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1.
The influence of sucrose (0-40 wt %) on the thermal denaturation and functionality of whey protein isolate (WPI) solutions has been studied. The effect of sucrose on the heat denaturation of 0.2 wt % WPI solutions (pH 7.0) was measured using differential scanning calorimetry. Sucrose increased the temperature at which protein denaturation occurred, for example, by 6-8 degrees C for 40 wt % sucrose. The dynamic shear rheology of 10 wt % WPI solutions (pH 7.0, 100 mM NaCl) was monitored as they were heated from 30 to 90 degrees C and then cooled to 30 degrees C. Sucrose increased the gelation temperature and the final rigidity of the cooled gels. The degree of flocculation in 10 wt % oil-in-water emulsions stabilized by 1 wt % WPI (pH 7.0, 100 mM NaCl) was measured using a light scattering technique after they were heated at fixed temperatures from 30 to 90 degrees C for 15 min and then cooled to 30 degrees C. Sucrose increased the temperature at which maximum flocculation was observed and increased the extent of droplet flocculation. These results are interpreted in terms of the influence of sucrose on the thermal unfolding and aggregation of protein molecules.  相似文献   

2.
Heat-induced gel formation by soy proteins at neutral pH   总被引：9，自引：0，他引：9  
Heat-induced gel formation by soy protein isolate at pH 7 is discussed. Different heating and cooling rates, heating times, and heating temperatures were used to elucidate the various processes that occur and to study the relative role of covalent and noncovalent protein interactions therein. Gel formation was followed by dynamic rheological measurements. Heat denaturation was a prerequisite for gel formation. The gelation temperature (84 degrees C) was just above the onset denaturation temperature of glycinin. The stiffness of the gels, measured as the elastic modulus, G', increased with the proportion of denatured protein. An increase in G' was also observed during prolonged heating at 90 degrees C. This increase is explained by the occurrence of rearrangements in the network structure and probably also by further incorporation of protein in the network. The increase in G' upon cooling was thermoreversible indicating that disulfide bond formation and rearrangements do not occur upon cooling.  相似文献   

3.
Whey protein isolate (WPI) gels were prepared from solutions containing ribose or lactose at pH values ranging from 6 to 9. The gels with added lactose had no color development, whereas the gels with added ribose were orange/brown. Lactose stabilized the WPI to denaturation, which increased the time and temperature required for gelation, thus decreasing the fracture modulus of the gel compared to the gels with added ribose and the gels with no sugar added. Ribose, however, favored the Maillard reaction and covalent cross-linking of proteins, which increased gel fracture modulus. The decreased pH caused by the Maillard reaction in the gels containing ribose occurred after protein denaturation and gelation, thus having little if any effect on the gelation process.  相似文献   

4.
Acid-induced cold gelation of soy protein hydrolysates was studied. Hydrolysates with degrees of hydrolysis (DH) of up to 10% were prepared by using subtilisin Carlsberg. The enzyme was inhibited to uncouple the hydrolysis from the subsequent gelation; the latter was induced by the addition of glucono-delta-lactone. Visual observations, confocal scanning laser microscopy images, and the elasticity modulus showed that hydrolysates gelled at higher pH values with increasing DH. The nonhydrolyzed soy protein isolate gelled at pH approximately 6.0, whereas a DH = 5% hydrolysate gelled at pH approximately 7.6. Gels made from hydrolysates had a softer texture when manually disrupted and showed syneresis below a pH of 5-5.5. Monitoring of gelation by measuring the development of the storage modulus could be replaced by measuring the pH onset of aggregate formation (pH(Aggr-onset)) using turbidity measurements. The rate of acidification was observed to also influence this pH(Aggr-onset). Changes in ionic strength (0.03, 0.2, and 0.5 M) had only a minor influence on the pH(Aggr-onset), indicating that the aggregation is not simply a balance between repulsive electrostatic and attractive hydrophobic interactions, but is much more complex.  相似文献   

5.
Thermal denaturation and aggregation abilities of salmon myofibrils and myosin were studied measuring turbidity, intrinsic fluorescence, 8-anilino-1-naphthalene sulfonic acid binding, and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide cross-linking. The thermal behaviors of protein preparation from white and red muscles were compared, and the relationship with thermal gelation properties is discussed. The low gelation ability of salmon muscle proteins was related to a limited extent of protein denaturation and aggregation upon heating. These properties seemed to be carried by myosin molecules as a similar behavior was observed for both myofibrils and myosin preparations. The higher thermal stability observed for red muscle proteins with higher transition temperatures in rheological profiles was related to a shift to higher temperature in denaturation and aggregation processes. The extent of denaturation and aggregation was very similar for both muscle types as was the final rigidity of the gels formed.  相似文献   

6.
The influence of sucrose (0--40 wt %) on the thermal denaturation and gelation of bovine serum albumin (BSA) in aqueous solution has been studied. The effect of sucrose on heat denaturation of 1 wt % BSA solutions (pH 6.9) was measured using ultrasensitive differential scanning calorimetry. The unfolding process was irreversible and could be characterized by a denaturation temperature (T(m)), activation energy (E(A)), and pre-exponential factor (A). As the sucrose concentration increased from 0 to 40 wt %, T(m) increased from 72.9 to 79.2 degrees C, E(A) decreased from 314 to 289 kJ mol(-1), and ln(A/s(-1)) decreased from 104 to 94. The rise in T(m) was attributed to the increased thermal stability of the globular state of BSA relative to its native state because of differences in their preferential interactions with sucrose. The change in preferential interaction coefficient (Delta Gamma(3,2)) associated with the native-to-denatured transition was estimated. The dynamic shear rheology of 2 wt % BSA solutions (pH 6.9, 100 mM NaCl) was monitored as they were heated from 30 to 90 degrees C, held at 90 degrees C for either 15 or 120 min, and then cooled to 30 degrees C. Sucrose increased the gelation temperature due to thermal stabilization of the native state of the protein. The complex shear modulus (G) of cooled gels decreased with sucrose concentration when they were held at 90 degrees C for 15 min because the fraction of irreversibly denatured protein decreased. On the other hand, G of cooled gels increased with sucrose concentration when they were held at 90 degrees C for 120 min because a greater fraction of irreversibly denatured protein was formed and the strength of the protein-protein interactions increased.  相似文献   

7.
Reconstituted skim milk was adjusted to pH values between 6.5 and 7.1 and heated (90 degrees C) for up to 30 min. The skim milk samples were then readjusted to pH 6.7. Acid gels prepared from heated milk had markedly higher G ' values, a reduced gelation time, and an increased gelation pH than those prepared from unheated milk. An increased pH at heating decreased the gelation time, increased the gelation pH, and increased the final G ' of acid set gels prepared from the heated milk samples. There were only small differences in the level of whey protein denaturation in the samples at different pH values, and these differences could not account for the differences in the G ' of the acid gels. The levels of denatured whey protein associated with the casein micelles decreased and the levels of soluble denatured whey proteins increased as the pH at heating was increased. The results indicated that the soluble denatured whey proteins had a greater effect on the final G ' of the acid gels than the denatured whey proteins associated with the casein micelles.  相似文献   

8.
The gelling characteristics of two vicilin fractions from pea (Pisum sativum L.) were compared over a range of pH and salt conditions after preliminary results showed that despite having equal opportunity to unfold, and expose hydrophobic residues, they had different minimum gelling concentrations (at pH 7.6). Furthermore, at this pH one fraction formed turbid gels and the other formed transparent gels. The fraction that formed transparent gels contained a substantial amount of the 70 kDa alpha-subunits of vicilin, and thus it was hypothesized that the highly charged N-terminal extension region on these 70 kDa subunits hinders gelation of this vicilin fraction at pH 7.6 and I = 0.2 due to repulsion of the net negative charge. The experiments designed to test this hypothesis are presented and discussed in this paper and prove that the hypothesis was true, which offers the possibility to control or modify the gelation behavior of vicilin on the basis of information of its subunit composition.  相似文献   

9.
Physicochemical changes of myosin during heating were investigated to elucidate the mechanism of heat-induced gelation of arrowtooth flounder (ATF) myosin at high ionic strength. Changes in dynamic properties indicated ATF myosin formed a gel in three different stages as shown by the first increase in G' (storage modulus) at 28 degrees C, followed by the decrease at 35 degrees C and the second increase at 42 degrees C. DSC thermogram showed the onset of myosin denaturation at 25 degrees C with two maximum transition temperatures at 30 and 36 degrees C. The decrease in alpha-helical content indicated ATF myosin began to unfold at 15 degrees C and the unfolding continued until it reached 65 degrees C. Turbidity measurement showed myosin began to aggregate at 23 degrees C and the aggregation was complete at 40 degrees C. Surface hydrophobicity increased consistently in the temperature range studied, 20-65 degrees C. Sulfhydryl contents decreased significantly at 20-30 degrees C due to the formation of disulfide linkages but remained constant at temperatures >30 degrees C. ATF myosin was shown to be extremely sensitive to heat, resulting in denaturation at lower temperature than other fish myosin. Denaturation was initiated by unfolding of the alpha-helical region in myosin followed by exposure of hydrophobic and sulfhydryl residues, which are subsequently involved in aggregation and gelation processes.  相似文献   

10.
Raman spectroscopic study of changes in fish actomyosin during setting.   总被引：2，自引：0，他引：2  
Actomyosins (AMs) isolated from tilapia, lemon sole, ling cod, and rock fish were heated at 40 degrees C, and structural changes in AMs were investigated using Raman spectroscopy to elucidate low-temperature gelling phenomenon, that is, "setting", of surimi. The following conformational transitions were observed in lemon sole, ling cod, and rock fish gels during setting: a slow unfolding of alpha-helix and exposure of hydrophobic amino acid residues occurring in long-time incubation at 40 degrees C, thereby forming hydrophobic interactions among AM molecules. In addition, the most frequent conformation in disulfide bonds was gauche-gauche-trans (g-g-t) form in the set gel. On the other hand, tilapia AM did not form a gel with heating at 40 degrees C, its alpha-helical structure in the myosin being far more stable than that of the other species. The heat resistance of the tight alpha-helix may prevent the gelation of tilapia AM. It is, therefore, likely that the unfolding of the alpha-helix of myosin is a prerequisite for gelation of AM during setting.  相似文献   

11.
The denaturation and aggregation of reagent-grade (Sigmaalpha-La), ion-exchange chromatography purified (IEXalpha-La), and a commercial-grade (Calpha-La) alpha-lactalbumin were studied with differential scanning calorimetry (DSC), polyacrylamide gel electrophoresis, and turbidity measurement. All three preparations had similar thermal denaturation temperatures with an average of 63.7 degrees C. Heating pure preparations of alpha-lactalbumin produced three non-native monomer species and three distinct dimer species. This phenomenon was not observed in Calpha-La. Turbidity development at 95 degrees C (tau95 degrees C) indicated that pure preparations rapidly aggregate at pH 7.0, and evidence suggests that hydrophobic interactions drove this phenomenon. The Calpha-La required 4 times the phosphate or excess Ca2+ concentrations to develop a similar tau95 degrees C to the pure preparations and displayed a complex pH-dependent tau95 degrees C behavior. Turbidity development dramatically decreased when the heating temperature was below 95 degrees C. A mechanism is provided, and the interrelationship between specific electrostatic interactions and hydrophobic attraction, in relation to the formation of disulfide-bonded products, is discussed.  相似文献   

12.
The influence of pH and ionic strength on gel formation and gel properties of soy protein isolate (SPI) in relation to denaturation and protein aggregation/precipitation was studied. Denaturation proved to be a prerequisite for gel formation under all conditions of pH and ionic strength studied. Gels exhibited a low stiffness at pH >6 and a high stiffness at pH <6. This might be caused by variations in the association/dissociation behavior of the soy proteins on heating as a function of pH, as indicated by the different protein compositions of the dissolved protein after heating. At pH 3-5 all protein seems to participate in the network, whereas at pH >5 less protein and especially fewer acidic polypeptides take part in the network, coinciding with less stiff gels. At pH 7.6, extensive rearrangements in the network structure took place during prolonged heating, whereas at pH 3.8 rearrangements did not occur.  相似文献   

13.
When turkey breast muscle and isolated myofibrillar protein and myosin of cod or turkey (pH approximately 7) were subjected to pressures up to 800 MPa for 20 min, DSC and electrophoresis (SDS-PAGE) indicated that high pressure-induced denaturation of myosin led to the formation of structures that contained hydrogen bonds and were additionally stabilized by disulfide bonds. Disulfide bonds were also important in heat-induced myosin gels. Hardness of whole cod muscle, estimated by texture profile analysis, showed pressure-treated samples (400 MPa) to be harder than cooked (50 degrees C) or cooked and then pressure-treated or pressure-treated and then cooked samples, supporting the suggestion that pressure induces the formation of heat labile hydrogen-bonded structures while heat treatment gives rise to structures that are primarily stabilized by disulfide bonds and hydrophobic interactions. As expected, turkey myosin is more stable than that of cod; however, it seems their pressure-induced gelation mechanisms are similar.  相似文献   

14.
The ability of alphas1/beta-casein and micellar casein to protect whey proteins from heat-induced aggregation/precipitation reactions and therefore control their functional behavior was examined. Complete suppression (>99%) of heat-induced aggregation of 0.5% (w/w) whey protein isolate (pH 6.0, 85 degrees C, 10 min) was achieved at a ratio of 1:0.1 (w/w) of whey protein isolate (WPI) to alphas1/beta-casein, giving an effective molar ratio of 1:0.15, at 50% whey protein denaturation. However, in the presence of 100 mM NaCl, heating of the WPI/alphas1/beta-casein dispersions to 85 degrees C for 10 min resulted in precipitation between pH 6 and 5.35. WPI heated with micellar casein in simulated milk ultrafiltrate was stable to precipitation at pH>5.4. Protein particle size and turbidity significantly (P相似文献   

15.
The effects of Trichoderma reesei tyrosinase-catalyzed cross-linking of isolated chicken breast myofibril proteins as a simplified model system were studied with special emphasis on the thermal stability and gel formation of myofibrillar proteins. In addition, tyrosinase-catalyzed cross-linking was utilized to modify the firmness, water-holding capacity (WHC), and microstructure of cooked chicken breast meat homogenate gels. According to SDS-PAGE, the myosin heavy chain (MHC) and troponin T were the most sensitive proteins to the action of tyrosinase, whereas actin was not affected to the same extent. Calorimetric enthalpy (DeltaH) of the major thermal transition associated with myosin denaturation was reduced and with actin denaturation increased in the presence of tyrosinase. Low-amplitude viscoelastic measurements at constant temperatures of 25 degrees C and 40 degrees C showed that tyrosinase substantially increased the storage modulus (G') of the 4% myofibrillar protein suspension in the 0.35 M NaCl concentration. The effect was the most pronounced with high-enzyme dosages and at 40 degrees C. Without tyrosinase, the G' increase was low. Tyrosinase increased the firmness of the cooked phosphate-free and low-meat chicken breast meat homogenate gels compared to the corresponding controls. Tyrosinase maintained gel firmness at the control level of the low-salt homogenate gel and weakened it when both salt and phosphate levels were low. Tyrosinase improved the WHC of the low-meat and low-salt homogenate gels and maintained it at the level of the corresponding controls of phosphate-free and low-salt/low-phosphate homogenate gels. Microstructural characterization showed that a collagen network was formed in the presence of tyrosinase. Keywords: Chicken myofibrillar proteins; protein modification; cross-linking; tyrosinase; gelation; thermal stability; texture; water-holding capacity; microstructure.  相似文献   

16.
Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) were used to study changes in the conformation of globulin from common buckwheat (Fagopyrum esculentum Moench) (BWG) under various environmental conditions. The IR spectrum of the native BWG showed several major bands from 1691 to 1636 cm(-1) in the amide I' region, and the secondary structure composition was estimated as 34.5% beta-sheets, 20.0% beta-turns, 16.0% alpha-helices, and 14.4% random coils. Highly acidic and alkaline pH conditions induced decreases in beta-sheet and alpha-helical contents, as well as in denaturation temperature (Td) and enthalpy of denaturation (DeltaH), as shown in the DSC thermograms. Addition of chaotropic salts (1.0 M) caused progressive decreases in ordered structures and thermal stability following the lyotropic series of anions. The presence of several protein structure perturbants also led to changes in IR band intensities and DSC thermal stabilities, suggesting protein unfolding. Intermolecular antiparallel beta-sheet (1620 and 1681 cm(-1)) band intensities started to increase when BWG was heated to 90 degrees C, suggesting the initiation of protein aggregation. Increasing the time of the preheat treatment (at 100 degrees C) caused progressive increases in Td and pronounced decreases in DeltaH, suggesting partial denaturation and reassociation of protein molecules.  相似文献   

17.
Structural and functional properties of two amaranth protein isolates as a function of pH were studied. Isolates, A9 and A11, were obtained by alkaline extraction at pH 9 and 11, respectively. Gel filtration chromatograms of A9 and A11 showed similar profiles. The A11 isolate contained mainly albumins and globulins, and a small proportion of globulin‐P aggregates, suggesting the presence of species with a higher degree of denaturation compared to A9. Differential scanning calorimetry (DSC) showed that A9 was characterized by two thermal transitions (65.8 and 98°C); A11 exhibited only a small endotherm (66.6°C) and a second, less defined one. DSC analysis of A9 at pH 2–4 did not show endotherms, but at pH 5, some protein structures were observed. A11 showed a greater degree of denaturation. FPLC results showed that the proteins in A9 are more folded and their conformation is closer to the native state than those in A11, which are more unfolded due to pH‐mediated denaturation, mainly in acid media. The surface hydrophobicity of the isolates in acid media was lower than in alkaline media. The fluorescence emission spectra of the isolates showed differences in acidic pH conditions. As expected, the highest solubility was at alkaline pH. The water‐holding capacity was similar for both isolates. The water‐imbibing capacity and speed of foaming was higher for A11 than for A9. In summary, intense pH treatment of amaranth isolates generated partial or total protein denaturation and differences in the functional properties.  相似文献   

18.
Extensive hydrolysis of whey protein isolate by Alcalase 2.4L produces a gel. The objectives of this study were to compare enzyme-induced gelation with the plastein reaction by determining the types of interactions involved in gelation. The average chain length of the peptides did not increase during hydrolysis and reached a plateau after 30 min to be approximately 4 residues, suggesting that the gel was formed by small molecular weight peptides held together by non-covalent interactions. The enzyme-induced gel network was stable over a wide range of pH and ionic strength and, therefore, showed some similarities with the plastein reaction. Disulfide bonds were not involved in the gel network. The gelation seems to be caused by physical aggregation, mainly via hydrophobic interactions with hydrogen bonding and electrostatic interactions playing a minor role.  相似文献   

19.
Denaturation of proteins from striated and smooth muscles of scallop (Zygochlamys patagonica) was studied with differential scanning calorimetry (DSC) by monitoring maximum temperatures of transition and denaturation enthalpies. DSC thermograms of both striated and smooth whole muscles showed two transitions: Tmax 55.0, 79.2 degrees C; and Tmax 54.7, 78.7 degrees C, respectively. The DSC thermograms of myofibrils and actomyosin were similar to those corresponding to their respective whole muscles. As pH and ionic strength increased, the thermal stability of whole muscles decreased. The pH increase (5.0-8.0) significantly (p < 0.01) decreased the denaturation enthalpies (deltaH total, deltaH peakI, and deltaH peakII) of whole striated muscles. A significant decrease (p < 0.05) in the deltaH total and the deltaH peakI was also observed in DSC thermograms of smooth muscles at pH 8.0. Denaturation enthalpies (deltaH total and deltaH peakI) significantly decreased (p < 0.01) when the ionic strength increased from 0.05 to 0.5 in both types of muscles. Striated muscles were more affected than smooth muscles by changes in the chemical environment.  相似文献   

20.
pH-Induced cold gelation of whey proteins is a two-step process. After protein aggregates have been prepared by heat treatment, gelation is established at ambient temperature by gradually lowering the pH. To demonstrate the importance of electrostatic interactions between aggregates during this latter process, beta-lactoglobulin aggregates with a decreased iso-electric point were prepared via succinylation of primary amino groups. The kinetics of pH-induced gelation was affected significantly, with the pH gelation curves shifting to lower pH after succinylation. With increasing modification, the pH of gelation decreased to about 2.5. In contrast, unmodified aggregates gel around pH 5. Increasing the iso-electric point of beta-lactoglobulin via methylation of carboxylic acid groups resulted in gelation at more alkaline pH values. Comparable results were obtained with whey protein isolate. At low pH disulfide cross-links between modified aggregates were not formed after gelation and the gels displayed both syneresis and spontaneous gel fracture, in this way resembling the morphology of previously characterized thiol-blocked whey protein isolate gels (Alting, et al., J. Agric. Food Chem. 2000, 48, 5001-5007). Our results clearly demonstrate the importance of the net electric charge of the aggregates during pH-induced gelation. In addition, the absence of disulfide bond formation between aggregates during low-pH gelation was demonstrated with the modified aggregates.  相似文献   

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