Characterization of gellan/gelatin mixed solutions and gels

The physico-chemical properties of gellan/gelatin mixed solutions and gels were examined at five different ratios of gellan to gelatin (100:0 (I), 80:20 (II), 60:40 (III), 40:60 (IV), 20:80 (V)) and four different NaCl levels (0–300 mmol/l). All mixed solutions exhibited the shear-thinning behavior, which decreased with increasing gelatin proportion, temperature, and NaCl level. Synergism on G′ was observed in mixed solution III and IV depending on NaCl level. Hardness of mixed gel decreased with increasing gelatin proportion and cohesiveness increased up to the gellan to gelatin ratio of 40–60 and then decreased. For gellan dominant gels, maximum hardness and cohesiveness were observed at NaCl level of 150 mmol/l. Increasing gelatin proportion caused an increase in gel turbidity at lower NaCl levels and a decrease in gel turbidity at higher NaCl levels. In general, WHC increased with increasing gelatin proportion and decreasing NaCl level. Color holding capacity significantly increased with increasing gelatin proportion. Flavor holding capacity increased by adding gelatin and then linearly decreased with increasing gelatin proportion. Therefore, this study suggests that there is an optimum NaCl concentration and gellan to gelatin ratio to enhance the physico-chemical properties of gellan/gelatin mixed solutions and gels.     

Effect of polymer ratio and calcium concentration on gelation properties of gellan/gelatin mixed gels

Gelation properties of gellan/gelatin mixed solutions were studied using dynamic viscoelastic testing at eight different ratios of gellan (1.6–0.2% w/v) to gelatin (0–1.4% w/v) and seven different calcium levels (0–30 mM). The gelation temperature and gelation rate of the mixed gels were significantly affected by the ratio of gellan to gelatin as well as concentration of calcium. Addition of calcium at low levels resulted in an increase in gelation temperature and gelation rate compared to gels with no added calcium. Further increases in calcium increased the gelation temperature, but caused a decrease in gelation rate of the mixed gels. In addition, the presence of gelatin generally had a negative influence on gelation rate, especially at high proportions and when the solution had a high gelling temperature, probably by physically hindering the growth and development of gellan crosslinks. It appeared that in the presence of calcium, gellan formed the continuous gel matrix, with gelatin present as a discontinuous phase. Gellan/gelatin mixtures can form gels over a wide temperature range by varying the ratio of the two polymers as well as the calcium concentration.     

结冷胶-羧甲基纤维素复配胶性能的研究

利用全质构分析、离心分离和分光光度法研究了结冷胶-羧甲基纤维素（carboxyl methyl cellulose,CMC）的配比、离子种类和离子浓度对结冷胶-CMC复配胶质构、持水力以及透明度的影响。结果表明：复配胶硬度随结冷胶比例降低而降低;在离子浓度达到临界水平前随离子浓度增加而增加,临界水平之后随离子浓度增加而降低;添加钙离子比钠离子更容易得到高硬度的凝胶。总的来说,添加离子会降低凝胶的弹性和内聚性;适当的结冷胶和CMC比例可提高凝胶弹性和内聚性。降低结冷胶-CMC复配比例可提高复配胶的透明度。     

低酰基结冷胶-果胶复配体系的性能

利用质构仪和流变仪对低酰基结冷胶(low acyl gellan,LA)和果胶复配体系的胶体质构和流变性能进行研究。考察不同种类离子(Ca~(2+)、K~+)、离子浓度(0、2、4、6、8、10、20、40 mmol/L)以及LA和果胶不同质量比(100:0、75:25、50:50、25:75)对复配体系质构特性的影响。结果表明,随着离子浓度的增加破坏应力先增大,当达到临界值后开始减小,二价离子(Ca~(2+))的作用效果要强于一价离子(K~+)。复配胶破坏应力随LA比例降低先增加后减少,质量比在75:25时具有显著协同效应。复配体系的黏度随剪切速率增大而减小;当剪切速率相同时,黏度随LA比例的增加而增加,离子种类和浓度对复配体系黏度的影响与质构特性一致。随着温度的升高,复配体系黏度减小,在相同温度条件下,结冷胶与果胶质量比75:25时,体系的黏度最大。储能模量G’大于相应质量比损耗模量G",且G’和G"随LA比例的增加而增大,说明体系的黏性和弹性随LA比例增加而变大。     

高酰基结冷胶/酪蛋白酸钠复合凝胶粘弹行为的研究

本文研究了酪蛋白酸钠浓度、结冷胶浓度、离子和测试条件对高酰基结冷胶/酪蛋白酸钠复合凝胶粘弹性的影响。结果表明:高酰基结冷胶/酪蛋白酸钠共混体系为典型的切力变稀流体,表观粘度随着酪蛋白酸钠浓度的升高而降低,而随着阳离子浓度的增大出现先增大后减小的变化趋势。压缩速度对复合凝胶硬度几乎无影响,而内聚性和弹性则随着压缩速度的增加而增大。内聚性随着压缩应变的增大出现先增大后减小的变化趋势。复合凝胶的硬度和弹性随着酪蛋白酸钠浓度的增加而下降,但复合凝胶的内聚性对酪蛋白酸钠浓度不敏感。高酰基结冷胶浓度越高,复合凝胶的硬度和弹性越大。相对于一价离子而言,二价离子形成的凝胶更强且用量更少。钾离子的添加对复合凝胶保水性影响较弱,而钙离子的添加则可以提高复合凝胶的保水性。     

Polymer and Ion Concentration Effects on Gellan Gel Strength and Strain

Failure stresses and strains were measured in compressive, tensile and torsional modes on gellan gels at four polymer (0.6–1.8% w/v) and seven Ca⁺⁺ (1.5–60 mM) concentrations. Shear stresses at failure were equal in all three testing modes and proportional to gellan content. Low calcium gels increased linearly in strength with Ca⁺⁺ concentration until it reached a level of about 0.5 calcium ions per repeat tetrasaccharide unit of gellan gum polymer. Gel strength decreased linearly with Ca⁺⁺ at higher concentrations. Low calcium gels were extensible with failure strains decreasing as the logarithm of Ca⁺⁺; whereas high calcium gels were brittle and failed at a constant strain, the value of which was twice as high in compression and torsion as in tension.     

Effect of salts and sugars on the clarity of gellan gels

The effect of glucose, fructose and sucrose (0–15 wt%) on the clarity of low acyl gellan gels (0.5 wt%) in the presence of potassium chloride (40–100 mm ), calcium chloride (10–80 mm ) and their mixtures at a total molar concentration of 80 mm , was investigated by light absorbance experiments (490 nm). The effect on gel clarity (at 100 mm KCl) of all sugar combinations, at a final concentration of 15 wt%, was also investigated. All gels with potassium were less turbid than the ones with calcium. In the case of salt mixtures, increasing concentration of calcium in the mixtures resulted in decreased clarity. In addition, turbidity increased with increasing salt concentration. Moreover, and at the higher salt concentrations, the presence of sugars resulted in clearer gels. When mixtures of sugars were added, all combinations resulted in increased clarity compared to the individual sugars.     

TEXTURE PROPERTIES OF GELLAN GELS AS AFFECTED BY TEMPERATURE

Gellan gels can be made very brittle, similar to agar gels, or very flexible, like gelatin gels. The entropy or enthalpy nature governing those gellan gel behaviors was studied by mechanical testing at temperatures varying from 2 to 62C. Both failure stress and strain for 1% low acyl and low acyl/high acyl mixed gellan gels decreased with increasing temperature, indicating that the hydrogen bonding contributed significantly to the stabilization of gellan gels in addition to the polyanion-calcium-polyanion bonding. Hydrophobic interactions were less important. The initial Young's modulus for two mixed high and low acyl gellan gels containing 2 mM Ca⁺⁺ increased with temperature from 2–42C, indicating entropy elasticity. Average molecular weight between adjacent crosslinks for these two mixed gels was larger than 10⁴. For other gels, the entropy elasticity was not a dominant mechanism for elastic force because of molecular weights between crosslinks and from the observation of negative temperature dependence of the modulus.     

Characterizations of fish gelatin films added with gellan and κ-carrageenan

Fish gelatin is known to be inferior to mammalian gelatins. Gellan and κ-carrageenan were added to improve properties of the fish gelatin films. Initially, polysaccharides were added to make fish gelatin gels, and tested for the melting point. Mechanical, barrier, color and microstructure properties, as well as Fourier transform infrared (FTIR) and thermal analysis (DSC) of the modified fish gelatin films were evaluated. The addition of gellan and κ-carrageenan increased the melting point of fish gelatin gels, gellan being more effective. Polysaccharides modified fish gelatin films by increasing tensile strength and barrier against water vapor, but made films slightly darker. Scanning electron microscopy (SEM) microstructure analysis revealed that gellan eliminated cracks present in the film matrix resulting in a more uniform structure. FTIR and DSC analyses showed that both polysaccharides effectively interacted with fish gelatin, and moreover, gellan being more effective. Overall, addition of gellan up to 2 g/100 g of gelatin performed better in enhancing fish gelatin films properties.     

EFECT OF pH BUFFERS ON MECHANICAL PROPERTIES OF GELLAN GELS

The strength and deformability of calcium cross-linked gellan gels as affected by pH 3.5 and 5.0 citrate and acetate buffers were measured by large compressive deformation test until failure. The trend of dependence of gel strength on polymer and calcium concentrations was similar to gels formed in distilled water without pH adjustment. A critical calcium concentration was observed for each gellan concentration. Gels formed at the critical calcium concentration exhibited the maximum strength. The chelating effect of pH 5.0 citrate buffer greatly increased the critical calcium concentration. The failure strain, representing the deformability, of gellan gels formed in buffers behaved differently from gels formed in distilled water. In the pH 3.5 buffer systems, gellan gels were brittle regardless of gellan and calcium concentrations. In the pH 5.0 buffer systems, gellan gels were brittle at high calcium concentrations and ductile at calcium concentrations less than 24 mM in citric buffer and less than 6 mM in acetate buffer.     

The effect of salts on the retention of ethyl butyrate by gellan gels

The effect of gellan gels with different texture on the retention of ethyl butyrate was investigated by Static Headspace Gas Chromatography. Calcium induced gels enriched with 400 ppm of the volatile showed not significantly different aroma release for calcium concentrations up to 40 mM, whereas higher concentrations exhibited greater partition coefficient values not significantly different from each other. Aroma release was not controlled by the mechanical properties when 1000 ppm of ethyl butyrate was added. When mixtures of calcium and potassium chloride, at a total molar concentration of 80 mM, were used to induce gelation, aroma release became greater with increasing calcium concentration in the mixtures. Moreover, elevated concentrations of the aroma compound (400-1000 ppm) added to gellan matrices, gelled by 10 mM calcium chloride, resulted in increased aroma release. For all samples, the percentage of retention was also calculated and both positive and negative values were determined.     

EFFECT OF ADDED SALT ON TEXTURAL PROPERTIES OF HEAT-INDUCED GELS MADE FROM GUM–PROTEIN MIXTURES

Large deformation rheological tests were employed to determine the textural differences in heat‐induced gel systems. Three different large deformation rheological methods (viscosity index and apparent elasticity, texture profile analysis (TPA) and torsional fracture) were employed to study the dependence of the ion type on the textural properties of heat‐induced mixed protein–gum gels. Protein–gum mixed solutions were prepared with bovine serum albumin (BSA) or egg white albumin (EWA) (20% w/v) with κ‐carrageenan (KCG), gellan (GLN) or xanthan gums (XNT) (0.5% w/v) at 0.1 M sodium chloride (NaCl), potassium chloride (KCl) or no added salt. Despite inherent differences in protein type, the main effect on the textural properties evaluated was for the kind of salt added, since potassium ions, with a strong influence on KCG and GLN gelation, affected the parameters related to the structure or hardness of the samples. There was no significant effect on parameters associated with sample ductility or elasticity. GLN formed stronger gels than KCG, whereas XNT did not perform as well in gel formation since it does not contribute to protein matrix formation. The results indicated that potassium may be substituted for sodium ions at low ionic strengths in foods where the incorporation of KCG or GLN helps to improve texture and related features.     

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.     

Gelling Temperatures of Gellan Solutions as Affected by Citrate Buffers

Effects of citrate buffers at pH 3.5 and 5.0 on gelling temperatures of gellan solutions with 0.4–1.8% gellan and 1.5–60 mM Ca²⁺were studied. Partial dissociation of the carboxyl groups in gellan polymer in pH 3.5 solutions resulted in weakened gels. The pH 3.5 buffer exhibited weak chelating ability for Ca²⁺. The gelling temperature of gellan solutions at pH 3.5 was quantitatively related to polymer and cation concentrations using a similar model to that for gellan water solutions. The pH 5.0 buffer exhibited strong chelating ability. Gelling temperatures at pH 5.0 were generally lower than those at pH 3.5, except at low calcium concentrations.     

Influence of pH and added gums on the properties of konjac flour gels

The gel strength and texture of konjac gel and mixed gels of konjac and various gums were measured after gelation at various concentrations of alkali. A selection of different alkaline reagents was used. Regardless of alkali concentration, increasing konjac levels caused a decrease in pH but an increase in hardness and strength of konjac/gellan gum mixed gels. The highest gel strength and hardness were given by mixed konjac/gellan gum gels using sodium carbonate as the gelling medium. Under similar gelling conditions, the addition of gellan gum resulted in the greatest gel hardness. Of the gums examined, a possible synergistic effect on konjac/gellan gum mixed gel texture was observed.     

Mechanical Properties of Gellan Gels in Relation to Divalent Cations

Mechanical behavior of gels formed with gellan polymer crosslinked by calcium and magnesium ions was studied to determine the influence of divalent ion type and polymer concentration. Failure strength and deformation were measured in compression and related to concentrations of gellan and bound cations in gel matrices. Insufficient cations formed weak, extensible gels. Maximum gel strength was achieved at 0.5 divalent cations/repeat tetrasaccharide unit, assumed to be the condition for maximal numbers of complete junction zones. At optimum cation levels gels with Ca⁺⁺ were about 1.2 times stronger than gels with Mg⁺⁺ at the same polymer concentration. Excessive cations weakened the gels. Twice as much reduction in gel strength resulted from additional Ca⁺⁺ as compared to the same additional amount of Mg⁺⁺. Differences between strengths of the gels may be attributable to polymer configurations at junction zones in relation to cation size.     

低酰基结冷胶-乳清蛋白混合凝胶的凝胶特性

研究了离子种类、离子浓度和基体浓度对低酰基结冷胶/乳清蛋白混合凝胶力学性质、保水性质、光学性质和网络孔径的影响。研究发现,乳清蛋白浓度对混合凝胶凝胶特性影响较小。混合凝胶的断裂应力、断裂应变、不透明性和保水性随着低酰基结冷胶浓度的增大而增大,混合凝胶网络孔径则随着低酰基结冷胶浓度的增大而减小。离子种类和离子浓度对混合凝胶凝胶特性影响显著,混合凝胶的断裂应力和孔径随着离子浓度的增加出现了先增大后降低的变化趋势,断裂应变和保水性则随着离子浓度的增大而减小。离子浓度增大,混合凝胶的不透明指数随之升高,当离子浓度超过某一定值后,不透明指数则基本保持恒定。相对于钠离子而言,钙离子形成的凝胶更强且用量更少。钠离子和钙离子在诱导凝胶形成上不存在协同效应。     

Rheological and sensory properties of fish gelatin gels as influenced by agar from Gracilaria tenuistipitata

Agar extracted from Gracilaria tenuistipitata and commercial agars were incorporated into fish gelatin at various levels (0, 5, 10, 15 and 20% gelatin substitution). G. tenuistipitata agar (GA) had lower failure stress (~16 kPa) than commercial agar (CA) (~20 kPa). However, the former showed higher failure strain (~30%) with lower melting temperature (65.9 °C). The critical linear stress and failure stress of agar/gelatin mixed gels increased with increasing agar levels (P < 0.05). At 15 and 20% of agar used, the mixed gels containing CA exhibited higher failure stress than those with added GA (P < 0.05). Two melting points of agar/gelatin mixed gels were observed, corresponding to the melting temperatures of gelatin and agar gels. Nevertheless, the incorporation of agar lowered the likeness score of gelatin gel. Thus, both GA and CA had the impact on rheological property and the selected sensory characteristics of fish gelatin, depending on the level of substitution.     

Gel Properties of Whey Protein Concentrates as Influenced by Ionized Calcium

Ionic calcium in four whey protein concentrates (WPC) was decreased using sodium tripolyphosphate (NaTPP) or ethylenediamine tetraacetic acid (EDTA) and effects on gel properties were determined. Total calcium ranged from 0.22–0.41 g/100g WPC and ionic calcium 2.98–47.25 mg/100g protein. Hardness was maximized and expressible moisture (EM) minimized in three WPC gels with 10 mM NaTPP. EDTA had a similar effect on one WPC gel. Addition of 10 mM NaTPP decreased ionic calcium to 5.23–10.31 mg/100g protein. NaTPP or CaCl₂ did not improve hardness or EM of one WPC gel which contained the lowest total and ionized calcium. Chelating agents were effective in improving gel properties of WPC containing higher than optimal calcium.     

Relaxation Time Spectrum of Hydrogels by CONTIN Analysis

CONTIN is a general-purpose program for inverting noisy linear algebraic and integral equations by means of inverse Laplace transform. This study explored the application of CONTIN analysis to determine the relaxation time distribution spectra for food gels, including gellan, carrageenan, whey protein, and gelatin gels, based on stress-relaxation data. CONTIN results represent the continuous relaxation time spectra when the number of the terms in the discrete Maxwell stress-relaxation model approached infinity. The CONTIN results for gellan gels were correlated to the texture properties of gels from compression tests with respect to the effects of calcium concentrations. CONTIN analysis may be a very effective tool in elucidating the microstructural properties of a hydrogel from mechanical testing.