Phospholipid/fatty acid-induced secondary structural change in beta-lactoglobulin during heat-induced gelation

Probing protein-membrane interactions using solid supported membranes

Tethered bilayer lipid membranes have been used as a model system to mimic the interactions between the whey protein β-lactoglobulin and a lipid interface. The approach allowed for a detailed study of the lipid-protein interactions, the results being of possible importance in food and cosmetic applications. For such applications, lipid-protein interactions and the interfacial behavior are vital factors in controlling and manipulating process conditions such as emulsion stabilization and gelification. Lipid composition as well as the structural properties of the protein governed their interactions, which were probed by a combination of surface plasmon spectroscopy, neutron reflectivity, and electrochemical impedance spectroscopy. Comparison of results obtained using native and a partially unfolded protein indicated that the protein preferentially forms loosely packed layers at the lipid interface.

On the supposed influence of milk homogenization on the risk of CVD, diabetes and allergy

Commercial milk is homogenized for the purpose of physical stability, thereby reducing fat droplet size and including caseins and some whey proteins at the droplet interface. This seems to result in a better digestibility than untreated milk. Various casein peptides and milk fat globule membrane (MFGM) proteins are reported to present either harmful (e.g. atherogenic) or beneficial bioactivity (e.g. hypotensive, anticarcinogenic and others). Homogenization might enhance either of these effects, but this remains controversial. The effect of homogenization has not been studied regarding the link between early cow's milk consumption and occurrence of type I diabetes in children prone to the disease and no link appears in the general population. Homogenization does not influence milk allergy and intolerance in allergic children and lactose-intolerant or milk-hypersensitive adults. The impact of homogenization, as well as heating and other treatments such as cheesemaking processes, on the health properties of milk and dairy products remains to be fully elucidated.

Liquid chromatographic analysis of milk phospholipids with on-line pre-concentration

Two methods have been developed for the analysis of bovine milk phospholipid (PL) classes by NP-HPLC with evaporative light scattering detection. In the first method, a PVA-Sil guard column was used for the rapid determination of the major milk PL, phosphatidylethathanolamine (PE), phosphatidylcholine (PC) and sphingomyelin (SM). In the second method, the guard column was used to pre-concentrate the PL, which were then transferred on-line onto a PVA-Sil analytical column by the use of column switching valves. This enabled separation of complete milk PL, including phosphatidylinositol (PI), phosphatidylserine (PS) and lysophosphatidylcholine (LPC).

Dynamic light scattering and circular dichroism studies on heat-induced gelation of hard-keratin protein aqueous solutions

Animal hairs consist of aggregates of dead cells filled with keratin protein gel. We succeeded in preparing water-soluble hard-keratin proteins and reconstructing the keratin gels by heat-induced disulfide linkages in vitro. Here, the roles of intermolecular hydrophobic interaction and disulfide bonding between the proteins in the gel were discussed. Water-soluble keratin proteins consisting of mixtures of type I ( approximately 48 kDa) and type II ( approximately 61 kDa) were prepared from wool fibers as S-carboxymethyl alanyl disulfide keratin (CMADK). The gelation was achieved by heating an aqueous solution containing at least 0.8 wt % CMADK at 100 degrees C. CMADK solutions with different urea or N-ethylmaleimide concentrations or pH were exposed to dynamic light scattering (DLS) and circular dichroism (CD). DLS clarified the gelation point of CMADK solutions and provided information on the changes in keratin cluster size. DLS suggested two types of gelation mechanism. One was the regenerated chemical disulfide bonding between keratins from CMAD parts of chains. After the gel formed, this bond became important to maintain the gel structure. The other was the physical assembly due to hydrophobic interaction between alpha-helix parts of keratin chains. This hydrophobic assembly also played an important role during gelation. CD confirmed a conformational change in the keratin protein, resulting heat-induced gelation. CD clarified the relationship between keratin protein conformation and gelation, i.e., a rodlike conformation with many alpha-helix structures was necessary to associate keratin chains and form a gel network.

Fine-stranded and particulate aggregates of heat-denatured whey proteins visualized by atomic force microscopy

beta-Lactoglobulin and whey protein isolate (WPI) were heated in aqueous solutions at pH 2 and 7 at 80 degrees C, spread onto freshly cleaved mica surfaces, and visualized under butanol using atomic force microscopy. Fine-stranded aggregates were formed at pH 2, the diameter of strands being ca. 4 nm for beta-lactoglobulin and 10 nm for WPI. At pH 7, aggregates were composed of ellipsoidal particles, regardless of the concentration of added NaCl. This observation supports the previously proposed two-step aggregation model at neutral pH (Aymard, P.; Gimel, J. C.; Nicolai, T.; Durand, D. J. Chim. Phys. 1996, 93, 987-997), consisting of the formation of primary globular particles and the subsequent aggregation of those primary particles. The AFM provides the first direct evidence for the anisotropic shape of these primary particles. The heights of primary particles increased from ca. 11 to 27 nm with increasing concentrations of added NaCl from 0 to 0.3 M in the case of WPI. The rate of aggregation was also accelerated with increasing NaCl concentrations, which appeared to induce transitions in gel networks from fine-stranded toward particulate networks. The present study provides structural information essential for understanding the diverse physical properties of heat-induced whey protein gels.

Intermolecular forces in bovine serum albumin solutions exhibiting solidlike mechanical behaviors

Mechanical properties of bovine serum albumin (BSA) solutions were analyzed to gain information on intermolecular forces that stabilize the system under normal physiological conditions. BSA solutions showed unexpectedly large zero shear viscosity values under steady shear flows but responded like solids to sinusoidal linear strains: the storage shear moduli were always larger than the loss shear moduli in the frequency range 1-100 rad/s. These results suggest that BSA solutions are so-called colloidal crystals in which colloidal particles are ordered in an array due to strong repulsive forces among particles. However, the pair potential between BSA molecules predicted based on the conventional Derjaguin-Landau-Verwey-Overbeek theory failed to explain these remarkable mechanical properties of BSA solutions. Additional repulsive forces other than electrostatic must be introduced to explain stability of BSA aqueous dispersions.

Analysis of free fatty acids in whey products by solid-phase microextraction

To evaluate the impact of Cheddar cheese starter cultures on the level of free fatty acids in liquid whey, a solid-phase microextraction (SPME) technique was utilized. The determination of response factors relative to an internal standard and the verification of linearity over a wide concentration range allowed for the quantitation of free fatty acids in experimentally produced liquid whey and in a wide range of dry whey ingredients. Results indicated that whey produced with a Lactococcus lactis subsp. lactis starter culture contained the highest level of total free fatty acids with significantly higher levels of lauric, myristic, and palmitic acids. Significant declines in oleic, linoleic, and palmitic acid occurred during storage. Commercial whey ingredients demonstrated a linear increase in free fatty acids with increasing percent protein, except for whey protein isolate, which had the lowest concentration. The use of SPME for the rapid analysis of free fatty acids in whey products should allow for further research to determine the importance of these compounds on the quality and stability of whey products.