Acid Gelation Research Articles

Salt/current-triggered stabilization of β-cyclodextrins encapsulated host-guest low-molecular-weight gels

Host-guest supramolecular gels were developed via the self-assembly of inclusion complexes (ICs) of β-cyclodextrins/phenylboronic acid gelator (PBA). Salts and current were involved in the self-assembly to stabilize the host-guest gels. The stability of the gels was greatly improved after salts were added. The stable time of gels was extended from 2.5 h to 120 h with the addition of NH4NO3 at the concentration of 2.5 × 10−2 g/mL. The morphology of the gel was affected by the concentrations of NH4NO3. SEM images revealed that the gels were three-dimensional nanofibrous networks, the sizes of fibers decreased with decreasing NH4NO3 concentrations, which affected the stability of gels, further proved by the rheological properties of gels. More stable gels were obtained with current stimulation, the stable time of the gel was increased from 2.5 h to 55 h with current by adding NaBF4. The current also exhibited significant influence on the aggregation as the voltage varied (0–500 mV) with a constant concentration of salts. The result showed the self-assembly process of host-guest gel could be well controlled via the addition of salts and current to desired morphology and stability.

All-small-molecule dynamic covalent gels with antibacterial activity by boronate-tannic acid gelation

Reversible boronate-catechol linkage was widely used to construct two-dimensional coatings and three-dimensional nanostructures or hydrogels. The construction of these functional materials usually requires the pre-synthesis of macromolecular building blocks, and direct gelation between natural polyphenols and small molecule boronic acids is yet to be investigated. In this study, we fabricated a family of all-small-molecule dynamic covalent gels consisting of tannic acid and boronic acids. Transparent and thixotropic gels were formed by boronate affinity towards catechol groups abundant on natural polyphenols. The gels showed multi-responsiveness, such as acid-, base-, reduction- and oxidant-sensitive depending on the used boronic acid building blocks. The chemistry for gel formation and stimuli-responsiveness was characterized by 11B NMR spectroscopy. The multi-stimuli responsiveness, green processing and facile modular design make the boronic acid-tannic acid gels promising candidates for the development of smart soft materials.

Impact of heat treatment and acid gelation on polyphenol enriched milk samples

The effects of heat treatment and the stage of polyphenol addition to milk before or after heat treatment on the total phenolic content (TPC), ferric-ion reducing antioxidant power (FRAP), pH, casein micelle size (CMS) and whey protein denaturation content of milk-polyphenol mixtures were investigated. Four sources of phenolic compounds (green tea, white grape, tannic acid, gallic acid) were incorporated into pasteurized-skim milk. A heat treatment (85 °C for 30 min) was applied to pasteurized-skim milk either before (MhP) or after polyphenols addition (MhPh). Acid milk gels were produced using MhPh samples, and their TPC and FRAP were determined. Heat treatment decreased the TPC and FRAP values of the samples, except for tannic acid, probably due to increased polyphenol-protein interactions. However, MhPh tannic acid sample resulted in significantly higher FRAP value than MhP. The addition of gallic acid before heat treatment (MhPh) significantly increased CMS due to the lower pH of this sample during heating. Acid gelation decreased the extractable polyphenols, however there was no significant different on FRAP between acid gel and MhPh milk samples. This study showed that the properties of phenolic source, particularly pH, and the stage of polyphenol addition to milk had an impact on selected properties.

Effect of transglutaminase and acidification temperature on the gelation of reconstituted skim milk

Effects of transglutaminase (TG), acidification temperature and total milk solids level on the acid gelation of skim milk were investigated. Despite similar acidification kinetics, TG-treated milk acidified at ≥35 °C showed differences in elastic modulus (Gʹ) with acidification time, particularly by inhibiting the formation of the peculiar shoulder (Gʹshoulder) observed at an early stage of gelation in the control milk. Regardless of the milk solids content, the G'shoulder was absent in both types of milk at 30 °C. However, control milk above 2.5% (w/w) milk solids showed the G'shoulder at 45 °C. G'shoulder is proposed as the transition from the first increase in Gʹ, due to aggregation of soluble protein complexes at the early stage of acidification, to the second, due to further aggregation of casein micelles (and aggregated soluble complexes) as acidification progresses. The G'shoulder was absent in acidified TG-treated milk due to the lack of soluble protein complexes containing caseins.

Physicochemical and rheological changes of acidified camel milk added with commercial low methoxyl-pectin.

The influence of the addition of low methoxyl amidated pectin (LMA) on acid milk gels from whole camel milk (WCM) on physicochemical and rheological proprieties were studied. The zeta potential, particle size, viscosity, dynamic oscillatory rheology and isothermal titration calorimetry were monitored. The presence of LM-pectin in milk had an impact on the average size of the casein micelles and a large and dominant influence on rheological behavior during acidification. Zeta potential and viscosity of gels with 0.5% pectin were not affected. However, milk gels containing 1%, 1.5% and 2% of LM-pectin showed highest values of particle size at pH4. This modification of the structure of the casein micelles induces a significant improvement (p<0.05) on its acid gelation behavior. Therefore, the addition of pectin enhanced the rheological proprieties. Higher pectin concentration led to a strong gel with higher G' values. This result could be attributed to the formation of complexes and the mechanical spectra prove the hypothesis that pectin forms strands with caseins micelles. Isothermal titration calorimetry results showed that pectin concentration had a marked influence on the gels structure and that this polysaccharide stabilizes caseins micelles in acidified camel milk gel due to electrostatic interaction and steric repulsion.

Acid gelation of mixed thermal aggregates of pea globulins and β-lactoglobulin

Abstract The acid gelation by glucono-δ-lactone of thermal protein aggregates from mixed pea globulin (Glob) and β-lactoglobulin (βlg), namely “mixed-aggregates”, was investigated at 25 °C in comparison to that of mixtures of thermal aggregates of each protein obtained beforehand separately (“mixtures of aggregates”). A phase diagram indicating thermal gelation and acid gelation conditions was obtained from mixed protein systems varying in concentration and composition. The minimum acid gelation concentration was 3% regardless of the protein aggregates system, a value about half the concentration threshold measured for thermal gelation. The rheological properties, the microstructure and the water holding capacity (WHC) of acid gels were then evaluated from 4 wt% protein aggregate solutions at different βlg/Glob weight ratios (0/100, 30/70, 50/50, 70/30 and 100/0). Acid gelation led to weak viscoelastic gels and the gel strength increased with βlg content. The formation of acid gel from “mixed-aggregates” resulted in more elastic gels and improved WHC compared to gels resulting from “mixtures of aggregates”. This behavior seemed to correlate with the regular filamentous and highly entangled gel network structure observed for mixed-aggregates. These properties were believed to originate from the initial structure of the thermal protein aggregates strongly influenced by βlg content.

Differences in the yoghurt gel microstructure and physicochemical properties of bovine milk containing A1A1 and A2A2 β-casein phenotypes

β-casein (β-CN) comprises a major portion of milk caseins and are present as several genetic variants. In this study, we investigated the effects of the β-CN A2 variant versus the A1 variant on the calcium distribution, acid gelation and foaming properties of milk, and the microstructure of acid milk gels. The results showed that milk containing the β-CN phenotype A2A2 had a higher proportion of free ionic calcium, enhanced foam formation capability and required a longer time for gelation to occur. The storage modulus at the end of fermentation was significantly lower for β-CN A2A2 milk gel compared with that from β-CN A1A1 milk. The microstructures of the gels characterised by both confocal microscopy and Cryo-SEM demonstrated the differences in the gel porosity and protein strand thickness. The more porous microstructure and thinner protein strands in the β-CN A2A2 milk gel resulted in a lower gel strength compared to β-CN A1A1 milk gel. These findings provide new insights to the subtle differences in the physical properties of milk containing β-CN A2A2 and A1A1 phenotypes, which could support dairy producers in the development of new dairy products with different functional properties.

Egg yolk gels: Sol-gel transition and mechanical properties as affected by oleuropein enrichment

The effect of oleuropein on the gelation properties of whole egg yolk at different pHs (2, 4.5 and 6.4) was studied. Egg yolk solutions were added with increasing amount of pure oleuropein, gelled by either thermal (pHs 4.5 and 6) or acid gelation (pH 2) and submitted to frequency sweep and stress relaxation tests. The experimental data obtained by the frequency sweep tests were modeled with a power law equation in order to get the coordination number z and the proportional coefficient A, which describe the network extension and its strength, while data obtained from stress-relaxation tests were fitted by a generalized Maxwell model with 5 elements. The addition of oleuropein affected the gelation behavior as well as the rheological properties of the gel network, and the effect was influenced by the pH of the systems. A shift of the gel point towards lower temperatures was observed in the presence of the phenolic compound, and this effect was significant in the systems at pH 4.5 (from 72.55 ± 0.07 °C to 70.43 ± 0.06 °C). According to the results obtained from the modelling of the frequency sweep test, gels at pH 6.4 and 2 were characterized by a weak network; on the contrary, at pH 4.5 the network was characterized by a low number of flow units (lower z, z = 16.52 on average) interacting with one another in a more strictly way (higher A, A = 5362 Pa s1/z on average). Such findings were successively confirmed by the stress relaxation results where it was shown that firmer gels were obtained at pH 4.5 after oleuropein enrichment.

Physicochemical properties of milk protein ingredients and their acid gelation behaviour in different ionic environments

Dispersions of whey protein isolate (WPI), microparticulated whey protein (MWP), nanoparticulated whey protein (NWP) and micellar casein isolate (MCI) were characterised in terms of protein denaturation and particle size. WPI exhibited high native whey protein content, while MWP and NWP showed low native whey protein content. The acid gelation behaviour of milk protein ingredients and mixtures thereof (5% protein) in different ionic environments (deionised water, whey permeate and dialysed against skimmed milk) was also investigated. The gelation curves of NWP showed no significant difference for in the three different ionic environments, and high gel strength after acidification was observed. WPI, MWP and MCI dispersed in deionised water gelled faster than in the other two ionic environments. MCI in whey permeate showed slower gelation and lower G’ than their dialysed samples.

Effect of fermentation temperature on the properties of exopolysaccharides and the acid gelation behavior for milk fermented by Streptococcus thermophilus strains DGCC7785 and St-143

We investigated the effect of fermenting milk with 2 strains (DGCC7785 and St-143) of Streptococcus thermophilus, which are known to produce different types of exopolysaccharide (EPS) structures. The yields and physical properties of these ropy EPS were monitored during the fermentation of milk at different temperatures. We wanted to understand how these types of EPS properties affected yogurt gelation. Reconstituted skim milk was fermented at 33, 39, or 45°C until pH values reached 5.2, 4.9, 4.7, and 4.5. Molar mass of ropy EPS samples was determined using size exclusion chromatography coupled with multiangle laser light scattering. Rheological properties of fermented milk gels were analyzed using small-strain dynamic oscillatory measurements. In both strains, concentrations of ropy EPS increased during fermentation and at all temperatures. Fermentation times, by both strains, were shortest at 45°C and longest at 33°C. For both strains, molar mass of ropy EPS ranged from 2 to 4 × 106 g/mol during fermentation. A major proteinaceous contaminant that was co-isolated with the ropy EPS fraction by our isolation method was identified as a milk-derived phosphoglycoprotein PP3. Increase in fermentation temperature from 33 to 45°C significantly decreased the storage modulus values (from 170 to 41 Pa) for milk gelled by strain DGCC7785, whereas the gels made with St-143 had very low storage modulus values (11-17 Pa) regardless of fermentation temperatures. For both strains, the values of maximum loss tangent in the milk gels increased with fermentation temperature; the maximum loss tangent occurred at higher pH values when milk was fermented by strain DGCC7785. The specific type of EPS produced appeared to be responsible for the differences in yogurt texture rather than the concentration or molar mass of the EPS.

Acid gelation of raw and reconstituted spray-dried dromedary milk: A dynamic approach of gel structuring

Dynamic oscillatory rheology and scanning electron microscopy (SEM) were used to further understand the formation of acid milk gels obtained from raw and reconstituted spray-dried dromedary milk. The influence of acidification temperature, concentration of acidifier (glucono-δ-lactone; GDL) and milk fat content on the rheological parameters of milk gelation were firstly investigated. Our results confirmed the poor ability of dromedary milk to be processed into acidic gels. Under optimised conditions (45 °C, 2.25% GDL), the gel point of dromedary milk occurred at a lower pH (4.46) than that of bovine milk (5.06). The resulting gel obtained after 3 h acidification had a storage modulus (G′) 35 times lower than that obtained from bovine milk. SEM observations of dromedary and bovine milk during acidification highlighted many important differences in the implementation of the microstructure of acid gels. These fundamental differences are discussed based on the literature knowledge of cows' milk acid gelation.

The study of curcumin interaction with micellar casein and lactic acid bacteria cell envelope

Curcumin interaction with micellar caseins (MC), Lactobacillus delbrueckii bulgaricus (Lb) and Streptococcus thermophilus (St) and its influence on the acid gelation were investigated. Epifluorescence microscopy evidenced the adsorption of curcumin on bacteria surfaces and its transfer from MC to bacteria. Fluorescence spectroscopy of curcumin and fluorescence quenching of caseins variations in presence of bacteria and MC were evaluated. The interaction between curcumin and St and Lb was confirmed due to the decrease of curcumin fluorescence intensity from 1.7 A.U to 1.20 A.U and to 1.40 A.U, respectively. The transfer of curcumin between MC and bacteria was confirmed. The curcumin interacted more with St than with Lb envelope with a fluorescence intensity decrease from 6.20 ± 0.02 A.U to 5.20 ± 0.08 A.U and to 4.60 ± 0.08 A.U, respectively. It also showed that St was more hydrophobic than Lb bacterial envelope leading curcumin to interact more with Streptococcus surface. Curcumin adsorption did not affect Lb and St growth nor its acidification rate of milk. The acid gelation study led to a typical gel yoghurt-like formation. For the first time, it was demonstrated that curcumin interacted with lactic bacteria without modifying its growth and milk acidic gelation property.

Insulin Inclusion into a Tragacanth Hydrogel: An Oral Delivery System for Insulin.

Nanoparticles or microparticles created by physical complexation between two polyelectrolytes may have a prospective use as an excipient for oral insulin administration. Natural polymers such as tragacanth, alginate, dextran, pullulan, hyaluronic acid, gelatin and chitosan can be potential candidates for this purpose. In this research, insulin particles were prepared by the inclusion of insulin into a tragacanth hydrogel. The effect of the pH and concentration relationship involving polyelectrolytes offering individual particle size and zeta potential was assessed by zetasizer and scanning electron microscopy (SEM). Insulin–tragacanth interactions at varying pH (3.7, 4.3, 4.6, or 6), and concentration (0.1%, 0.5%, or 1% w/w) were evaluated by differential scanning calorimetry (DSC) and ATR Fourier transform infrared (ATR-FTIR) analysis. Individual and smaller particles, approximately 800 nm, were acquired at pH 4.6 with 0.5% of tragacanth. The acid gelation test indicated that insulin could be entrapped in the physical hydrogel of tragacanth. DSC thermograms of insulin–tragacanth showed shifts on the same unloaded tragacanth peaks and suggested polyelectrolyte–protein interactions at a pH close to 4.3–4.6. FTIR spectra of tragacanth–insulin complexes exhibited amide absorption bands featuring in the protein spectra and revealed the creation of a new chemical substance.

Effect of whey protein addition and pH on the acid gelation of heated skim milk

The impact of pH and whey protein addition on protein distributions and the properties of acid gels prepared from heated milk was studied. Adding whey protein to milk at its natural pH before heating (80 °C/30 min) increased serum-phase κ-casein and whey protein levels and produced acid gels with higher stiffness, yield stresses and yield strains. For skim milk and skim milk with added whey protein, samples heated at pH 6.5 had low levels of serum-phase protein; however, the level increased as the pH increased to 7.1. At each pH, lower levels of serum-phase proteins were in skim milk than in those with added whey protein. Acid gels had higher stiffness and yield strains as the pH of the milk at heating increased. At each milk pH, the acid gels from milk with added whey protein had higher gel stiffness and yield stresses than those without added whey protein.

Development of the composition and technology of a soap with sapropel

Hygienic treatment of hands is an effective means of reducing the spread of transient microflora and is considered as one of the most important procedures for preventing the infection transmission by the contact-household way. Hygienic treatment of hands is necessary in public catering establishments, treatment and prevention institutions, for immunity compromised patients, in the period of seasonal diseases, etc. Most antibacterial soaps are based on synthetic surfactants using such synthetic antibacterial additives as triclosan, triclocarbanum, farnesol, benzalkonium chloride, pyroctonolamine. Therefore, development of soaps containing natural substances in their composition is relevant.Aim. To develop a soap with sapropel, which would have the antibacterial effect and satisfactory sensory properties.Materials and methods. To make the soap the liquid corn and castor oils, solid palm and coconut oils, as well as argan oil were selected as a superfatting agent. Oil saponification was carried out with sodium hydroxide solution. The quality of the samples obtained was assessed by the organoleptic and physicochemical parameters (the qualitative number (the mass of fatty acids calculated with reference to the nominal weight of a soap bar of 100 g), the mass fraction of free caustic alkali, the mass fraction of sodium products calculated with reference to Na2O, the temperature of fatty acid (titer) gelation, the mass fraction of sodium chloride).Results and discussion. By the indicators of the saponification value of the selected components of the soap fatty base containing 43.8 % of palm oil and 25 % of coconut oil, 6.2 % of corn oil and 25 % of castor oil, and with the necessary consistency properties, soap ductility, high solubility and foaming in cold water the amount of alkali needed for making soap has been calculated. According to the results of determination of the soap consumer properties the concentration of argan oil has been chosen. As a result of studying the organoleptic and physicochemical parameters of the soap with 5 and 10 % of sapropel it has been found that the soap obtained by the cold method meets the standards for soap of the brand “Special” with disinfectants or therapeutic additives.Conclusions. The soap composition containing 5 and 10 % of sapropel has been proposed. In the laboratory conditions the cold and hot methods for obtaining the soap with sapropel have been tested. It has been found that the samples obtained by the cold method meet the requirements of the normative documentation in contrast to the soap with the unsatisfactory organoleptic parameters obtained by the hot method.

Effect of temperature on casein micelle composition and gelation of bovine milk

Variation in temperature causes changes in the composition of the casein micelles. Milk samples with 10, 20, 30, 40, and 50% reduction of total β-CN from casein micelles were prepared and analysed. The micelle structure was maintained with minor changes in z-average diameters, zeta potentials, and micellar calcium. During acid gelation decreases in maximum storage modulus, fracture force and firmness were observed with gradual removal of β-CN, but with no significant differences regarding the gelation time among the samples. Decreases in gelation time, maximum G′, fracture force and firmness occurred with gradual removal of β-CN during rennet-induced gelation. Thus, the changes in composition of casein micelle during low temperature treatments had an influence on the gelation properties of bovine milk.

Properties of acid gels made from sodium caseinate-maltodextrin conjugates prepared by a wet heating method

Covalent attachment of polysaccharides to proteins (conjugation) via the Maillard reaction has been extensively studied. Conjugation can lead to a significant improvement in protein functionality (e.g., solubility, emulsification, and heat stability). Caseins have previously been successfully conjugated with maltodextrin (Md), but the effect on the detailed acid gelation properties has not been examined. We studied the effect of conjugating sodium caseinate (NaCN) with 3 different sized Md samples via the Maillard reaction in aqueous solutions. The Md samples had dextrose equivalents of 4 to 7, 9 to 12, and 20 to 23 for Md40, Md100, and Md200, respectively. The conjugation reaction was performed in mixtures with 5% NaCN and 5% Md, which were heated at 90°C for 10 h. The degree of conjugation was estimated from the reduction in free amino groups as well as color changes. Sodium dodecyl sulfate-PAGE analysis was performed to confirm conjugation by employing staining of both protein and carbohydrate bands. The molar mass of samples was determined by size-exclusion chromatography coupled with multi-angle laser light scattering. After the conjugation reaction, samples were then gelled by the addition of 0.63% (wt/vol) glucono-δ-lactone at 30°C, such that samples reached pH 4.6 after about 13 h. The rheological properties of samples during acidification was monitored by small-strain dynamic oscillatory rheology. The microstructure of acid gels at pH 4.6 was examined by fluorescence microscopy. Conjugation resulted in a loss of 10.8, 8.8, and 11.9% of the available amino groups in the protein for the NaCN-Md40 conjugates (C40), NaCN-Md100 conjugates (C100), and NaCN-Md100 conjugates (C200), respectively. With a decrease in the size of the type of Md, an increase occurred in the molar mass of the resultant conjugate. The weight average molar masses of NaCN-Md samples were 340, 368, and 425 kDa for the conjugates C40, C100, and C200, respectively. Addition of Md to NaCN dispersion resulted in slightly shorter acid gelation times even without the conjugation reaction. The storage modulus (G') of acid gels was significantly lower in conjugated samples compared with the corresponding (unreacted) mixtures of Md and NaCN. The sample with the lowest G' value at pH 4.6 was the C40 conjugate. Fluorescence microscopy showed that gels made by conjugates had slightly larger pores. These results indicate that conjugation of casein modified its acid gelation properties, presumably by the Md polysaccharide moiety hindering aggregation and rearrangements of the casein network.

An amino acid-based gelator for injectable and multi-responsive hydrogel

Hydrogels formed by gelators have attracted growing attention for their promising application in biomaterials and biotechnology. We describe in this paper the generation and characterization of a novel photo-, thermal- and pH-responsive hydrogel based on an amino acid gelator AA-Azo-EG6. Specifically, the gelator bears an amino acid head, an azobenzene (Azo) linker, and a short oligoethylene glycol tail (EG6). The resulting AA-Azo-EG6 hydrogel is injectable and exhibits interesting helical self-assembled structures. Meanwhile, the hydrogel is able to experience a gel-sol or gel-precipitate phase transition responding to external stimuli. Thus, this AA-Azo-EG6 gelator is a promising building block for intelligent materials and drug delivery.

Interactions in casein micelle - Pea protein system (Part II): Mixture acid gelation with glucono-δ-lactone

In our preceding study (Part I), the thermal denaturation and aggregation of enriched pea protein fractions, namely vicilin/convicilin 7S (Vic) and legumin 11S (Leg), were investigated in the absence or in presence of casein micelles (CM) at pH ≈ 7.1. The present report (Part II) focuses on the glucono-δ-lactone (GDL) acid-induced gelation of either co-heated proteins in admixture (namely route 1) or thermally-aggregated pea proteins mixed with unheated CM (route 2), while applying a pea protein (either non fractionated PP or Vic or Leg) - to - CM weight ratio of 1: 1 and total protein concentration of 3.6 wt%. The pea protein thermal aggregates obtained by route 1 were of lower size and less soluble than those yielded in isolation. For route 1, gelation of the heated CM – Vic mixture was triggered at a higher pH value and led to higher final storage modulus G′ than corresponding protein samples in isolation. In contrast, the presence of large and sedimentable aggregates in the case of the CM – PP and CM – Leg mixtures impaired gelation. Concerning route 2, either the PP or Vic aggregates mixed with unheated CM resulted in rapid gelation and higher final G’ values than those measured for their single-protein sample counterparts. Viscoelastic properties of the mixed gel depended on the pea protein fraction used, thermal aggregation route, extent of physical interactions between pea proteins during acidification and further involvement of CM. Hence, route 2 would be more reliable than route 1 to produce a “mixed” dairy-like gelled product containing pea proteins with improved texture properties.

Functional and in vitro gastric digestibility of the whey protein hydrogel loaded with nanostructured lipid carriers and gelled via citric acid-mediated crosslinking

Nanostructured lipid carriers (NLCs) with mean size of 347nm were fabricated and added into a heat-denatured whey protein solution. The subsequent crosslinking of proteins by citric acid or CaCl2 resulted in the formation of cold-set hydrogels. Fourier transform infrared spectroscopy (FTIR) proposed formation of more hydrogen bonds in gel due to NLC loading or citric acid-mediated gelation. It was also found based on FITR spectroscopy that citric acid crosslinking disordered whey proteins. Scanning electron microscopy (SEM) imaging showed a non-porous and finely meshed microstructure for the crosslinked gels compared to non-crosslinked counterparts. Crosslinking also increased the firmness and water-holding capacity of gels.In pepsin-free fluid, a strong correlation existed between reduction in gel swellability and digestibility over periods up to 60min due to NLC loading and citric acid gelation. However, in peptic fluid, NLC loading and citric acid crosslinking brought about much higher decrease in digestibility than swellability.