牦牛奶中不同酪蛋白的提取及其功能特性研究

本研究以新鲜牦牛乳为原料,添加钙盐后利用其等电点分离提取牦牛奶中的不同酪蛋白,利用单因素实验筛选牦牛奶中α-、β-、κ-酪蛋白的提取分离参数;并利用电泳(SDS-PAGE)分析牦牛乳不同酪蛋白纯度。同时对牦牛乳中α-、β-、κ-酪蛋白的溶解性、乳化性、发泡性进行分析比较。结果表明:牦牛奶不同酪蛋白分离的最佳Ca~(2+)浓度为0.08 mol/L,最佳温度2℃,此时牦牛乳中α-、β-酪蛋白分离效果较好。牦牛乳不同酪蛋白在等电点(p I)附近时,α-、β-、κ-酪蛋白溶解性、乳化性、发泡能力最低,泡沫稳定性最好。等电点附近酪蛋白的溶解度最低,α-、β-酪蛋白pH值在2.1时溶解度最高(分别为60%、80%),而k-酪蛋白在pH值6.22时最高(43%);在40～80℃范围内,κ-酪蛋白溶解度受温度影响显著;当pH值为2.1时,α-、β-、κ-酪蛋白的乳化能力和发泡能力都最大,其乳化稳定性则是远离等电点后逐渐上升;本研究旨在为牦牛乳酪蛋白的利用和深加工提供思路,为云南特色乳制品的开发利用提供科学依据。     

酪蛋白湿法糖基化改性研究

研究了酪蛋白糖基化湿法改性工艺,分析了底物配比、浓度以及pH值对酪蛋白接枝度的影响,并确定了酪蛋白-葡聚糖接枝反应的优化工艺参数。研究结果表明:pH增大有助于该反应的进行,底物的配比与浓度对反应的影响是双向的。正交分析表明,最佳反应条件为:酪蛋白浓度2mg/mL,底物配比(酪蛋白与葡聚糖的质量比)0.1,pH7.8,接枝度(DG)可达到35.10%,且褐变指数仅为0.198。乳化性能测定结果表示随着接枝度的增大,乳化性逐渐增大,当接枝度为35%时,酪蛋白的乳化性提高了约1.5倍。SDS-PAGE结果证实酪蛋白经湿法改性生成了接枝共聚物。     

羊乳酪蛋白的乳化性质和热稳定性研究

试验采用等电点沉淀法和高速离心法提取崂山奶山羊原料乳的酪蛋白,并对其等电点、SDS-PAGE图谱、乳化活力、乳化稳定性、浊度和热稳定性进行分析。结果表明:崂山奶山羊乳酪蛋白的等电点pH为4.10,分子量为20~40 kDa,包括α-酪蛋白、β-酪蛋白和κ-酪蛋,且β-酪蛋白含量最多。两种提取方法所得酪蛋白溶液的乳化性质和浊度具有极显著性差异(p0.01);酪蛋白的热稳定性随温度的升高而降低。相比于高速离心法,等电点沉淀法制备的酪蛋白电泳图谱条带清晰、纯度较高、热稳定性较好,且酪蛋白溶液的乳化性均一性好。     

糊精改性酪蛋白的制备及乳化特性研究

酪蛋白是一种优良的乳化剂,在等电点PI=4.6附近容易凝沉,采用糊精对酪蛋白进行接枝改性,拓宽酪蛋白在酸性食品中的应用范畴。作者以目标蛋白乳化性为指标进行正交试验设计,确定了糊精改性酪蛋白制备的最佳工艺,并研究了该改性酪蛋白在不同pH条件下的乳化性和乳化稳定性。结果表明,在pH为7.8,温度为110℃,酪蛋白质量浓度为20 mg/m L,酪蛋白与糊精质量比为3∶1,反应时间为60 min条件下,制备得到的改性酪蛋白乳化性达到0.887。此外,发现改性酪蛋白在pH 4和pH 5的条件下,乳化性较改性前分别提高了50.9%和110.7%,乳化稳定性与改性前相比,在pH 4和pH 5时分别提高了54.8%和108.7%。     

牦牛酪蛋白理化及功能特性的研究

研究了不同温度、pH值条件下牦牛酪蛋白的溶解度、表观黏度、疏水性以及乳化、发泡等功能特性.结果表明,在等电点范围之外,牦牛酪蛋白具有较理想的溶解性、乳化性、起泡性,其功能特性受pH值、温度影响较显著.     

燕麦β-葡聚糖的乳化性研究

以裸燕麦麸皮为原料，提取得到β-葡聚糖。对不同质量分数燕麦β-葡聚糖的乳化性、乳化稳定性以及温度和DH值对它们的影响进行了研究，同时比较了燕麦β-葡聚糖和几种不同食品胶体的乳化性能。结果显示：燕麦β-葡聚糖是有良好的乳化性和乳化稳定性，且浓度、温度、pH对其有一定影响。     

水牛奶酪蛋白胶束结构的荧光光谱研究

利用内源荧光团色氨酸（Trp）和外源荧光探针8-苯胺基-1-萘磺酸（8-anilino-1-naphthalenesulfonic acid,ANS）的荧光特性对水牛奶酪蛋白胶束结构进行研究。结果表明：当蛋白质质量浓度较低时,酪蛋白胶束结构变化不明显,而较高的蛋白质质量浓度会破坏其胶束结构,使其疏水基团暴露;此外,在离子强度较大、pH值较低条件下,对酪蛋白胶束结构影响较大,使酪蛋白疏水基团暴露,酪蛋白微球先膨胀后聚集并形成酪蛋白胶束。     

酪蛋白磷酸肽(CPP)理化特性的研究

研究了CPP的溶解性、起泡性及乳化性。CPP在pH2.0～10.0范围内,溶解性除在pH4.0约为90％外,其它均高于90％,且随pH增加而增大。与酪蛋白相比,CPP具有更好的起泡性和泡沫稳定性。CPP对热及Ca~(2 )具有较好的稳定性。CPP乳化力较酪蛋白下降约20％,乳化性下降2.89％,乳化稳定性下降1.45倍。     

乳酸饮料与稳定剂

乳酸饮料是一种悬浮液,主体是乳蛋白,其稳定性受蛋白质的影响。乳蛋白中80％为酪蛋白,是一种高分子两性电解质,在其等电点(pH=4.6)时,蛋白质上的正负电荷相等、完全沉淀,当pH值3.5～3.6时,氨基占优势,蛋白质带正电荷,形成不稳定的溶胶。而乳酸饮料的酸度基本上就在等电点附近,因此,必须借助饮料自身的胶或添加稳定剂来维持其稳定性。     

美拉德反应改善苦荞蛋白水解物乳化性的研究

采用碱性蛋白酶水解1 h制备的苦荞蛋白水解物(tartary buckwheat protein hydrolysates,TBPH)与3种还原糖(葡萄糖、麦芽糊精、葡聚糖)进行温和的美拉德反应,对反应温度、反应时间、反应物的浓度和pH值条件进行优化,最终确定反应条件为pH 7.0、反应浓度为10%、反应时间为3 d。3种还原糖中葡聚糖与苦荞蛋白水解物反应生成的共聚物游离氨基含量最高,具有最好的乳化活性和乳化稳定性,形成的乳状液具有最小的粒径、负电荷最多和抑制脂质氧化能力最强(P 0.05)。而葡萄糖与苦荞蛋白水解物形成的共聚物乳化效果和氧化稳定性均相对较差。试验结果也表现出还原糖的分子链越长,与苦荞蛋白水解物形成共聚物的乳化性能就越好。     

Emulsifying properties of soy whey protein isolate–fenugreek gum conjugates in oil-in-water emulsion model system

Soy whey protein isolate (SWPI)–fenugreek gum conjugates were prepared by Maillard type reactions in a controlled dry state condition (60 °C, 75% relative humidity for 3 days) to improve emulsification properties. SDS-PAGE electropherogram showed that conjugation formed high molecular weight products with the disappearance of 7S fraction, acidic subunits of the 11S fractions and protein band at molecular weight 21 and 24 kDa. However, the amount of protein at molecular weight 30 kDa remained unchanged. The protein solubility of SWPI–fenugreek gum conjugates improved as compared to SWPI and SWPI–fenugreek gum non-conjugated mixture especially at isoelectric point of protein when assessed in the pH range 3–8 at 22 °C. In comparison to SWPI, fenugreek gum and non-conjugated SWPI–fenugreek gum, SWPI–fenugreek gum conjugates had better emulsifying properties near the isoelectric pH of protein. Emulsification at near the isoelectric pH of protein was chosen as at this pH the proteins are prone to aggregate, which could destabilize the emulsion. Heating solutions of the conjugates prior to emulsification further improved their emulsification properties. The conjugates also showed a better emulsifying property at high salt concentration as compared to SWPI alone.     

pH dependence of structure and surface properties of microbial EPS

The flocculation of microorganisms plays a crucial role in bioreactors, and is substantially affected by pH. However, the mechanism for such an effect remains unclear. In this work, with an integrated approach, the pH dependence of structure and surface property of microbial extracellular polymeric substances (EPS), excreted from Bacillus megaterium TF10, and accordingly its flocculation is elucidated. From the Fourier transform infrared spectra and acid-base titration test results, the main functional groups and buffering zones in the EPS responsible for the microbial flocculation are indentified. The laser light scattering analysis reveals that the deprotonated or protonated states of these functional groups in EPS result in more dense and compact structure at a lower pH because of hydrophobicity and intermolecular hydrogen bonds. The zeta potential measurements identify the isoelectric point and indicate that the electrostatic repulsion action of EPS is controlled by pH. The highest flocculation efficiency is achieved near the isoelectric point (pH 4.8). These results clearly demonstrate that the EPS structure, surface properties, and accordingly the microbial flocculation are dependent heavily on pH in solution.     

Stability of casein micelles cross-linked with genipin: A physicochemical study as a function of pH

Chemical or enzymatic cross-linking of casein micelles (CMs) increases their stability against dissociating agents. In this paper, a comparative study of stability between native CMs and CMs cross-linked with genipin (CMs-GP) as a function of pH is described. Stability to temperature and ethanol were investigated in the pH range 2.0–7.0. The size and the charge (ζ-potential) of the particles were determined by dynamic light scattering. Native CMs precipitated below pH 5.5; CMs-GP precipitated from pH 3.5 to 4.5, whereas no precipitation was observed at pH 2.0–3.0 or pH 4.5–7.0. The isoelectric point of CMs-GP was determined to be pH 3.7. Highest stability against heat and ethanol was observed for CMs-GP at pH 2, where visible coagulation was determined only after 800 s at 140 °C or 87.5% (v/v) of ethanol. These results confirmed the hypothesis that cross-linking by GP increased the stability of CMs.     

Interaction between proteins and acid polysaccharides]

The intereaction between proteins and acid polysaccharides is electrostatic in nature and leads to the formation of soluble charged and insoluble neutral complexes. The complex formation in the system casein-dextran sulphate is followed by means of turbidimetric titration. It depends on the pH value and the electrolyte concentration. On free electrophoresis, complexes formed below the isoelectric point of the protein exhibit anodic mobility, whereas pure casein migrates to the cathode. The protein in the complex is not able to bind amido black. Consequently, it cannot be detected electrophoretically by dyebinding. The results from viscosity and diffusion measurements are indicative of an increased hydrodynamic volume of the complexes.     

Binding,stability, and antioxidant activity of curcumin with self-assembled casein–dextran conjugate micelles

This work studied the potential of self-assembled micelles of casein–dextran conjugates (CD) as a protective carrier for bioactive curcumin. CD was prepared through the Amadori rearrangement of the Maillard reaction. In casein micelles and CD micelles, the addition of curcumin can quench the intrinsic fluorescence of casein gradually. According to the double logarithm equation, the binding constants were determined as 3.9 × 10⁴ and 1.8 × 10⁵ M^?1 for the binding of curcumin with casein micelles and CD micelles, respectively. The higher affinity of hydrophobic binding of curcumin with CD micelles was ascribed to the more hydrophobic and compact structure of casein core surrounded by the hydrophilic dextran shell, which was supported by the fluorescence spectrum and fluorescence anisotropy of curcumin. Furthermore, CD micelles were found to be superior to casein micelles for improving the stability and radical scavenging activity of curcumin.     

Physico-chemical factors controlling the foamability and foam stability of milk proteins: Sodium caseinate and whey protein concentrates

We explored the foaming behavior of the two main types of milk proteins: flexible caseins and globular whey proteins. Direct foam comparison was complemented with measurements in model experiments such as thin foam films, dynamic surface tension, and protein adsorption. Foaming was studied as a function of pH (from below to above isoelectric point, pI) and range of ionic strengths. Maximum foamability was observed near pI ≈ 4.2 for WPC in contrast to sodium caseinate which had minimum foaming near pI = 4.6. Good foamability behavior correlated well with an increased adsorption, faster dynamic surface tension decrease and increased film lifetime. Differences in the stability of the foams and foam were explained with the different molecular structure and different aggregation behavior of the two protein types. Far from its isoelectric pI, casein adsorption layers are denser and thicker thus ensuring better stabilization. Added electrolyte increased further the adsorption and the repulsion between the surfaces (probably by steric and/or osmotic mechanism). In contrast the globular molecules of WPC probably could not compact well to ensure the necessary films and foams stabilization far from pI, even after electrolyte addition.     

Improved emulsion stabilizing properties of whey protein isolate by conjugation with pectins

Functional properties of glyco-protein conjugates of the anionic polysaccharide pectin with whey protein isolate, obtained by dry heat treatment at 60 °C for 14 days, have been investigated in O/W emulsions containing 20% (w/w) soybean oil and 0.4% (w/w) protein both at pH 4.0 and 5.5. Emulsion stabilizing properties of mixtures and conjugates were compared at five protein to pectin weight ratios by determining changes in droplet size distribution and extent of serum separation with time. The results indicated that the dry heat-induced covalent binding of low methoxyl pectin to whey protein, as shown by SDS-PAGE, led to a substantial improvement in the emulsifying behaviour at pH 5.5, which is near the isoelectric pH of the main protein β-lactoglobulin. At pH 4.0, however, a deterioration of the emulsifying properties of whey protein was observed using either mixtures of protein and pectin or conjugates.The observed effects could be explained by protein solubility and electrophoretic mobility measurements. The protein solubility at pH 5.5 was hardly changed using mixtures of protein and low methoxyl pectin or conjugates, whereas at pH 4.0 it was decreased considerably. Electrophoretic mobility measurements at pH 5.5 revealed a much more pronounced negative charge on the emulsion droplets in the case of protein–pectin conjugates, which clearly indicated that conjugated pectin did adsorb at the interface even at pH conditions above the protein's iso-electric point. Hence, the improved emulsifying properties of whey protein isolate at pH 5.5 upon conjugation with low methoxyl pectin may be explained by enhanced electrosteric stabilization.Comparing two different commercial pectin samples, it was clearly shown that the dextrose content during dry heat treatment of protein–pectin mixtures should be as low as possible since protein–sugar conjugates not only resulted in increased brown colour development, but also gave raise to a largely decreased protein solubility which very badly affected the emulsifying properties.     

RHEOLOGY OF ACID-INDUCED SODIUM CASEINATE STABILIZED EMULSION GELS

The storage modulus G', loss modulus G", and phase angle δ of acidinduced sodium caseinate emulsion gels were measured at 25C for a certain period of time after the addition of glucono-δ-lactone (GDL). Comparison between pure protein gels and emulsion gels revealed that the presence of emulsion droplets greatly enhanced the gel strength. Acidification by mixing an emulsion with a GDL solution caused immediate gelation but the emulsion gel had similar mechanical properties to the gel formed by direct addition of GDL granules. The viscoelasticity of the gel was strongly related to the pH value of the system. There was no evidence for a three-dimensional network when the pH value was higher than 5.8 or lower than 3.2. The largest storage and loss moduli were observed for gels formed at pH values near the isoelectric point of sodium caseinate (pH 4.6). Rheological differences for gels made at different pH values became distinguishable at low frequencies, where a much smaller phase angle was determined for a gel made at a pH value below the isoelectric point. Partial recovery of the three-dimensional gel network was observed for disrupted gels formed at a pH near the isoelectric point.     

羊皮胶原蛋白乳化性与乳化稳定性的研究

以实验室自制羊皮胶原蛋白为原料,利用柠檬酸和胃蛋白酶进行胶原蛋白的提取,并研究不同浓度、pH和电解质对胶原蛋白乳化性和乳化稳定性的影响。结果表明,所提取的胶原蛋白保持良好的结构,其乳化性随蛋白浓度增加而增大。在远离等电点时,有良好的乳化性;接近等电点pH=5时乳化性最差。在较低NaCl浓度范围内,NaCl的加入对乳化性有促进作用。乳化稳定性的变化较为复杂,有时与乳化性的变化相反,乳化性较差的点表现出了较好的乳化稳定性,反之亦然,建议综合考虑乳化性和乳化稳定性的变化获得最佳改性效果。     

脱脂花生粉特性对其挤压产品结构的影响

以低温脱脂花生粉为原料,采用DSE-25型双螺杆挤压实验室工作站,在花生粉高水分挤压组织化工艺优化的基础上,研究了原料特性(粒度、pH值和油脂含量)对挤压组织化产品结构的影响规律。结果表明,原料特性对脱脂花生粉挤压组织化产品的质量和结构有明显影响。粒径在0.2~0.3mm范围内的原料,其挤压产品既可保持适当的硬度和咀嚼度,又有较好的组织化结构,其微观结构细致,气腔小且分布均匀。原料的pH值对产品的流变学性质和微观结构影响较大;在花生蛋白等电点附近(pH值4.5),挤压产品的硬度和咀嚼度最高,具有较致密的组织结构;在接近中性时,组织化度最高;偏碱性条件不利于产品组织化结构的形成。随原料中油脂含量的增加,挤压产品的硬度、咀嚼度和剪切力均明显下降,扭矩显著降低;综合挤压产品的感官质量和质构分析结果,认为原料中油脂含量在3%~6%范围内,有利于挤压产品组织结构的改善和感官质量的提高。