The effect of stirring and seeding on the formation of fibrils in whey protein isolate (WPI) solutions was studied. More fibrils of a similar length are formed when WPI is stirred during heating at ...pH 2 and 80 °C compared to samples that were heated at rest. Addition of seeds did not show an additional effect compared to samples that were stirred. We propose a model for fibril formation, including an activation, nucleation, growth, and termination step. The activation and nucleation steps are the rate-determining steps. Fibril growth is relatively fast but terminates after prolonged heating. Two processes that possibly induce termination of fibril growth are hydrolysis of nonassembled monomers and inactivation of the growth ends of the fibrils. Stirring may break up immature fibrils, thus producing more active fibrils. Stirring also seems to accelerate the kinetics of fibril formation, resulting in an increase of the number of fibrils formed. Keywords: Heat-induced aggregation; fibrils; whey proteins; stirring; seeding
Fibril formation of individual pure whey proteins and whey protein isolate (WPI) was studied. The heat-induced conversion of WPI monomers into fibrils at pH 2 and low ionic strength increased with ...heating time and protein concentration. Previous studies, using a precipitation method, size-exclusion method, or proton NMR spectroscopy, reported a wide range of values for the conversion. An alternative method was developed, namely centrifugal filtration, giving a consistent picture of the conversion. The present results help to explain the disparities reported in literature. No fibrils formed upon heating pure
α-lactalbumin or pure BSA at pH 2, whereas fibrils formed in pure
β-lactoglobulin (
β-lg) and WPI solutions. Experiments indicate that
β-lg was the only whey protein involved in fibril formation. In all whey protein samples, hydrolysis occurred during heating at pH 2, as determined by HPLC and SDS-PAGE. When WPI fibrils formed at pH 2 were stored at pH 7 or 10, disulphide bonds were formed in the samples.
Fibril Assemblies in Aqueous Whey Protein Mixtures Bolder, Suzanne G; Hendrickx, Hanneke; Sagis, Leonard M. C ...
Journal of agricultural and food chemistry,
06/2006, Letnik:
54, Številka:
12
Journal Article
Recenzirano
Fibril formation in mixtures of whey proteins upon heating at pH 2 was investigated. Fibrils were found to coexist with other structures, such as spherulites. These spherulites consist of radially ...oriented fibrils. At total protein concentrations above 6 wt %, transparent gels were formed. Changing the ratio between the various whey proteins did not affect this gelation concentration as long as β-lactoglobulin (β-lg) was present, suggesting that β-lg was dominant in the gelation. Pure α-lactalbumin and pure bovine serum albumin did not form fibrils, nor did they gel upon heating at pH 2 and 80 °C for up to 10 h. They did however induce a decrease in the β-lg concentration needed for gel formation upon heating at pH 2. Our results suggest that β-lg is the only fibril forming protein at the conditions used and that no mixed fibrils are formed. Keywords: Heat-induced aggregation; heat-set gelation; fibrils; whey proteins; β-lactoglobulin
The conversion of protein monomers into fibrils can be determined using the centrifugal filtration method. The results of this method were used to calibrate steady-shear birefringence and Thioflavin ...T fluorescence measurements. For both measurements, a linear correlation with the fibril concentration was extracted, resulting in two fast assays to determine the fibril concentration quantitatively. From birefringence measurements and the conversion determined using the centrifugal filtration method, we were able to calculate more precise values for the birefringence per unit length of the fibrils (M) and the flexibility of the fibrils (β).
In this paper we describe the rheological behaviour of Ca
-induced cold-set gels of whey protein mixtures. Cold- set gels are important applications for products with a low thermal stability. In ...previous work 1, we determined the state diagram for whey protein mixtures that were heated for 10 h at pH 2 at 80°C. Under these conditions, the major whey protein, β-lactoglobulin (β-lg), forms fibrils. When whey protein mixtures are heated at protein concentrations in the liquid solution regime of the state diagram, cold-set gels can be formed by adding Ca
ions at pH
. We studied the rheological behaviour of cold-set gels for various sample compositions for whey protein mixtures. When keeping the total whey protein concentration constant, the elastic modulus,
, for the cold-set gels decreased for increasing a-lactalbumin and bovine serum albumin ratios, because less material (β- lg fibrils) was available to form a gel network. In the cold-set gels the interactions between the β-lg fibrils induced by the calcium ions are dominant. The β-lg fibrils are forming the cold-set gel network and therefore determine the gel strength. a-Lactalbumin and bovine serum albumin are not incorporated in the stress-bearing structure of the gels.
In this paper, we have investigated the mesoscopic structure and rheological properties of heat-set β-lactoglobulin gels at low pH and low ionic strength. We have determined the scaling of the ...elastic or storage modulus of the gel with protein concentration. Four batches with a pH of 2 and an ionic strength of 13
mM showed a wide range of scaling behavior. In two of the batches, two concentration regimes with distinctive exponents could be detected. We constructed the phase diagram for this system as a function of pH and ionic strength, and were able to show that the wide spread in scaling behavior between the batches is the result of differences in the mescopic structure of the gels. These differences in mesoscopic structure were the result of small variations in pH and ionic strength between the batches. Our experiments show that when characterizing gel properties of β-lactoglobulin solutions at very acidic conditions and low ionic strength, extreme care has to be taken to stabilize the pH of these systems. Small variations in pH have a drastic effect on the structure of the gels, and on the macroscopic viscoelastic properties.
Abstract
In this paper we describe the rheological behaviour of Ca
2+
-induced cold-set gels of whey protein mixtures. Cold- set gels are important applications for products with a low thermal ...stability. In previous work 1, we determined the state diagram for whey protein mixtures that were heated for 10 h at pH 2 at 80°C. Under these conditions, the major whey protein, β-lactoglobulin (β-lg), forms fibrils. When whey protein mixtures are heated at protein concentrations in the liquid solution regime of the state diagram, cold-set gels can be formed by adding Ca
2+
ions at pH
7
. We studied the rheological behaviour of cold-set gels for various sample compositions for whey protein mixtures. When keeping the total whey protein concentration constant, the elastic modulus,
G’
, for the cold-set gels decreased for increasing a-lactalbumin and bovine serum albumin ratios, because less material (β- lg fibrils) was available to form a gel network. In the cold-set gels the interactions between the β-lg fibrils induced by the calcium ions are dominant. The β-lg fibrils are forming the cold-set gel network and therefore determine the gel strength. a-Lactalbumin and bovine serum albumin are not incorporated in the stress-bearing structure of the gels.
The objective of this thesis was to study fibril assembly in mixtures of whey proteins. The effect of the composition of the protein mixture on the structures and the resulting phase behaviour was ...investigated. The current work has shown that β-lactoglobulin is responsible for the fibril assembly in whey protein mixtures upon heating at pH 2 and low ionic strength. To determine the efficiency of fibril formation, a method was developed to measure the conversion of whey protein monomers into fibrils. This method was used to study the effect of heating time, protein concentration, and stirring and seeding on the formation of fibrils in whey protein isolate solutions. The results have provided a better understanding of the mechanism of fibril formation and a model for fibril formation is proposed, including an activation, nucleation, growth, and termination step. Based on the method to measure the conversion two fast assays were found to determine the fibril concentration quantitatively. These two fast assays can be used for studying the kinetics of protein assembly in general, once they have been calibrated for a certain protein.
