Surface functional properties of blood plasma protein fractions

The solubility, foaming and emulsifying properties of porcine blood plasma and its main protein fractions (serum, globulins and albumin) were investigated at pH 4.5, 6.0 and 7.5 in order to clarify the contribution of each fraction and encourage the optimisation of plasma-derived products. Soluble protein contents above 85% were obtained in all samples. Plasma, serum and albumin showed good foaming capacities, reasonably similar at different pH conditions, although the highest foam stability corresponded to both albumin and plasma at pH 4.5 and 6.0. All protein fractions showed good emulsifying activities, but the stability of the formed emulsions decreased with acidification, being emulsions of albumin and globulins at pH 7.5 the most stable ones. In addition, the interaction indexes calculated to investigate protein–protein interactions revealed synergistic interactions between albumin and globulins when in co-occurrence in their foaming capacity at pH 6.0 and 7.5, and in the stability of emulsions at pH 4.5 and 6.0, but slightly negative effects in the solubility of the mixture, and a great decrease in the stability of emulsions at pH 7.5. On the other hand, the elimination of fibrinogen improved the stability of emulsions and foams at acidic conditions.     

Study of the denaturation/aggregation behaviour of whole porcine plasma and its protein fractions during heating under acidic pH by variable-temperature FTIR spectroscopy

Variable-temperature (VT) Fourier transform infrared (FTIR) spectroscopy was employed to examine changes in secondary structure of whole plasma proteins as well as of plasma protein fractions (serum, serum albumin and globulins) upon heating at pH 4.5 and to establish their kinetics of thermally induced protein aggregation through formation of non-native intermolecular beta-sheets. A detailed analysis of the amide I′ band in the VT-FTIR spectra indicated that plasma proteins were more thermally sensitive at pH 4.5 than at pH 7.5 both when found as mixtures and in monomolecular systems, with the thermal aggregation being strongly enhanced under acidic conditions, particularly in the case of serum albumin. Comparison of the spectral changes of plasma and serum (fibrinogen-depleted plasma) during heating indicated that fibrinogen has no role in protein aggregation under acidic conditions, in contrast to findings at pH 7.5. Considering the particular characteristics of the different plasma proteins, the strong predominance of positive charges in the plasma as a whole at pH 4.5 along with the effects of these pH conditions on the conformation of fibrinogen could be suggested as the main factors responsible for the lack of a contribution by fibrinogen to protein aggregation. Moreover, 2D correlation spectroscopy indicated that the sequence of structural changes occurring during heating was practically identical among the different protein fractions examined and completely different from that established at pH 7.5, with the native beta-sheets being now more heat-sensitive than the alpha-helical structures and with protein aggregation through the formation of intermolecular beta-sheets beginning after native beta-sheets started to unfold.     

Influence of green tea polyphenols on the colloidal stability and gelation of WPC

Green tea extracts are being widely used in food products due to their health-promoting properties. Polyphenols can interact with food proteins leading to the formation of soluble or insoluble complexes; therefore they could alter functional properties of proteins. The objective of the present work was to study the colloidal stability and gelation characteristics of a whey protein concentrate (WPC) in the presence of green tea polyphenols. Mixtures of WPC35 (8 and 30% w/v) and green tea polyphenols (0.25–1% w/v) were prepared at pH 4.5 and 6.0. The size of particles formed was analyzed by light scattering, while gelation was characterized by means of dynamic rheometry and texture analysis of gels. At pH 6.0, the particles were smaller and had a higher net charge than at pH 4.5, which accounted for by a less precipitation of the system at pH 6.0. The G′ parameters of gels upon cooling at 35 °C increased with increasing polyphenols concentration at both pH values. However, the relative viscoelasticity decreased. The texture analysis indicated that the addition of polyphenols improved the firmness and adhesiveness of the gels at pH 6.0, while no significant differences were seen at pH 4.5. The results obtained in this work indicate that pH-dependent interaction between green tea polyphenols and WPC induces the formation of aggregates that modifies the viscoelastic and texture properties of the gels.     

Heat-induced denaturation/aggregation of porcine plasma and its fractions studied by FTIR spectroscopy

The aim of the present work is the in depth study of the protein aggregation mechanisms of whole porcine plasma and its fractions (serum, albumin and globulins) during heating using FTIR spectroscopy. Also, 2D correlation spectroscopy (2D COS) was used to establish the sequence of events during heat-induced gelation for all fractions. The results indicate that serum albumin quickly aggregates from 70 °C through non-native intramolecular β-sheets while globulins show lower susceptibility to protein aggregation. When found together, the aggregation pattern strongly depends on the composition of the protein mixture. That makes the great difference between plasma (serum albumin + globulins + fibrinogen) and serum (serum albumin + globulins) behavior, with the aggregation degree at the end of the thermal process being enhanced in the presence of fibrinogen - and achieving a similar level to that of serum albumin - while minimized in its absence. Attending on the low content of fibrinogen in plasma, our results suggest a great fibrinogen ability to alter the thermal serum albumin and globulins behavior by modifying the negative interactions established between them when no more proteins are found in the media. Moreover, it is noteworthy the slow plasma aggregation pattern at the beginning of the thermal process relative to serum albumin, this way allowing a higher protein unfolding. This could be related to the high heat-induced gel properties of plasma. Also, 2D COS indicates that the sequence of events is very similar for the all analyzed samples, with α-helix being more thermo-labile than native β-sheet structure.     

Effect of partial hydrolysis with an immobilized proteinase on thermal gelation properties of beta-lactoglobulin B

We have investigated the influence of partial hydrolysis with an immobilized proteinase from Bacillus licheniformis on the thermal gelation of isolated beta-lactoglobulin B. Gelation behaviour was determined by dynamic rheological measurements (small deformation) and the gels were characterized with respect to microstructure and water-holding properties. A fine-stranded gel with a complex modulus of approximately 2000 Pa was formed from beta-lactoglobulin (50 g/l in 75 mM-Tris-HCl, pH 7.5). Limited hydrolysis prior to thermal gelation resulted in coarser gels with thicker protein strands and larger pores. Gel structure correlated with its permeability, proton mobility and water-holding capacity. Total stiffness gel increased with low degrees of hydrolysis, but decreased after prolonged hydrolysis. Maximal gel stiffness was 1.5-fold that gels made from of unhydrolysed beta-lactoglobulin. This was much lower than the stiffening effect obtained after partial hydrolysis of whey protein isolate, showing that the gel strengthening effect of partial hydrolysis was depedent on the protein composition and/or the hydrolysis and gelatin conditions. A mechanism to explain the observed effects of hydrolysis on gelation and gel properties is presented.     

血清蛋白质加热凝胶的形成

家畜血液中含有丰富的蛋白质。血液的主要成分——血清或血浆与卵白都有经加热可以形成凝胶的相同性质。食品中蛋白质凝胶的形成能力直接影响着食品的品质。通过对一定浓度的牛和猪的血清蛋白质加热形成凝胶强度的评价,分析了血清中主要蛋白质成分在凝胶形成过程中的作用及其机理。结果表明:牛血清的凝胶强度高于猪血清;牛血清中的白蛋白在血清加热凝胶形成过程中起着重要作用;通过加热血清蛋白,蛋白质分子间的二硫键交联形成凝胶;利用血清蛋白质经加热可以形成凝胶的性质将血清添加到食品中,不仅可以改善食品的品质,而且还可以提高食品中蛋白质的含量。     

Heat-Induced Gelation of Individual Whey Proteins A Dynamic Rheological Study

Heat-induced gelation of the bovine whey proteins [serum albumin (BSA), β-lactoglobulin (β-Lg) and α-lactalbumin (α-La)] has been studied individually and in mixture at different conditions by a dynamic rheological method. Values in the shear stiffness modulus (/G*/) appeared on heating at low protein concentration for BSA (～2%) and at intermediate concentration for β-Lg (～ 5%). α-La did not form a heat-induced gel of concentrations up to 20% (w/v). The ratio of viscous to elastic properties (loss factor) at maximum possible measuring temperature was below 0.07 for the BSA gels and 0.1–0.3 for the β-Lg gels. The temperature of gelation was highly dependent on pH. In mixture one protein could not be exchange for another without changing the gelation behavior of the mixture.     

Simultaneous application of microbial transglutaminase and high hydrostatic pressure to improve heat induced gelation of pork plasma

The effects of treating porcine plasma with microbial tranglutaminase (MTGase) under high hydrostatic pressure (HHP) were studied as a means of improving its gel-forming properties when subsequently heated at pH 5.5, near the pH of meats. Plasma containing varying levels of commercial MTGase was pressurized (400MPa, room temperature, pH 7) for different times, and adjusted to pH 5.5 prior to heating to induce gelation. MTGase-treatment under HHP led to greater enhancement of heat-induced plasma gel properties as compared to control samples. The greatest improvements were achieved by pressurising plasma with 43.3U MTGase/g protein for 30min, thereby achieving recoveries of 49% and 63% in fracture force (gel strength) and fracture distance (gel deformability) of the subsequently heat-induced gels, respectively, relative to gel properties obtained by heating untreated plasma at physiological conditions (pH 7.5).     

Formation of Elastic Whey Protein Gels at Low pH by Acid Equilibration

Abstract: Whey protein gels have a weak/brittle texture when formed at pH ≤ 4.5, yet this pH is required to produce a high-protein, shelf-stable product. We investigated if gels could be made under conditions that produced strong/elastic textural properties then adjusted to pH ≤ 4.5 and maintain textural properties. Gels were initially formed at 15% w/w protein (pH 7.5). Equilibration in acid solutions caused gel swelling and lowered pH because of the diffusion of water and H⁺ into the gels. The type and concentration of acid, and presence of other ions, in the equilibrating solutions influenced pH, swelling ratio, and fracture properties of the gels. Swelling of gels decreased fracture stress (because of decreased protein network density) but caused little change to fracture strain, thus maintaining a desirable strong/elastic fracture pattern. We have shown that whey protein isolate gels can be made at pH ≤ 4.5 with a strong/elastic fracture pattern and the magnitude of this pattern can be altered by varying the acid type, acid concentration, pH of equilibrating solution, and equilibrating time. Practical Application: Low-pH shelf-stable whey protein gels having the strong/elastic texture can be made by forming gels at high pH and equilibrating in acid solutions. Acid equilibration causes the gel to swell and lower the gel pH. Moreover, gel properties can be altered by varying the acid type, acid concentration, pH of equilibrating solution, and equilibrating time.     

pH Induced Aggregation and Weak Gel Formation of Whey Protein Polymers

Whey protein polymers were formed by heating (80 °C) a 4% (w/v) whey protein (WP) isolate dispersion at pH 8.0 for 15, 25, 35, 45, or 53 min. Dispersions were adjusted to pH 6.0, 6.5, 7.0, 7.5, or 8.0 after heating and the rheological properties were determined. Viscosity increased with increased heating time and decreased pH. At pH 7.0 and 7.5, high-viscosity dispersions with pseudoplastic and thixotropic flow behavior were formed, while weak gels were formed at pH 6.0 and 6.5. The storage (elastic) and loss (viscous) moduli of pH-induced gels increased when temperature was increased from 7 °C to 25 °C, suggesting that hydrophobic forces are responsible for gelation. Key Words: weak-gels, whey proteins, polymers, gelation, functionality     

Gelation of casein-whey protein mixtures

Heated milk consists of a mixture of whey protein-coated casein micelles and soluble whey protein aggregates. The acid-induced gelation properties of heated milk are consistently different from those of unheated milk—i.e., a shift in gelation pH, stronger gels, and a different microstructure of the gels. In this study we investigated the role of the different fractions of denatured whey proteins on the acid-induced gelation, the gel hardness, and the microstructure. Both whey protein fractions contribute to the observed shift in gelation pH, although by a different mechanism. Obtaining gels with high gel hardness occurs most effectively when all denatured whey proteins are present as whey protein aggregates. It was observed that disulfide bridge exchange reactions during the acid-induced gelation at ambient temperature play an important role for both whey protein fractions. Additionally, disulfide interactions seem to occur between the aggregates and the casein micelles during the gel state. In this study, we show the development of a new approach for confocal scanning laser microscopy measurements—i.e., separate staining of the proteins in milk. By using this method, we were able to determine that, although whey protein aggregates are not linked to the casein micelles, they nevertheless gel at the same moment. This work adds to a better understanding of the role of denatured whey proteins during acid-induced gelation and could improve the effective use of whey proteins.     

Heat-induced gel formation of plasma proteins: New insights by FTIR 2D correlation spectroscopy

Generalized 2D correlation spectroscopy (COS) has been applied to FTIR spectra of porcine plasma proteins to elucidate the sequence of events leading to pH- and/or thermal-induced protein unfolding and aggregation. Changes in the amide I′ region of the infrared spectra (in the pH range between 7.5 and 4.5, at 0.5 pH intervals) at 30 °C were especially evident as the pH approached the pI of serum albumin (4.8), with the globulin fraction in the plasma proteins undergoing denaturation prior to serum albumin. The effect of increasing temperature (from 30 to 90 °C, in increments of 5 °C) on the secondary structure of the plasma proteins at pHs in the range of 7.5–6.0 revealed that a decrease in alpha-helical structures is taken place previously to diminish native beta-sheets. So, the overall results of this study demonstrate that serum albumin and the globulin fraction differ in their sensitivity to pH and temperature.     

Physicochemical Properties of 7S and 11S Protein Mixtures Coagulated by Glucono-δ-lactone

ABSTRACT: Blends of 7S and 11S proteins with added glucono-δ-lactone were investigated to study the effects of protein composition on gelation. The pH, water-holding capacity, textural, and color properties of the gels formed were studied at a constant temperature as a function of time. Generally high 11S to 7S ratios produced gels of higher hardness, cohesiveness, gumminess, and L values than those of the rest. 11S formed faster acid-induced gels compared with those containing low proportions of 11S. From the data, it was predicted that fractions of 7S:11S differing by 1:10 will form gels with similar physicochemical properties when the coagulating times (at 60 °C) differed by 20 min.     

Chicken muscle homogenate gelation properties: effect of pH and muscle fiber type

Gelation properties of chicken breast and thigh muscle homogenates at a protein concentration of 4.5% under different pH conditions (5.80–6.60) and those of myofibrillar proteins at a protein concentration of 2% were compared to determine the influence of muscle fibre types on gelation. The optimal gelling pH for breast muscle homogenates (pH 6.30) was slightly higher than that for thigh muscle homogenates (pH 5.80–6.30), a similar trend was found for the isolated chicken myofibrillar proteins (pH 6.00 for breast and 5.50 for leg). Similarly, the pH values at which breast muscle homogenate gels were weaker (pH<6.20) or stronger (pH6.20) than thigh muscle homogenate gels were higher when compared with chicken breast and leg myofibrillar protein gels (pH<5.80 and pH>5.90, respectively).     

Dextran modulates physical properties of rennet-induced milk gels

Physical properties of rennet-induced milk gels as core intermediates of cheese production are mainly affected by milk composition, type and amount of coagulation enzyme and starter culture activity. We investigated model systems of reconstituted milk and dextran, which triggers effects on milk gels similar to exopolysaccharides from lactic acid bacteria. Furthermore, clotting activity (0.02 or 0.04 IMCU mL⁻¹) and milk pH adjusted prior to renneting (6.5–5.7) were studied. A lower pH at renneting resulted in an earlier gelation onset, a higher gelation velocity and gel stiffness. The addition of dextran stabilised the gels especially at higher pH, and microstructural analysis revealed larger, more interconnected protein aggregates. However, at pH 5.7, a reverse effect was observed, indicating a destabilisation of the casein network. The current study indicates that altering milk pH and addition of polysaccharides gives the potential to change textural properties of cheese by affecting rennet-induced gelation.     

Functional aspects of blood plasma proteins.

The viscosity of solutions (6% w/w protein in 45% sucrose) of porcine and bovine whole blood plasma, porcine serum and porcine plasma fractions exhibited Newtonian behaviour between 20°C and 73°C. On further heating, the viscosity of these solutions increased exponentially producing a reversible gel structure at 76°C and an irreversible gel at 79°C. In contrast, egg albumen protein solutions indicated a relatively low initial viscosity but started to thicken at 65°C, forming reversible gels at 73°C and irreversible gels at 76°C. The viscosity of the egg albumen protein solutions was similar to that of the serum proteins and plasma protein fractions and less than that of whole plasma protein solutions. Viscosities lower than expected were exhibited by mixtures of whole porcine or bovine plasma and egg albumen proteins. However, synergistic interaction between the proteins of egg albumen and porcine plasma fraction I and fraction III (albumin) and porcine serum occurred at high temperatures of heating.     

Mixtures of whey protein microgels and soluble aggregates as building blocks to control rheology and structure of acid induced cold-set gels

Whey proteins (WP) today offer an extremely high potential for innovative development of functional and nutritious food products. Acid cold-set gels present an interesting approach of gelation at low temperature upon acidification of preformed whey protein (WP) aggregates. In the present work, we aimed to demonstrate how structure and rheological properties of acid gels can be controlled by combining two types of WP aggregates with different structural and chemical properties. Whey protein microgels (WPM) and soluble aggregates (WPSA) were generated upon heating WP isolate in specific pH conditions and temperature, leading to Z-average hydrodynamic diameters close to 270 nm for WPM and 100 nm for WPSA. Mixtures of WPM and WPSA were prepared at different weight ratios ranging from 100% WPM to 100% WPSA. The total protein concentration was set to 4 or 8%wt. Acidification was performed at 40 °C by addition of 1%wt glucono-δ-lactone (GDL). Gelation was followed using turbidimetry and small deformation rheology as function of pH. Microstructures of the gel were investigated at different length scales using various microscopy techniques (CLSM, SEM, AFM). When the WPM/WPSA ratio decreased, the pH of gelation and the gel strength increased because of the different structure and chemical reactivity of the two types of WP aggregates. The final pH had a strong impact on the structure of the gels. When final pH decreased below pH 4.3, a structure change was suggested by turbidimetry measurements. This resulted in a non self-supporting gel or in a decrease of gel strength. For pH above 4.3, self supporting gel were obtained. The rheological properties of the gel could therefore be modulated depending on the properties of the building blocks used (WPM versus WPSA). Interestingly, the gel microstructures observed for WPM/WPSA mixtures or WPM were comparable to those of acidified skimmed milk gels ranging from coarse structures with clumps of aggregates or to homogeneous fine networks (WPSA only) that have been described for WP gels obtained upon direct heating at various pH.     

Gelation mechanism of milk as influenced by temperature and pH; studied by the use of transglutaminase cross-linked casein micelles

Casein micelles in milk are colloidal particles consisting of four different caseins and calcium phosphate, each of which can be exchanged with the serum phase. The distribution of caseins and calcium between the serum and micellar phase is pH and temperature dependent. Furthermore, upon acidification casein micelles lose their colloidal stability and start to aggregate and gel. In this paper, we studied two methods of acid-induced gelation, i.e., 1) acidification of milk at temperatures of 20 to 50 degrees C and 2) decreasing the pH at 20 degrees C to just above the gelation pH and subsequently inducing gelation by increasing the temperature. These two routes are called T-pH and pH-T, respectively. The gelation kinetics and the properties of the final gels obtained are affected by the gelation route applied. The pH-T milks gel at higher pH and lower temperature and the gels formed are stronger and show less susceptibility to syneresis. By using intramicellar cross-linked casein micelles, in which release of serum caseins is prevented, we demonstrated that unheated milk serum caseins play a key role in gelation kinetics and characteristics of the final gels formed. This mechanism is presented in a model and is relevant for optimizing and controlling industrial processes in the dairy industry, such as pasteurization of acidified milk products.     

Effects of heating rate and pH on fracture and water-holding properties of globular protein gels as explained by micro-phase separation

The effect of heating rate and pH on fracture properties and held water (HW) of globular protein gels was investigated. The study was divided into 2 experiments. In the 1st experiment, whey protein isolate (WPI) and egg white protein (EWP) gels were formed at pH 4.5 and 7.0 using heating rates ranging from 0.1 to 35 °C/min and holding times at 80 °C up to 240 min. The 2nd experiment used one heating condition (80 °C for 60 min) and probed in detail the pH range of 4.5 to 7.0 for EWP gels. Fracture properties of gels were measured by torsional deformation and HW was measured as the amount of fluid retained after a mild centrifugation. Single or micro-phase separated conditions were determined by confocal laser scanning microscopy. The effect of heating rate on fracture properties and HW of globular protein gels can be explained by phase stability of the protein dispersion and total thermal input. Minimal difference in fracture properties and HW of EWP gels at pH 4.5 compared with pH 7.0 were observed while WPI gels were stronger and had higher HW at pH 7.0 as compared to 4.5. This was due to a mild degree of micro-phase separation of EWP gels across the pH range whereas WPI gels only showed an extreme micro-phase separation in a narrow pH range. In summary, gel formation and physical properties of globular protein gels can be explained by micro-phase separation. PRACTICAL APPLICATION: The effect of heating conditions on hardness and water-holding properties of protein gels is explained by the relative percentage of micro-phase separated proteins. Heating rates that are too rapid require additional holding time at the end-point temperature to allow for full network development. Increase in degree of micro-phase separation decreases the ability for protein gels to hold water.     

Chemical Forces,Structure, and Gelation Properties of Sweet Potato Protein as Affected by pH and High Hydrostatic Pressure

Sweet potato is one of cheap sources for starch industries worldwide, and exploiting starch wastewater as an alternative protein source is mainly environmental and economic concerns. In this study, the effects of high hydrostatic pressure (HHP; 250, 400, and 550 MPa) on chemical forces, structure, and gelation properties of sweet potato protein (SPP) at pH 3.0, 6.0, and 9.0 were investigated. The values of surface hydrophobicity (H_o) and absolute value of zeta potential of SPP significantly increased from 250 to 550 MPa (p?<?0.05) at all three pH conditions. The total amount of sulfhydryl (-SH-) groups in SPP decreased after HHP at pH 9.0, whereas the amount of free -SH- increased. High molecular mass aggregates (>?180 kDa) were observed in SPP after HHP at pH 6.0 and 9.0 by SDS-PAGE. Regarding elastic rheological behaviors, storage modulus (G′) values of SPP were significantly strengthened after HHP treatment. In addition, textural properties and water-holding capacity of gels made from SPP after 250 and 400 MPa at pH 9.0 were significantly improved, and the gels showed a compact and uniform gel network with the contribution of immobilized water fractions. The gel properties exhibited by SPP after HHP treatment at different pH levels, in particular after 400 MPa at pH 9.0, suggested that it could be potential protein resources as new gelling reagent in the food system.