Whey Protein/Arabic Gum Gels Formed by Chemical or Physical Gelation Process

Whey proteins (WP) gelation process with addition of Arabic gum (AG) was studied. Two different driving processes were employed to induce gelation: (1) heating of 12% whey protein isolate (WPI) solutions (w/w) or (2) acidification of previous thermal denatured WPI solutions (5% w/w) with glucono-δ-lactone (GDL). Protein concentrations were different because they were minimal to form gel in these two processes, but denaturation conditions were the same (90 °C/30 min). Water-holding capacity and mechanical properties of the gels were evaluated. The BST equation was used to evaluate the nonlinear part of the stress–strain data. Cold-set gels were weaker than heat-set gels at the pH range near the isoelectric point (pI) of the main whey proteins, but heated gels were more deformable (did not exhibit rupture point) and showed greater elasticity modulus. However, gels formed by heating far from the pI (pH 6.7 or 3.5) showed more fragile structure, indicating that, in these mixed gels, there are prevailing biopolymers interactions. Cold-set and heat-set gels at pH near or below the WP pI showed strain-weakening behavior, but heated gels at neutral pH showed strong strain-hardening behavior. Such results suggest that differences in stress–strain curve at the nonlinear part of the data could be correlated to structure particularities obtained from different gelation processes.     

Effect of Soy Protein Subunit Composition on the Rheological Properties of Soymilk during Acidification

The effect of soy protein subunit composition on the acid-induced aggregation of soymilk was investigated by preparing soymilk from different soybean lines lacking specific glycinin and β-conglycinin subunits. Acid gelation was induced by glucono-δ-lactone (GDL) and analysis was done using diffusing wave spectroscopy and rheology. Aggregation occurred near pH 5.8 and the increase in radius corresponded to an increase in the elastic modulus measured by small deformation rheology. Diffusing wave spectroscopy was also employed to follow acid gelation, and data indicated that particle interactions start to occur at a higher pH than the pH of onset of gelation (corresponding to the start of the rapid increase in elastic modulus). The protein subunit composition significantly affected the development of structure during acidification. The onset of aggregation occurred at a higher pH for soymilk samples containing group IIb (the acidic subunit A₃) of glycinin, than for samples prepared from Harovinton (a commercial variety containing all subunits) or from genotypes null in glycinin. The gels made from lines containing group I (A₁, A₂) and group IIb (A₃) of glycinin resulted in stiffer acid gels compared to the lines containing only β-conglycinin. These results confirmed that the ratio of glycinin/β-conglycinin has a significant effect on gel structure, with an increase in glycinin causing an increase in gel stiffness. The type of glycinin subunits also affected the aggregation behavior of soymilk.     

Impact of incorporations of various polysaccharides on rheological and microstructural characteristics of heat-induced quinoa protein isolate gels

This study aimed to investigate the properties of heat-induced gels (85 °C for 30 min) of quinoa protein isolate (QPI) in the presence and absence of various polysaccharides including guar gum (GG), locust bean gum (LBG), and xanthan gum (XG) at pH 7. For this purpose, samples with three gum concentrations (0.05, 0.1, and 0.2 wt%) at a fixed QPI concentration (10 wt%) and a fixed ionic strength (50 mM NaCl) were studied in terms of their gelation behaviour, small and large deformation rheological properties, water holding capabilities, and microstructural characteristics. Rheological measurements revealed that all polysaccharides incorporation could improve gel strength (complex modulus, G*) and breaking stress, accelerate gel formations, and more stiffer gels were obtained at greater polysaccharide concentrations. The XG exhibited the most gel strengthening effect followed by LBG and GG. Incorporation of 0.2 wt% XG led to a 15 folds increase in G* compared to the control. Confocal laser scanning microscopy observation revealed that the polysaccharides also altered gel microstructures, with the gels containing XG showing the most compact gel structures. The findings of this study may provide useful information for the fabrication of novel QPI based food gel products with improved texture.

     

Gelation by phase separation in a whey protein system: in-situ kinetics of aggregation

The aggregation and gelation properties of beta-lactoglobulin (BLG), a globular protein from milk, was studied in aqueous ethanol solutions at room temperature. The phase state diagrams as a function of pH and ethanol concentration showed that a gel structure appeared after a period ranging from 1 min to 1 week, depending on the physico-chemical conditions. The in-situ kinetics of aggregation were followed by several methods in order to obtain a better understanding of the building of aggregates by the addition of ethanol. It was shown that the aggregation kinetics highly depended upon the pH, the process being fastest at pH 7. Viscoelasticity and infrared measurements indicated that alcohol-induced gelation would proceed via a two-step mechanism: small aggregates loosely connected between them were first built up; a real network took place in a second step. The coarse and irregular structures formed in aqueous ethanol gels revealed by confocal laser scanning microscopy could be analysed in terms of a phase separation. This observation was supported by a syneresis phenomenon visible in the final gel state. BLG in water-ethanol solution would undergo either an inhibition of the demixing by gelation or a binary phase separation accompanied by an irreversible gelation transition.     

Whey protein coating bead improves the survival of the probiotic Lactobacillus rhamnosus CRL 1505 to low pH

Aims: To evaluate the efficacy of a novel microencapsulation procedure using whey protein and pectin to improve the survival rate of Lactobacillus rhamnosus CRL 1505 to low pH and bile. Methods and Results: Lactobacillus rhamnosus CRL 1505 was encapsulated by ionotropic gelation using pectin (PE) and pectin–whey protein (PE–WP). Both types of beads (MC_PE/WP and MC_PE–WP/WP) were covered with a layer of whey protein by complex coacervation. The noncapsulated lactobacilli were not sensitive to bile salts but to acid. Both microparticles protected Lact. rhamnosus CRL 1505 at pH 2·0, but only MC_PE/WP was effective at pH 1·2. Conclusions: The combination of ionotropic gelation and complex coacervation techniques is efficient to obtain microcapsules of pectin covered with whey proteins. The MC_PE/WP beads were more stable than the MC_PE–WP/WP beads in simulated gastric conditions, thus offering better protection to Lact. rhamnosus CRL 1505 at low pH. Significance and Impact of the Study: Pectin beads with a whey protein layer (MC_PE/WP) could be used as probiotic carrier in functional foods of low pH (e.g. apple juice), thus protecting Lact. rhamnosus CRL 1505 against the stressful conditions of the gastric tract.     

Structural investigation of beta-lactoglobulin gelation in ethanol/water solutions.

The aggregation and gelation properties of beta-lactoglobulin (BLG), a globular protein from milk, was studied in hydro-ethanolic solutions (50/50% (v/v)) at room temperature. The phase state diagrams as a function of pH and ethanol concentration showed that a gel structure appeared after a period ranging from 1 min to 1 week depending on the physico-chemical conditions. The aggregation kinetics, studied by infrared spectroscopy and dynamical rheological measurements, highly depended upon the pH; the process being the fastest at pH 7. Alcohol-induced aggregation of BLG was characterized by the formation of intermolecular hydrogen bonded beta-sheet structures. Small angle neutron scattering indicated that the aggregates structures in the final gels were similar at pH 7, 8 and 9. Through the data obtained at the molecular and macroscopic levels, it can be concluded that the kinetics of gelation were pH dependent while the spatial arrangements of the aggregates were similar in the final structures. The heterogeneous structures formed in hydro-ethanolic gels could be analysed in terms of a phase separation, the syneresis being the final visible state.     

Use of hydrophilic natural gums in formulation of sustained-release matrix tablets of tramadol hydrochloride

The objective of this work was to develop matrix sustained-release tablets of highly water-soluble tramadol HCl using natural gums (xanthan [X gum] and guar [G gum]) as cost-effective, nontoxic, easily available, and suitable hydrophilic matrix systems compared with the extensively investigated hydrophilic matrices (ie, hydroxypropyl methylcellulose [HPMC]/carboxymethyl cellulose [CMC] with respect to in vitro drug release rate) and hydration rate of the polymers. Matrix tablets of tramadol (dose 100 mg) were produced by direct compression method. Different ratios, of 100∶0, 80∶20, 60∶40, 20∶80, 0∶100 of G gum (or X):HPMC, X gum:G gum, and triple mixture of these polymers (G gum, X gum, HPMC) were applied. After evaluation of physical characteristics of tablets, the dissolution test was, performed in the phosphate buffer media (pH 7.4) up to 8 hours. Tablets with only X had the highest mean dissolution time (MDT), the least dissolution efficiency (DE₈%), and released the drug following a zero-order model via swelling, diffusion, and erosion mechanisms. Guar gum alone could not efficiently control the drug release, while X and all combinations of natural gums with HPMC could retard tramadol HCl release. However, according to the similarity factor (f ₂), pure HPMC and H₈G₂ were the most similar formulations to Topalgic-LP as the reference standard. Published: March 17, 2006     

A New Preparation Method of Coenzyme A with Microorganisms

The structure of aggregates formed by heating dilute BSA solution was analyzed with the fractal concept using light scattering methods. BSA was dissolved in HEPES buffer of pH 7.0 and acetate buffer of pH 5.1 to 0.1% and 0.001% solutions, respectively, and heated at 95°C, varying the heating time t_a. The fractal dimension D_f of the aggregate in the solution was evaluated from static light scattering experiments. The polydispersity exponent τ and the average hydrodynamic radius <R_h> of the aggregates were calculated from dynamic light scattering experiments using master curves obtained by Klein et al. The values of D_f and τ of heat-induced aggregates of BSA at pH 7.0 were about 2.1 and 1.5, respectively, the values of which agreed with those predicted by the reaction-limited cluster–cluster aggregation (RLCCA) model. On the other hand, D_f of heat-induced aggregates at pH 5.1 was about 1.8, which agreed with that predicted by the diffusion-limited cluster–cluster aggregation (DLCCA) model. The dependence of <R_h> for the sample of pH 7.0 on t_a was similar to that of the polystyrene colloids reported previously.     

Enzymatic Cross-Linking of Casein Facilitates Gel Structure Weakening Induced by Overacidification

Casein (α_S1, α_S2, β, κ) is the major protein fraction in milk and, together with heat denatured whey proteins, responsible for gel network formation induced by acidification. Rheological measurements during gelation typically reveal a maximum storage modulus (G') at a pH close to the isoelectric point (pI) of casein (~4.6). With further decreasing pH gel stiffness decreases because of increased electrostatic repulsion, which is referred to as overacidification. In this study we investigated the effect of casein cross-linking with microbial transglutaminase on gel structure weakening induced by acidification to pH below the pI. Although enzymatic cross-linking increased the maximum stiffness (G' _MAX ) of casein gels the reduction of G' during overacidification, expressed as ratio of the plateau value (G' _FINAL ) to G' _MAX , was more pronounced. Almost no soluble protein was detected in the serum of gels from cross-linked casein, whereas considerable amounts of α_S- and κ-casein were released from reference gels below the pI. This suggests that covalent cross-linking of casein retains charged molecules within the gel network and therefore causes a higher reduction of protein-protein interactions because of higher electrostatic repulsion. Furthermore, higher amounts of uncross-linked β-casein, which was the only casein type not found in the serum, resulted in higher G' _FINAL to G' _MAX ratios, underlining the important contribution of β-casein to acid gel formation and prevention of gel structure weakening.     

The causes for barley root growth retardation in the presence of ammonium and glutamate

In barley (Hordeum vulgare L.) seedlings, the rate of root growth, osmotic pressure (Π), hydraulic conductance (L _p), and longitudinal (δl) and transverse (δD/D) extensibility of root cells were measured. The seedlings were grown on Knop solution with nitrate or without nitrate with addition of 5–10 mM NH₄⁺ or 0.5–1.0 mM glutamate. Root growth retardation on the 1st–4th days of exposure to NH₄⁺ was determined by a decrease in δl in the zone of elongation, whereas root thickening was evidently related to an increase in Π. Biphasic dynamics of δl in the presence of NH₄⁺ was imitated by medium acidification near the root surface to pH 3.7, which confirms a conclusion, we have done earlier, about a non-monotonous pH-dependence of longitudinal extensibility. Root growth retardation during the first day of exposure to Glu was also determined by a decrease in the δl, which was, however, accompanied by an increase in the δD/D and L _p. Fast Glu-induced changes of measured root parameters were imitated by root exposure to oryzalin, ionomycin, and inhibitors of the H⁺-pump. It was supposed that a decrease in δl in the presence of NH₄⁺ and Glu was related to cortical microtubule disorganization with the involvement of cytosolic calcium Ca_cyt²⁺. A decrease in the δD/D and L _p in the presence of NH₄⁺ was related to apoplast acidification and a high activity of the plasmalemmal H⁺-pump. An increase in the δD/D and L _p in the presence of Glu indicates the inhibition of the plasmalemmal H⁺-pump. On the 2nd–4th days of exposure to Glu, root growth ceased, as distinct from treatment with NH₄⁺. This complete root growth inhibition by Glu was possibly related to a rapid uptake of Ca²⁺ through Glu-sensitive Ca²⁺-channels, Ca²⁺-dependent inhibition of the plasmalemmal H⁺-pump, and a decrease in mitotic activity.     

Alginate-based solid media for plant tissue culture

Summary A new method for solid medium plant tissue culture based on in situ gelation of alginate is proposed as an alternative to agar-based media. In situ gelation by the use of dispersed CaCO₃ and the slowly hydrolysing acid glucono--lactone (GDL) was the basis for the use of alginate as a gelling agent. Inexpensive alginate-based media can be made in a wide range of pH values. Biological tests of these gels, concerning sterile seed growth and microcalli plating of Brassica napus (cv. Topas) and biomass production of Panax ginseng callus, showed results equal to those achieved with agar-based gels.     

Coral calcification responds to seawater acidification: a working hypothesis towards a physiological mechanism

The decrease in the saturation state of seawater, Ω, following seawater acidification, is believed to be the main factor leading to a decrease in the calcification of marine organisms. To provide a physiological explanation for this phenomenon, the effect of seawater acidification was studied on the calcification and photosynthesis of the scleractinian tropical coral Stylophora pistillata. Coral nubbins were incubated for 8 days at three different pH (7.6, 8.0, and 8.2). To differentiate between the effects of the various components of the carbonate chemistry (pH, CO₃²⁻, HCO₃⁻, CO₂, Ω), tanks were also maintained under similar pH, but with 2-mM HCO₃⁻ added to the seawater. The addition of 2-mM bicarbonate significantly increased the photosynthesis in S. pistillata, suggesting carbon-limited conditions. Conversely, photosynthesis was insensitive to changes in pH and pCO₂. Seawater acidification decreased coral calcification by ca. 0.1-mg CaCO₃ g⁻¹ d⁻¹ for a decrease of 0.1 pH units. This correlation suggested that seawater acidification affected coral calcification by decreasing the availability of the CO₃²⁻ substrate for calcification. However, the decrease in coral calcification could also be attributed either to a decrease in extra- or intracellular pH or to a change in the buffering capacity of the medium, impairing supply of CO₃²⁻ from HCO₃⁻.     

Instability of Pressure-Treated Reconstituted Skim Milk to Acidification

When high-pressure (HP)-treated reconstituted skim milks (200–600 MPa/5–30 min) were acidified with glucono-δ-lactone, the elastic modulus (G′) displayed atypical behaviour with pH, increasing as the pH decreased between 6.0 and 5.3, indicating that a weak gel had formed as soon as the pH of the milk decreased. The formation of a weak gel at pH 6.0 to 5.3 indicates that HP milks are more unstable to acidification than untreated or heated milks; this effect has not been previously reported. Untreated and heated (90 °C/30 min) milks did not show an increase in G′ until the pH was below 4.9 and 5.3, respectively. Frequency sweeps confirmed that the HP-treated milks formed weak gels at pH much higher than where typical acid gelation of milk occurs. Microstructural and particle size analyses indicated that the HP-treated milks started aggregating as soon as the pH declined whereas the heated milks did not aggregate until the pH was below 5.5. Heat treatment of milk either before or after HP treatment completely eliminated the weak gelation as these samples did not form gels until the pH decreased below pH 5.3. It is apparent that the restructured colloidal particles formed when milk is HP treated are unstable to acidification, and it is proposed that the redistributed κ-casein cannot stabilize these particles when the milk is acidified. The role of denatured whey proteins in the weak gelation phenomenon is unclear.     

“Melt-in-the-mouth” gels from mixtures of xanthan and konjac glucomannan under acidic conditions: A rheological and calorimetric study of the mechanism of synergistic gelation

The effect of acidification on a typical commercial xanthan and on pyruvate-free xanthan (PFX), alone and in gelling mixtures with konjac glucomannan (KGM), has been studied by differential scanning calorimetry (DSC) and small-deformation oscillatory measurements of storage modulus (G′) and loss modulus (G″). For both xanthan samples, progressive reduction in pH caused a progressive increase in temperature of the disorder–order transition in DSC, and a progressive reduction in gelation temperature with KGM. This inverse correlation is interpreted as showing that synergistic gelation involves disruption of the xanthan 5-fold helix, probably by attachment of KGM to the cellulosic backbone of the xanthan molecule (as proposed previously by a research group in the Institute of Food Research, Norwich, UK). Higher transition temperature accompanied by lower gelation temperature for PFX in comparison with commercial xanthan at neutral pH is explained in the same way. However, an additional postulate from the Norwich group, that attachment of KGM (or galactomannans) can occur only when the xanthan molecule is disordered, is inconsistent with the observation that gelation of acidified mixtures of KGM with PFX can occur at temperatures more than 60 °C below completion of conformational ordering of the PFX component (as characterised by DSC). Increase in G′ on cooling for mixtures of commercial xanthan with KGM at pH values of 4.5 and 4.25 occurred in two discrete steps, the first following the temperature-course observed for the same mixtures at neutral pH and the second occurring over the lower temperatures observed for mixtures of KGM with PFX at the same values of pH. These two “waves” of gel formation are attributed to interaction of KGM with, respectively, xanthan sequences that had retained a high content of pyruvate substituents, and sequences depleted in pyruvate by acid hydrolysis. At pH values of 4.0 and lower, gelation of mixtures of KGM with commercial xanthan followed essentially the same temperature-course as for mixtures with PFX, indicating extensive loss of pyruvate under these more strongly acidic conditions. Mixtures prepared at pH values in the range 4.0–3.5 gave comparable moduli at room temperature (20 °C) to those obtained at neutral pH, but showed substantial softening on heating to body temperature, suggesting possible applications in replacement of gelatin in products where “melt-in-the-mouth” characteristics are important for acceptability to the consumer.     

Rheology and Confocal Reflectance Microscopy as Probes of Mechanical Properties and Structure during Collagen and Collagen/Hyaluronan Self-Assembly

In this work, the gelation of three-dimensional collagen and collagen/hyaluronan (HA) composites is studied by time sweep rheology and time lapse confocal reflectance microscopy (CRM). To investigate the complementary nature of these techniques, first collagen gel formation is investigated at concentrations of 0.5, 1.0, and 1.5 mg/mL at 37°C and 32°C. The following parameters are used to describe the self-assembly process in all gels: the crossover time (t_c), the slope of the growth phase (k_g), and the arrest time (t_a). The first two measures are determined by rheology, and the third by CRM. A frequency-independent rheological measure of gelation, t_g, is also measured at 37°C. However, this quantity cannot be straightforwardly determined for gels formed at 32°C, indicating that percolation theory does not fully capture the dynamics of collagen network formation. The effects of collagen concentration and gelation temperature on k_g, t_c, and t_a as well as on the mechanical properties and structure of these gels both during gelation and at equilibrium are elucidated. Composite collagen/HA gels are also prepared, and their properties are monitored at equilibrium and during gelation at 37°C and 32°C. We show that addition of HA subtly alters mechanical properties and structure of these systems both during the gelation process and at equilibrium. This occurs in a temperature-dependent manner, with the ratio of HA deposited on collagen fibers versus that distributed homogeneously between fibers increasing with decreasing gelation temperature. In addition to providing information on collagen and collagen/HA structure and mechanical properties during gelation, this work shows new ways in which rheology and microscopy can be used complementarily to reveal details of gelation processes.     

Relationship between the Rheological Properties of Thickener Solutions and Their Velocity through the Pharynx as Measured by the Ultrasonic Pulse Doppler Method

The dependence of the dynamic viscoelastic parameters of carboxymethylcellulose (CMC), xanthan gum, and guar gum solutions on the angular frequency (ω) was compared with that of their viscosity (μ) on the shear rate (γ). In addition, the effect of these rheological properties on the maximum velocity through the pharynx, V _max, as measured by the ultrasonic pulse Doppler method, was investigated. The CMC and guar gum solutions examined were taken as a dilute solution and a true polymer solution, respectively. The xanthan gum solution was taken as a weak gel above 0.5% and a true polymer solution below 0.2%. The maximum velocity, V _max, of the thickener solutions correlated well with μ, the dynamic viscosity η′, and the complex viscosity η^*, especially those measured at γ or ω of 20–30 s^?1 (or rad/s) and above, suggesting that μ, η′, and η^* are suitable indexes for care foods of the liquid type for dysphagic patients.     

Some comments on the conductance of slow inward current in cardiac muscle

Voltage clamp experiments, which determine the kinetic parameters of calcium conductance of cardiac muscle, (d _∞,f _∞, τ _d and τ _f ) are analyzed with a generally accepted expression for slow inward currentI _s=g _sdf (E-E _R). Activation (d) and inactivation (f) reach the final valuesd _∞ andf _∞ with time constants τ _d and τ _f respectively. The analysis indicates that the measuredf _∞ agrees with the theoreticalf _∞, but the measuredd _∞ differs from the theoreticald _∞ by a factor which depends on τ _d . The peak tension can be made to correlate closely with the theoreticald _∞ after a correction factor is applied to the raw measurements of activation. It can be shown that experiments designed to measure τ _f can also be used to determine τ _d with greater accuracy.     

Feather deuterium as an indicator of age‐class in the Pectoral Sandpiper Calidris melanotos

We measured deuterium isotope ratios (δD_f) in primary feathers to distinguish first‐year from older Pectoral Sandpipers Calidris melanotos captured in Barrow, Alaska, during the breeding season. δD_f showed a distinct bimodal distribution, and model‐based clustering placed the δD_f values into two non‐overlapping groups. More negative δD_f corresponded to Arctic areas, probably identifying first‐year birds with Arctic‐grown juvenile feathers retained from the previous year. The more positive values corresponded to lower latitudes, possibly identifying older birds that grew their feathers at non‐Arctic latitudes.     

Vapor Phase Deposition, Structure, and Plasmonic Properties of Polymer-Based Composites Containing Ag–Cu Bimetallic Nanoparticles

Nanocomposite (NC) thin films with noble metal nanoparticles embedded in a dielectric material show very attractive plasmonic properties due to dielectric and quantum confinement effects. For single component nanoparticles (NPs), the plasmon resonance frequency can only be tuned in a narrow range. Much interest aroused in bimetallic nanoparticles (BNPs), however many wet chemical approaches do not allow large variation of the NP alloy composition and filling factor. Here, we report a vapor phase co-deposition method to produce polymer–metal NCs with embedded Ag_1 − x Cu _x alloy particles. The method allows production of NPs with controlled alloy composition (x), metal filling (f) and nanostructure in a protecting Teflon AF matrix. The nanostructure size and shape were characterized by transmission electron microscope. Energy dispersive X-ray spectroscopy was used to determine x and f. The optical properties and the position of surface plasmon resonances were studied by UV–Vis spectroscopy. The plasmon resonances can be tuned over a large range of the visible spectrum associated with the change in x, f, and nanostructure. For low filling factors and small particle sizes, only one resonance peak was observed. This is attributed to enhanced miscibility at the nanoscale. Double plasmon resonances were seen for larger particle sizes in accord with phase separation expected from the bulk phase diagram and were explained in terms of the formation of core-shell structures with Cu core and Ag shell. Changes upon annealing at 200 °C are also reported.