Influence of pectin characteristics on complexation with ß-lactoglobulin

Pectin and proteins are both common food constituents. The type of pectin that forms complexes with protein is known to be of great influence on the structure and stability of liquid foods. Therefore, the aim of this thesis is to investigate the influence of the overall charge and local charge density of pectin on the formation of soluble complexes with β-lactoglobulin (β-lg). Combination of state diagrams and binding isotherms shows that a high local charge density of pectin is a prerequisite to form soluble complexes with β-lg at higher ionic strength. A high overall charge of pectin results in the close proximity of the GalA blocks. Therefore, β-Lg neighbours bind close together on pectin with a high overall charge, which leads to lateral repulsion and hence, maxima in the binding constant and the pH where insoluble complexes form with increasing ionic strength. The formation of soluble complexes has an enthalpic driving force from electrostatic attraction and an entropic driving force from the release of small counterions from the electric double layer and water molecules from hydrophobic surroundings. A high local charge density, at low ionic strength results in complex formation dominated by an enthalpic driving force. A low local charge density gives a more even distribution between enthalpic and entropic contributions. An increase in ionic strength decreases the enthalpic contribution, with a relative increase in the entropic contribution, supporting the idea of water release from hydrophobic surroundings. Adsorption from β-lg–pectin mixtures to a hydrophobic surface leads to low adsorption rates due to a low concentration of free protein. Sequential adsorption of β-lg and pectin shows that low overall charge pectin protrudes more into the solution than high overall charge pectin, resulting in a more negative ζ-potential for low overall charge pectin. After sequential adsorption, β-lg is most stable against wash-out with a terminal pectin layer. <br/

Ketens groene grondstoffen

De centrale kennisvraag van deze studie is hoe het Nederlandse overheidsbeleid beter kan bijdragen aan de totstandkoming van de biobased economy (BBE) in Nederland. Om deze vraag te beantwoorden is het noodzakelijk om ontwikkelingen te schetsen die nodig zijn om de productieketens in de BBE tot volledige commerciële ontwikkeling te brengen, en hoe nieuw of bestaand beleid daarop in kan spelen. De uiteindelijke doelstelling van het project is om een onderbouwing en verdere uitwerking te ontwikkelen voor een strategie die tot doel heeft de productie van grondstoffen voor de BBE in Nederland te stimuleren. Dit project is geen nieuwe verkenning van welke biomassa stromen er in Nederland beschikbaar zijn of komen, of welke half- of eindproducten in de BBE geproduceerd kunnen worden; hiertoe zijn bestaande studies beschikbaar die het vertrekpunt vormen van dit project. Deze studie heeft als doelstelling om kansrijke grondstof-productketens voor de BBE in Nederland te definiëren, op basis van bestaande studies en een aantal van deze ketens verder in detail uit te werken

Rheology and structure of ovalbumin gels at low pH and low ionic strength

The objective of this study was to relate the rheological behavior of ovalbumin gels at low pH and low ionic strength to their mesoscopic structure, using rheological measurements and Transmission Electron Microscopy (TEM). Varying pH, ionic strength and protein concentration, we obtained transparent solutions, transparent gels, opaque gels, or turbid gels, upon heating and subsequent cooling of the ovalbumin solutions. At equal pH and increasing salt concentration we found an increase in turbidity, suggesting that the structure of the aggregates changed from linear or branched to more clustered aggregates. The gel strength increased with increasing salt concentration. A similar trend was observed at equal ionic strength and increasing pH. TEM micrographs of ovalbumin aggregates at pH 2 and 15 mM NaCl show that the ovalbumin aggregates are linear. Increasing the pH from 2 to 3.5 results in structures that are still linear, but with a higher degree of clustering. No random aggregates were observed. TEM micrographs of the gel phase at pH 3.5 and 30 mM NaCl (turbid) show that the structure consists of linear aggregates organized in large clusters of approximately 230–350 nm. At pH 2 and 30 mM NaCl (transparent) the gel consists of single strands with a diameter of about 3.3–3.9 nm, i.e. one or at most two monomers thick

Binding of ß-lactolobulin to pectins varying in their overall and local charge density

The formation of complexes between proteins and polysaccharides is of great importance for many food systems like foams, emulsions, acidified milk drinks, and so on. The complex formation between ß-lactoglobulin (ß-lg) and pectins with a well-defined physicochemical fine structure has been studied to elucidate the influence of overall charge and local charge density of pectin on the complex formation. Binding isotherms of ß-lg to pectin are constructed using fluorescence anisotropy, which is shown to be an excellent technique for this purpose, as it is fast and requires low sample volumes. From the binding isotherms the maximal adsorbed amount, binding constant (kobs) and the cooperativity of binding are obtained at different ionic strengths. The Hill model is used to fit the binding isotherms and is shown to be preferable over a Langmuir fit. At pH 4.25, kobs shows a maximum at an ionic strength of 10 mM when using a low methyl esterified pectin (LMP) due to the balance of attractive and repulsive electrostatic forces between ß-lg and pectin and ß-lg neighbors. For two high methyl esterified pectins, one with a blockwise distribution of methyl esters (HMPB) and one with a random distribution (HMPR), this ionic strength maximum is absent and kobs decreases with increasing ionic strength. kobs is found to be largest for LMP and HMPB and considerably lower for HMPR. A positive cooperativity is observed for both LMP (above an ionic strength of 45 mM) and HMPR (above an ionic strength of 15 mM) but not for HMPB. Positive cooperativity is thought to be caused by a rearrangement of the pectin helix structure caused by binding of ß-lg, thus creating new or binding sites with a higher affinity. To attain strong binding of ß-lg to pectin it is preferable to use a pectin with a blockwise distribution of methyl esters. When complex formation takes place in high ionic strength media an LMP gives the best results, while at low ionic strength a high methyl esterified pectin with blockwise distribution may give better results, due to reduced electrostatic repulsion between both pectin and ß-lg and ß-lg neighbor

Rheology and structure of ovalbumin gels at low pH and low ionic strength

The objective of this study was to relate the rheological behavior of ovalbumin gels at low pH and low ionic strength to their mesoscopic structure, using rheological measurements and Transmission Electron Microscopy (TEM). Varying pH, ionic strength and protein concentration, we obtained transparent solutions, transparent gels, opaque gels, or turbid gels, upon heating and subsequent cooling of the ovalbumin solutions. At equal pH and increasing salt concentration we found an increase in turbidity, suggesting that the structure of the aggregates changed from linear or branched to more clustered aggregates. The gel strength increased with increasing salt concentration. A similar trend was observed at equal ionic strength and increasing pH. TEM micrographs of ovalbumin aggregates at pH 2 and 15 mM NaCl show that the ovalbumin aggregates are linear. Increasing the pH from 2 to 3.5 results in structures that are still linear, but with a higher degree of clustering. No random aggregates were observed. TEM micrographs of the gel phase at pH 3.5 and 30 mM NaCl (turbid) show that the structure consists of linear aggregates organized in large clusters of approximately 230–350 nm. At pH 2 and 30 mM NaCl (transparent) the gel consists of single strands with a diameter of about 3.3–3.9 nm, i.e. one or at most two monomers thick

Overall Charge and Local Charge Density of Pectin Determines the Enthalpic and Entropic Contributions to Complexation with ß-Lactoglobulin

The complex formation between ß-lactoglobulin and pectins of varying overall charge and local charge density were investigated. Isothermal titration calorimetry experiments were carried out to determine the enthalpic contribution to the complex formation at pH 4.25 and various ionic strengths. Complex formation was found to be an exothermic process for all conditions. Combination with previously published binding constants by Sperber et al. (Sperber, B. L. H. M.; Cohen Stuart, M. A.; Schols, H. A.; Voragen, A. G. J.; Norde, W. Biomacromolecules 2009, 10, 3246-3252) allows for the determination of the changes in the Gibbs energy and the change in entropy of the system upon complex formation between ß-lactoglobulin and pectin. The local charge density of pectin is found to determine the balance between enthalpic and entropic contributions. For a high local charge density pectin, the main contribution to the Gibbs energy is of an enthalpic nature, supported by a favorable entropy effect due to the release of small counterions. A pectin with a low local charge density has a more even distribution of the enthalpic and entropic part to the change of the Gibbs energy. The enthalpic part is reduced due to the lower charge density, while the relative increase of the entropic contribution is thought to be caused by a change in the location of the binding place for pectin on the ß-lactoglobulin molecule. The association of the hydrophobic methyl esters on pectin with an exposed hydrophobic region on ß-lg results in the release of water molecules from the hydrophobic region and surrounding the methyl esters of the pectin molecule. An increase in the ionic strength decreases the enthalpic contribution due to the shielding of electrostatic attraction in favor of the entropic contribution, supporting the idea that the release of water molecules from hydrophobic areas plays a part in the complex formation

Influence of the overall charge and local charge density of pectin on the complex formation between pectin and beta-lactoglobulin

The complex formation between ß-lactoglobulin (ß-lg) and pectin is studied using pectins with different physicochemical characteristics. Pectin allows for the control of both the overall charge by degree of methyl-esterification as well as local charge density by the degree of blockiness. Varying local charge density, at equal overall charge is a parameter that is not available for synthetic polymers and is of key importance in the complex formation between oppositely charged (bio)polymers. LMP is a pectin with a high overall charge and high local charge density; HMPB and HMPR are pectins with a low overall charge, but a high and low local charge density, respectively. Dynamic light scattering (DLS) titrations identified pHc, the pH where soluble complexes of ß-lg and pectin are formed and pH, the pH of phase separation, both as a function of ionic strength. pHc decreased with increasing ionic strength for all pectins and was used in a theoretical model that showed local charge density of the pectin to control the onset of complex formation. pH passed through a maximum with increasing ionic strength for LMP because of shielding of repulsive interactions between ß-lg molecules bound to LMP, while attractive interactions were repressed at higher ionic strength. Potentiometric titrations of homo-molecular solutions and mixtures of ß-lg and pectin showed charge regulation in ß-lg¿pectin complexes. Around pH 5.5¿5.0 the pKas of ß-lg ionic groups are increased to induce positive charge on the ß-lg molecule; around pH 4.5¿3.5 the pKa values of the pectin ionic groups are lowered to retain negative charge on the pectin. Since pectins with high local charge density form complexes with ß-lg at higher ionic strength than pectins with low local charge density, pectin with a high local charge density is preferred in food systems where complex formation between protein and pectin is desired