Properties Of Gels Research Articles

Silver carp swim bladder collagen derived from deep eutectic solvents: Enhanced solubility against pH and NaCl stresses

Deep eutectic solvents (DESs) are renowned in chemical and food industries for their eco-friendly extraction efficacy. Silver carp swim bladders, a collagen-rich byproduct of surimi production, are underutilized, resulting in considerable protein waste. Traditionally, collagen extraction has relied on harsh acids, contributing to environmental pollution and producing collagens with limited solubility, thus constraining their applications. This study evaluated DESs compared to conventional acids in extracting collagen, focusing on structural and solubility variations. Notably, urea-based DESs (urea-lactic acid: U-LA, 1:10, w/v) achieved the highest hydroxyproline recovery rates (∼ 94 %), comparable to acetic acid (AA, 1:20, w/v), but with half the solid-liquid ratio. Unlike acid-extracted collagen, which preserved the triple-helical structure, urea-based DESs partially disrupted this configuration by reducing intramolecular hydrogen bonding within collagen. However, these solvents simultaneously increased intermolecular hydrogen bonding. This alteration significantly enhanced collagen's solubility, maintaining over 60 % across a broad pH range (1−10) and various NaCl concentrations (0–6 %, w/v). Furthermore, urea-acetic acid (U-AA) extracted collagen exhibited the highest maximum transition temperature (solid state, Tmax = 101.94 °C) and gel strength (165 g). The findings suggest that urea-based DESs not only enhance collagen recovery rates but also its solubility and gelation properties, broadening its potential applications in cosmetics, food products, and biomaterials.

Composite starch/fermented protein emulsion gels for plant-based cheese applications

This work investigated the effect of incorporating lipids and proteins on the properties of starch-based gels relevant for plant-based cheese analogues. Gels were prepared by first fermenting protein-based emulsions to create primary gels, and subsequently using the water of these primary gels as starch hydration media to produce secondary, starch-based, pea protein-rich composite emulsion gels. Fermentation appeared to increase gel hardness compared to non-fermented protein addition. Compared to natural and crosslinked/acetylated starches, the acid-hydrolysed/acetylated starch produced firmer gels and was selected for further experimentation. Gels were macroscopically homogeneous but microscopically showed lipid coalescence at increasing lipid content (>8%), which was reduced by ultrasonicating the emulsion prior to fermentation. Dry matter content was key in determining textural and rheological properties of the gels, with starch/protein gels of 36% dry matter showing hardness and viscoelastic properties similar to those of the dairy cheese Danbo. However, the composite gels did not melt on increasing temperature up to 80 °C (delta below 20°). Proton dynamics were found to depend on gel properties and lipid addition. Overall, this work contributes towards the development of plant-based cheeses with desired characteristics by increasing their protein content and modulating their properties through fermentation and syneresis.

Gelatin-based hydrogels with tunable network structure and mechanical property for promoting osteogenic differentiation

Osteoarthritis (OA) is a joint disease involving all joint components, including cartilage, calcified cartilage, and subchondral bone. The repair of osteochondral defects remains a significant challenge in orthopedics. Development of new strategies is essential for effective osteochondral injury repair. In this study, gelatin (Gel), polyethylene glycol diglycidyl ether (PEGDGE), hydroxyethyl cellulose (HEC) and chitosan (CS) were used to prepare semi-IPNs and IPNs hydrogels. Mechanical properties of Gel based hydrogels significantly improved with the semi-IPN and IPN structures. Tensile strength ranges from 238.7 KPa to 479.5 KPa, and its compressive strength ranges from 35.6 KPa to 112.7 KPa. Additionally, the stress relaxation rate increased with higher CS concentrations, ranging from 25 % to 35 %. The network structure of Gel-based hydrogels was a key factor in regulating stress relaxation. Viscoelasticity was adjusted by its network structures. Swelling and degradation behaviors of Gel based hydrogels were systematically investigated. Gel based hydrogels had good cytocompatibility. Both semi-IPN and IPN structures Gel based hydrogels could promote cell spreading and osteogenic differentiation. G10HEC1 and G10CS1 hydrogels show promise as candidates for osteochondral tissue regeneration, offering a new strategy for osteochondral tissue engineering.

Improved qualities of cod-rice dual-protein gel as affected by rice protein: Insight into molecular flexibility, protein interaction and gel properties

Blending plant-based proteins with animal-based proteins to achieve adequate dietary protein intake is a strategy to address dietary deficiencies in the elderly. This research systematically investigated the effect of the ratio of cod protein/rice protein (21:0, 21:1.5, 21:3, 21:4.5, 21:6, 21:7.5, and 21:9) on the gelation properties of dual-protein gels and the underlying dual-protein interaction mechanisms. The results indicated that the myosin heavy chain (MHC) of cod and the glutelin in rice protein are primarily linked by hydrogen bonds, particularly involving Tyr residues, as evidenced by molecular docking and fluorescence quenching results. The addition of rice protein in cod protein promoted α-helix transforming into β-sheet, β-turn and random coil of the original protein solution, which was significantly correlated with molecule flexibility increasing. The decrease in the dual-protein particle size, and rice protein uniformly distributed in a cod protein-based gel network, which promoted the compactness and density of the gel structure. It was found that the hardness and springiness of 21:6 cod-rice protein gel increased by 73.96% and 17.28% compared to single cod gel, respectively. This study provides theoretical basis to the mechanism of dual-protein interaction affecting gel properties from the molecular level.

Modification of gel properties of Meretrix meretrix (clam) with polysaccharides: physical characterization and interaction mechanism.

This study investigated the contribution of 11 polysaccharides (2%, w/w), including pectin (PC), κ-carrageenan (KC), ι-carrageenan (IC), gellan gum (GG), guar gum (GM), sodium alginate (SA), konjac gum (KG), gum arabic (GA), fucoidan (FC), locust bean gum (LBG), and curdlan (CD), to the gel and microstructural properties of Meretrix meretrix clam gel (MMG). The hardness, springiness and chewiness of MMG with KC, IC, GG, SA and FC addition increased by ~10%-250%, while PC, GM, KG and LBG groups decreased by ~0.6% to 69%. KC, IC, SA, GG and FC decreased the cooking loss rate (CLR) by 69.4% to 88.7% and correspondingly enhanced the water holding capacity (WHC) by 10.2% to 21.4%, which was accompanied by an increased bound water and immobilized water area and high hydrogen proton density. The addition of KC transformed the MMG microstructure from a loose network with large pores to a compact, dense network, reducing lacunarity by 57.9%. The primary intermolecular forces in MMG with the incorporation of KC, IC, GG, SA and FC were hydrophobic interactions and disulfide bonds, which increased by 32.8%-105.3% and 45.6%-114.5% than MMG alone, respectively. Collectively, KC, IC, GG, SA and FC could improve the gel properties of MMG and the strongest synergistic combination was found in the MMG/KC system. This study suggests that the incorporation of polysaccharides is a strategy with potential for modifying the gel properties of shellfish surimi products. © 2024 Society of Chemical Industry.

Insight into the digestion mechanism of proteins in silver carp (Hypophthalmichthys molitrix) surimi by different alginates

This study aimed to evaluate the impact of potassium alginate (PA), propylene glycol alginate (PGA), and calcium alginate (CA) on the gel properties of silver carp (Hypophthalmichthys molitrix) surimi (control group) throughout gastrointestinal digestion. The findings revealed that the protein digestibility of the PA/PGA and CA groups was found to be lower compared to the control group. Among these groups, the CA group had the lowest digestibility rate at 82.49 ± 3.50 %. The study revealed a reduction in the number of peptides found in the surimi group treated with alginate compared to the control group. Alginate was discovered to have inhibitory effects on proteolysis by forming a robust cross-linked network that obstructs pepsin from accessing its substrates. This research provides valuable insights into the potential application of alginate for improving the digestibility of surimi proteins and creating commercial surimi products.

Effect of ultrasound combined with highland barley dietary fiber on gel properties of reduced-salt chicken breast myofibrillar protein.

This study aimed to investigate the effect of ultrasound combined with highland barley dietary fiber (HBDF) on the quality of reduced-salt chicken breast myofibrillar protein (MP) gel. The molecular forces maintaining gel structure, the gelling formation process, and gel microstructure of different groups, two control groups (2% sodium chloride [NaCl] group, 1% NaCl group), and four treatment groups (0.3% HBDF+U5, 0.3% HBDF+U10, 0.5% HBDF+U5, and 0.5% HBDF+U10) were examined. Results indicated significant improvements (p<0.05) in gel properties such as water-holding capacity, textural characteristics, and color of the MP gel of the four treatment groups compared to Control 2 (1% NaCl) group. Furthermore, the second structural alterations were characterized by increase β-sheet, β-turn, and random coil structure contents in treatment groups, especially in 0.3% HBDF+U5 and 0.5% HBDF+U5 groups; in addition, the exposure of more hydrophobic groups and the formation of disulfide bonds and hydrogen bonds were promoted in treatment groups, thus enhancing protein aggregation and gel quality. Finally, compared to Control 2 (1% NaCl) group, more compact and uniform gel network structures and pores inside the composite gels were observed in treatment groups. In conclusion, the findings demonstrated that the application of ultrasound in combination with HBDF improved the gelling characteristics of reduced-salt chicken breast MP gel, especially 0.3% HBDF+U5 and 0.5% HBDF+U5 groups.

Effect of three polysaccharides with different charge characteristics on the properties of highland barley starch gel

Highland barley, a nutritious whole grain, faces limited market utilization due to the poor heating stability of its starch. The aim of this study was to investigate the effects of three differently charged ionic polysaccharides—guar gum (GG), xanthan gum (XG), and carboxymethyl chitosan (CMC)—on the gel properties of highland barley starch (HBS). GG and XG notably increased pasting viscosity, viscoelasticity, hardness, and strength of HBS gels. Conversely, CMC resulted in decreased gel properties. All three polysaccharides enhanced OH tensile vibration (3000–3800 cm−1), with GG and XG promoting denser honeycomb network structures and lower spin–spin relaxation time (T2), indicating improved structural integrity. In contrast, low concentrations of CMC led to disorder and loose structure. Hydrogen bonding and electrostatic interactions were the main forces by which polysaccharides influenced the properties of starch gels. This research contributes to enhancing the properties of HBS gel during heating and expanding its commercial applications. It also provides some insights to understand the interaction between different charged polysaccharides and starch.

Time-varying moisture characteristics and C-S-H gel properties of concrete in dry and hot environments

The construction of deeply buried tunnels has intensified concerns regarding shrinkage cracking and the long-term performance deterioration of lining concrete in dry-hot environments. These issues are related to the dynamic interconversion of internal moisture states, the changing moisture behavior over time, and the evolution of calcium silicate hydrate (C-S-H) properties. However, the mechanisms underlying these phenomena are not fully understood. This study investigates the dynamics of moisture content across different states, the source and composition of evaporated water, as well as the mechanism of change in the microscopic properties of C-S-H in concrete subjected to dry-hot conditions (40℃ and 60℃) and standard curing environments through tests on water loss rates, chemically bound water, and low-field nuclear magnetic resonance. The findings reveal that at 40℃ and 60℃, hydration products were generated rapidly before mold removal, leading to significant increases in chemically bound water, interlayer water, and gel water, while capillary water was notably consumed. After mold removal, the rate of chemically bound water increase slowed, while interlayer water, gel water, and capillary water evaporated significantly, with gel water comprising 87.27 % and 83.51 % of the evaporated water at 40℃ and 60℃, respectively. From day 1 to day 28, interlayer water, gel water, and capillary water contents decreased significantly, with reductions of up to 85.98 % at 60℃. In dry-hot environments, moisture primarily exists as chemically bound water and gel water, contrasting with standard curing conditions where capillary water is also present. The study concludes that drying of C-S-H gels leads to both positive effects, such as interlayer space compression and increased indentation modulus, and negative effects, including increased microporosity and reduced matric densification, ultimately contributing to the deterioration of long-term mechanical properties.

Effect of chitosan oligosaccharides with different molecular weight in alleviating textural deterioration of chicken myofibrillar protein gel with high-temperature treatment

The molecular weight (MW) of oligosaccharides on gel properties of myofibrillar protein (MP) at high temperature remains unclear. In this study, it was found that chitosan oligosaccharides (CO) with different MW all significantly alleviated the textural deterioration of MP gel with high-temperature treatment. Moreover, MP-CO gel with the largest MW had the highest breaking force and the lowest cooking loss. Low-field NMR results further indicated that MP-CO gel with larger MW of CO had gradually increased relaxation rate, thus binding water more tightly. Rheological and microrheological tests suggested the addition of CO with larger MW resulted in much tighter gel network. These results indicated that CO with larger MW improved the quality of MP gel more effectively, which was because CO with larger MW inhibited aggregation of MP to a larger extent, resulting in smaller MP aggregates. Then MP-CO gel with much denser and more homogeneous structure was formed. Besides, MP-CO gel with larger MW of CO had higher content of β-sheet, resulting in MP gel with more ordered structure and better gel quality. Therefore, this study provided theoretical guidance for choosing the appropriate CO in improving texture of high temperature meat products.

Mechanism of Toona sinensis seed polyphenols inhibiting oxidation and modifying physicochemical and gel properties of pork myofibrillar protein under oxidation system

This research aimed to elucidate how Toona sinensis seed polyphenols (TSSs) inhibited pork myofibrillar protein (MP) oxidation and modified physicochemical and gel properties in the Fenton oxidation system. Our result displayed that TSSs had a lower carbonyl (from 1.38 to 0.59 nmol/mg) and dityrosine contents (from 47.22 to 25.07), and higher free amino content (from 386.99 to 485.00 nmol/mg), followed by ellagic acid (EA) and quercetin-3-O-rhamnoside (QC). Meanwhile, the incorporation of phenolic compounds changed secondary and tertiary structure of proteins and then increased solubility and absolute value of zeta potential, as well as decreased turbidity and average particle size. Molecular docking indicated that MP interacted with EA primarily via hydrogen bonds and hydrophobic interaction, and with QC mostly through hydrogen bonds and electrostatic interaction. Otherwise, the incorporation of EA promoted MP gel to form a honeycomb-like microstructure after oxidation. Therefore, TSSs, EA, and QC could significantly inhibit MP oxidation as well as modify its physicochemical properties, but only EA enhanced its gel properties after oxidation. The results could be useful to improve the comprehensive utilization of plant by-products and provide theoretical references for the role of TSSs in the improvement of meat products.

Improving gel properties of low-salt silver carp surimi through single-mode microwave-assisted processing

Surimi, a product derived from low-value fish, is widely used to produce high-value items such as imitation crabsticks and fish balls. However, surimi from freshwater fish like silver carp generally exhibits inferior gel properties compared to marine fish. This study investigated the effects of 915 MHz single-mode microwave-assisted processing on the gel properties of low-salt silver carp surimi. Traditional water bath processing served as the control for comparison. Gel strength, texture, water-holding capacity (WHC), whiteness, protein secondary structure, and microstructure, along with thermal processing efficiency for microbial inactivation were evaluated. The results demonstrated that single-mode microwave processing significantly improved gel properties compared to water bath processing. Compared with the optimal water bath treatment, the gel strength of microwave processed samples increased significantly from 479.32 g·cm to 821.74 g·cm, while WHC increased from 0.77 to 0.87. The microwave processed surimi exhibited a more uniform microstructure. Because it enhanced the dissolution and cross-linking of myofibrillar proteins, leading to superior gel formation characterized by a lower α-helix content (10.31%) and higher β-sheet content (61.70%). Furthermore, microwave processing achieved faster heating rates, reducing the time spent in the critical gel deterioration temperature range, thereby improving overall gel quality. Single-mode microwave-assisted gelation also provided better thermal processing uniformity and efficiency, achieving pasteurization standards. The study highlights the potential of 915 MHz single-mode microwave processing to produce high-quality, low-salt surimi products, meeting contemporary health and convenience demands. These findings offer valuable insights for the industrial application of single-mode microwave technology in surimi production, promoting the development of innovative processing techniques to enhance the quality of surimi-based food products.

Structure formation mechanism of pectin-soy protein isolate gels: Unraveling the role of peach pectin fractions.

This study investigated the macro & micro properties of the composite gels formed by soy protein isolate (SPI) and peach pectin fractions: water-soluble pectin (WSP), chelator-soluble pectin (CSP), and sodium carbonate soluble pectin (NSP). Specially, the interaction between pectin fractions and SPI was studied to explain the formation mechanism of the composite gels. WSP, as a high methoxyl pectin, exhibited rich branching (sugar ratio B=3.10). CSP, as a low methoxyl pectin, depicted a high linearity. NSP, with low linearity (sugar ratio A=6.14), contained numerous side chains. Due to the strong interaction between pectin fractions and SPI, the new composites with excellent dense network microstructures were formed, accompanied by increased apparent viscosity, higher G' and G'', and reduced particle size. XRD and FT-IR analysis highlighted the modifications in gel structures. SEM-dispersive X-ray spectroscopy observed elemental distribution and framework composition in pectin-SPI gels. Hydrophobic interaction was the most important chemical force in pectin-SPI binding. Molecular docking results indicated that galacturonic acid in pectin bound more strongly to 7S than to 11S, with tighter hydrogen bonds. Notably, WSP-SPI showed the lowest turbidity, indicating enhanced solubility and particle dispersion, which helped prevent aggregation. CSP-SPI demonstrated the highest G' and G'', ascribing to the high linearity and abundant carboxyl groups in CSP. NSP-SPI showed the highest apparent viscosity and irregular structure. Overall, the texture properties of pectin-SPI gels were driven by pectin's structure properties, which would provide new and valuable information for texture control in gel formulation.

Prediction of 3D printability and rheological properties of different pineapple gel formulations based on LF-NMR

In this study, a pineapple-starch-xanthan gum system was prepared using fresh pineapple juice, maize starch, and xanthan gum (XG). The feasibility of using low-field nuclear magnetic resonance (LF-NMR) to predict pineapple gels' rheological properties and printability was evaluated. Results indicated that as maize starch and XG increased, the gel transformed from unable to support printed models to a stable shape, eventually becoming too viscous for printing. Principal component analysis and Fisher discriminant analysis classified the gels into four categories based on their rheological properties, aligning with the actual printing results. Pearson correlation analysis showed a strong correlation between the LF-NMR parameters and the rheological properties of gels. The partial least squares (PLS) and back-propagation artificial neural network (BP-ANN) models constructed using the LF-NMR parameters can effectively predict the rheological properties of pineapple gels. Therefore, LF-NMR is a valuable, non-destructive method for quickly assessing pineapple gels' 3D printing suitability.

Effect of β-sitosterol and oil phase on gel properties, microstructure and sensory characteristics of pea protein/carrageenan emulsion gels as solid fat substitutes

Effect of β-sitosterol and oil phase on gel properties, microstructure and sensory characteristics of pea protein/carrageenan emulsion gels as solid fat substitutes

Development and characterisation of a novel bigel based on pea protein hydrogel and rice bran wax oleogel: Enhancement of rheological properties and freeze-thaw stability

In this study, a novel pea protein (PP)-based bigel was developed, featuring a high internal phase emulsion. The impact of gelling agent concentration on the gel properties and freeze-thaw stability of the bigel was investigated. The bigel was comprised of two distinct gel phases: an aqueous-phase gel with a covalent network formed by PP and transglutaminase (TGase), and an oil-phase gel with a crystal network structure of rice bran wax (RBW). Microstructural analysis revealed a bi-continuous network structure in the bigel, with network density increasing as TGase and RBW concentrations rose. Rheological analysis showed that storage modulus (G′), apparent viscosity, and structural recovery of the bigel increased with higher TGase and RBW concentrations. Temperature scanning experiments confirmed that the bigel maintained its elastic solid behavior even at elevated temperatures. Optimal sensory properties and low coefficient of friction were achieved at 0.4 % TGase and 7 % RBW concentrations. Additionally, the bigel exhibited notable freeze-thaw stability at TGase and RBW concentrations exceeding 0.2 % and 5 %, respectively. These findings highlight the excellent gelation properties and stability of the PP-RBW-based bigel, suggesting its potential as a fat substitute in the food industry.

Understanding the role of CaCl2 in salt substitute for low-salt and high-quality surimi products

Salt substitute has been widely used to prepare low-salt foods due to potential health benefits, though the role of CaCl2 in salt substitute and its unique impacts on food quality have been rarely investigated. In this study, comprehensive research has been conducted to elucidate the effects of replacing NaCl with varying concentrations of CaCl2 on the surimi gel characteristics. The introduction of CaCl2 interacted with surimi proteins differently from NaCl, thus leading to difference in protein aggregation behaviors and surimi gel properties. It has been found that a proper proportion of CaCl2 for NaCl substitution could create salt bridges between surimi proteins more effectively, resulting in an ordered, smooth and dense gel network with an increased water holding capacity (WHC) and improved gel strength. Furthermore, TGase activated by Ca2+ boosted the formation of ε-(γ-glutamyl) lysine bonds, which cross-linked surimi proteins to form a firm gel with a better three-dimensional structure. However, replacing NaCl with excessive amount of CaCl2 as divalent salts induced more serious protein aggregation, leading to water loss and gel properties deterioration. More specially, replacing NaCl with CaCl2 at 50% showed the best performance, as evidenced by the most abundant disulfide bonds and hydrophobic interactions, highest hardness and chewiness, and greatest storage modulus. This study provided new insights on developing high-quality surimi gels with significantly reduced salt concentration and improved gel characteristics.

Extraction technology determines the properties of bamboo shoots dietary fiber concentrate and its application in chicken mince gels: Systematic analysis

This study investigated how physical, chemical, and enzymatic extraction technologies affected the physicochemical properties of bamboo shoot insoluble dietary fiber (IDF). Additionally, the effects of IDF extracted by these techniques on the gel properties, water retention, microstructure, and digestibility of chicken mince gels were evaluated. Results showed that IDFs extracted by physical (P-IDF), chemical (C-IDF), and enzymatic (E-IDF) exhibited a typical cellulose polysaccharide structure and strong water and oil retention capabilities, with the highest values reaching 14.18 g/g and 6.74 g/g, respectively. Compared to P-IDF, C-IDF and E-IDF displayed a rougher porous structure and stronger adsorption capacities for glucose (409.91 mg/g and 604.95 mg/g), cholesterol (54.46 mg/g and 64.06 mg/g), sodium cholate, and nitrite. The addition of IDF improved the water retention, water stability, texture, and rheological properties of chicken mince gels, and had no negative effect on the digestion of chicken mince proteins. On the one hand, the hydrophilic groups exposed in IDF absorb water and fill the gel networks, reducing the formation of water channels in the gel and improving water stability. Furthermore, the hydrophilic groups on the glucose unit that makes up the IDF interacts with the proteins through the hydrogen bond and the transformation of protein β-structure to α-structure, promoting the formation of gel network structure and improving gel texture and rheological properties. E-IDF and C-IDF showed better overall gel performances, but E-IDF had better dry matter digestibility, protein digestibility, and gastrointestinal digestive effects. Therefore, IDF prepared by enzymatic hydrolysis is more suitable for chicken mince processing.

Effect of gelled phases on the relevance of double emulsion gels for probiotics encapsulation, rheological properties, and stability

This study aimed to develop a stable double emulsion gel (DEG) based on whey protein cold-set gelation of the external water phase and carnauba wax-based solidification of the oil phase as a potential delivery system for probiotics in high-protein food products. The effect of the gelation of different phases on the microstructure, gel properties, and stability of DEGs under storage and freezing–thawing conditions and the viability of L. plantarum loaded in the inner water phase of DEGs were evaluated. The oil phase crystallization allowed the formation of stable DEGs during storage, showing negligible changes in consistency index, G′, and particle size after 56 d of storage and under four freeze–thaw cycles. The gelation of both the oil and external water phases allowed a stronger gel network formation, with a six times lower creaming index than non-gelled double emulsions. All samples demonstrated that the encapsulation of L. plantarum into the inner water phase of the DEG is beneficial for the storage stability of probiotics.

Keratin Gel From Chicken Feathers Waste Obtained by Mercaptoethanol Extraction

ABSTRACTProtein gels prepared by keratin extracted from chicken feathers show potential applications as engineered materials. Feathers are an abundant waste material, whose principal component is keratin, which may have gelling properties not yet sufficiently studied so far which are strongly dependent on the extraction method adopted. The aim of the study is to explore the properties of gels obtained through mercaptoethanol extraction and dialysis process and to evaluate their structural characteristics. The keratin hydrogels were characterized with Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy through which it was possible to identify the secondary structure of the protein on hydrated and dry gel. Moreover, the morphological analysis by scanning electron microscopy (SEM) combined with the rheological analysis showed how the consistency of the gels is maintained on a wide range of loads and frequencies. Furthermore, the biocompatibility of the gels was investigated for the release of subcutaneous drugs using curcumin, an antioxidant polyphenol compound. The fastest release was obtained at pH 7.4, corresponding to physiological conditions.