Influence of pH on the dry heat-induced denaturation/aggregation of whey proteins

The effect of pH on the heat-induced denaturation/aggregation of whey protein isolate (WPI) in the dry state was investigated. WPI powders at different pH values (6.5, 4.5, and 2.5) and controlled water activity (0.23) were dry heated at 100 °C for up to 24 h. Dry heating was accompanied by a loss of soluble proteins (native-like β-lactoglobulin and α-lactalbumin) and the concomitant formation of aggregated structures that increased in size as the pH increased. The loss of soluble proteins was less when the pH of the WPI was 2.5; in this case only soluble aggregates were observed. At higher pH values (4.5 and 6.5), both soluble and insoluble aggregates were formed. The fraction of insoluble aggregates increased with increasing pH. Intermolecular disulphide bonds between aggregated proteins predominated at a lower pH (2.5), while covalent cross-links other than disulphide bonds were also formed at pH 4.5 and 6.5. Hence, pH constitutes an attractive tool for controlling the dry heat-induced denaturation/aggregation of whey proteins and the types of interactions between them. This may be of great importance for whey ingredients having various pH values after processing.     

Acid-induced aggregation of actomyosin from silver carp (Hypophthalmichthys molitrix)

Acid-induced denaturation and aggregation properties of silver carp actomyosin added with d-gluconic acid-δ-lactone (GDL) during incubation at chilled temperature (4 °C) were studied. Actomyosin underwent aggregation with decreasing pH, as evidenced by increased turbidity and decreased protein solubility in 0.6 M NaCl solution. Ca²⁺-ATPase activity decreased continually with increasing incubation time in the presence of GDL, accompanied by an initial increase in surface reactive sulphydryl (SH) content, indicating the conformation changes of actomyosin during acidification. Protein solubility in selected solvents and electrophoresis analysis showed that the major myofibrillar proteins, particularly, myosin heavy chain and actin were involved in the formation of protein aggregates mainly through noncovalent bonds including hydrophobic interactions and hydrogen bonds during acidification. The slight decrease in total SH content during acidification suggested that disulphide bonds were also involved in acid-induced aggregation of silver carp actomyosin to a lesser extent.     

Effects of non-covalent interactions between the milk proteins on the rheological properties of acid gels

When skim milk (SM) and whey-protein-enriched skim milk (WPE-SM) were heated (80 °C, 30 min) with N-ethylmaleimide (NEM, 0–0.8 mm), the levels of residual native whey proteins increased to ∼70%, whereas the levels of disulphide-bonded whey proteins decreased to <10%. Acid gels prepared from heated SM with added NEM had slightly lower firmness than those made from control heated SM because the former gels lacked intermolecular disulphide bonds. In contrast, acid gels made from heated WPE-SM with added NEM had higher firmness than those prepared from control heated WPE-SM. This implies that, even without intermolecular disulphide bonds, non-covalent interactions can be sufficient to produce acid gels with firmness higher than gels with disulphide bonds. Nevertheless, disulphide interactions can be more important than non-covalent interactions in influencing the yield properties of the gels because acid gels without disulphide bonds can be fractured more easily than those with disulphide bonds.     

Effects of catechins on the polymerisation behaviour,conformation and viscoelasticity of wheat gluten

The effects of catechins on the structural properties of wheat gluten were investigated in terms of their polymerisation behaviour, conformation and viscoelasticity. Depolymerisation of glutenin macropolymers was analysed by size-exclusion high-performance liquid chromatography (SE-HPLC) of gluten with catechins, revealing major variation in sulfhydryl (SH) content. Compared with controls, the SH content of catechin-enhanced gluten was significantly increased due to breakage of disulphide (SS) bonds. Reversed-phase HPLC revealed that catechins had minimal effect on subunit distribution of gliadin and glutenin. Fourier transform infrared (FTIR) spectroscopy showed that catechins induced the transformation of α-helices to β-sheets, β-turns and antiparallel β-sheets. Rearrangement of secondary structure when supplemented with catechins might be a consequence of altered non-covalent interactions. Catechins increased tan δ, indicating enhanced gluten fluid properties. These results imply that catechins might weaken the network structure of gluten and change the rheological properties by inhibiting SS cross-linkage formation and influencing non-covalent bonds.     

The use of whey protein particles in gluten-free bread production,the effect of particle stability

Wheat dough has unique properties for bread making due to its elastic and strain hardening behaviour. A mesoscopically structured whey protein particle system possesses those elastic and strain hardening properties when mixed with starch to a certain extent. However, the extensibility is lower and the particles are more stable than gluten particles upon kneading, probably due to a too high degree of internal crosslinking. This study describes the relation between the number of disulphide bonds of a mesoscopic whey protein particle suspension blocked by NEM treatment and the resulting properties of a dough and bread prepared with that suspension. This study shows that the properties of the particle network are influenced by the ability to form disulphide bonds. Our study shows that a certain amount of disulphide bonds is essential, but too many disulphide bonds can lead to too stiff dough and poorer bread properties.     

FAILURE MECHANISMS OF TOMATO PERICARP TISSUE SUGGESTED BY LARGE AND SMALL DEFORMATION TESTS

Failure mechanisms of tomato pericarp tissue were investigated by measuring puncture and viscoelastic properties of tissue from fruit stored at 22C (nonchilled) or 5C (chilled) for 28 days, or at 5C for 15 days prior to transfer to 22C for an additional 13 days (prechilled) to facilitate ripening. The puncture properties of tissue remained unchanged during 28 days of chilling. However, tissue shear strength and rigidity increased with chilling, as demonstrated by increases in storage and loss moduli, and decreases in oscillatory strain and the loss tangent. Puncture failure force, failure firmness and storage and loss moduli of nonchilled and prechilled tissues decreased to low, constant levels by 12 and 9 days, respectively. Simultaneously, pseudoplastic and failure strains and oscillatory strain each increased. These data suggest that the loss of tissue compressive strengths and transition in the dominant mode of failure from cell relaxation and rupture to cell debonding occurred at a faster rate during ripening of prechilled (i.e., chill-injured) compared to nonchilled fruit.     

CONTROL OF DEFORMATION IN TEXTURE TESTS *

Abstract. The motion of the Instron crosshead during small deformations (e. g. 0.25 mm) in tensile tests performed at the available range of deformation rates was examined. Large errors are introduced in the deformation readings if it is assumed that the deformation rate was constant at the rate selected because, as with any instrument of this type, the crosshead requires time to accelerate to the selected speed. The characteristic behaviour of the food is subject to misinterpretation if the applied deformation profile is unknown. It was found that the errors are larger when the instrument is used incorrectly by performing tensile tests below the crosshead. The work demonstrated some of the limitations that must be recognized when applying the Instron, or any deformation mechanism, to certain texture tests. It is concluded that food behaviour, under small deformations, could be recorded precisely and interpreted correctly providing that both the force and deformation are recorded against time. Control of the crosshead stopping and reversal points was also examined, particularly in relation to the instrumental texture profile analysis. Crosshead acceleration affects the readings and interpretation of the data at the reversal point. If the deformation rate is changed, the stopping point must be adjusted.     

The effect of washing water parameters (pH, hardness and sodium pyrophosphate content) on the water-holding capacity and gelation characteristics of sardine (Sardina pilchardus) mince

In order to determine the effect of washing water parameters [pH, hardness and tetrasodium pyrophosphate (TSPP) content] on the water-holding capacity (WHC) and gelation properties of sardine (Sardina pilchardus) mince, two experiments designed by response surface methodology were carried out. The WHC of the mince was maximum when washed in the pH range of 6.5–7.0, with TSPP>1.7?g???l^-1, and CaCO₃ between 30 and 60?mg???l^-1. With the exception of breaking deformation and the folding test, where the highest values were obtained at 60?mg???l^-1 CaCO₃, the variation of water hardness did not induce any significant changes in the other rheological parameters. The use of relatively high concentrations of TSPP in the leaching water contributed to the achievement of high values of breaking deformation, though beyond 2.5?g???l^-1 a detrimental effect on breaking force was observed, and consequently also on gel strength.     

Physical and chemical changes of silver carp sausages during fermentation with Pediococcus pentosaceus

Changes in protein composition and physicochemical properties of silver carp sausages during fermentation were investigated. As fermentation progressed, the amount of salt-soluble and water-soluble proteins decreased gradually with a concomitant increase in insoluble proteins and non-protein constituents. The rapid reduction in pH to 4.5 within 48 h of fermentation coincided with a progressive increase in titratable acidity. The increasing content of TCA-soluble peptides during fermentation indicated intensive degradation of muscle proteins. SDS–PAGE showed that muscle proteins tended to aggregate into large polymers through disulphide bonds and non-disulphide covalent bonds during fermentation, which may be responsible for the formation of superior textural properties of silver carp sausages.     

Protein molecular interactions involved in the gel network formation of fermented silver carp mince inoculated with Pediococcus pentosaceus

The types of interactions and the proteins involved in the formation of gel network of fermented silver carp mince were investigated. Results showed that the amount of each type of protein interactions varied significantly depending on fermentation time. The content of sulphydryl groups decreased continually with a concomitant increase in disulphide bonds during fermentation. The higher gel strength and more compact fibrous network of fermented silver carp mince corresponded to a higher percentage of hydrophobic interactions, disulphide bonds and non-disulphide covalent bonds. SDS–PAGE showed that the major myofibrillar proteins, particularly myosin heavy chain were involved in the formation of aggregates through hydrophobic interactions and disulphide bonds during fermentation. Furthermore, low molecular weight proteins probably produced by proteolysis were also involved in the formation of gel matrix.     

INFLUENCE OF WASHING PARAMETERS ON THE CHARACTERISTICS OF HORSE MACKEREL (TRACHURUS TRACHURUS) MINCE

The objective of the present work was to establish the optimal conditions of pH, hardness and tetrasodium pyrophosphate (TSPP) in the leaching water used in the preparation of washed horse mackerel (Trachurus trachurus) mince to obtain a product with the best gelation characteristics. Total protein content and water holding capacity were affected by the wash water pH. The folding test, the breaking deformation and the water holding capacity declined considerably for pH values less than about 6.5. On the other hand, gel strength, breaking force and hardness were significantly influenced by TSPP concentration but not by pH water hardness influenced the breaking deformation and protein content of minces but not in its hardness, cohesiveness and elasticity. From these results the best conditions for leaching water to be used with horse mackerel mince seem to be pH 6.5, 0.0–2.Og TSPP/L and 50mg CaCO₃/L.     

ANALYSIS OF SURIMI GEL PROPERTIES BY COMPRESSION AND PENETRATION TESTS

Gel properties of surimi of different quality were analyzed using compression and penetration tests. Parameters used to vary the surimi quality were species, freshness, freeze-abuse and added ingredients. Compressive force at failure was more discriminative than penetration force. The result of a failure response analysis suggests that the compression test is useful in assessing the cohesive property, while the penetration test is better for assessing density and compactness. Good correlations between compression and penetration force values were seen in surimi gels prepared from the same species with and without ingredients, while poor correlations were observed in the surimi which either underwent freeze-abuse or was prepared from different species.     

A CONSTITUTIVE RELATIONSHIP FOR GELS UNDER LARGE COMPRESSIVE DEFORMATION

A three-parameter constitutive relationship was developed based on a strain energy function to describe the mechanical behaviour of gels subjected to large deformation under compressive load. The parameters in the relationship can be estimated from experimental data using regression techniques. Alternatively, these unknowns may be determined directly from three compression parameters σ_m, ɛ_m, β, which represent the strength, ductility and nonlinearity of the mechanical behaviour, respectively. The new constitutive relationship fits experimental data, for both brittle and ductile gels, up to the point of compressive failure.     

THE EFFECTS OF pH AND TIME ON RHEOLOGICAL CHANGES DURING EARLY CHEESE MATURATION

Changes in rheological properties of cheese curd during the initial stages of ripening, with pH (5.45–6.05) and with time (2–14 days), were evaluated using a small strain oscillatory test and a newly designed large strain deformation test. The new test method developed for evaluation of grated cheese employed extrusion flow technique and was carried out using the Instron Universal Testing Machine. The values for the storage modulus (G') measured using the small strain test increased with an increase in the pH of cheese curds from 5.45 to 5.90. The results from the large strain test showed essentially similar trends. The samples of curd also showed an increased solid-like behaviour as the pH increased and appeared to become more elastic with time during the two-week period of analysis.     

An assessment of the texture of acidified sodium caseinate gels with added inulin using response surface methodology

This work evaluated the effect of inulin concentration (0–6.0% w/w) on the mechanical properties and microstructure of gels composed of sodium caseinate (CN) (1.5–6.5% w/w) at pH 4.6. Inulin did not statistically affect the elastic modulus and rupture properties of the studied gels but affected the nonlinear mechanical behaviour. Samples with higher inulin concentrations (6% w/w) presented strain‐weakening deformation behaviour, whilst gels with lower inulin concentrations were strain hardening. The presence of inulin resulted in a more closed‐pores network. Gel deformability increased as CN concentration was enhanced, reaching a maximum of 0.60 at 5.58% (w/w) CN, but was not affected by inulin concentration.     

Effect of solution pH on solubility and some structural properties of soybean protein isolate films

Changes in solubility and molecular properties of protein films obtained from soy protein isolate (SPI) solutions at different pH values (2, 8 and 11) were investigated to study protein behavior during film formation. Proteins retained their native conformation in films at pH 8, but were partially or extensively denatured at pH 11 and 2. Although film protein networks were maintained by the same type of interactions at different pH values—covalent (disulfide bonds) and non‐covalent bonds (especially hydrophobic interactions and hydrogen bonds)—the intensity of each type of interaction (predicted from solubility tests in buffers with different chemical action) depended on the pH of the initial solution. Films obtained at pH 8 presented the highest solubility in all the buffers, whereas films at formed pH 2 presented the lowest, except in the buffer of pH 8 that contained urea, SDS and 2‐mercaptoethanol, which totally dissolved 100% of the film proteins. Films prepared at extreme pH values had a denser microstructure than those at pH 8. SDS–PAGE patterns indicated that films were mainly formed by β‐conglycinin and glycinin, which aggregated in different forms during film formation, depending on the pH of the initial solutions. Some of these proteins remained weakly bonded and/or were held by the network of films. These differences in the protein networks structure would affect the physical, mechanical and barrier properties of the films. Copyright © 2006 Society of Chemical Industry     

Influence of sugars on the characteristics of glucono-δ-lactone-induced soy protein isolate gels

The effects of the reducing sugars (glucose and lactose) and the non-reducing sugar (sucrose), heated in combination with soy protein isolate (SPI) at neutral pH, on the physicochemical and rheological properties of SPI were determined. After formation of gels induced by glucono-δ-lactone (GDL), the textural profile and physicochemical bonds of the non-heated and heated SPI gels were investigated. The gelation of SPI was induced in three stages of processing that is similar to some tofu-making procedures. First, SPI was heated in the presence of sugars at neutral pH above the denaturation temperature of SPI; then gelation was induced by GDL at iso-electric pH and finally the acidic gels were heat treated again. Heat treatment with glucose at neutral pH resulted in SPI with higher glycation degree than with lactose, whereas SPI heat treated in the presence of sucrose was not glycated. GDL-induced gels of SPI glycated with glucose was more soluble in water than gels of SPI reacted with lactose, which in turn was more soluble than the control and gels of SPI heated in the presence of sucrose. This indicates a change in the net charge of proteins caused by the glycation reaction. Glucose and lactose had a protective effect on protein denaturation at neutral pH, albeit less than sucrose, resulting in GDL-induced gels with increased water holding capacity and reduced gel hardness than sucrose. Chemical analysis indicated that disulphide bonds were involved in maintaining the structure of the gels, and solubility profiles of gels in different buffers indicate that other types of covalent bonds besides disulphide bonds were formed in gels of glycated SPI, resulting in reduced gel elasticity.     

Rheological studies on mixtures of agar (Gracilaria changii) and κ-carrageenan

The rheological properties of agar and κ-carrageenan mixtures (with a total polysaccharide concentration of 1.5% w/w) were investigated using dynamic oscillatory measurements, creep compliance tests and compressive deformation measurements. The effects of different agar/κ-carrageenan ratios (100/0, 80/20, 60/40, 40/60 and 20/80) on the formation and properties of the gel mixtures were investigated at different pH values (pH 3.5, 4.5 and 5.5). Temperature dependence of G′ of mixtures showed one step like change during cooling, however, two-steps like change was observed on heating trace. Significant thermal hysteresis was observed in all mixtures, moreover, the observed hysteresis was influenced by those characteristics of κ-carrageenan. The gel point as determined by oscillatory measurements of storage and loss modulus (G′ and G″) of these mixtures show a monotonic decrease in temperature as the proportion of κ-carrageenan increased. Additionally, the gelling temperature of the mixtures resembled that of pure agar system. Incorporation of κ-carrageenan caused a large reduction in gel rigidity, which arise from sharp drop in G′ determined by mechanical spectra. Reduction in gel rigidity has also been demonstrated by a marked increased in instantaneous compliance evaluated by creep compliance tests. Young's modulus, stress and strain at failure were also monitored. In comparison with agar gel, the mixed gels were much more deformable, with a higher failure strain, but had lower strength indicated by a marked decrease in Young's modulus and failure stress. On the other hand, a decrease in pH below 4.5 caused a sudden drop in the gelling temperature, G′ and instantaneous compliance. In general, reduction in pH caused gel to be weaker and more brittle, as shown by lower values for Young's modulus, failure stress and failure strain. These effects were attributed to an increase in the number of shorter chains, which disfavored the formation of junction zones. The results of this study suggest that the gelation of the mixtures appeared to have occurred through a segregative phase separation where agar-rich phase formed a continuous phase and κ-carrageenan rich-phase formed a discontinuous gelled phase.     

A review on fabric bagging: the concept and measurement methods

Fabric bagging is a three-dimensional permanent deformation which can deteriorate aesthetical appearance and mechanical properties of garments. This review presents a brief look at the concept of bagging deformation consisting basic definitions, theories and modelling, objective and subjective evaluation and the parameters affecting fabric bagging. The effects of both fabric structural properties and bagging test parameters have been also explored. A thorough knowledge of bagging measurement by different laboratory tests such as unidirectional tensile, bursting strength, ball penetration tests and devices resembling an arm having an elbow point for simulating human movements in static or dynamic conditions was reviewed in details.     

23—STUDIES OF THE SULPHTTOLYSIS OF WOOL FIBRES PREVIOUSLY TREATED WITH IONIC SURFACTANTS

A study is reported in which wool fibres were pre-treated with ionic surfactants and subsequently subjected to sulphitolysis at different pH values in order to follow the chemical behaviour of cystine disulphide bonds in relation to that of bisulphite ions when surfactant molecules had previously been fixed on the protein. The influence of several parameters, such as the pH, concentration, temperature, and time of treatment of wool fibres with ionic surfactants, on the cystine reactivity was studied. Investigations of how treatment with alkylsulphates in the chain-length range C₆–C₁₆ modifies the reactivity of the native protein to sodium bisulphite are also reported.