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1.
Isothermal titration calorimetry (ITC) was used to determine the binding constant, stoichiometry, enthalpy, and entropy of beta-lactoglobulin/low- and high-methoxyl pectin (beta-lg-LM- and HM-pectin) complexes at 22 degrees C and at pH 4. The binding isotherms revealed the formation of soluble intrapolymer complexes (C1) further followed by their aggregation in interpolymer complexes (C2). The interaction between beta-lg and LM- or HM-pectin in C1 and C2 occurred spontaneously with a Gibbs free energy around -10 kcal/mol. The C1 were enthalpically driven, whereas enthalpic and entropic factors were involved in the C2 formation. Because ITC did not allow the dissociation of different enthalpic contributions, the values measured as pectin and beta-lg interacted could partially be attributed to conformational changes. The C1 had a binding stoichiometry of 8.3 and 6.1 beta-lg molecules complexed per LM- or HM-pectin molecule, respectively. The C2 had about 16.5 and 15.1 beta-lg molecules complexed per LM- and HM-pectin, respectively.  相似文献   

2.
The oxidation of fatty acids can be inhibited by engineering the surface of oil-in-water emulsion droplets to decrease interactions between aqueous phase prooxidants and lipids. The objective of this research was to evaluate whether emulsions stabilized by a multilayer emulsifier systems consisting of beta-lactoglobulin and citrus or sugar beet pectin could produce fish oil-in-water emulsions that had good physical and oxidative stability. Sugar beet pectin was compared to citrus pectin because the sugar beet pectin contains the known antioxidant, ferulic acid. A primary Menhaden oil-in-water emulsion was prepared with beta-lactoglobulin upon which the pectins were electrostatically deposited at pH 3.5. Emulsions prepared with 1% oil, 0.05% beta-lactoglobulin, and 0.06% pectins were physically stable for up to 16 days. As determined by monitoring lipid hydroperoxide and headspace propanal formation, emulsions prepared with the multilayer system of beta-lactoglobulin and citrus pectin were more stable than emulsions stabilized with beta-lactoglobulin alone. Emulsions prepared with the multilayer system of beta-lactoglobulin and sugar beet pectin were less stable than emulsions stabilized with beta-lactoglobulin alone despite the presence of ferulic acid in the sugar beet pectin. The lower oxidative stability of the emulsions with the sugar beet pectin could be due to its higher iron and copper concentrations which would produce oxidative stress that would overcome the antioxidant capacity of ferulic acid. These data suggest that the oxidative stability of oil-in-water emulsions containing omega-3 fatty acids could be improved by the use of multilayer emulsion systems containing pectins with low metal concentrations.  相似文献   

3.
Oil-in-water emulsions containing droplets stabilized by beta-lactoglobulin (beta-Lg)-pectin membranes were produced using a two-stage process. A primary emulsion containing small droplets (d(32) approximately 0.3 microm) was prepared by homogenizing 10 wt % corn oil with 90 wt % aqueous solution (1 wt % beta-Lg, 5 mM imidazole/acetate buffer, pH 3.0) using a high-pressure valve homogenizer. The primary emulsion was then diluted with pectin solutions to produce secondary emulsions with a range of pectin concentrations (5 wt % corn oil, 0.45 wt % beta-Lg, 5 mM imidazole/acetate buffer, 0-0.22 wt % pectin, pH 3.0). The electrical charge on the droplets in the secondary emulsions decreased from +33 +/- 3 to -19 +/- 1 mV as the pectin concentration was increased from 0 to 0.22 wt %, which indicated that pectin adsorbed to the droplet surfaces. The mean particle diameter of the secondary emulsions was small (d(32) < 1 microm) at relatively low pectin concentrations (<0.04 wt %), but increased dramatically at higher pectin concentrations (e.g., d(32) approximately 13 microm at 0.1 wt % pectin), which was attributed to charge neutralization and bridging flocculation effects. Emulsions with relatively small mean particle diameters (d(32) approximately 1.2 microm at 0.1 wt % pectin) could be produced by disrupting flocs formed in secondary emulsions containing highly negatively charged droplets, for example, by sonication, blending, or homogenization. The particles in these emulsions probably consisted of small flocs containing a number of protein-coated droplets bound together by pectin molecules. These emulsions had good stability to further particle aggregation up to relatively high ionic strengths (< or =500 mM NaCl) and low pH (pH 3). The interfacial engineering technology used in this study could lead to the creation of food emulsions with improved physicochemical properties or stability.  相似文献   

4.
Interfacial protein-polysaccharide complexes can be used to improve the physical stability of oil-in-water emulsions. The purpose of this study was to examine the impact of ionic strength on the formation and stability of oil-in-water emulsions containing polysaccharide-protein-coated droplets. Emulsions were prepared that contained 0.1 wt % corn oil, 0.05 wt % beta-lactoglobulin, and 0.02 wt % pectin at pH 7. The emulsions were then adjusted to pH 4 to promote electrostatic deposition of the pectin molecules onto the surfaces of the protein-coated droplets. The salt concentration of the aqueous phase (0 or 50 mM NaCl) was adjusted either before or after deposition of the pectin molecules onto the droplet surfaces. We found that stable emulsions containing polysaccharide-protein-coated droplets could be formed when the salt was added after pectin adsorption but not when it was added before pectin adsorption. This phenomenon was attributed to the ability of NaCl to promote droplet flocculation in the protein-coated droplets so that the pectin molecules adsorbed onto the surfaces of flocs rather than individual droplets when salt was added before pectin adsorption. We also found that polysaccharide-protein-coated droplets had a much improved stability to salt-induced flocculation than protein-coated droplets with the same droplet charge (zeta-potential). Theoretical predictions indicated that this was due to the ability of the adsorbed polysaccharide layer to strongly diminish the van der Waals attraction between the droplets.  相似文献   

5.
The effect of ionic strength ( I) on the interfacial interactions between pectin and the bovine serum albumin (BSA) surface has been investigated using the quartz crystal microbalance with dissipation monitoring (QCM-D). As I increases from 0.01 to 0.02 M, the frequency shift (Delta F) decreases, whereas the energy dissipation shift (Delta D) changes toward a higher value. Further increase of I from 0.02 to 0.5 M causes both Delta F and Delta D to gradually return to almost zero. The adsorbed mass and thickness of the pectin adlayer estimated from the Voigt model confirm that the adsorption of pectin and the formation of thicker pectin adlayers on a BSA surface are favored by the increase of ionic strength at I = 0.01 approximately 0.02 M. An increase of I above 0.02 M hinders pectin adsorption and causes the formation of a thinner pectin adlayer. The ionic strength-enhanced effect at I values lower than 0.02 M is explained as an increase of ionic strength that can screen the electrostatic repulsion to a larger extent than the electrostatic attraction between pectin and BSA. However, when I is higher than 0.02 M, both electrostatic repulsion and attraction can be significantly screened by the increasing ionic strength, resulting in the ionic strength-reduced effect. On the other hand, the high viscoelasticity of the pectin adlayer revealed by the Voigt model suggests the formation of a network-structured pectin adlayer on the BSA surface, which contains two steps for higher pectin adsorptions at I = 0.0125 approximately 0.1 M by the indication of two slopes in Delta D-Delta F plots.  相似文献   

6.
Molecular complexes based on proteins and ionic polysaccharides have considerable potential for encapsulation of functional food components, but their widespread utilization is limited because their structure is highly sensitive to pH and ionic strength. We have investigated the possibility of creating stable hydrogel particles by thermal treatment of protein (beta-lactoglobulin) and cationic polysaccharide (chitosan) mixtures. Mixed solutions of beta-lactoglobulin (0.5 wt %) and chitosan (0.1 wt %) were prepared at various pH's (3-8) and were heated (80 degrees C for 20 min). Prior to heating, the biopolymer mixtures formed molecular complexes at pH values where there was an electrostatic attraction between the protein and the polysaccharide: soluble complexes at pH 4.5; complex coacervates at pH 5.0 and 5.5; precipitates at pH>5.5. After heating, relatively small (d approximately 140 nm) and cationic (zeta>+20 mV) hydrogel particles were formed at pH 4.5, but much larger aggregates were formed at pH 5.0 and higher (d>1000 nm). The thermally treated hydrogel particles formed at pH 4.5 maintained their initial particle size when the pH was subsequently adjusted within the range pH 3-5, but they aggregated when the pH was adjusted to >pH 5 because of a reduction in the magnitude of their electrical charge. This study suggests that hydrogel particles can be formed by heating mixed protein-polysaccharide systems under controlled conditions. These hydrogel particles may be useful for encapsulation of functional food components.  相似文献   

7.
The binding processes of thearubigin, which is one of the two major polyphenols (the other one is theaflavin) that gives black tea its characteristic color and taste, to the bovine serum albumin (BSA) surface have been investigated by quartz crystal microbalance with dissipation monitoring (QCM-D). The mass and thickness of the thearubigin adlayer on BSA surfaces at various thearubigin concentrations, salt concentrations, and pH values have been determined by QCM-D using the Voigt model. Our results show that the adsorption isotherm of thearubigin on the BSA surface can be better described by the Langmuir model than the Freundlich model, suggesting that the thearubigin adsorption on the BSA surface is dominated by specific interactions, such as electrostatic interaction and hydrogen bonding, as evidenced by the stronger thearubigin adsorption at pH below the isoelectric point (pI) of BSA and shifts in the positions of both amide bands in the FTIR spectra of the BSA surface with and without thearubigin adsorption. The addition of salt can also influence the thearubigin binding to BSA surfaces. The salt concentration-enhanced effect at a salt concentration lower than 0.1 M is explained as that an increase of salt concentration can screen the electrostatic repulsion to a larger extent than the electrostatic attraction between thearubigin and BSA. On the other hand, when the salt concentration is higher than 0.1 M, both electrostatic repulsion and attraction can be significantly screened by the higher salt concentration, resulting in the salt concentration-reduced effect. However, when the salt concentration is further increased to 0.4 M, the addition of thearubigin may promote the formation of a certain type of complex with BSA, resulting in the increases of both thickness and mass of the thearubigin adlayer.  相似文献   

8.
The changes in molecular masses of pectin in 0.5% pectin-pectinesterase (PE) mixtures (2 units/mL) incubated at various temperatures, pH values, and NaCl levels for 30 min were observed by a Toyopearl TSK HW-65 (F) gel permeation chromatography. The molecular mass of pectin was remarkably increased from 103 to 266 kDa when the incubation temperature of pectin-tomato PE was increased from 25 to 45 degrees C. A further increase in molecular mass was observed when a pectin-citrus PE mixture was incubated at 65 degrees C. The values of pH and NaCl levels were also crucial to the transacylation activity of PEs. Reaction at pH 7.5 with tomato PE and citrus PE remarkably expanded the molecular mass of pectin to 410 and 670 kDa, respectively. The NaCl level of 0.3-0.5 and 0.3 M was favorable for the transacylation reaction of tomato PE and citrus PE, respectively. Only high methoxylpectin was the suitable substrate for PE to conduct the transacylation reaction.  相似文献   

9.
The formation of conjugates between beta-lactoglobulin and acacia gum based on electrostatic complexes formed at pH 4.2 was investigated upon dry-state incubation for up to 14 days at 60 degrees C and 79% relative humidity (RH). By means of SEC-HPLC and RP-HPLC, it was shown that the beta-lactoglobulin incubated alone was able to form polymers with molecular masses higher than 200 kDa until 50% of the initial monomeric protein disappeared after 14 days. In the presence of acacia gum at initial protein to polysaccharide weight mixing ratios of 2:1 and 1:2, only 35% of the initial beta-lactoglobulin monomers disappeared after 14 days. Using RP-HPLC, an apparent reaction order of 2 was found for the disappearance of monomeric beta-lactoglobulin both in the presence or absence of acacia gum. However, the reaction rate was faster in the absence of acacia gum. SDS-PAGE electrophoresis with silver staining confirmed the formation of beta-lactoglobulin/acacia gum conjugates. The solubility curves of the incubated beta-lactoglobulin showed a minimum around pH 4-5. By contrast, the minimum of solubility of the beta-lactoglobulin/acacia gum incubated mixtures shifted to lower pH values compared to initial mixtures. The conjugates exhibited higher foam capacity than the incubated protein as well as lower equilibrium air/water surface tension. Conjugation at ratio 1:2 led to increased interfacial viscosity (300 mN s m(-1) at 0.01 Hz) compared to beta-lactoglobulin alone (100 mN s m(-1) at 0.01 Hz), but similar interfacial elasticity (30-40 mN m(-1)). The foam capacity of the conjugates was significantly higher than that of the incubated beta-lactoglobulin as well as foam expansion and drainage time, especially at pH 5.3, i.e., higher than the pH of formation of the conjugates.  相似文献   

10.
This paper deals with the influence of different levels of three pectins, low-methylated pectin (LMP), high-methylated pectin (HMP), and low-methylated and amidated pectin (LMA), on the in vitro gastric hydrolysis of beta-lactoglobulin (beta-lg). Proteolysis by pepsin consisted of a 2-h progressive reduction of pH. A turbidity measurement of beta-lg-pectin mixtures was carried out during the proteolysis. The influence of pectins on pepsin enzymatic activity was also evaluated. beta-Lg was resistant to peptic digestion. The presence of each of the three pectins at a concentration of 50 wt % increased the N release at all pH values considered, despite a significant inhibition of the pepsin enzymatic activity with the pectins. The turbidity of beta-lg solutions during proteolysis was reduced by the addition of pectins, because of the formation of electrostatic complexes between this protein and pectins. The increase of N release could be a false positive result due to the difficulty of precipitating protein by trichloroacetic acid because of the formation of electrostatic complexes demonstrated by the decrease of turbidity.  相似文献   

11.
A pectin methylesterase (PME) from sweet orange fruit rag tissue, which does not destabilize citrus juice cloud, has been characterized. It is a salt-dependent PME (type II) and exhibits optimal activity between 0.1 and 0.2 M NaCl at pH 7.5. The pH optimum shifted to a more alkaline range as the salt molarity decreased (pH 8.5-9.5 at 50 mM NaCl). It has an apparent molecular mass of 32.4 kDa as determined by gel filtration chromatography, an apparent molecular mass of 33.5 kDa as determined by denaturing electrophoresis, and a pI of 10.1 and exhibits a single activity band after isoelectric focusing (IEF). It has a K(m) of 0.0487 mg/mL and a V(max) of 4.2378 nkat/mg of protein on 59% DE citrus pectin. Deblocking the N-terminus revealed a partial peptide composed of SVTPNV. De-esterification of non-calcium-sensitive pectin by 6.5% increased the calcium-sensitive pectin ratio (CSPR) from 0.045 +/- 0.011 to 0.829 +/- 0.033 but had little, if any, effect on pectin molecular weight. These properties indicate this enzyme will be useful for studying the PME mode of action as it relates to juice cloud destabilization.  相似文献   

12.
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13.
The structure of aggregates and gels formed by heat-denatured whey protein isolate (WPI) has been studied at pH 7 and different ionic strengths using light scattering and turbidimetry. The results were compared with those obtained for pure beta-lactoglobulin (beta-Lg). WPI aggregates were found to have the same self-similar structure as pure beta-Lg aggregates. WPI formed gels above a critical concentration that varied from close to 100 g/L in the absence of added salt to about 10 g/L at 0.2 M NaCl. At low ionic strength (<0.05 M NaCl) homogeneous transparent gels were formed, while at higher ionic strength the gels became turbid but had the same self-similar structure as reported earlier for pure beta-Lg. The length scale characterizing the heterogeneity of the gels increased exponentially with increasing NaCl concentration for both WPI and pure beta-Lg, but the increase was steeper for the former.  相似文献   

14.
Biopolymer interactions have many potential applications in pharmaceutical, cosmetic, nutraceutical, and functional food industries. Attractive interactions between proteins and polysaccharides can lead to the formation of complexes. Binding parameters of beta-lactoglobulin (beta-lg)/pectin complexes were determined using frontal analysis continuous capillary electrophoresis and the overlapping binding site model. At pH 4, approximately 23 beta-lg molecules were cooperatively complexed on low-methoxyl pectin, where each beta-lg molecule covered an average of 12 galacturonic acid residues. The calculated binding constant was 1431 M(-1). The interactions between pectin and four selected peptides located on the outer surface of the beta-lg were investigated in order to identify which part of the protein was likely to interact with the pectin. The peptide beta-lg 132-148, which corresponds to the alpha-helix zone, and the peptides beta-lg 76-83, 41-60, and 1-14 would be involved in the interaction with the pectin.  相似文献   

15.
The electromigration behavior of chitosan D-glucosamine and oligomers with a degree of polymerization from 1 to 6 in dilute aqueous systems containing either NaCl or KCl salt at 0.01, 0.05, and 0.1 M at pH values from 2 to 9 was evaluated. The results showed that the electromigration of the chitosan D-glucosamine and oligomers did not change by changing the type of salt in the running medium and that the pH had a significant effect on the direction of migration under an external electric field. In addition, the increase in the ionic strength of the medium caused a significant decrease on the absolute value of the electrophoretic mobility, and the highest values of the electromobility were observed in water. However, the ionic strength had no significant effect on the electrophoretic mobilities at pH 2 in comparison with the other pH values. The dimer showed the highest electrophoretic mobility in the alkaline zone of the pH. At pH values lower than the pKa of the D-glucosamine, the chitosan D-glucosamine, and oligomers migrated toward the anode, where the amine groups are protonated and carry positive charge. At higher pH values, chitosan D-glucosamine and oligomers migrated toward the anode, even though they did not carry any electric charge. The contribution of the difference in the dielectric constants between the solvent and the solute to this phenomenon was highlighted. It was shown that the glucose moiety contributes to the direction of migration of the chitosan D-glucosamine and oligomers under alkaline conditions and that the difference in the dielectric constant of glucose and the solvent accounts for the direction and the extent of electromobility.  相似文献   

16.
The objective of this study was to characterize the changes in peptide solubility resulting from changing some physicochemical conditions in a tryptic hydrolysate of beta-lactoglobulin (beta-LG). The turbidity (500 nm) of a 1% solution of tryptic peptides was measured at pH 3-10, at 5, 25, and 50 degrees C, in the presence of different salt concentrations (0, 0.5, and 1 M NaCl), in the presence of denaturing and reducing agents (6 M urea, 5% SDS, or 5% beta-mercaptoethanol), and under an electric field (isoelectric focusing). The results reveal an increase in turbidity of the peptide solution at pH 4, but a slight increase in turbidity was also observed at pH 8, which is attributable to peptides linked by disulfide bridges. The effect of temperature and ionic strength on the turbidity occurring at pH 4 indicates that mainly hydrophobic interactions are involved in the aggregation process. The material in the precipitate at pH 4 was identified as the peptides beta-LG 1-8, 15-20, and 41-60 and non-hydrolyzed alpha-lactalbumin. These results suggest that a limited number of peptides are involved in the aggregation process observed at pH 4, some of which having bioactive (beta-LG 15-20, ACE inhibitor, and opioid) or emulsifying properties (beta-LG 41-60). Aggregation of these peptides at acidic pH indicates that a simple acidification step could represent an easy process for isolating peptidic fractions enriched in bioactive or functional peptides.  相似文献   

17.
湿筛过程中分散液的质量对土壤团聚体稳定性影响   总被引：3，自引：1，他引：2  
【目的】土壤团聚体是土壤结构的基本单元,决定着土壤在生态系统中的作用和功能。保持良好的结构状态和稳定的结构性能是确保土壤水、 肥、 气、 热肥力因子协调的基本条件。现代土壤管理中大量使用的化肥无机盐以及灌溉水体富含的离子都会造成对团聚体的破坏,本文以富含有机质的黑土为研究对象,探索湿筛过程中分散溶液的质量对团聚体稳定性的影响。【方法】以氯化钠、 氯化钙和氯化铝3种不同价态的阳离子溶液为介质,采用改进的萨维若夫（Саввинов）法进行湿筛,设置0（不加盐去离子水,CK）、 0.1、 0.3、 0.5 mol/L 4个浓度水平,研究直径0.25 mm的土壤团聚体在不同阳离子盐溶液作用下的稳定性。【结果】不同盐溶液中黑土团聚体的稳定性差异很大。在氯化钠盐溶液中,黑土团聚体的盐破坏率（PAD0.25/w）均为正值,且随钠盐溶液浓度的增大而增加,在浓度为0.1、 0.3和0.5 mol/L的氯化钠溶液中分别为0.21%、 10.7%和14.4%。在氯化钙溶液和氯化铝溶液中,黑土PAD0.25/w均为负值, 氯化钙溶液中PAD0.25/w的绝对值随着盐浓度的增大而减小,氯化铝溶液中PAD0.25/w的绝对值均在51%以上。【结论】一价盐溶液加剧了0.251 mm黑土团聚体的分散,但能够保护直径1 mm以上的团聚体,而二价盐和三价盐溶液对团聚体的分散有明显的抑制和保护作用。  相似文献   

18.
Borate sorption by a soil was measured with 0.01, 0.1, or 1.0 M sodium chloride as background electrolyte and samples of soil with a range of pH values achieved by incubating with either calcium carbonate or hydrochloric acid for 24 h at 60°C.
Borate sorption generally increased with increasing pH. The more concentrated the electrolyte, the steeper the increase. At low pH, increasing the salt concentration decreased borate sorption; at high pH, it increased sorption. There was an intermediate pH at which salt had no effect on borate sorption. The point of zero salt effect on borate sorption was at a higher pH than the point of zero salt effect on pH. This result was explained by a mechanism in which borate ions react with variable charge surfaces which are heterogeneous and for which part of the heterogeneity is in the electric potential of the surfaces. It cannot be explained by mechanisms which do not take into account the effects of the electric potential of the reacting surfaces on the reaction with borate ions. Although the behaviour of borate was broadly consistent with that of other anions, it differed in that about half of the heterogeneity had to be allocated to the binding constant for borate ions. It was suggested that this was because reaction with organic matter was more important for borate than for other anions.  相似文献   

19.
The fraction of sugar beet pectin (SBP) adsorbed onto limonene oil droplets during emulsification has been isolated, and its chemical and physicochemical characteristics have been determined. While the SBP sample itself was found to contain 2.67 and 1.06% protein and ferulic acid, respectively, the adsorbed fraction contained 11.10% protein and 2.16% ferulic acid. The adsorbed fraction was also found to have a higher degree of acetylation, notably at the C2 position on the galacturonic acid residues, and was also found to contain a higher proportion of neutral sugars, which are present in the ramified side chains of the pectin molecules. The thickness of the layer of SBP adsorbed onto polystyrene latex particles was studied by dynamic light scattering and was found to increase with increasing surface coverage. It was found to have a value of approximately 140 nm at plateau coverage, which closely corresponded to the hydrodynamic diameter of the pectin chains. The adsorbed layer thickness was found to be sensitive to pH and the presence of electrolyte. The thickness at a surface coverage of approximately 20 mg/m(2) in the absence of electrolyte at pH approximately 4 was 107 nm and at pH 8.8 was 70 nm, while at pH approximately 4 in the presence of 10 mM NaCl the thickness was found to be 70 nm. It was concluded that the SBP molecules form multilayers at the surface due to electrostatic interaction between the positively charged protein moieties and the galacturonic acid residues. The removal of calcium from the SBP had no effect on the adsorbed layer thickness; hence, multilayer formation due to calcium ion cross-linking was considered unlikely.  相似文献   

20.
Pectin gels were induced by monovalent salts (0.2 M) concurrently with deesterification of high methoxy pectin using a salt-independent orange pectin methylesterase (PME). Constant pH was maintained during deesterification and gelation. If salt or PME was absent, the pectin did not form a gel. The gel strength was influenced by both pH and species of monovalent cation. At pH 5.0, the pectin gel induced by KCl was significantly stronger than the NaCl-induced gel. In contrast, a much stronger gel was produced in the presence of NaCl as compared to KCl at pH 7.0. LiCl did not induce pectin gelation at either pH. Molecular weights of pectins increased from 1.38 x 10(5) to 2.26 x 10(5) during NaCl-induced gelation at pH 7. One proposal to explain these pectin molecular weight changes is a hypothetical PME transacylation mechanism. However, these pectin molecular weight changes can also be explained by metastable aggregation of the enzymatically deesterified low methoxy pectin. We postulate that gelation was induced by a slow deesterification of pectin under conditions that would normally salt out (precipitate) low methoxy pectin in the absence of PME.  相似文献   

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