Ryoya Niki

Critical behavior of modulus of gel

The critical behavior of the shear modulus of casein gel is studied. The shear modulus of casein gel scales with the conductivity exponent in the immediate vicinity of the sol–gel transition point. The asymptotic behavior of the modulus in the region far above the transition point is governed by a different exponent which is much larger than the conductivity exponent. These results are explainable by the crossover behavior of the percolation process. This study shows that the gelation of the casein micelle solution is a realization of the percolation process.

Micellar structure of β-casein observed by small-angle X-ray scattering

Abstract The small-angle X-ray scattering was observed from β-casein micelles in 0.2 M phosphate buffer (pH 6.7) with varying temperatures. An oblate ellipsoid of a rigid core with a thin soft layer was proposed as a probable model of the β-casein micellar structure, according to the results of the model optimization with simple triaxial bodies. Here the axial ratio was found to decrease and the micelle to become spherical when the polymerization proceeds with temperature. The consistency of the present model was examined with the results of hydrodynamic measurements published previously.

Percolation Theory and Elastic Modulus of Gel

Dynamic shear modulus of casein gel was measured slightly above the gelation concentration as functions of frequency and concentration. The results show that the shear modulus G is expressed by G = C · e t , where e is the reduced concentration in reference to the gelation concentration. The exponent t is found to be constant at lower frequency region and is close to the value expected from the percolation theory.

Rheological study on the rennet-induced gelation of casein micelles with different sizes

Abstract Casein forms a gel in the presence of rennet. Kappa-casein existing on the surface of casein micelles liberates glycomacropeptide (GMP), and para -casein micelles aggregate to form a three-dimensional network. The effects of micellar size on the gelation were examined in the present work. Casein micelles with different sizes were prepared from skimmed milk by differential centrifugation. Micelle sizes were determined by turbidity measurements. The amount of GMP was estimated by high-performance liquid chromatography (HPLC). Complex rigidity G ∗ = G ′ + iG ″ was observed as a function of time after rennet was added to casein micelle solutions. Small micelles liberated more GMP per unit weight than large micelles. The growth of the storage modulus was well approximated by first order kinetics. The gel time for small micelles was shorter than that for large micelles, and was a decreasing function of temperature. The gelation rate constant K g was proportional to the concentration of rennet, and larger for small micelles. It is increased with increasing temperature. The final value of G ′ was proportional to the square of concentration, and showed a maximum at 27°C as a function of temperature. It is suggested that whilst hydrogen bonding is not negligible in the gelation of casein micelles, this event is governed mainly by hydrophobic interaction. Smaller micelles from a more solid-like gel than larger micelles.

A sedimentation equilibrium study of the temperature-dependent association of bovine β-casein

The temperature-dependent association of beta-casein was studied by the sedimentation equilibrium method. The weight-average molecular weight of the protein was determined in 0.2 M sodium phosphate buffer (pH 6.7) at 20, 15, 10 and 2 degrees C, and was found to be dependent markedly on both temperature and protein concentration. The data were well fitted by a monomer-n-mer association scheme, and the values of n, the second viral coefficient, and the free energy change for the association (association constant) were evaluated. The results show that temperature not only shifts the equilibrium but alters the polymer size: the values of n are 49, 22, and 12 at 20, 15, and 10 degrees C, respectively; the free energy change per monomer becomes more negative with increasing temperature, indicating that the attraction between the monomers in a polymer is stronger at higher temperature. This effect of temperature is in contrast to that of ionic strength which affects mainly the equilibrium. The enthalpy change, entropy change, and heat capacity change for the association were also estimated and were indicative of the formation of a considerable amount of hydrophobic bonds upon association.

Interaction of sodium dodecyl sulfate and of non-ionic detergents with S-carboxyamidomethyl-κ-casein

Sodium dodecyl sulfate binds to S-carboxyamidomethyl-k-casein in a highly cooperative manner at a concentration near the critical micelle concentration, showing a strong dependence on ionic strength. The maximum number of sodium dodecyl sulfate molecules bound is attained above the critical micelle concentration, and is very close to the micelle aggregation number in the absence of protein. The binding sites on the protein for sodium dodecyl sulfate are localized mainly on para-k-casein part, which is a hydrophobic fragment of k-casein produced by rennin attack. The mode of the action of sodium dodecyl sulfate on S-carboxyamidomethyl-k-casein resembles that of several integral membrane proteins, rather than of water soluble proteins. On considering possible situations, it is suggested that the unusual interaction of S-carboxyamidomethyl-k-casein with sodium dodecyl sulfate is responsible for an anomalous migration of reduced k-casein observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis. Further, the suggestion was made by the binding studies of sodium dodecyl sulfate and non-ionic detergents that the sites which were involved in self-association of S-carboxyamidomethyl-k-casein participated in the binding sites of detergents.

Application of the theory of gelation to enzymatic clotting process of casein micelle solution

Dynamic mechanical measurements were carried out to clarify the mechanism of the clotting process of casein micelle solution. It was found that the clotting process of casein micelle solution was formally expressed by a first order reaction. The enzyme concentration dependence of the latent time tL and the rate constant of gelation Kg were found to be tL alpha [E]-1.1, and Kg alpha [E]1.0, respectively. These results were intrepreted on the basis of the theory of gelation. The results obtained here were found to agree with the theoretical conjectures. The casein micelle concentration dependence of the complex rigidity was also studied.

Effect of dietary protein level on intestinal aminopeptidase activity and mRNA level in rats

Abstract To investigate the mechanism of adaptive response of intestinal aminopeptidase on the dietary protein level, we determined the aminopeptidase activity and mRNA level in the intestinal mucosa of rats with different dietary protein levels. In the first experiment, the specific activity of aminopeptidase in the homogenate of intestinal mucosa from rats fed a 60% casein diet for 7 days was significantly higher than that from rats fed a 20% casein diet. A larger difference between the two groups was observed in the ileum than in the jejunum. In contrast to aminopeptidase, the specific activity of sucrase in the ileum of rats fed a 60% casein diet was significantly lower than that of rats fed a 20% casein diet. The activities of sucrase in the jejunum and alkaline phosphatase in the jejunum and ileum did not change between the two groups. Northern blot analysis of intestinal RNA using 1.1 kb aminopeptidase N cDNA as a probe did not show a significant difference in the level of aminopeptidase N mRNA between two dietary groups. In the next experiment, the aminopeptidase activity and mRNA level in the ileal mucosa were determined in rats fed 0, 10, 20, 40, and 60% casein diets for 7 days. While the specific activity of aminopeptidase increased in response to the dietary protein level, sucrase and alkaline phosphatase activities did not change. However, we did not observe any change in the level of ileal aminopeptidase N mRNA in all groups, and there was no significant correlation between the specific activity and the mRNA abundance. These data indicate that the catalytic activity of intestinal aminopeptidase responds to the dietary protein level, but the level of mRNA is preserved even under the protein malnourished condition. The data also suggest that translational and/or posttranslational regulations for the expression of intestinal aminopeptidase are necessary for the rapid response to changes in intraluminal nutrients.

Size effects of casein micelles on rennet gels in the presence of β-lactoglobulin

The influence of casein micelle size on zeta potential and rennet gelation properties is studied. Heating these micellar suspensions in the presence of β-lactoglobulin is also found to influence rennet gel properties.

Expression of apolipoprotein A-I mRNA in liver and intestine of cecectomized rats fed beet fiber

Sham-operated and cecectomized rats were fed a cholesterol-free diet with no added fiber (fiber-free) for 10 days, followed by the diet of 15% beet fiber for 10 days. The plasma cholesterol concentrations in rats fed the fiber-free diet were not significantly different between sham-operated and cecectomized groups. Plasma cholesterol concentrations in sham-operated rats were decreased by feeding the beet fiber diet, whereas those in cecectomized rats did not change. Final plasma total cholesterol concentrations in cecectomized rats were significantly higher than in sham-operated rats. This difference was due mainly to lower HDL cholesterol concentrations. The cecectomized rats also tended to have higher apolipoprotein A-I concentrations in plasma. Northern blot analysis revealed that the relative concentrations of ileal apolipoprotein A-I mRNA were the same in the two groups, while hepatic apolipoprotein A-I mRNA levels were significantly higher in cecectomized rats than in sham-operated rats. These data demonstrate that the cecectomy abolished the hepatic apolipoprotein A-I mRNA-lowering effect of dietary beet fiber, and it is suggested that the cecum plays an important role in the regulation of hepatic apolipoprotein A-I expression which seems to be responsible for the hypocholesterolemic effect of dietary beet fiber.