Heat-Induced Gelation of β-Lactoglobulin/α-Lactalbumin Blends at pH 3 and pH 7

Co-aggregation between whey proteins and carotenoids from yellow mombin (Spondias mombin): Impact of carotenoids' self-aggregation

The interaction between whey proteins and carotenoid is reported to improve carotenoid solubility and stability, however, the strong trend of carotenoids to aggregate when in polar systems is often neglected in papers addressing their molecular interaction. Therefore, this study focused on characterizing the aggregative behavior of the carotenoids from yellow mombin (Spondias mombin) and to understand how these carotenoids behave when added to aqueous dispersions of whey proteins. Carotenoids-rich extract, containing mainly β-cryptoxanthin and lutein, was obtained from freeze-dried yellow mombin pulp and its aggregative behavior in ethanol/water medium was studied. By increasing the medium polarity, carotenoids trend to form J-aggregation, causing a drop in the color intensity of the solution. When added to whey protein aqueous dispersions, rather than a protein-carotenoid bimolecular interaction, the formation of co-aggregates between carotenoids and whey proteins was evidenced by preparative size exclusion chromatography. These results may contribute to the developing functional food products.

Ionic strength and buffering capacity of emulsifying salts determine denaturation and gelation temperatures of whey proteins

Ionic conditions affect the denaturation and gelling of whey proteins, affecting the physical properties of foods in which proteins are used as ingredients. We comprehensively investigated the effect of the presence of commonly used emulsifying salts on the denaturation and gelling properties of concentrated solutions of β-lactoglobulin (β-LG) and whey protein isolate (WPI). The denaturation temperature in water was 73.5°C [coefficient of variation (CV) 0.49%], 71.8°C (CV 0.38%), and 69.9°C (CV 0.41%) for β-LG (14% wt/wt), β-LG (30% wt/wt), and WPI (30% wt/wt), respectively. Increasing the concentration of salts, except for sodium hexametaphosphate, resulted in a linear increase in the denaturation temperature of WPI (kosmotropic behavior) and an acceleration in its gelling rate. Sodium chloride and tartrate salts exhibited the strongest effect in protecting WPI against thermal denaturation. Despite the constant initial pH of all solutions, salts having buffering capacity (e.g., phosphate and citrate salts) prevented a decrease in pH as the temperature increased above 70°C, resulting in a decline in denaturation temperature at low salt concentrations (≤0.2 mol/g). When pH was kept constant at denaturation temperature, all salts except sodium hexametaphosphate, which exhibited chaotropic behavior, exhibited similar effects on denaturation temperature. At low salt concentration, gelation was the controlling step, occurring up to 10°C above denaturation temperature. At high salt concentration (>3% wt/wt), thermal denaturation was the controlling step, with gelation occurring immediately after. These results indicate that the ionic and buffering properties of salts added to milk will determine the native versus denatured state and gelation of whey proteins in systems subjected to high temperature, short time processing (72°C for 15 s).

Gelation of food protein-protein mixtures

Gelation of proteins is one of the principal means to give desirable texture to food products. Gelation of individual proteins in aqueous solution has been investigated intensively in the past, but in most food products the system contains mixtures of different types of proteins. Therefore one needs to consider interaction between different proteins both before and during gelation. Most food proteins can be classified as globular proteins, but casein and gelatin are also important food proteins. In this review the focus is on gelation induced by heating or cooling, which is the most commonly used method. After briefly discussing general features of protein aggregation and gelation, the literature on gelation of mixtures of different types of globular proteins is reviewed as well as that of mixtures of globular proteins with gelatin or with casein. The effect on the gel stiffness and the microstructure of the gelled mixtures will be discussed in terms of different scenarios that can be envisaged: independent aggregation and gelation, co-aggregation and phase separation.

Alkali cold gelation of whey proteins. Part I: sol-gel-sol(-gel) transitions

The cold gelation of preheated whey protein isolate (WPI) solutions at alkaline conditions (pH>10) has been studied to better understand the effect of NaOH in the formation and destruction of whey protein aggregates and gels. Oscillatory rheology has been used to follow the gelation process, resulting in novel and different gelation profiles with the gelation pH. At low alkaline pH, typical sol-gel transitions are observed, as in many other biopolymers. At pH>11.5, the system gels quickly, after approximately 300 s, followed by a slow degelation step that transforms the gel to a viscous solution. Finally, there is a second gelation step. This results in a surprising sol-gel-sol-gel transition in time at constant gelation conditions. At very high pH (>12.5), the degelation step is very severe, and the second gelation step is not observed, resulting in a sol-gel-sol transition. The first quick gelation step is related to the quick swelling of the WPI aggregates in alkali, as observed from light scattering, which enables the formation of new noncovalent interactions to form a gel network. These interactions are argued to be destroyed in the subsequent degelation step. Disulfide cross-linking is observed only in the second gelation step, not in the first step.

Fibril assemblies in aqueous whey protein mixtures

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 beta-lactoglobulin (beta-lg) was present, suggesting that beta-lg was dominant in the gelation. Pure alpha-lactalbumin and pure bovine serum albumin did not form fibrils, nor did they gel upon heating at pH 2 and 80 degrees C for up to 10 h. They did however induce a decrease in the beta-lg concentration needed for gel formation upon heating at pH 2. Our results suggest that beta-lg is the only fibril forming protein at the conditions used and that no mixed fibrils are formed.

Rheology of protein gels synthesized through a combined enzymatic and heat treatment method

Whey protein gels prepared under acidic conditions (pH<4.6) remain largely unutilized because of their weak and brittle nature in contrast to the favorable elastic gels produced at neutral or basic conditions. However, such usage is important, as low pH food products are desirable due to their shelf stability and less stringent sterilization processes. In this study, we use a two-step process involving enzyme followed by heat treatment to produce whey protein gels at low pH (4.0). Dynamic rheological measurements reveal that the gel elastic modulus and yield stress increase substantially when heat treatment is supplemented with enzyme treatment. Both the elastic modulus and yield stress increase with increasing enzyme concentration or treatment time. In contrast, the dynamic yield strain decreases with enzyme concentration but increases with time of enzyme treatment. These results are explained in terms of the enzyme treatment time affecting the diffusion of the enzyme within the gel. This in turn leads to two types of gel microstructure at short and long enzyme treatment times, with the extent of enzyme diffusion modulating the structure at intermediate times.

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.

Mechanical characterization of network formation during heat-induced gelation of whey protein dispersions

The formation of gel network structures during isothermal heating of whey protein aqueous dispersions was probed by mechanical spectroscopy. It was anticipated that the pathway of the sol-to-gel transition of whey protein dispersions is quite different from that of ordinary cross-linking polymers (e.g., percolation-type transition), since aqueous solutions of native whey proteins have been shown to be highly structured even before gelation, in our previous study. At 20 degrees C, aqueous dispersions of beta-lactoglobulin, the major whey protein, and those of whey protein isolate (WPI), a mixture of whey proteins, exhibited solid-like mechanical spectra, i.e., the predominant storage modulus G' over the loss modulus G", in a certain range of the frequency omega (1-100 rad/s), regardless of the presence or absence of added NaCl. The existence of the added salt was, however, a critical factor for determining transitions in mechanical spectra during gelation at 70 degrees C. beta-Lactoglobulin dispersions in 0.1 mol/dm(3) NaCl maintained the solid-like nature during the entire gelation process and, after passing through the gelation point, satisfied parallel power laws (G' approximately G" approximately omega(n)) that have been proposed for a critical gel (i.e., the gel at the gelation point) that possesses a self-similar or fractal network structure. In contrast, beta-lactoglobulin dispersions without added salt exhibited a transition from solid-like [G'(omega) > G"(omega)] to liquid-like [G'(omega) < G"(omega)] mechanical spectra before gelation, but no parallel power law behavior was recognized at the gelation point. During extended heating time (aging), beta-lactoglobulin gels with 0.1 mol/dm(3) NaCl showed deviations from the parallel power laws, while spectra of gels without added NaCl approached the parallel power laws, suggesting that post-gelation reactions also significantly affect gel network structures. A percolation-type sol-to-gel transition was found only for WPI dispersions without added salt.