Heat-induced aggregation of β-lactoglobulin AB at pH 2.5 as influenced by ionic strength and protein concentration

Heat-induced (80°C) aggregation of β-lactoglobulin AB at pH 2.5 was studied using size-exclusion chromatography in combination with multi-angle laser light scattering, dynamic light scattering and electrophoretic techniques. Upon heating, large aggregates with molar masses of 10⁶–10⁷ Da were formed, whereas the concentration of intermediate-sized aggregates was very low. The rate of disappearance of native-like β-lactoglobulin increased with increasing protein concentration (reaction order 2) and ionic strength. Aggregate size increased slightly with heating time and ionic strength, but was independent of protein concentration. Aggregates were held together entirely with non-covalent bonding.     

Emulsion properties of algae soluble protein isolate from Tetraselmis sp.

To study possible applications of microalgae proteins in foods, a colourless, protein-rich fraction was isolated from Tetraselmis sp. In the present study the emulsion properties of this algae soluble protein isolate (ASPI) were investigated. Droplet size and droplet aggregation of ASPI stabilized oil-in-water emulsions were studied as function of isolate concentration (1.25–10.00 mg/mL), pH (3–7), and ionic strength (NaCl 10–500 mM; CaCl₂ 0–50 mM). Whey protein isolate (WPI) and gum arabic (GA) were used as reference emulsifiers. The lowest isolate concentrations needed to reach d₃₂ ≤ 1 μm in 30% oil-in-water emulsions were comparable for ASPI (6 mg/mL) and WPI (4 mg/mL). In contrast to WPI stabilized emulsions ASPI stabilized emulsions were stable around pH 5 at low ionic strength (I = 10 mM). Flocculation only occurred around pH 3, the pH with the smallest net droplet ζ-potential. Due to the charge contribution of the anionic polysaccharide fraction present in ASPI its droplet ζ-potential remained negative over the whole pH range investigated. An increase in ionic strength (≥100 mM) led to a broadening of the pH range over which the ASPI stabilized emulsions were unstable. GA emulsions are not prone to droplet aggregation upon changes in pH or ionic strength, but much higher concentrations are needed to produce stable emulsions. Since ASPI allows the formation of stable emulsions in the pH range 5–7 at low protein concentrations, it can offer an efficient natural alternative to existing protein–polysaccharide complexes.     

热诱导菜豆属7S球蛋白纤维聚集研究

研究了菜豆属球蛋白(芸豆和绿豆)在低pH低离子强度条件下热诱导形成的自组装纤维聚集机理。通过对制备的球蛋白进行热性质及临界形成凝胶浓度分析,选定85℃和0.5%作为自组装纤维化的参数。通过硫代黄色素T(ThT)荧光法,动态光散射(DLS)和原子力显微镜(AFM)表征蛋白纤维化聚集程度及形态。结果表明:芸豆和绿豆球蛋白的Th T最大荧光强度在1h之内剧烈增大(分别是765和1093),表明芸豆和绿豆球蛋白自组装纤维化的"构筑单元"主要产生于加热的起始阶段。DLS结果显示芸豆自组装纤维聚集的能力比绿豆强。但是二者的自组装机理有所不同。AFM图清晰地显示了芸豆球蛋白在加热12 h时自组装形成规则的高度有序的"念珠串状"长线性纤维,绿豆则形成无规则的短棒状纤维和大量碎片状纤维。这为研究进一步研究超低固形物下基于自组装技术的纤维型植物蛋白凝胶的制备提供参考。     

Complex coacervation of chitosan and soy globulins in aqueous solution: a electrophoretic mobility and light scattering study

The effect of pHs and heating on the protein–polysaccharide complexation between the 0.5 wt% soy globulin (7S or 11S) and 0.1 wt% chitosan was studied. Electrophoretic and light scattering techniques were used to examine the electrical charge and aggregation of the individual biopolymers and complexes. At pH 3.0–6.5, 7S (or 11S) globulin in the presence of chitosan had significantly higher ζ‐potentials and lower particles size than 7S (or 11S) globulin alone did (e.g. 600–6000 nm at pH 5.5), indicating the formation of complexes. After heating 7S (or 11S)–chitosan mixtures had higher positive value of ζ‐potential. 7S (or 11S)–chitosan mixtures exhibited a significant increase in positive value of ζ‐potential and stability after heating at lower pH values (pH 3.3 instead of pH 4.5). Compared with other mixtures, at pH 2.5–6.0, the most remarkable decrease in aggregation was obtained for 11S–chitosan mixtures after heating at pH 3.3.     

Influence of environmental stresses on the physicochemical stability of orange oil bilayer emulsions coated by lactoferrin–soybean soluble polysaccharides and lactoferrin–beet pectin

Based on layer-by-layer electrostatic deposition, orange oil bilayer emulsions stabilized with lactoferrin (LF)–soybean soluble polysaccharides (SSPS) and lactoferrin (LF)–beet pectin (BP) were prepared. The effect of environmental stresses (ionic strength, pH, freeze–thaw and light) on the physicochemical stability of primary and secondary emulsions was investigated. In the absence of anionic polysaccharides, orange oil emulsion was highly unstable and aggregated at pH 7–9 and NaCl of 0.1–0.5 M. The droplets in LF–SSPS coated emulsion were stable against aggregation at pH range of 3–10 and NaCl concentration less than 0.3 M, while the droplets in LF–BP coated emulsion were stable against aggregation at pH 4–9 and NaCl concentrations of 0–0.5 M. All the primary and secondary emulsions showed the instability after the freeze–thaw treatment and the stability could be improved in the presence of maltodextrin. During the light exposure (0.35 W/m², 45 °C) for 8 h, the bilayer emulsions could protect key volatile compounds (decanal, octanal and geranial) from the oxidation compared with the primary emulsions. These results suggested that the layer-by-layer electrostatic deposition could improve the stability of LF-coated emulsion to environmental stresses.     

DSC在大豆蛋白功能性质研究中应用

大豆蛋白因其独特的蛋白含量及其较高营养价值,多年来倍受关注。该文综述近十年来有关DSC在大豆蛋白功能性质研究中应用,其中主要涉及大豆蛋白11S和7 S成分热属性及pH、离子强度、钙离子、乙醇、2-巯基乙醇对大豆蛋白热稳定性及功能性质的影响;有关大豆蛋白结合水及其分散相的热属性和流变性质研究,为在酸性条件下制备含大豆蛋白的半固体食品提供一定的理论基础。     

Stability of protein‐stabilised β‐carotene nanodispersions against heating,salts and pH

BACKGROUND: Milk proteins are used in a wide range of formulated food emulsions. The stability of food emulsions depends on their ingredients and processing conditions. In this work, β‐carotene nanodispersions were prepared with selected milk‐protein products using solvent‐displacement method. The objective of this work was to evaluate the stability of these nanodispersions against heating, salts and pH. RESULTS: Sodium caseinate (SC)‐stabilised nanodispersions possessed the smallest mean particle size of 17 nm, while those prepared with whey‐protein products resulted in larger mean particle sizes (45–127 nm). Formation of large particles (mean particle size of 300 nm) started after 1 h of heating at 60 °C in nanodispersions prepared with SC. More drastic particle size changes were observed in nanodispersions prepared with whey protein concentrate and whey protein isolate. The SC‐stabilised nanodispersions were fairly stable against Na⁺ ions at concentrations below 100 mmol L^?1, but drastic aggregation occurred in ≥ 50 mmol L^?1 CaCl₂ solutions. Aggregation was also observed in whey protein‐stabilised nanodispersions after the addition of NaCl and CaCl₂ solutions. All sample exhibited the smallest mean particle size at neutral pH, but large aggregates were formed at both ends of extreme pH and at pH around the isoelectric point of the proteins. CONCLUSION: The nanodispersions prepared with SC were generally more stable against thermal processing, ionic strength and pH, compared to those prepared with whey proteins. The stable β‐carotene nanodispersions showed a good potential for industrial applications. Copyright © 2008 Society of Chemical Industry     

ROLE OF IONIC STRENGTH IN BIOCHEMICAL PROPERTIES OF SOLUBLE FISH PROTEINS ISOLATED FROM CRYOPROTECTED PACIFIC WHITING MINCE

Biochemical characteristics of Pacific whiting muscle proteins extracted at acidic, neutral and alkaline conditions were investigated as affected by various ionic strength levels. The protein solubility at pH 4 declined, as NaCl was added up to 200 mM, due to protein aggregation through hydrophobic interactions. In contrast, at pH 7 and 10, solubility increased as NaCl was added up to 400 mM after which it remained constant. Changes in total SH content and S_owere highly related to the different molecular weight distributions of the soluble proteins. At pH 4, myosin heavy chain (MHC) was soluble as evidenced by the presence of MHC in the soluble fraction, even though degraded molecules were shown at IS 10–100 mM, and became completely insoluble at IS ≥ 150 mM. At pH 10, the density of the MHC band gradually increased as IS increased and the formation of high MW polymers was observed at IS ≥ 150 mM.     

Ability of flaxseed and soybean protein concentrates to stabilize oil-in-water emulsions

The ability of flaxseed protein concentrate (FPC) to stabilize soybean oil-in-water emulsion was compared with that of soybean protein concentrate (SPC). The stability of emulsions increased with increase in protein concentration. The FPC-stabilized emulsions had smaller droplet size and higher surface charge, but worse stability at the same protein concentration compared to SPC-stabilized emulsions. Oil-in-water emulsions stabilized by both proteins were diluted and compared at different pH values (3–7), ionic strength (0–200 mM NaCl) and thermal treatment regimes (25–95 °C for 20 min). Considerable emulsion droplet flocculation occurred around iso-electric point of both proteins: FPC (pH 4.2) and SPC (pH 4.5). FPC and SPC-stabilized emulsions remained relatively stable against droplet aggregation and creaming at NaCl concentration below 100 and 50 mM, respectively. The emulsions stabilized by both proteins were fairly stable within these thermal processing regimes. FPC appears to be less effective as an emulsifier compared to SPC due to its lower emulsion viscosity. Hence, FPC could be more effective in emulsions that are fairly viscous.     

Disappearance of intermolecular beta-sheets upon adsorption of beta-lactoglobulin aggregates at the oil–water interfaces of emulsions

Native proteins usually undergo structural modification upon adsorption at interface. Heat treatments are commonly applied at the industrial scale and lead to aggregation of proteins. We characterized nanometric aggregates of β-lactoglobulin by infrared spectroscopy in solutions, in hexadecane oil-in-water emulsions and at the air–water interface at low and high (0.1 M) ionic strengths and at pH 7. In solutions, on the contrary to native β-lactoglobulin, all aggregates prepared with or without salt possessed intermolecular β-sheets evidenced by two strong absorption bands at 1614 cm⁻¹ and 1682 cm⁻¹. In emulsions, at low ionic strength, they lose their intermolecular β-sheets once they are adsorbed at the oil–water interface. At high ionic strength, most of aggregates are localized at the interfaces where they lose their intermolecular β-sheets in direct contact with the surface and only partially when they are farther from the interface. The loss of intermolecular β-sheets was similarly observed at the air–liquid interface.     

SOLUBILITY OF THE PROTEINS OF MACKEREL LIGHT MUSCLE AT LOW IONIC STRENGTH

Over 90% of the proteins of mackerel light muscle were soluble in solutions of physiological ionic strength or less. To accomplish this solublization, it was necessary to extract certain proteins at moderate ionic strength and neutral pH before extracting the rest of the myofibrillar and cytoskeletal proteins in water. Six proteins were favorably solubilized by sodium chloride solutions of moderate ionic strength at neutral pH under conditions that allowed later dissolution of myofibrillar and cytoskeletal proteins in water. The possibility is suggested that three of these proteins were involved in preventing the solubilization in water of other myofibrillar and cytoskeletal proteins of mackerel light muscle. Based on molecular masses and relative abundance, these proteins could possibly be M-protein (166 kDa), α-actinin (95 kDa) and desmin (56 kDa).     

Ultracentrifugal and polyacrylamide gel electrophoretic studies of extractability and stability of almond meal proteins

Solubility and stability properties of almond proteins were determined using ultracentrifugation and gel electrophoresis to gain a better insight into the complexity of these proteins. Ultracentrifugal analyses of the water-extractable proteins of defatted almond meal revealed four fractions of 2S, 9S, 14S and 19S. The 14S fraction corresponds to amandin, the classical globulin isolated earlier, and constitutes 65–70% of the extractable proteins. Variation of ionic strength from 0 to 1·0 at pH 6–8 showed no evidence of association–dissociation reactions that are typical of many oilseed and legume proteins. Polyacrylamide gel electrophoresis of the water-extractable proteins under reducing conditions separated two pairs of major polypeptides of 44 and 42 kDa and 27 and 25 kDa that appeared to be the respective acidic and basic polypeptides of amandin corresponding to the classical legumin model. Sodium chloride had no effect on total protein extractability but variation of extraction pH showed a broad minimum in extractability at pH 3–5. In contrast, when a pH 9 extract was lowered in pH, the minimum in protein solubility was narrower and shifted upward to pH 5 largely as a result of the precipitation of amandin. Interaction of amandin with phytate may explain the lower pH of minimum solubility when the meal was extracted directly as opposed to lowering the pH of an alkaline extract. Amandin is a cryoprotein and was obtained in 90% purity by cooling a water extract of defatted meal. Incubation of a water extract of meal in the presence of azide for about 12 days revealed proteolytic nicking of the acidic polypeptides of amandin apparently as a result of attack by endogenous proteinase(s). © 1998 Society of Chemical Industry.     

离子强度和pH值对肌原纤维蛋白热诱导凝胶特性的影响

以肌原纤维热诱导凝胶的保水性和质构特性,包括硬度、弹性、内聚性、胶黏性、回复性为指标,考察离子强度和pH值对猪背最长肌肌原纤维热诱导凝胶特性的影响.结果表明,离子强度和pH对凝胶不同特性的影响存在差异,综合各种指标,离子强度为0.5,pH值在6.5和7.0之间有利于形成较好的凝胶.不同条件下凝胶形成的机制可能存在差异.凝胶的硬度与弹性、胶黏性和保水性相关性显著(P＜0.05),可以作为重点考察的指标.     

辛烯基琥珀酸纳米淀粉酯颗粒的制备及其食品级Pickering乳液的特性

利用醇沉法结合辛烯基琥珀酸酐酯化反应,成功制备能够稳定食品级Pickering乳液的纳米淀粉酯颗粒。以纳米淀粉酯粒径、Zeta电位、光学和荧光显微镜观察为指标,研究颗粒添加量、pH值和离子强度对Pickering乳液稳定性的影响。结果表明,体系pH值和离子强度在一定范围内改变了纳米淀粉酯的电位值,其中在极端pH值或者高离子强度条件下,纳米淀粉酯的电位绝对值最低。研究发现,当纳米淀粉酯添加量为2.0 g/100 mL时,制备的Pickering乳液具有较强的稳定性;此外,在体系pH 6.0并且KCl浓度为0.005 mol/L条件下,Pickering乳液分散相油滴的直径最小并且分布均匀,油滴不容易发生聚结,Pickering乳液的稳定性最高。     

Foam properties of algae soluble protein isolate: Effect of pH and ionic strength

In this study the foam properties of algae soluble protein isolate (ASPI), a mixture of mainly proteins and polysaccharides, were investigated as function of isolate concentration (0.1–1.0 mg/mL) and pH (3.0–7.0) at 10 mM and 200 mM NaCl. In addition, adsorption kinetics and dilatational elasticity at the air–water interface were studied. Whey protein isolate (WPI) and egg white albumin (EWA) were used as reference proteins. The consistent dilatational behaviour of ASPI at all pH values and ionic strengths tested indicated a similar interfacial composition at all these conditions. Adsorption kinetics, in contrast, were influenced by varying environmental conditions. At increased ionic strength and close to the theoretical isoelectric point calculated based purely on the amino acid composition of ASPI (pH 7) adsorption increased. Since similar adsorption behaviour was also observed for WPI and EWA, the interfacial properties of ASPI are most likely dominated by its protein fraction. This is further confirmed by the fact that ζ-potential measurements suggested an overall isoelectric point of ASPI below pH 3, while adsorption kinetics varied between pH 5 and pH 7 (the theoretical protein-based isoelectric point of ASPI). The overall foam stability of ASPI stabilized foams was superior to those of WPI and EWA in the pH range 5–7. In conclusion, the molecular and interfacial properties of ASPI, a mixture of proteins and polysaccharides, seem to favour the production of very stable foams in this pH range by the selective adsorption of its protein fraction to the air–water interface.     

Mechanical Properties of Ovalbumin Gels Formed at Different Conditions of Concentration,Ionic Strength,pH, and Aging Time

Ovalbumin gels were prepared by heat treatment at constant pH and ionic forces. Ovalbumins are widely utilized as emulsifying or binding agents. However, due to their protein origin, mechanical properties of ovalbumins are enclosed in a wide range of rheological responses depending on concentration, ionic strength, pH, and aging time. The objective of this work was to study the effect of processing conditions (pH, ionic strength, and protein content) on the textural attributes of an ovalbumin protein system by means of uniaxial compression. Gels were prepared by dispersing proteins (purity 98%) (8.3–12.5% w/w) until complete dissolution in deionized water at 90°C by 45 min, pH (6.3–9.1) was adjusted using citric acid, and the ionic strength (0–100 mM of NaCl) was adjusted using NaCl. The storage of gels was done at 63°C (24–168 h). The rheological tests of gels were done by uniaxial compression. A rupture force peak was observed at high protein content together with an increase in the Young’s modulus. At fixed conditions of ion content (NaCl 50 mM) and pH of 7, the gels presented a maximum in fracture force and Young's modulus after 7 days of storage. The addition of minimum amount of citric acid increases the stability of ovalbumin gels. This information is useful to ensure that the final product will remain stable during storage time at longer shelf lives.     

Thermal aggregation and gelation of kidney bean (Phaseolus vulgaris L.) protein isolate at pH 2.0: Influence of ionic strength

Thermal aggregation and gelation of kidney bean protein isolate (KPI) at pH 2.0 and varying ionic strengths (0–300 mM) were investigated using dynamic light scattering (DLS), atomic force microscopy (AFM), and turbidity and dynamic oscillatory measurements. DLS and AFM analyses showed that the extent of thermal aggregation at pH 2.0, or contour length of the worm-like and fine-stranded aggregates, progressively increased with increasing ionic strength. Turbidity and dynamic rheological analyses indicated that, the turbidity and mechanical moduli of the formed gels also increased with the increase in both ionic strength and protein concentration (c). The c dependence of the elastic modulus G′ could be well described using both fractal and percolation models, though in the case of fractal model applied, two distinct scaling regimes were observed. These results suggest that at pH 2.0, the thermal aggregation and gelation behaviors of the proteins in KPI could be remarkably affected by a change in electrostatic repulsion, and homogenous fine-stranded gels formed at ionic strengths in the 0–300 mM range.     

Interactions and gel strength of mixed myofibrillar with soy protein, 7S globulin and enzyme-hydrolyzed soy proteins

The mixed protein gels were prepared adding soy protein isolate (SPI), 7S globulin, enzyme-hydrolyzed soy proteins, 10- to 100-kDa ultrafiltration fraction and 0.5- to 10-kDa ultrafiltration fraction to myofibril protein isolate (MPI) gels, and five chemical interactions namely nonspecific associations, ionic bonds, hydrogen bonds, hydrophobic interactions and disulfide bonds in these gels were investigated by means of determining gel solubility within 20–75 °C. Furthermore, correlations between gel strength and different chemical interactions were evaluated statistically by Pearson’s correlation test. The gels with 0.5- to 10-kDa fraction presented the biggest gel strength below 60 °C, and the gels with SPI had better gel strength above 65 °C. At different endpoint temperatures, nonspecific associations decreased in order of MPI mixed with 0.5- to 10-kDa fraction, 10- to 100-kDa fraction, enzyme-hydrolyzed soy proteins, 7S globulin and SPI. Gels with ultrafiltration fractions had higher ionic bonds. Hydrogen bonds fluctuated in small scale below 55 °C and reduced at higher temperature. Hydrophobic interactions increased to maximum before decreasing slowly as the temperature went on. In short, both hydrophobic interactions and ionic bonds had significantly positive correlation with gel strength for mixed gels with enzyme-hydrolyzed soy proteins, whereas for the other four mixed gels, it was hydrophobic interactions and nonspecific associations.     

Effect of ionic strength and/or pH on Extractability and physico-functional characterization of broad bean (Vicia faba L.) Protein concentrate

Protein extractability studies showed that the protein of broad bean (Vicia faba L.) was extractable at both acidic and basic pH. The percentage of pH-12 extractable protein that was precipitated at pH 4 (isoelectric point) from protein concentrate, dehulled full-fat seed flour and whole seed flour are 62.0%, 61.2% and 71.6%, respectively. Low ionic strength (μ0.4) increased the solubility of the protein in the bean concentrate at acidic pH, while at alkaline pH, increase in ionic strength (0.1–2.0) had an inverse relationship on the concentrate protein solubility. The capacity to form protein-stabilized foam was least (34%) at pH 4 and highest (97%) at pH 12. These were increased to 62% (pH 4) and 139% (pH 12) in medium with ionic strengths of 0.2 and 0.4, respectively. The foam formed was more stable at pH 4 than at the other pHs. Low ionic strength of 0.1 improved water absorption capacity but reduced it at ionic strength of 0.6.     

Role of pH and Ionic Strength on Water Relationships in Washed Minced Chicken-breast Muscle Gels

ABSTRACT The relationship between pH, ionic strength, and water balance of chicken-breast muscle gels was investigated. An increase in gel pH (pH 6.4 to 7.4) without added NaCl led to dramatic increases in water-holding capacity and water uptake (P < 0.05). Gels at 150 mM NaCl exhibited less ability to adsorb water than salt-free gels (P < 0.05 at pH 6.8 to 7.4) and had lower water-holding capacities (P < 0.05) and fold scores at and below pH 7. Varying salt concentration of the gel-bathing solutions had dramatic effect on the water uptake of the gels. The results show that strong water-absorbing gels can be produced at low ionic strengths and suggest that the negative charge of the muscle proteins is the driving force for water uptake and retention.