Impact of cosolvents (polyols) on globular protein functionality: Ultrasonic velocity, density, surface tension and solubility study

The objective of this study was to understand how cosolvents influence the molecular and functional properties of globular proteins in aqueous solutions. The ultrasonic velocity, density and adiabatic compressibility of cosolvent solutions (0–50 wt% sorbitol or glycerol) were measured in the absence and presence of a globular protein (1 wt% β-lactoglobulin) at 30 °C. These measurements were used to calculate the partial specific apparent volume and adiabatic compressibility of the protein. The protein's volume decreased and its compressibility increased in the presence of high cosolvent concentrations, which were attributed to changes in the properties of the protein interior and solvation layer. Sorbitol was more effective than glycerol at decreasing the protein volume at high cosolvent concentrations, which may be because glycerol has some surface activity and may therefore accumulate around hydrophobic regions on the protein surface. Our data were used to account for the observation that sorbitol is more effective than glycerol at increasing the thermal stability and self-association of the β-lactoglobulin. A better understanding of the influence of protein–cosolvent–solvent interactions on the functionality of globular proteins may facilitate the design of protein-based products.     

Specific Disulfide Bond in α-Lactalbumin Infiuences Heat- Induced Gelation of α-Lactalbumin-Ovalbumin-Mixed Gels

The gel strength of ovalbumin mixed with α-lactalbumin (α-La) was determined after heating at 80°C for 15 min at pH 7.0 Gel strength of the mixture of 4%α-La and 4% ovalbumin was two times that of 8% ovalbumin. Modified α-La at Cys6-Cys120 (3 SSα-La) had low enhancement effects on ovalbumin gelation. Competitive ELISA using monoclonal antibody to α-La showed decreased binding reactivities after heating α-La with ovalbumin at specific concentrations. The decrease of total SH groups in the gel mixed with 3SS α-La was much less than when mixed with α-La. A disulfide bond of Cys6-Cys120 in α-La contributed to interactions between ovalbumin and α-La in heat-induced gels.     

INFLUENCE OF PROTEIN CHARGE ON THERMAL PROPERTIES AS WELL AS MICROSTRUCTURE AND RHEOLOGY OF HEAT INDUCED NETWORKS FOR OVALBUMIN AND VICILIN

The storage modulus (G') and loss modulus (G") for both ovalbumin and vicilin exhibited bimodal responses with respect to pH; lower values were obtained around the isoelectric points (IEP) and at pH extremes. Around the IEP, protein networks were characterized as highly aggregated with high tan δ (G"/G') values. At pH values away from the IEP, the microstructure appeared as well crosslinked networks with corresponding lower tan δ values. In most cases, (except ovalbumin in acid conditions) this trend was not reversed at pH extremes. Inclusion of sodium dodecylsulfate (SDS) at pH 8.5 had an effect similar to increasing pH. For both proteins, network deterioration at high SDS concentrations was associated with protein unfolding and increased protein solvent interactions. For networks with low tan δ values (e.g. ovalbumin in alkaline conditions) the G moduli in the final network were influenced by changes during the initial cooling phase, whereas for aggregated products and most vicilin networks, changes in the G moduli during the final cooling phase were prominent. The contribution of electrostatic interactions to well crosslinked networks from both vicilin and ovalbumin was as a repulsive force in the balance required for network formation. For vicilin, electrostatic interactions also appeared to serve as an attractive force even under conditions of reasonable network formation. This may account for the difference in network quality for the two proteins.     

Effects of Temperature, Concentration and Solute Structure on the Acoustic Properties of Monosaccharide Solutions

A sound velocimeter was used to study the physical structure of aqueous solutions (0.0–1.20 molal) of five monosaccharides, fructose, galactose, glucose, mannose and xylose. Over most of the temperature range employed (20–80°C) adiabatic compressibility of the solutions was the dominant factor in defining sound velocity through and structural rigidity of solution. Analysis of the data also indicated that differences in the solution rigidity of the various monosaccharides was attributable to structural differences among the sugars. At temperatures less than 30°C, where foods are often stored or consumed, fructose produces the most rigid structure followed in descending order by galactose, glucose, mannose and xylose.     

Influence of sodium chloride and glucose on the aggregation behavior of heat-denatured ovalbumin investigated with a multiangle laser light scattering technique

ABSTRACT:  The molecular characteristics of ovalbumin aggregates formed by heating with NaCl and glucose were investigated with a multi-angle laser light scattering system. The presence of NaCl and glucose affected the formation and molecular structure of the aggregates. Specifically, glucose increased the denaturation temperature of ovalbumin due to thermal stabilization of the native state of ovalbumin, regardless of the content of added NaCl. The surface hydrophobicity of the aggregates was increased by the addition of NaCl, which induced the denaturation of ovalbumin at a lower temperature. Aggregates with a larger weight-average molar mass ( M_w ) and root mean square radius ( R_g ) formed from heat-denatured ovalbumin with NaCl and glucose. The presence of NaCl during heat denaturation caused the formation of aggregates with a larger M_w (1.9 × 10⁵ and 3.5 × 10⁶ g/mol for 0 and 10 mM NaCl, respectively) and R_g (14.8 and 80.4 nm for 0 and 10 mM NaCl, respectively). Over a certain amount of NaCl, the addition of more glucose resulted in the formation of more aggregates with greater M_w and R_g values. In sum, the thermostability of ovalbumin was affected primarily by glucose, but the molecular characteristics of the soluble aggregates formed by heat denaturation varied primarily with NaCl content.     

Impact of pH on the interactions between whey and egg white proteins as assessed by the foamability of their mixtures

The impact of protein–protein interactions on foaming properties of mixtures consisting of egg white proteins (EWP) and whey proteins (WP) with total protein content of 60 g/L was examined at pH 5, 7 and 9. The ratio between EWP and WP in the mixtures was varied between 67:33, 50:50 and 33:67 (in %; w/w). The ionic strength was adjusted to that of milk (I = 176 mM). The foamability of the protein products was characterized by the foam capacity, stability and firmness. In addition, the hydrophobicity in the protein solutions was assessed as a measure for the physical behaviour and ability of proteins to adsorb at the air–water interface.The individual egg white proteins and whey proteins each showed the best foaming properties at pH 5 and pH 9, respectively. At pH 9 a synergism was observed in the capacity and stability of the foams from EWP/WP-mixtures. This effect appears to be caused by the electrostatic interactions between egg white and whey proteins which occur in the bulk solution after the pH adjustment prior to the foaming. In contrast, at pH 5 no positive influence of foaming the components in a mixture as well as no indication of intermolecular interactions was found. At pH value near the pI of ovalbumin the protein interactions occur when the proteins have adsorbed at the air–water interface. The protein systems foamed at pH 7 showed intermediate foamability compared to the values obtained at alkaline and acidic pH.     

Ovalbumin thermal Gelation Prediction by Application of Temperature-Time History

A model for characterizing changes in viscosity of soy protein during extrusion was adapted to predict apparent viscosity ratio of gelling ovalbumin. For ovalbumin (87% purity) the temperature time histories of 3,5, and 7% gel formation were determined at 85, 90 and 95°C. A back-extrusion method was used to determine apparent viscosity ratios during gelation. Activation energy for denaturation/gelation was estimated at 159 kJ/mol. The model parameter a (degree of molecular entanglement) was 0.75, 0.75, and 1.0 for 3, 5, and 7% gels, respectively. Reaction rate constants were similar for samples with different protein concentrations. Maximum apparent viscosity ratio increased as protein concentrations increased. The mathematical model was verified by determining apparent viscosity ratios at 86 and 93°C and water holding at 90°C, resulting in R²-0.93. This model may be useful in predicting ovalbumin thermal gelation apparent viscosity ratios.     

Immunochemical Properties of Proteins Glycosylated Through Maillard Reaction: β-Lactoglobulin-Lactose and Ovalbumin-Glucose Systems

β-Lactoglobulin and ovalbumin were glycosylated through Maillard reaction with glucose and lactose, respectively. The immunoreactivity between β-lactoglobulin-lactose and its specific antibody was inhibited strongly by free galactose and lactose, whereas free glucose did not at all inhibit the reaction between ovalbumin-glucose and its specific antibody. Furthermore, the antibody directed against β-lactoglobulin-lactose reacted also with other lactosylated proteins but the antibody to ovalbumin-glucose had no reactivity with the other proteins glycosylated with glucose, indicating that lactose attached to carrier proteins through Maillard reaction could be a hapten-like antigen but glucose could not.     

A differential scanning calorimetric study of conversion of ovalbumin to S-ovalbumin in eggs.

The presence of S-ovalbumin, a more heat-stable form of ovalbumin formed on storage of eggs, can be determined by differential scaning calorimetry of egg white. At a heating rate of 10°C/min, at pH 9, the characteristic denaturation temperature of ovalbumin is 84.5°C, that of S, ovalbumin, 92.5°C. The formation of S-ovalbumin proceeds through a previously unrecognised intermediate species having a denaturation temperature of 88.5°C. The kinetics of the conversion on storage of eggs at 4, 22 and 37°C have been determined. Differential scanning calorimetry is a rapid and convenient method of determining the quality of eggs held in storage. Freeze-dried preparations of ovalbumin stored in the cold for 20 years showed partial conversion to the intermediate, but not to S-ovalbumin.     

Protection of ovalbumin against irreversible heat denaturation by a cationic amphiphile at high concentration

The interaction of the cationic amphiphile cetylpyridinium chloride (CPC) with the egg white protein ovalbumin, was studied at various temperatures and pH values. The influence of different concentrations of CPC on the thermal behaviour of the protein was followed by specific optical rotation measurements in the pH interval 3-9. No increase in optical rotation was found at any pH when CPC was added to ovalbumin at room temperature. When heated, a gradual increase in optical rotation was registered, which became smaller the higher the pH used. At molar ratios of CPC :ovalbumin of above 70 this increase was reversed on cooling. The reversibility of the thermal unfolding on cooling was not affected by a variation in protein concentration between 0.5 and 10%. Circular dichroism measurements before and after a heating-cooling cycle confirmed the results obtained on optical rotation. When CPC was exchanged against the anionic amphiphile, sodium dodecylsulphate (SDS), a denaturing effect was achieved at neutral and alkaline pH values already at room temperature. At the acidic side of the isoelectric point, however, there was no major effect of SDS. No thermal aggregation of ovalbumin occurred near the isoelectric point when CPC was present and complete resolubility was found after thermal treatment and drying.     

Some Functional Properties under Heating of the Globin Prepared by Carboxymethyl Cellulose Procedure

Conditions for gel formation of heated globin and inhibition of thermocoagulation of ovalbumin and bovine serum albumin by globin were investigated. Globin was highly soluble even when heated at 100°C at pH below 5. Globin also considerably inhibited thermocoagulation of ovalbumin and serum albumin near their isoelectric point. A great increase of viscosity was observed when 1% globin solution was heated at above 80°C in the narrow pH range between pH 5.2 and 5.4. Heated globin formed a transparent gel at concentration above 3% under well controlled heat conditions.     

Effect of heat treatment on foaming properties of ostrich (Struthio camelus) egg white proteins

The effect of heat-treatment on foaming properties of ostrich albumen was investigated. At pH 7.5, coagulation of ovotransferrin (OT) resulted in minimum foaming capacity (FC) and maximum foam density (FD). While both exposure of sulfhydryl groups of ovalbumin (OV) and OT-OV interactions improved FC at lower pH, neither of them had beneficial effect on FC at pH 9. At pH 9, FC and FD were mildly affected by heat-treatment because of less aggregation of OT, then FC decreased due to covalent aggregates composed of ovomucin and OV, but at higher temperatures, higher level of OV unfolding resulted in enhanced FC.     

A study of the solution properties of selected binary mixtures of bulk and intense sweeteners in relation to their psychophysical characteristics

The solution properties of binary mixtures of two bulk sweeteners (sucrose and maltitol) and three intense sweeteners (acesulfame K, aspartame and sodium cyclamate) were studied. The parameters measured were apparent specific volumes, isentropic compressibilities and compressibility hydration numbers. An attempt has been made to correlate the solution properties of some of the mixtures with their sweetness responses. Both specific volume and isentropic compressibility data have been used to interpret the possibility of synergism or suppression in the mixtures in terms of the affinity of the solutes for the surrounding solvent structure, and hence the effectiveness of transport of the molecules to their appropriate receptor sites. However, an understanding of the nature and stereochemistry of the individual components in the mixtures is necessary for predicting the packing characteristics of molecules in water and their accession to receptor sites. The intense sweeteners seem to play an important role in modifying the structure of water in solution. Although there is no clear proof of how this affects perception, fine differences in solution behaviour can be observed with isentropic apparent specific compressibility measurements. The study supports the idea that receptors lie at different depths in the lingual epithelium, and also that in two-component systems, one species may alter the packing efficiency of the other in solution sufficiently to effect a change in taste quality or intensity.     

Apparent molar volumes and 1H-NMR relaxation values of glucose syrups

The apparent molar volumes and proton magnetic resonance ¹H-NMR pulse relaxation values of D-glucose and four commercial glucose syrups were determined in the concentration range 5–50% (w/w). The apparent molar volumes increased linearly with average degree of polymerisation (DP) of the syrups and were thus in excellent agreement with average molecular weight. These increases contrasted markedly with the known increases of specific rotation of maltooligosaccharides which reach a limiting value at about DP 7 or 8. Apparent specific volumes were all confined to the range 0.60–0.64 cm³g^?1 as expected for carbohydrates. They decreased sharply in the DP range 1–3 and showed little change thereafter. Apparent specific volumes also increased with increasing concentration of solute, as expected. Spin-spin relaxation values (T₂-values) decreased with increasing DP or increasing concentration of solute, as the proportion of ordered protons in solution increased. Consequently, within one glucose syrup, T₂ decreased with increasing apparent specific volume (increasing concentration) and, between glucose syrups, T₂ decreased with increasing DP. These results reflect the packing characteristics of the solutes among water molecules and accord with their known sweetness properties. They also underline the value of solution properties for the rapid characterisation of glucose syrups.     

温度对碱作用下卵白蛋白-葡萄糖体系色泽、理化特性及抗氧化活性的影响

探讨温度（4、25、37 ℃）对碱作用下卵白蛋白-葡萄糖体系色泽、理化特性及其抗氧化活性的影响。结果表明,随着孵育时间延长和温度升高,卵白蛋白-葡萄糖体系色泽不断加深,棕色产物以及荧光化合物含量不断增加,其美拉德反应中间产物含量在4 ℃和25 ℃时不断上升,而在37 ℃时先上升后下降,其pH值在整个孵育期间呈现出先上升后下降的趋势。傅里叶变换红外光谱分析表明,卵白蛋白和葡萄糖发生了缀合反应。氨基酸分析表明在碱诱导下卵白蛋白中的丝氨酸、组氨酸和半胱氨酸具有较高的反应性,其中反应性最高的是半胱氨酸。孵育后卵白蛋白-葡萄糖体系的抗氧化活性与卵白蛋白葡萄糖混合物相比有了显著提升,但随孵育时间的延长有轻微下降。     

Thermal Denaturation of Hen Egg White Studied by Chromatographic and Immunochemical Techniques

The course of thermal denaturation of hen egg white at 70°C was studied in detail by fast gel chromatography on Superose 12 and by immunochemical techniques. The mechanism of thermal denaturation differed depending on the pH of the environment. The aggregation of protein at the beginning of heat treatment was typical at pH 5 while at pH 9 the breakdown to smaller fragments prevailed. The chromatographic method for the determination of thermally undamaged (native) ovalbumin correlated well with the immunochemical method.     

Modelling the interactions between Lactobacillus curvatus and Enterobacter cloacae. I. Individual growth kinetics

Mixed cultures of Lactobacillus curvatus and Enterobacter cloacae were chosen as a model system to quantitatively study microbial interactions involved in food spoilage and food preservation. In this paper models were developed to predict the individual behaviour of L. curvatus and E. cloacae in pure suspension cultures as a function of the glucose and lactate concentration and the pH. In a second paper these models will be used to predict the moment of interaction in mixed suspension cultures and the cell concentrations at this moment. The effect of pH on the maximum specific growth rate could be described by a parabolic equation for L. curvatus and E. cloacae. E. cloacae is clearly more sensitive to a pH decrease than L. curvatus, as may be concluded from the theoretical minimum pH for growth of 5.6 for E. cloacae and 4.3 for L. curvatus. For both organisms no effect of glucose on the maximum specific growth rate could be detected and Monod kinetics with a low Monod constant was assumed. For L. curvatus the effect of lactate on the maximum specific growth rate at different initial pH values could be described by a linear relationship. For E. cloacae a slight effect of lactate on the maximum specific growth rate could only be detected at pH 7. However, this effect was negligible compared to the effect of the pH. For both organisms the lag time was modelled by the inverse of the specific growth rate. It can be concluded that, with regard to interactions between L. curvatus and E. cloacae, the pH is likely to be the most important factor in the case where L. curvatus is the dominant organism. Substrate limitation is likely to become important in the case where E. cloacae is the dominant organism or for media with a high buffer capacity.     

High pressure processing effects on the molecular structure of ovalbumin,lysozyme and β-lactoglobulin

Structural changes to the three sensitive food proteins, ovalbumin, β-lactoglobulin and lysozyme were examined following a series of high pressure processing experiments. The proteins prepared at specific pH were pressurized up to 600 MPa and held for periods of up to 30 min. Changes to the secondary and tertiary by examination of circular dichroism spectra revealed that the structure of the three proteins behaved differently to both the applied pressure and holding time. The notable effects were found to occur to both ovalbumin and β-lactoglobulin while lysozyme was found to be the most pressure resistant. For each of the proteins, it also appears that the processing conditions applied at specific pH act in combination to bring about structural change.     

Paricá (Schizolobium amazonicum) and embaúba (Cecropia sp.) as new raw materials for particleboards

In this work two potential new species for forest plantation projects in Northern Brazil were evaluated for their suitability in particleboard production. The main evaluated parameters of the laboratory boards were pH value and buffer capacity of the particles, compaction ratio, thickness swelling, modulus of elasticity (MOE), modulus of rupture (MOR) and internal bond (IB). The results showed that both species are highly suitable for particleboard production as they presented low densities (400–430 kg/m³), light color, good compressibility and no apparent adverse reactions after UF-resination.     

Influence of sodium chloride and glucose on acid-induced gelation of heat-denatured ovalbumin

ABSTRACT:  We examined the effects of NaCl and glucose on cold-set ovalbumin gelation. Cold-set gels were prepared by adding glucono-δ-lactone (GDL) to a 2% heated ovalbumin solution. For the gel prepared from ovalbumin heat-denatured with NaCl and glucose, the gel with 10 mM NaCl was most transparent and had high gel strength. Its maximum complex shear modulus ( G *) and turbidity were 2.5 times greater and 3 times lower, respectively, than those of the gel without NaCl. The turbidity of the gel with the higher NaCl content increased steeply after the addition of GDL and did not change during the experimental period. The maximum G * of the gel exhibited positive correlations with the molar mass, radius, and surface hydrophobicity of soluble aggregates and the NaCl content, but the turbidity exhibited negative correlations with these factors. The presence of glucose did not significantly affect the turbidity or rheological properties of the gel. For the gel prepared by adding NaCl and glucose with GDL, the presence of glucose did not affect the turbidity, but the maximum G * decreased in inverse proportion to the glucose content. The turbidity of the gel with higher NaCl content (≥ 50 mM) was the greatest among all samples, and the increased turbidity was maintained throughout the measurements. The gels with 50 and 100 mM NaCl exhibited thixotropy during shearing at a constant shear rate. Therefore, the presence of NaCl and glucose during cold gelation could facilitate the preparation of cold-set gels having various properties for food applications.