琥珀酰化修饰改善牦牛乳酪蛋白胶束结构及疏水性

琥珀酰化是改善食品蛋白质功能性质的常用手段,牦牛乳是青藏高原特色乳资源.为了揭示琥珀酰化对牦牛乳酪蛋白胶束结构的修饰作用,以牦牛乳酪蛋白为原料,研究了琥珀酰化修饰对其结构及疏水性的影响.分别通过傅里叶变换红外光谱分析了修饰前后酪蛋白胶束二级结构变化,采用动态光散射技术分析了修饰前后酪蛋白胶束粒径变化,采用扫描电镜技术观察了酰化反应对酪蛋白的表面形貌的影响.结果显示,牦牛乳酪蛋白胶束二级结构为33.1%β-转角、23.2%无规卷曲,32.0%β-折叠和11.7%大环结构,α-螺旋结构未检出.琥珀酰化修饰后,α-螺旋结构形成,含量为12.4%,其他二级结构含量变化不大.琥珀酰化修饰使酪蛋白胶束粒径减小,由不规则的球形转变为规则的球形,疏水性显著降低.研究结果可为牦牛乳酪蛋白的改性提供参考依据.     

pH值和温度对燕麦蛋白溶解与聚集特性的影响

为研究燕麦蛋白的理化性质,本研究采用碱提酸沉法提取燕麦蛋白,并分析不同pH值与温度对燕麦蛋白溶解性、ζ-电位和粒径分布的影响。结果表明,在等电点附近(pH值5.0),燕麦蛋白的表面电荷数最低,蛋白粒度较大,溶解性和乳化性较低。当pH值远离等电点时,燕麦蛋白的表面电荷增加,蛋白粒度逐渐减小,溶解度升高,乳化性也随之升高;随着温度升高(55~95℃),燕麦蛋白聚集体解离,粒度变小,溶解度增加,稳定性变好;温度进一步升高(121、130℃),燕麦蛋白重新聚集形成粒径较大的聚集体,溶解度降低,稳定性变差。本研究结果为燕麦蛋白资源的利用提供了重要的理论指导。     

蚕豆蛋白的提取及NaCl浓度和pH值对其溶解性和乳化性的影响

蚕豆蛋白富含微量元素和人体必需的8种氨基酸,具有较高的开发价值。采用超声提取和水提取的方法提取蚕豆蛋白,研究了NaCl浓度和pH对蚕豆蛋白提取率、沉淀、功能性的影响。在pH值为8～12,超声提取和水提取均有较高的提取率。蚕豆蛋白的等电点在pH4.0～4.2之间。在pH4溶解性和乳化性最低,pH值在4～12时溶解性和乳化性随pH值的升高而升高, pH值为12时溶解性和乳化性最高。NaCl浓度从0 到 1.0 mol/L,溶解性和乳化性升高;当NaCl浓度继续增加,在pH 4、pH 7溶解性和乳化性随之下降。在相同的NaCl浓度和pH值时,超声提取比水提取蛋白的溶解性和乳化性高。该研究为蚕豆蛋白的提取工艺的确定及其在食品中的应用提供依据。     

高凝胶性蛋白粉制取工艺条件优化

为了进一步改善高凝胶蛋白粉的品质,探讨了利用酶交联反应制取高凝胶蛋白粉的新方法,在进行了琥珀酰化酪蛋白添加量、反应pH值、转谷氨酰胺酶添加量对蛋白粉凝胶强度影响的单因素试验基础上,该文通过响应面分析法对制取高凝胶性蛋白粉的酶交联反应条件进行了优化,并通过SDS-PAGE电泳、扫描电镜等手段探讨了酶交联蛋白粉凝胶性能增加的机理。结果表明,最佳工艺条件为:琥珀酰化酪蛋白添加量2.29%、反应pH值5.87、转谷氨酰胺酶添加量1.06%、反应温度40℃、反应时间40min。理论最大凝胶值为1072.8g/cm2,验证值为1054.5g/cm2,说明预测模型可靠性高,可用于反应条件的优化。     

挤压温度对大豆分离蛋白与原花青素复合物结构和功能特性的影响

为了探究不同挤压温度（40、60、80、100和120℃）对大豆分离蛋白（Soy Isolate Protein，SPI）与葡萄籽原花青素（Grape Seed Proanthocyanidin Extract，GSPE）复合物功能性质及结构特性的影响。该研究以溶解度、乳化性、乳化稳定性、ζ-电位、粒度为指标，利用荧光光谱、红外光谱分析该复合体系中大豆分离蛋白功能性质及结构的变化。结果表明：相较于挤压SPI，经过挤压处理的SPI-GSPE复合物的溶解度、乳化活性指数、乳化稳定性指数、ζ-电位绝对值及持水性均显著提高（P<0.05），其表面疏水性、持油性显著下降（P<0.05）。随着挤压温度的升高，SPI-GSPE复合物的溶解度、持油性及乳化活性均先增大后减小且在80℃达到最大值，而其表面疏水性先减小后增大且最小值在80℃，ζ-电位绝对值、乳化稳定性及持水性均随温度的升高而降低。粒径分析结果表明，挤压处理后SPI与GSPE形成了更加致密的复合物；荧光光谱及红外光谱结果表明，与GSPE的复合及挤压处理使SPI氨基酸残基所处微环境发生变化，蛋白结构发生变化。以上结果表明挤压温度为80℃时SPI-GSPE复合物功能性质提高幅度最大，为GSPE与SPI复合提高SPI的功能性质提供参考。     

琥珀酰化和蛋白酶改性对小麦面筋蛋白功能性质的影响

对小麦面筋蛋白进行琥珀酰化和蛋白酶复合改性以提高其溶解性及其他特性。对复合改性面筋蛋白与原面筋蛋白、琥珀酰化面筋蛋白、碱性蛋白酶改性面筋蛋白进行了比较。结果表明:在pH 3~11、水解度4%~12%的范围内,复合改性面筋蛋白的溶解度亦随着水解度的增大而增大,比原面筋蛋白、酰化面筋蛋白、中性蛋白酶和碱性蛋白酶的改性产物都高。起泡性和起泡稳定性则先增加后降低,在水解度4%时具有较佳值,但在各水解度下较单一改性的面筋蛋白产物都要低。添加复合改性面筋蛋白面团黏弹性和面包口感较好,内部结构均匀、细腻。     

IOS和MOS对牦牛肌原纤维蛋白生化与功能特性的影响

氧化是调控肉类品质的一种潜在工具,肉类蛋白氧化调控理论的完善有助于其在食品工程中的应用。为了研究不同氧化体系下,牦牛肌原纤维蛋白化学修饰与功能性质之间关系,该研究分析了离子氧化体系(Iron Oxidizing System,IOS)和高铁肌红蛋白(Metmyoglobin Oxidizing System,MOS)氧化体系下在特定氧化程度(蛋白羰基含量)下牦牛肌原纤维蛋白的生化特性、二级结构和功能特性。结果发现,随着氧化强度的提升,牦牛肌原纤维蛋白会发生显著的巯基丧失、二硫键生成、二酪酸增加以及表面疏水性下降现象(P0.05)。当羰基浓度达到(1.51±0.13) nmol/mg以后,MOS体系的二硫键和表面疏水性变化比IOS系统更明显。此外,随着氧化程度的增加,α-螺旋结构比例会持续减少,而β-折叠与β-转角结够会持续增加。通过对蛋白质功能性功能性研究,还发现适当的氧化对牦牛肌原纤维蛋白的溶解性和乳化性有一定改善作用,其改善峰值和范围因氧化体系不同而有所差异,MOS的功能特性影响对氧化强度更敏感。通过相关性分析可以发现,二硫键与溶解度、浊度、乳化性等功能特性相关性显著,二级结构大多数指标则与溶解性和浊度相关性显著。综上所述,不同氧化体系对于肌原纤维蛋白的生化及功能特性的影响存在差异,MOS系统能够在较小的氧化程度下对功能特性产生改善作用,这与其同时带来的二硫键和表面疏水性更显著的变化有关。     

超声处理改善不同比例大豆-乳清混合蛋白理化性质

为了探究超声作用对不同比例大豆-乳清混合蛋白功能性质的影响。该试验以大豆蛋白与乳清蛋白为原料,对粒径、ζ-电位、内源性荧光光谱、扫描电子显微镜等结构性质,以及乳化活性、乳化稳定性、质构、持水性等理化特性和功能特性进行研究。结果表明:当SPI-WPI(soy protein isolate-whey protein isolate)质量比为5:5时,乳化活性与乳化稳定性最大(65.5 m2/g,16.3 min),同时粒径分布由双峰转为单峰,体积平均粒径D[4,3]达到最小值(205.6 nm)、ζ-电位绝对值达到最大(21.4m V),此时混合体系稳定性最好。内源性荧光光谱显示有荧光物质释放,荧光强度持续增强,说明超声处理改变了混合体系蛋白结构。超声处理后混合蛋白比例在5:5时,具有最佳的凝胶性质,硬度达到最高(475.61 N),持水性达到最大(85.32%),与扫描电子显微镜的结果一致,显示此时混合蛋白体系形成致密、均一、有规则凝胶网络结构。该研究可为大豆-乳清混合蛋白的应用提供技术支撑。     

壳聚糖与牛血清白蛋白作用机制及其复合物乳化性的研究

为探究蛋白质与多糖之间的相互作用及其复合物的研究乳化性质,本试验以牛血清白蛋白(BSA)-水溶性壳聚糖(WCS)复合体系为研究对象,采用紫外吸收光谱、荧光发射光谱、电位粒径分析、乳液储藏稳定性观察等方法研究在微观环境下蛋白质与多糖的相互作用机理,并进一步探究两者对水包油型乳液乳化性能的影响。结果表明,多糖和蛋白质的相互作用随着pH值和BSA/WCS复合比的变化而不同,混合溶液的紫外吸收峰和荧光发射峰发生偏移,且当复合比小于1∶2时发生荧光淬灭。混合溶液的Zeta电位和平均粒径测定结果表明,BSA与WCS之间主要通过静电相互作用形成复合物,且该复合物的电荷性质、尺寸大小具有pH值和复合比依赖性。研究混合溶液乳化性发现,当pH值为3.0、4.0、复合比为4∶1~1∶1时,形成的乳液液滴粒径较小,性质稳定;当pH 值大于5.0时,乳液液滴粒径大,出现相分离、乳液高度不稳定现象。本研究为设计具有可预测稳定性或刺激响应性的乳液配方及新型功能特性材料提供了理论依据。     

超声处理对牦牛乳酶促凝胶流变特性的影响研究

为探究超声牦牛乳的酶促凝胶流变特性,并进一步分析超声后牦牛乳的蛋白疏水性和脂肪球粒径如何影响凝胶网络的形成,本研究以牦牛乳为原料,进行超声处理(时间5 min和10 min,功率300、400和500 W)后,编号为US1(300 W,5 min)、US2(300 W,10 min)、US3(400 W,5 min)、US4(400 W,10 min)、US5(500 W,5 min)和US6(500 W,10 min),以未处理的牦牛乳作为对照(CK),通过小振幅振荡时间扫描获取凝胶形成的流变信息。结果表明,超声处理后牦牛乳的脂肪含量、蛋白质含量以及pH值无显著变化(P>0.05),而均质效率和乳液稳定性显著提高(P<0.05);超声处理能够增加凝胶的储能模量,使得牦牛乳酶促凝胶的弹性增加。US5和US6的储能模量增幅最多(分别为6 824和7 189 Pa),但凝胶时间长。与CK相比,超声牦牛乳US1、US2、US3和US4的凝胶时间分别缩短了1.78、3.72、5.75和4.74 min;超声处理在改变乳蛋白表面疏水性的同时减小了脂肪球尺寸。与CK相比,US3最大荧光强度显著增加了51.22%(P<0.05),脂肪球粒径分布范围为0.300~4.002 μm,集中在1.974 μm处。US4最大荧光强度显著降低了5.31%(P<0.05),脂肪球粒径呈双峰分布,范围为0.131~4.502 μm。综上所述,经过不同时间和不同功率的超声处理后,US1~US6乳蛋白表面疏水性和脂肪球尺寸受到影响的程度不同,使得凝胶网络体系存在差异,从而表现出不同的凝胶特性。本研究为超声技术在牦牛乳凝胶类制品加工中的应用提供了理论基础。     

Solvent-mediated disruption of bovine casein micelles at alkaline pH

The disruption of casein micelles at alkaline pH was investigated using turbidity measurements. The rate and extent of disruption of casein micelles at alkaline pH (8.0-11.0) increased with pH. Furthermore, the extent of alkaline disruption increased with increasing temperature (5-40 degrees C). Preheating milk for 10 min at 90 degrees C did not influence the extent of alkaline disruption of casein micelles, suggesting that whey proteins do not influence the alkaline disruption process. Levels of ionic calcium and serum calcium and phosphate decreased in a logarithmic fashion with increasing pH, indicating precipitation of calcium phosphate onto the casein micelles. A mechanism for alkaline disruption of casein micelles is proposed, in which increasing the milk pH improves the solvent quality for the caseins, thereby leading to the disruption of casein micelles into their constituent nanoclusters; increases in the net-negative charge on the caseins on increasing pH may contribute to micellar dissociation.     

Effect of yak milk casein hydrolysate on TH1/TH2 cytokines production by murine spleen lymphocytes in vitro

Yak casein hydrolysate was derived from the enzymatic alcalase-hydrolysate of a typical northwestern China milk product called Qula. An in vitro study was conducted to examine their immunoregulatory effects on murine T cells, including Con-A-induced lymphoproliferation and splenocyte cell cycle, production, and messenger RNA (mRNA) expression of interleukin-2 (IL-2), interferon-gamma (IFN-gamma), and interleukin-4 (IL-4). The results showed that yak casein hydrolysate had lymphoproliferation activity on murine splenocytes and induced their cell cycle from the G1 to the S phase. It could increase Con-A-induced IL-2 and IFN-gamma production in spleen cells, but a very weak or no effect was observed in the absence of Con-A. The present study also showed that it could markedly increase the production and mRNA expression of IFN-gamma and IL-2, which are key cytokines for T helper 1 cell (Th1) cell development, in a dose-dependent manner. However, their effects on IL-4 secretion were not obvious, and the enhancement was much lower than that of IFN-gamma and IL-2. All of these demonstrated that yak casein hydrolysate could increase type 1 cytokine production, thereby shifting the Th1/T helper 2 cell (Th2) balance toward a Th1-dominant phenotype, which meant that yak casein hydrolysate could indeed not only modulate the differentiation of helper T cell but also has the capacity to modulate the Th1/Th2 balance. Therefore, yak casein hydrolysate may be useful for the treatment of cell-mediated immune diseases.     

适宜物理-酶联合改性提高酸性条件下大豆分离蛋白乳化性

为提高酸性条件下大豆分离蛋白（soy protein isolates,SPI）的乳化性能,该文研究了物理-酶联合改性对SPI（pH值为4）的乳化性能影响,通过对比确定了物理-酶联合改性,即超声波-酶复合改性和挤压膨化-酶复合改性两种改性方法在酸性条件下的乳化性能效果最好;并通过对改性后 SPI（pH 值为4）进行溶解性、游离巯基、二硫键、粒径、扫描电镜（scanning electron microscope,SEM）和激光共焦扫描显微镜（confocal laser scanning microscopy,CLSM）分析,从蛋白结构变化上进一步揭示了乳化性能提高现象的原因。结果表明：超声波联合植酸酶-酸性蛋白酶改性的 SPI （Uphy-aci-SPI）的乳化活性（emulsifying activity index,EAI）为0.53 m2/g,比未改性SPI（0.18 m2/g）显著提高了196%（P<0.05）,乳化稳定性（emulsifying stability index,ESI）为17 min,比未改性SPI（13.5 min）显著提高了25.9%（P<0.05）;挤压膨化联合菠萝蛋白酶改性的SPI（Ebro-SPI）的EAI为0.46 m2/g,比未改性SPI显著增加了155%（P<0.05）,ESI为17 min,比未改性SPI显著增加了25.9%（P<0.05）。在pH值为4的条件下对物理-酶联合改性的SPI的性质分析发现,物理-酶联合改性的SPI与未改性SPI相比,物理-酶联合改性的SPI的溶解性显著增加（P<0.05）;物理-酶联合改性的SPI的乳状液平均粒径减小,CLSM观察乳状液中油与蛋白溶液稳定共融,改善了油滴之间的空间排斥力。物理-酶联合改性的SPI游离巯基的含量显著增加（P<0.05）,二硫键含量显著降低（P<0.05）。SEM观察物理-酶联合改性的SPI为结构松散、破碎均一的微观结构。由此可见,乳化性能的提高是通过深层改变蛋白的结构来实现的。该研究可为探索提高酸性条件下SPI的乳化性能的方法提供理论依据。     

Acid gelation properties of heated skim milk as a result of enzymatically induced changes in the micelle/serum distribution of the whey protein/kappa-casein aggregates

Changes in the acid gelation properties of skim milk as a result of variations in the micelle/serum distribution of the heat-induced whey protein/kappa-casein aggregates, induced by the combination of heat treatment and limited renneting, were investigated. No dramatic change in the zeta potential or the isoelectric point of the casein micelles was suggested, whether the aggregates were all attached to the casein micelle or not. Fluorescence intensity measurement using 8-anilino-1-naphthalenesulfonic acid (ANS) showed that the heat-induced aggregates were highly hydrophobic. Dynamic oscillation viscosimetry showed that acid gelation using glucono-delta-lactone (GDL) started at a higher pH value in prerenneted milk. However, no change in the gelation profile of skim milk could be related to the proportion of aggregates bound to the surface of the casein micelles. The results support the idea of an early interaction between the serum aggregates and the casein micelles on acidification.     

Physicochemical and functional properties of buckwheat protein product

This study was conducted to compare the physicochemical and functional properties of buckwheat protein product (BWP), soy protein isolate (SPI), and casein. BWP was prepared from buckwheat flour by the method including alkaline extraction and isoelectric precipitation. The amino acid composition of BWP was very similar to that of buckwheat flour. The protein solubility (PS) of BWP was much greater than that of SPI at all pH levels (pH 2-10) but lower than that of casein at pH 7-10. The isoelectric point of BWP was around pH 4. The higher aromatic hydrophobicities (ARH) of BWP, SPI, and casein were obtained at lower pH levels (pH 2-3). The emulsifying stability (ES) of BWP was lower than those of SPI and casein at high pH levels (pH 7-10). At all pH levels, BWP formed a thin emulsion. Regression analysis showed that the ARH of BWP was significantly associated with the ES. Although the water holding capacity of BWP was quite lower than that of SPI, its fat absorption capacity was slightly higher than those of SPI and casein. These results indicated that the physicochemical properties of BWP were different from those of SPI or casein. Thus, BWP is a potential source of functional protein for possible food application.     

Charge modifications to improve the emulsifying properties of whey protein isolate

Whey protein isolate was modified by ethylene diamine in order to shift its isoelectric point to an alkaline pH. The extent of the modification was studied using SDS-PAGE and MALDI-TOF mass spectrometry. The modified whey proteins were used as an emulsifier to stabilize oil-in-water emulsions at acidic and neutral pH ranges, and their emulsifying properties were compared with that of the unmodified whey proteins and with the previously studied ethylene diamine modified sodium caseinate. The emulsifying activity of the modified whey proteins was similar to that of the unmodified ones, but the stability of an emulsion at pH 5 was significantly improved after the modification. Charge and coverage of droplet surface and the displacement of the interfacial proteins by surfactant Tween 20 were further studied as a function of pH. As compared with the unmodified whey proteins, the modified ones were proven to cover the interface more efficiently with extensive surface charge at pH 5, although the interfacial layer was less resistant to the surfactant displacement.     

Effect of Modified Oat Starch and Protein on Batter Properties and Quality of Cake

Starch and protein separated from oat were chemically modified using cross‐linking and acetylation protocols for starch, and deamidation and succinylation for protein isolate. Cross‐linking decreased swelling power of starch, whereas syneresis increased, but cross‐linking does not have a significant effect on gelatinization temperature. Acetylation increased swelling power, but gelatinization temperature and syneresis diminished. Deamidation and succinylation increased nitrogen solubility index, emulsion activity, foaming capacity, and water and oil binding capacity. Emulsion stability did not change with deamidation and it diminished with succinylation, while foaming stability decreased with both treatments. Acetylated starch and two types of modified proteins were substituted for 5, 10, 15, and 20% of oat flour to bake cake samples and then physical properties of the cakes were measured. Acetylated starch increased batter viscosity, cake volume, and whiteness of cake crust. Increased level of deamidated protein produced cakes with lower batter viscosity, higher volume, and darker color (increase in redness). Application of higher levels of succinylated protein led to higher batter viscosity and lightness, and lower cake volume. Therefore, substitution of deamidated protein and acetylated starch can improve cake properties.     

Conformational changes and some functional characteristics of gelatin esterified with fatty acid.

This study investigated the effects of attachment of fatty acid chains to gelatin molecules on their conformation and some functional properties in order to determine the effectiveness of this procedure for improving the emulsifying properties of gelatin. The esterification conferred upon the gelatin molecules a folded configuration resulting in a lambda(max) of fluorescence emission shift to lower wavelengths and decreases in solubility, intrinsic viscosity, and gelling ability. In contrast, surface hydrophobicity increased with extent of esterification, and the esterified derivatives exhibited higher emulsion stabilities. These results indicated that the oligomeric structure and functional attributes of the gelatin were altered with the modification.     

Reversible precipitation of casein micelles with a cationic hydroxyethylcellulose

The cationic hydroxyethylcellulose Polyquaternium 10 (PQ10) was found to produce a dose-dependent destabilization of casein micelles from whole or skim milk without affecting the stability of most of the whey proteins. The anionic phosphate residues on caseins were not determinant in the observed interaction since the destabilization was also observed with dephosphorylated caseins to the same extent. However, the precipitation process was completely inhibited by rising NaCl concentration, indicating an important role of electrostatic interactions. Furthermore, the addition of 150 mM NaCl solubilized preformed PQ10-casein complexes, rendering a stable casein suspension without a disruption of the internal micellar structure as determined by dynamic light scattering. This casein preparation was found to contain most of the Ca2+ and only 10% of the lactose originally present in milk and remained as a stable suspension for at least 4 months at 4 degrees C. The final concentration of PQ10 determined both the size of the casein-polymer aggregates and the amount of milkfat that coprecipitates. The presence of PQ10 in the aggregates did not inhibit the activity of rennet or gastrointestinal proteases and lipases, nor did it affect the growth of several fermentative bacteria. The cationic cellulose PQ10 may cause a reversible electrostatic precipitation of casein micelles without disrupting their internal structure. The reversibility of the interaction described opens the possibility of using this cationic polysaccharide to concentrate and resuspend casein micelles from whole or skim milk in the production of new fiber-enriched lactose-reduced calcium-caseinate dairy products.