Decrease In Gelation Time Research Articles

Thermoresponsive and Injectable Pluronic F127 Hydrogel for Loading Adipose-Derived Mesenchymal Stem Cells.

Stem cell-based therapies display immense potential in regenerative medicine, highlighting the crucial significance of devising efficient delivery methods. This study centers on a pioneering approach that utilizes Pluronic F127 (PF127) as a thermoresponsive and injectable hydrogel designed for the encapsulation of adipose-derived mesenchymal stem cells (AdMSCs). The degradation profile, gelation time, and microstructure of the PF127 hydrogel were thoroughly examined. AdMSCs were isolated, expanded, and characterized based on their multi-lineage differentiation potential. AdMSCs from the third passage were specifically employed for encapsulation within the PF127 hydrogel. Subsequently, the cytotoxicity of the AdMSC-loaded PF127 hydrogel was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and apoptosis assays. Characterized by scanning electron microscopy (SEM), the PF127 hydrogel exhibited a porous structure, indicating its suitability for accommodating AdMSCs and facilitating wound healing. The PF127 hydrogel demonstrated reversible phase transitions, rendering it suitable for in vivo applications. Studies on the gelation time of PF127 hydrogel unveiled a concentration-dependent decrease in gelation time, offering adaptability for diverse medical applications. Analysis of the degradation profile showcased a seven-day degradation period, leading to the decision for weekly topical applications. Cytotoxicity assessments confirmed that AdMSCs loaded into the PF127 hydrogel maintained heightened metabolic activity for up to one week, affirming the safety and appropriateness of the PF127 hydrogel for encapsulating cellular therapeutics. Furthermore, cell apoptosis assays consistently indicated low rates of apoptosis, emphasizing the viability and robust health of AdMSCs when delivered within the hydrogel. These findings underscore the vast potential of PF127 hydrogel as a versatile and biocompatible delivery system for AdMSCs in the realm of regenerative medicine. Boasting adjustable gelation properties and a remarkable capacity for cell encapsulation, this pioneering delivery system presents a promising path for applications in tissue engineering and wound healing. Ultimately, these advancements propel and elevate the landscape of regenerative medicine.

Kinetic Analysis of the Reaction between Tannic Acid (TA) and L-Lysine Diisocyanate (LDI) Systems

The kinetics of the synthesis of green polyurethane from the reaction between tannic acid (TA) and L-lysine diisocyanate (LDI) were investigated using the differential scanning calorimeter (DSC) technique and dynamic rheological tests. The evaluation of the reaction behavior of the prepared samples was carried out using nonisothermal conditions at dynamic heating rates of 5, 10, 15, and 20°C/min. The evolution of the activation energy with conversion was computed through the five isoconversional methods of Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose (KAS), the Ozawa-Flynn-Wall method (OFW), Friedman (FR), Starink, and Vyazovkin. The average activation energy calculated from these methods was estimated at 46.5, 46.8, 47.2, 47.3, and 51.4 KJ/mol, respectively. The preexponential factor was evaluated at 5.04 × 10 5 1/s. The overall reaction order ( n + m ) was also found to be around 1.8912. The results of the combination of the model-free method and model-fitting approach exhibited that the reaction mechanism was an autocatalytic type, implying the autocatalytic effect of the urethane groups formed during the reaction. The obtained kinetic for TA/LDI was verified through its good agreement with the experimental data. Moreover, the results found from the isothermal rheological test show that with increasing temperature, the gelation time decreases.

Elastin-Plasma Hybrid Hydrogels for Skin Tissue Engineering.

Dermo-epidermal equivalents based on plasma-derived fibrin hydrogels have been extensively studied for skin engineering. However, they showed rapid degradation and contraction over time and low mechanical properties which limit their reproducibility and lifespan. In order to achieve better mechanical properties, elasticity and biological properties, we incorporated a elastin-like recombinamer (ELR) network, based on two types of ELR, one modified with azide (SKS-N3) and other with cyclooctyne (SKS-Cyclo) chemical groups at molar ratio 1:1 at three different SKS (serine-lysine-serine sequence) concentrations (1, 3, and 5 wt.%), into plasma-derived fibrin hydrogels. Our results showed a decrease in gelation time and contraction, both in the absence and presence of the encapsulated human primary fibroblasts (hFBs), higher mechanical properties and increase in elasticity when SKSs content is equal or higher than 3%. However, hFBs proliferation showed an improvement when the lowest SKS content (1 wt.%) was used but started decreasing when increasing SKS concentration at day 14 with respect to the plasma control. Proliferation of human primary keratinocytes (hKCs) seeded on top of the hybrid-plasma hydrogels containing 1 and 3% of SKS showed no differences to plasma control and an increase in hKCs proliferation was observed for hybrid-plasma hydrogels containing 5 wt.% of SKS. These promising results showed the need to achieve a balance between the reduced contraction, the better mechanical properties and biological properties and indicate the potential of using this type of hydrogel as a testing platform for pharmaceutical products and cosmetics, and future work will elucidate their potential.

Polyvinylpyrrolidone-resorcinol-formaldehyde hydrogel system reinforced with bio-synthesized zinc-oxide for water shut-off in heterogeneous reservoir: An experimental investigation

This work aims at evaluating advancement in water shut-off performance using nanocomposite hydrogel (PVP-ZnO:RF) prepared from PolyVinylPyrrolidone (PVP); used as polymer, Resorcinol-Formaldehyde (RF); used as a crosslinker and nano Zinc Oxide (ZnO); used as strength modifier and it was compared with conventional hydrogel (PVP:RF)i.e., hydrogel without ZnO nanofiller. The ZnO, used as a nanofiller in this work, was successfully bio-synthesized (i.e., green route synthesized) from plant extract (Moringa oleifera leaves) and the average size was found to be 10 nm. In this research work, the effect of ZnO nanofiller on gelation time, gel strength, thermal stability, rheological properties and water shut-off performance was systematically evaluated. On the incorporation of ZnO nanofiller, gelation time decreases but gel strength increases. The thermal stability of hydrogel was studied using a Differential Scanning Calorimeter (DSC) that depicts maximum tolerable temperature increases from 86 °C to 92 °C at 0.5 wt.% of ZnO concentration in nanocomposite hydrogel (PVP). The mechanical stability of the nanocomposite hydrogel (PVP-ZnO:RF) demonstrates that infusion of ZnO nanofiller has significantly enhanced the dynamic moduli (i.e., storage modulus (G′) and loss modulus (G″)). Moreover, the optimum results of storage modulus (G′) and loss modulus (G″) are found at 0.5 wt.% of ZnO nanofiller. The water shut-off performance in the high permeable streak, in terms of percentage reduction in permeability, was 97% and 92% for nanocomposite hydrogel (PVP-ZnO:RF) and conventional hydrogel (PVP:RF), respectively. Also, the residual resistance factor is found to be 31.31 and 12.71 for PVP-ZnO:RF and PVP:RF hydrogels. Thus, the developed nanocomposite hydrogel (PVP-ZnO:RF) may be a promising solution to excessive water production in mature oil fields.

Compositions of unsaturated polyester resins used for facing layers of laminates

Hybrid composites of unsaturated polyester resin with the addition of glass microspheres and bentonite modified with a quaternary ammonium salt were obtained. The content of bentonite in composites was 3 wt %, while glass microspheres varied in the range of 5–15 wt %. The effect of the addition of glass microspheres on the rheological and mechanical properties of unsaturated polyester resin composites containing modified bentonite was investigated. On the basis of the obtained results, it was found that the optimum content of glass microspheres in polyester composites at which a clear increase in the value of most mechanical parameters was achieved, is 15 wt %. The Young’s modulus, flexural modulus, tensile strength, and Charpy impact strength increased by 14.7, 38.7, 17.7 and 25%, respectively, compared to the unmodified resin. On the other hand, with relation to the sample containing 3 wt % of bentonite, a significant improvement was achieved only in the case of the flexural modulus and impact strength, amounting to 16.5 and 36.0%, respectively. In addition, composite with 15 wt % of glass microspheres was characterized by a smaller, by about 25%, flexural strength in relation to the unfilled resin and resin with 3 wt % of bentonite. In turn, rheological studies have shown that as the content of glass microspheres increased, there was an increase in viscosity and a decrease in gelation time in relation to the reference sample and the sample with 3 wt % of bentonite.

Effect of Inorganic Polymer Gel Systems on Residual Resistance Factor in Fractured Core Model

Excessive water production through fractures become an important problem in oil exploration of fractured reservoirs. For this purpose, polymer gels were prepared by crosslinking of aqueous solutions of polymer and crosslinker for the purpose of water management in high water cut fractured reservoirs. A copolymer of sulfonated polyacrylamide was used as polymer and chromium triacetate (Cr(OAc)3) and aluminum nitrate nonahydrate (Al(NO3)3.9H2O) were used as inorganic crosslinkers at 90oC. Two quadratic models were presented for the two inorganic polymer gel systems to predict the gelation time by using a central composite design which showed highly significant results. The results also showed that polymer concentration was the main effect on gelation time. Increasing polymer concentration leads to accelerate the gelation process and then decrease of gelation time. Based on the gelation time and strength of three dimensional structure of polymer gel, the selected polymer gels of Cr(OAc)3 and Al(NO3)3.9H2O were applied to study the performance of polymer gel system in fractured core with the same polymer concentration of 37071 ppm and the crosslinker concentration of 13096 ppm and 2707 ppm, respectively. Also, the gelation time of these polymer gels was determined 12 h and 34 h, respectively. For this purpose, the coreflooding test was carried out to measure the output flow rate before and after polymer gel treatment in order to calculate the Residual Resistance Factor (RRF). As a result, after polymer gel treatment, the output flow rate decreased intensively and by increasing injection pressure, the RRF decreased gradually. The polymer gels of Cr(OAc)3 and Al(NO3)3.9H2O in the fracture were renitent up to 70 and 60 bar against the water pressure drop, while these polymer gels were renitent up to 60 and 40 bar against the oil pressure drop, respectively. The polymer gel of Cr(OAc)3 demonstrated higher residual resistance factor than the polymer gel of Al(NO3)3.9H2O.

Evaluation of Terpolymer-Gel Systems Crosslinked by Polyethylenimine for Conformance Improvement in High-Temperature Reservoirs

SummaryA terpolymer-gel system using low toxic polyethylenimine (PEI) as the crosslinker was developed for conformance improvement in high-temperature reservoirs. Suitable gelation time (GT), gel strength, and thermal stability could be obtained by selecting PEI molecular weight and adjusting terpolymer concentrations. With the increase of terpolymer concentration, GT decreases and the gel strength increases. However, in this research, the effect of PEI concentration on the gelation performance was much less obvious than that of the polymer concentration. Very low concentrations of sodium chloride (NaCl) can slightly shorten the GT. After critical concentrations were reached, the authors determined that the ions will delay the crosslinking reaction. Moreover, the addition of sodium carbonate (Na2CO3) can also lengthen GT. The gel systems were able to maintain thermal stability at 150°C. Uniformly distributed 3D network microstructures and the small size of the gel-grid pores made the network structure maintain thermal stability. The use of the terpolymer-gel-system gelation mechanism crosslinked by PEI can help petroleum engineers better understand and apply this terpolymer-gel system.

Experimental research of hydroquinone (HQ)/hexamethylene tetramine (HMTA) gel for water plugging treatments in high‐temperature and high‐salinity reservoirs

ABSTRACTPolymer gel, as a water plugging treatment agent, has been successfully used in enhanced oil recovery (EOR) of mature oil fields. A new thermal‐resistance and salt‐tolerance polymer gel was developed based on HPAM and HQ/HMTA under the condition of high‐temperature (100.8 °C) and high‐salinity (up to 19.8 × 104 mg/L and Ca2+&Mg2+ 0.8 × 104 mg/L). The influence factors of gelling performance and coreflood performances were studied, the microstructure of the gel was observed with the environmental scanning electron microscopy, and gelation mechanism was proposed to illuminate the detailed gelation process. The gelation time decreases and the gel strength increases with the increase of polymer concentration, crosslinker concentration, or temperature. Although shearing had a negative effect on the viscosity of gelling solution, the gel strength, and the stability of gel have not been affected. The gelling solution has a good ability of injection and could selectively flow into high permeable zone. Additionally, the plugging rate increases and stays above 85% with the increase of the permeability or the gel strength. The microstructure of the gel confirms that the gel formed a three‐dimensional network structure. Based on the microstructure and the reaction process of the gel, a possible gelation mechanism is proposed. This study suggests that the gel system can be used in harsh reservoir conditions and the gelation time and gel strength can be controlled with adjusting the formation rate and the concentration of crosslinking agents. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44359.

The influence of gel-casting parameters on the preparation of Si porous bodies

This study aimed to investigate and optimise the rheological behaviour and gel-casting parameters of Si suspensions prepared to fabricate RBSN bodies. Si green bodies with 30vol% solid content possessing roughly 40MPa flexural strength and 50vol% porosity. The gelation time decreases by heating, and the Si gel-cast body dried in PEG1000 solution shows less than 2% shrinkage. Based on FESEM analysis and adsorption-desorption of nitrogen gas, the final Si bodies show open pores and uniform distribution.

New insights into the hydroquinone (HQ)–hexamethylenetetramine (HMTA) gel system for water shut-off treatment in high temperature reservoirs

Aiming at water shut-off in high temperature reservoirs, the hydroquinone (HQ)–hexamethylenetetramine (HMTA) gel system was studied. The gelation performance and effects of various parameters were systematically evaluated. With increase of the concentrations and temperature, gelation time decreases and gel strength increases. In addition, gelation rate and gel strength were enhanced on adding acid. Moreover, the gel system shows good salt tolerance. The thermal stability measurement indicates that the gel can maintain stability up to 140°C. Uniformly distributed three-dimensional network structure was formed in the gel. At last, the gelation mechanism was proposed, illuminating the detailed gelation process clearly.

Study on strength and gelation time of polyacrylamide/polyethyleneimine composite gels reinforced with coal fly ash for water shut‐off treatment

ABSTRACTThis study investigates the influence of coal fly ash (CFA) as a reinforcing material on the strength and the gelation of polyacrylamide (PAM)/polyethyleneimine (PEI) composite gels. Pure PAM/PEI gel and PAM/PEI gels containing CFA up to 2 wt % were synthesized via the cross‐linking reaction between PAM and PEI solutions at room temperature (25°C) in distilled water dispersed with CFA. The strength of each composite gel was measured at temperature of 80°C, while the gelation was determined from 80°C to 95°C. Rheological measurements indicated that the strength of PAM/PEI composite gels filled with CFA contents was significantly rigid and stronger than that of pure PAM/PEI gel as a result of the enhanced interfacial interaction of well‐dispersed CFA contents in PAM/PEI gel matrix. The gelation times of PAM/PEI gels containing CFA up to 2 wt % deviated from the gelation time of pure PAM/PEI gel. It was further observed that an increase in temperature resulted in a decrease in gelation time of PAM/PEI gel containing 2 wt % CFA. The scanning electron microscopy revealed the surface micrographs of PAM/PEI gels filled with CFA to be very dense without any noticeable micropores. The micropores were absent as scanning was performed on the dried composite gels. It also establishes the strong interaction between CFA and PAM/PEI gel matrix. Experimental findings showed that PAM/PEI composite gels reinforced with CFA are potential candidates for total water shut‐off treatment in oilfields. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41392.

Nanostructure Formation and Transition from Surface to Bulk Degradation in Polyethylene Glycol Gels Chain-Extended with Short Hydroxy Acid Segments

Degradable, in situ gelling, inert hydrogels with tunable properties are very attractive as a matrix for cell encapsulation and delivery to the site of regeneration. Cell delivery is generally limited by the toxicity of gelation and degradation reactions. The objective of this work was to investigate by simulation and experimental measurement gelation kinetics and degradation rate of star acrylated polyethylene glycol (PEG) macromonomers chain-extended with short hydroxy acid (HA) segments (SPEXA) as a function of HA monomer type and number of HA repeat units. HA monomers included least hydrophobic glycolide (G), lactide (L), p-dioxanone (D), and most hydrophobic ε-caprolactone (C). Chain extension of PEG with short HA segments resulted in micelle formation for all HA types. There was a significant decrease in gelation time of SPEXA precursor solutions with HA chain-extension for all HA types due to micelle formation, consistent with the simulated increase in acrylate-acrylate (Ac-Ac) and Ac-initiator integration numbers. The hydrolysis rate of SPEXA hydrogels was strongly dependent on HA type and number of HA repeat units. SPEXA gels chain-extended with the least hydrophobic glycolide completely degraded within days, lactide within weeks, and p-dioxanone and ε-caprolactone degraded within months. The wide range of degradation rates observed for SPEXA gels can be explained by large differences in equilibrium water content of the micelles for different HA monomer types. A biphasic relationship between HA segment length and gel degradation rate was observed for all HA monomers, which was related to the transition from surface (controlled by HA segment length) to bulk (controlled by micelle equilibrium water content) hydrolysis within the micelle phase. To our knowledge, this is the first report on transition from surface to bulk degradation at the nanoscale in hydrogels.

Effect of alkali cation on irreversible gel formation in basic medium

This study is based on the understanding of the behavior of alkali silicates in a basic medium and aims to identify the mechanisms responsible for the formation of an irreversible gel and its consolidation. Commercial lithium, sodium and potassium silicate solutions were used to reveal the effect of the cation nature on the gelation process. Gels are obtained by acidifying alkaline silicate solutions with hydrochloric acid. A syneresis phenomenon during ripening that leads to the formation of a strongly consolidated solid has been observed. Whatever the cation, the gelation or syneresis mechanism would be similar. However, gelation time decreases and syneresis increases with the cation size. Gelation results from formation of small particles which grow in number and size and then gather to fill the available space. Ripening takes place through a dissolution/precipitation mechanism.

Gelation time and degradation rate of chitosan-based injectable hydrogel

Gelation time and degradation rate of thermally-sensitive aqueous solutions of chitosan/Gp (glycerophosphate disodium salt) have been studied. The effects of different parameters such as Gp salt concentration, solution temperature, degree of deacetylation of chitosan (DDA) and drug loading on the gelation time have been investigated. Gravimetric analysis, gel permeation chromatography and FTIR spectrophotometry were used to investigate the influence of the DDA and concentration of chitosan solution on hydrogel degradation. The presented results indicated that gelation time decreases by increasing Gp salt concentration, temperature, concentration and DDA of chitosan solutions, while drug loading has no significant effect on gelation time. Slower degradation profile was recorded for hydrogel with the higher DDA and concentration of chitosan in the primary solution. FTIR studies indicated that the chemical structure of chitosan macromolecules does not change significantly during the degradation. It could be concluded that biodegradation of chitosan hydrogel occurred via its surfaces.

In-Situ Formation of Biodegradable Hydrogels by Stereocomplexation of PEG−(PLLA)8 and PEG−(PDLA)8 Star Block Copolymers

Eight-arm poly(ethylene glycol)-poly(L-lactide), PEG-(PLLA)(8), and poly(ethylene glycol)-poly(D-lactide), PEG-(PDLA)(8), star block copolymers were synthesized by ring-opening polymerization of either L-lactide or D-lactide at room temperature in the presence of a single-site ethylzinc complex and 8-arm PEG (M(n) = 21.8 x 10(3) or 43.5 x 10(3)) as a catalyst and initiator, respectively. High lactide conversions (>95%) and well-defined copolymers with PLLA or PDLA blocks of the desired molecular weights were obtained. Star block copolymers were water-soluble when the number of lactyl units per poly(lactide) (PLA) block did not exceed 14 and 17 for PEG21800-(PLA)(8) and PEG43500-(PLA)(8), respectively. PEG-(PLA)(8) stereocomplexed hydrogels were prepared by mixing aqueous solutions with equimolar amounts of PEG-(PLLA)(8) and PEG-(PDLA)(8) in a polymer concentration range of 5-25 w/v % for PEG21800-(PLA)(8) star block copolymers and of 6-8 w/v % for PEG43500-(PLA)(8) star block copolymers. The gelation is driven by stereocomplexation of the PLLA and PDLA blocks, as confirmed by wide-angle X-ray scattering experiments. The stereocomplexed hydrogels were stable in a range from 10 to 70 degrees C, depending on their aqueous concentration and the PLA block length. Stereocomplexed hydrogels at 10 w/v % polymer concentration showed larger hydrophilic and hydrophobic domains as compared to 10 w/v % single enantiomer solutions, as determined by cryo-TEM. Correspondingly, dynamic light scattering showed that 1 w/v % solutions containing both PEG-(PLLA)(8) and PEG-(PDLA)(8) have larger "micelles" as compared to 1 w/v % single enantiomer solutions. With increasing polymer concentration and PLLA and PDLA block length, the storage modulus of the stereocomplexed hydrogels increases and the gelation time decreases. Stereocomplexed hydrogels with high storage moduli (up to 14 kPa) could be obtained at 37 degrees C in PBS. These stereocomplexed hydrogels are promising for use in biomedical applications, including drug delivery and tissue engineering, because they are biodegradable and the in-situ formation allows for easy immobilization of drugs and cells.

Effect of pH on the Gelation Properties of Skim Milk Gels Made From Plant Coagulants and Chymosin

The effect of three milk pH values, 6.0, 6.3 and 6.7, on gelation properties was monitored by small amplitude oscillatory rheology as well as a large deformation (yield) test for gels induced by the plant coagulants, Cynara cardunculus L. and Cynara humilis L., and chymosin. Gel microstructure was studied using confocal scanning laser microscopy. For each coagulant, a decrease in pH of milk resulted in a decrease in gelation time (tg), and an increase in the rate of increase in storage modulus (G′). At pH 6.0 and 6.3, plant coagulant-induced gels reached a maximum value in G′ (G′max) followed by a decrease in G′ values during the rest of the experiment, reflecting higher proteolytic activity of plant coagulants towards caseins as determined by SDS-PAGE. Gels produced at pH 6.0 and 6.3, exhibited a minimum in loss tangent (tan δ) followed by slight increase in tan δ during gel aging, that may have been associated with faster rearrangements of the gel network structure. In gels aged for ∼6h, the values for G′, tan δ at low frequency (0.006Hz) and yield stress were higher for chymosin than for plant-induced gels. For gels with the same pH value, no major differences were observed in microstructure between coagulants. Gels made at low pH values (6.3 and 6.0) appeared to have a denser or more interconnected structure than gels made at pH 6.7. Our results suggest that, at a low pH, the type of coagulant used in gelation is likely to have a considerably impact on gel/cheese structure.

Characterization of gelation time and texture of gelatin and gelatin–polysaccharide mixed gels

The effects of cooling rate, holding temperature, pH and polysaccharide concentration on gelation characteristics of gelatin and gelatin–polysaccharide mixtures were investigated using a mechanical rheometer which monitored the evolution of G′ and G″. At low holding temperatures of 0 and 4 °C, elastic gelatin gels were formed whereas a higher holding temperature of 10 °C produced less elastic gels. At slow cooling rates of 1 and 2 °C/min, gelling was observed during the cooling phase in which the temperature was decreased from room temperature to the holding temperature. On the other hand, at higher cooling rates of 4 and 8 °C/min, no gelation was observed during the cooling phase. Good gelling behavior similar to that of commercial Strawberry Jell-O ® Gelatin Dessert was observed for mixtures of 1.5 and 15 g sucrose in 100 ml 0.01 M citrate buffer containing 0.0029–0.0066 g low-acyl gellan. Also, these mixed gels were stronger than Strawberry Jell-O ® Gelatin Desserts as evidenced by higher G′ and gel strength values. At a very low gellan content of 0.0029 g, increasing pH from 4.2 to 4.4 led to a decrease in the temperature at the onset of gelation, G′ at the end of cooling, holding and melting as well as an increase in gel strength. The gelation time was found to decrease to about 40 min for gelatin/sucrose dispersions in the presence of 0.0029 g gellan at pH 4.2 whereas the corresponding time at pH 4.4 was higher (79 min). In general, the gelation time of gelatin/sucrose dispersions decreased by a factor of 2 to 3 in the presence of low-acyl gellan. The addition of low-acyl gellan resulted in an increase in the gelation rate constant from 157.4 to 291 Pa. There was an optimum low-acyl gellan content for minimum gelation time, this optimum being pH dependent. Addition of guar gum also led to a decrease in gelation time to 73 min with a corresponding increase in the gelation rate constant to 211 Pa/min though these values were not sensitive to guar gum content in the range of 0.008–0.05 g. The melting temperature of gelatin/sucrose/gellan as well as gelatin/sucrose/guar mixtures did not differ significantly from that of pure gelatin or Strawberry Jell-O ® Gelatin Desserts. At pH 4.2, the melting rate constant was highest at a low-acyl gellan content of 0.0029 g whereas the rate constant was insensitive to low-acyl gellan content at pH 4.4. Addition of guar did not seem to affect the melting temperature or the melting rate constant.

Rennet coagulation properties of skim milk concentrated by ultrafiltration: effects of heat treatment and pH adjustment

Skim milks were ultrafiltered to produce concentrates with protein levels of ∼6 and ∼9%, and were then heated at 140°C for 4 s in a UHT plate heat exchanger. In some cases, skim milks were heated at 140°C for 4 s prior to ultrafiltration. Rennet coagulation properties were assessed using low amplitude oscillatory rheometry by measurements of storage modulus ( G′) as a function of time after rennet addition. Concentration of skim milk by UF had no effect on gelation time ( GT) when measurements were made at the natural pH. However, when measurements were made at pH 6.5, concentration of milk resulted in a decrease in GT. Curd firming rates and the final G′ values increased with increase in milk concentration by UF. UHT-treatment of UF concentrated milk resulted in increased GT, decreased curd firming rates and decreased G′ values; the effects of heat treatment on these properties were greater when UHT treatment was given to skim milk prior to UF. Acidification of UHT-treated skim milk and UF concentrates to pH 5.5, followed by neutralization to pH 6.5 (‘pH cycling’), resulted in an improvement of rennet coagulation properties of normal milk, but had no effect on 2X concentrated milk. In contrast, the rennet coagulation properties of 3X concentrated milks were adversely affected by pH cycling. Similar trends were noted when pH cycling was carried out on heated skim milk prior to UF. ©

Phase separation in silica sol-gel system containing polyacrylic acid. III. Effect of catalytic condition

Phase separations of gelling solutions have been investigated in systems containing polyacrylic acid (HPAA) and alkoxysilane over the wide range of acid catalyst concentration. Gels with micrometer-range interconnected pores as previously reported in highly acidic systems have been obtained also in the pH condition higher than the isoelectric point of silica. Decreased acid catalyst concentration leads to both an increase and a decrease in gelation time depending on pH range, and average domain sizes in resultant gels decreased and increased, respectively. The differences in the weight- and number-average molecular weights of silica polymers at gel formation well explained the changes in the relations between reaction conditions and gel morphology with pH of the solution.

Hydrolysis-condensation reactions in the acid step of a two-step silica sol-gel process, investigated with 29Si NMR at −75°C

The relation between the hydrolysis time in the acid step and the gelation time in the basic step of a two-step silica sol-gel process was investigated using different HCl concentrations in tetraethoxysilane (TEOS), ethanol and water mixtures with a molar ratio of 1:4:( a + 0.5), where a was in the range of 0.25 and 4. At values of a < 3 a minimum in gelation time was followed by a maximum and subsequently by a gradual decrease; at ⩾ 3 a strong decrease in gelation time, followed by a nearly constant value, was found as a function of the hydrolysis time. For all the mixtures, the hydrolysis times associated with the minima in gelation time are found to be inversely proportional to the HCl concentration. The hydrolysis-condensation reactions in the acid step at 50°C were observed by 29Si NMR spectroscopy at −75°C using samples drawn from the hydrolysing mixtures and immediately quenched to temperatures below −100°C. At any desired moment in the course of the hydrolysis, an accurate spectrum with a good signal-to-noise ratio can thus be obtained, corresponding with a well-defined hydrolysis time.