≥0.25 wt% SSPS improved emulsion stability by inhibiting protein–protein interactions thus limiting increases in oil droplet diameter over time. Overall, both dispersion and emulsion stability greatly depended on pH, ionic strength and SSPS concentration. These results demonstrated that SSPS could effectively stabilize acidic SPI dispersions and that SPI–SSPS interactions may be used as a tool to improve the kinetic stability of SPI-based O/W emulsions.     

Membrane fractionation of a β-lactoglobulin tryptic digest: Effect of the pH

The tryptic digestion of β-lactoglobulin leads to the release of a large range of biologically active peptides. Ultrafiltration/nanofiltration technology can be used to fractionate protein hydrolysates in order to obtain permeate products with increased functionality and free from intact proteins and enzymes. The influence of the pH in the fractionation of the hydrolysate through a polyethersulfone membrane (MWCO 5 kDa) was investigated in this work. In this case peptide transmission was mainly governed by charge mechanisms and reached its maximum value when the pH value is close to the peptide isoelectric point. Almost complete rejection of acidic peptides was achieved at basic pH values due to electrostatic repulsive forces with the negatively charged membrane and, in agreement with Donnan theory, positively charged peptides shown lower transmission than the neutral species. The highest peptide recovery and the best separation factor between bioactive and non-bioactive peptides were obtained following nanofiltration at pH 8.0.     

Characterization of low-phytate soy protein isolates produced by membrane technologies

Soy protein isolates (SPI) produced by combining electro-acidification and tangential ultrafiltration/diafiltration (UF/DF) (pH 6), were compared in terms of composition and proteins solubility with isolates produced by UF/DF (pH 9) and isoelectric precipitation (pH 4.5). Mineral and phosphorus (phytic acid) removal was enhanced for the SPI pH 6. Whey-like proteins (M.W. < 66 kDa) were also found in higher concentration for the SPI produced by membrane technologies. This difference in composition resulted in improved solubility characteristics for the SPI pH 6 by as high as 25% and 60%, when compared to the SPI pH 4.5 and SPI pH 9, respectively. Improvement in solubility was most important between pH 2 and 4.5. The quantity of H⁺ ions added to the soy protein extract (SPE) and SPI to reduce the pH from 9 to 4.5, during solubility measurement, was related to the degree of proteins aggregation, as determined by size-exclusion high-performance liquid chromatography, and at a lesser extent to their phytic acid content. For the pH range of 4.5 to 2, the degree of proteins aggregation alone determines the quantity of H⁺ ions added.Industrial relevanceSoy protein production is one of the major agricultural sectors of significant importance to North America and soy proteins represent 69% of global plant protein consumption in the world. Soy protein concentrates and isolates are produced at the industrial scale by isoelectric precipitation. This process has a high productivity, however, it also generates large volumes of effluent. The final products also have significant contents of minerals and of phytic acid, the latter of which is well known to decrease the proteins and minerals adsorption in the intestine.We were the first group to combine bipolar membrane electrodialysis (BMED) and ultrafiltration (UF) (dead-end) for the production of soy protein concentrates (Mondor, Ippersiel, Lamarche & Boye, 2004). The new approach resulted in a significant decrease of the volumes of effluent due to the use of BMED to adjust the pH of the extract prior to UF and by improving the protein washing step using diafiltration (DF). It was also shown that for the pH range 6–9, minerals and phytic acid removal was improved with a decrease in pH. In this work, we present the characteristics of a soy protein isolate with a low phytic acid/protein ratio (SPI pH 6) produced by BMED and tangential flow UF/DF applying an optimal VCR5, re-VCR 5 sequence at pH 6. The SPI pH 6 shows an improved solubility by as high as 25% and 60%, when compared to an isolate produced by isoelectric precipitation at pH 4.5 and to one produced by UF/DF at pH 9, respectively. Improvement in solubility was most important between pH 2 and 4.5 indicating that this isolate could be considered as a valuable ingredient for the formulation of fruit juice beverages or power juices, considering that the pH of these liquid food products is around 3.5.     

Physicochemical and functional properties of acidic and basic polypeptides of soy glycinin

The amino acid composition, some selected physicochemical and functional properties of acidic and basic polypeptides of soy glycinin were investigated and compared. Large amount of these polypeptides were obtained by DEAE-Sepharose fast flow column chromatography. Free sulphydryl contents, surface hydrophobicity, solubility and emulsifying activities (at different pH values) were evaluated. Different polypeptides had different patterns of amino acid composition, especially contents of acidic (and basic) and hydrophobic amino acids. The free sulphydryl contents (including total and exposed) and surface hydrophobicity considerably varied with the type of polypeptides. Compared with glycinin, isoelectric point (pI) of individual polypeptides shifted towards a more acidic pH. At a given pH value (e.g. above or below pI), the solubility and emulsifying ability index of these polypeptides were closely related to their relative contents of acidic (and basic) amino acids. The results indicated that glycinin polypeptides with different amino acid character have different physicochemical and functional properties, especially solubility and emulsifying ability.     

酱油渣中蛋白的乳化特性研究

对比分析了酱油渣中蛋白(SRP)、糖基化大豆分离蛋白(GSP)和大豆分离蛋白(SPI)的ξ-电位、疏水性以及乳化活性和乳化稳定性。发现SRP的总糖和蛋白含量比为1∶1.4,其中含有大量的糖蛋白;SRP疏水性是SPI的2.5倍。SRP等电点接近pH=3.5,在酸性环境下溶解性较好。在酸性、高盐环境下SRP乳化能力高于GSP和SPI,在pH=5时,SRP的乳化活性(EAI)是SPI的8.1倍,在NaCl浓度为0.3mol/L时,SRP的EAI是SPI的1.77倍。     

紫苏分离蛋白功能性研究

为了开发紫苏蛋白在食品工业中的应用,以大豆分离蛋白为对照,研究紫苏分离蛋白的功能特性。结果表明：紫苏分离蛋白的溶解性与大豆分离蛋白的溶解性随pH值变化的趋势基本一致,但在等电点时紫苏分离蛋白的溶解性高于大豆分离蛋白。在pH7.0时,紫苏分离蛋白的持水性、起泡性及泡沫稳定性、乳化性和凝胶性均不及大豆分离蛋白。但紫苏分离蛋白的吸油性仅稍小于大豆分离蛋白,此外,在紫苏分离蛋白的蛋白质质量浓度为3g/100mL以后,其乳化稳定性与大豆分离蛋白的乳化稳定性基本相当。紫苏分离蛋白在食品加工中作为一种蛋白质强化剂具有一定潜力。