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.     

Gelation behaviour of gelatin and alginate mixtures

Mixtures of alginate and gelatin were studied by rheology as a function of different parameters, such as temperature, biopolymer concentrations, calcium concentration and ionic strength. In particular conditions, the formation of a mixed gel of alginate and gelatin is obtained. A slow release of calcium ions leads first to an irreversible alginate gel and cooling results in a reversible gelatin gel. Depending on experimental conditions, non-linear behaviours upon gelation of alginate occur and a collapse of alginate gel is directly observable by rheology. These trends are favoured between 35 and 45 °C, by a high total biopolymer concentration or a high calcium concentration and ionic strength. Different mechanisms could be responsible for this collapse, such as a competition between alginate gelation and phase separation in the biopolymer mixture or an over-association of alginate chains at high Ca²⁺ concentration, favoured by the presence of gelatin.     

高酰基结冷胶对乳清分离蛋白热诱导凝胶特性的影响

蛋白质-多糖凝胶具有良好的稳定性和机械强度,在稳定和传递生物活性物质、营养强化剂方面的应用前景广阔。该研究以乳清分离蛋白、高酰基结冷胶为原料制备热诱导混合凝胶,分析高酰基结冷胶对乳清分离蛋白-高酰基结冷胶混合凝胶的凝胶强度、保水性及显微结构等,揭示乳清蛋白-高酰基结冷胶凝胶形成机理。结果表明,高酰基结冷胶促使蛋白质巯基暴露从而使凝胶形成稳定结构,提高混合凝胶的凝胶强度和保水性,且随着高酰基结冷胶含量增加而显著增大,其质量浓度为4 g/L时,复合凝胶的凝胶强度最大,为26.97 g;保水性最好,为97.41%;透光率最低,为1.87%。温度扫描结果表明,增加高酰基结冷胶可以提高乳清分离蛋白的相转变温度,傅里叶红外光谱显示,乳清分离蛋白与高酰基结冷胶存在分子间作用力,扫描电子显微镜表明高酰基结冷胶诱导混合凝胶形成结构紧密的三维网络结构。该研究为拓展乳清分离蛋白和结冷胶的新型凝胶食品,提高传统食品的质量,改善食品的加工工艺提供基础理论数据。     

Conformational ordering and gelation of gelatin in mixtures with soluble polysaccharides

Previous studies have shown that conformationally disordered, soluble biopolymers cause large enhancements in self-association of calcium pectinate, thermally denatured whey protein and gelling maltodextrin under conditions where the pre-gel solutions remained as a single phase. The purpose of the present work was to screen for similar enhancements in gel strength and rate of conformational ordering of gelatin on incorporation of soluble, disordered polysaccharides. Type B gelatin was used at a fixed concentration of 5.0 wt%, at a pH well above its isoelectric point of ∼4.5. Mixtures were prepared at 45 °C, where the gelatin is in the disordered coil form. Gel strength was characterised as Young's modulus (E), derived from the initial slope of compression curves for gels formed by holding for 16 h at 5 °C. Rate of conformational ordering of gelatin (triple-helix formation) was monitored by changes in optical rotation during and after rapid quenching from 45 to 5 °C. The polysaccharides studied were carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), guar gum, gum arabic, dextran and inulin, all of which were used in compression testing. Combinations of gelatin with HPMC and guar gum became immediately turbid on mixing and could not, therefore, be included in the samples characterised by optical rotation. The other mixtures remained homogeneous (i.e. single-phase) on holding at 45 °C. The results obtained were entirely negative. Incorporation of the soluble polysaccharides caused no significant changes in Young's modulus, and optical rotation values remained within ∼10% of those for gelatin alone. It is suggested that the enthalpic advantage of segregation is sufficient to overcome the small reduction in entropy from association of conformationally immobile subunits in gelation of calcium pectinate, whey protein and maltodextrin, but not sufficient to overcome the much greater loss of entropy from conversion of flexible gelatin coils to rigid triple helices.     

Textural properties of legume protein isolate and polysaccharide gels

The influence of legume proteins from lupin, pea and fababean on the formation of gels prepared by heat treatment in the absence or presence of xanthan gum, locust bean gum and NaCl was investigated. The resulting fracture and texture properties of gels not only are associated with the heating process used to form the gel but also depend on the conformational aspects of xanthan–locust bean gum in admixture with legume proteins, which after 10 days of aging reinforce the system. The fracture and textural properties are explained in terms of the effect of the protein–polysaccharide molecular structure and physicochemical conditions applied in the gel system during the gel preparation and measurements. Copyright © 2006 Society of Chemical Industry     

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.     

Quantitative assessment of phase composition and morphology of two-phase gelatin–pectin gels using fluorescence microscopy

A technique for quantitative determination of the concentrations of polysaccharide and protein in two-phase mixtures by fluorometry has been developed and compared with chemical analysis. In the first case, a general method for fluorescent labeling of carbohydrate polymers was developed. For the latter purpose, two micro-assays were developed on the basis of recent polymer macro-assays. A blend of low-methoxyl pectin and gelatin B was used as a model system. The commercial components were subjected to multi-step purification procedures, and phase separation was initiated by the addition of NaCl to aqueous solutions containing the two polymers. Samples were withdrawn for microscopy after various holding times at 60°C. Tie-lines were determined using both the fluorescent and chemical methods. The results from these methods were in fair agreement with each other and with literature data. A three-phase region was discovered in the pseudo-ternary phase diagram. The morphology of double labeled gels was also studied in two and three dimensions using confocal scanning laser microscopy. The results show promise for the quantitative assessment of phases that contain carbohydrate polymers and in the study of morphological changes that occur during thermo-mechanical processing.     

Physicochemical properties of gelatin gels from walleye pollock (Theragra chalcogramma) skin cross-linked by gallic acid and rutin

Gelatin gels were cross-linked by gallic acid and rutin. The gel strength, viscoelastic properties, thermal stability, swelling property, ultrastructure, X-ray diffraction patterns and FTIR spectra were determined to evaluate the physicochemical properties of the modified gels. The gel strength increased with increasing gallic acid concentration up to 20 mg/g dry gelatin, and then decreased at further elevated gallic acid concentration, while it continuously increased with increasing levels of rutin. Either cross-linking agent could enhance the elastic modulus (G′) and the viscous modulus (G″) of hydrogels, but the gelling and melting points didn’t show a notable improvement. Rutin boosted the thermal stability of xerogels, but decreased the equilibrium swelling ratio significantly, while as for gallic acid, there were no obvious effects on the thermal stability and equilibrium swelling ratio of xerogels. Scanning electron microscopy (SEM) was applied to observe the ultrastructure changes of the modified xerogels suggesting that gelatin xerogel at rutin concentration of 8 mg/g dry gelatin showed the highest cross-linking density. X-ray diffraction revealed that both gallic acid and rutin could enter the spacing of polypeptide chains of gelatin to reinforce the intermolecular interaction. And FTIR spectra verified that gallic acid and rutin molecules mainly interacted with skeletal C–N–C group and carboxyl group of gelatin molecules in the formation of gels. The results suggested that rutin was a better cross-linking agent for gelatin, and gels treated with rutin could be found with different physicochemical properties.     

Associative and segregative interactions between gelatin and low-methoxy pectin: Part 2—co-gelation in the presence of Ca

The effect of segregative interactions with gelatin (type B; pI=4.9; 0–10 wt%) on the networks formed by low-methoxy pectin on cooling in the presence of stoichiometric Ca²⁺ at pH 3.9 has been investigated by rheological measurements under low-amplitude oscillatory shear. Samples were prepared and loaded at 85 °C, cooled (1 °C/min) to 5 °C, held for 100 min, and re-heated (1 °C/min) to 85 °C, with measurement of storage and loss moduli (G′ and G″) at 10 rad s⁻¹ and 2% strain. The final values of G′ at 5 °C for mixtures prepared at the same pH without Ca²⁺ were virtually identical to those observed for the same concentrations (0.5–10.0 wt%) of gelatin alone, consistent with the conclusion from the preceding paper that electrostatic (associative) interactions between the two polymers become significant only at pH values below 3.9. Increases in moduli on cooling in the presence of Ca²⁺ occurred in two discrete steps, the first coincident with gelation of calcium pectinate alone and the second with gelation of gelatin. Both processes were fully reversible on heating, but displaced to higher temperature (by 10 °C), as was also observed for the individual components. The magnitude of the changes occurring over the temperature range of the gelatin sol–gel and gel–sol transitions demonstrates that the gelatin component forms a continuous network; survival of gel structure after completion of gelatin melting shows that the calcium pectinate network is also continuous (i.e. that the co-gel is bicontinuous). On progressive incorporation of NaCl (to induce phase separation before, or during, pectin gelation) the second melting process, coincident with loss of calcium pectinate gel structure, was progressively abolished, indicating conversion to a gelatin-continuous network with dispersed particles of calcium pectinate. These qualitative conclusions are supported by quantitative analyses reported in the following paper.     

Effects of various plasticizers on mechanical and water vapor barrier properties of gelatin films

Different kinds of plasticizers were chosen to study the effects of plasticizer composition, size and shape on the mechanical properties and water vapor permeability (WVP) of gelatin films in this paper. Firstly, oligosaccharides – sucrose, and some organic acids such as oleic acid, citric acid, tartaric acid, malic acid (MA) were added to gelatin. It was found that only MA could improve the ductility of gelatin film, and the visual appearance of MA modified gelatin film was better. Secondly, polyethylene glycols (PEG) with different molecular weights (300, 400, 600, 800, 1500, 4000, 10?000, 20?000) were used to plasticize gelatin films. This showed that PEG of lower molecular weights exhibited better plasticizing effect for gelatin films, and such films had better visual properties. This shows that mannitol (Man) and sorbitol (Sor) could make gelatin films flexible, whereas Man could crystallize from gelatin film. Following this, the plasticization of ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG) series and ethanolamine (EA), diethanolamine (DEA), triethanolamine (TEA) series was studied. At last, suitable plasticizers (MA, PEG300, sorbitol, EG, DEG, TEG, EA, DEA, TEA) for gelatin were selected to investigate the WVP and water content of these plasticized gelatin films. The mechanical properties of these films were also compared.     

Gelation mechanism and network structure of mixed solution of low- and high-acyl gellan studied by dynamic viscoelasticity, CD and NMR measurements

The gelation mechanism and the change of the network structure during cooling of the mixed solution of high-acyl (HA) and low-acyl (LA) gellan (containing 0.5% HA gellan and 0.5% LA gellan; hereafter called “mixed solution”) were elucidated on the basis of the results of dynamic viscoelasticity, circular dichroism (CD), and NMR measurements, which provide information about the network formation, the structural change due to random coil-double helix (C–H) transition, and the chain mobility of gellan, respectively. It was demonstrated that HA gellan chains in the mixed solution underwent C–H transition individually to form a network structure at the transition temperature for 1% HA gellan solution (75 °C), where storage modulus G′ and loss modulus G″ were steeply increased and the chain mobility of the HA gellan was restricted. The structural change of the HA gellan chains proceeded gradually with further cooling. At 25 °C, which is the C–H transition temperature for 1% LA gellan solution, LA gellan chains in the mixed solution formed a double helix, where G′ and G″ were slightly increased and the chain mobility of LA gellan was restricted. The results suggest that the double helix formation involves only the same kind of gellan chains even in the mixed solution, and that LA gellan chains decrease the mobility and promote the double helix formation of HA gellan chains, and vice versa.     

Rheological and structural characterization of gels from whey protein hydrolysates/locust bean gum mixed systems

The gelling ability of whey proteins can be changed by limited hydrolysis and by the addition of other components such as polysaccharides. In this work the effect of the concentration of locust bean gum (LBG) on the heat-set gelation of aqueous whey protein hydrolysates (10% w/w) from pepsin and trypsin was assessed at pH 7.0. Whey protein concentrate (WPC) mild hydrolysis (up to 2.5% in the case of pepsin and 1.0% in the case of trypsin) ameliorates the gelling ability. The WPC synergism with LBG is affected by the protein hydrolysis. For a WPC concentration of 10% (w/w), no maximum value was found in the G′ dependence on LBG content in the case of the hydrolysates, unlike the intact WPC. However, for higher protein concentrations, the behaviour of gels from whey proteins or whey protein hydrolysates towards the presence of LBG becomes very similar. In this case, a small amount of LBG in the presence of salt leads to a big enhancement in the gel strength. Further increases in the LBG concentration led to a decrease in the gel strength.     

亚麻籽胶对猪、兔肉混合肉糜凝胶特性的影响

将亚麻籽胶添加至猪、兔肉混合肉糜凝胶中,测定肉糜凝胶的凝胶强度、白度、持水力、全质构等,研究不同的亚麻籽胶添加量对猪、兔肉混合肉糜凝胶品质特性的影响,并用变焦显微镜观察凝胶的立体微观结构。变焦显微镜观察结果表明,不同的亚麻籽胶添加量可显著改变猪、兔肉混合肉糜凝胶的微观结构,且添加量为0.2%时凝胶结构最为致密均一。同时,添加0.2%的亚麻籽胶可显著提高凝胶的白度,增加持水能力,改善凝胶的质构特性,但并不能提升凝胶强度。      

Associative and segregative interactions between gelatin and low-methoxy pectin. Part 3. Quantitative analysis of co-gel moduli

The experimental moduli (G′ at 5 °C) reported in the preceding paper for gelatin–calcium pectinate co-gels (pH 3.9; 1.0 wt% pectin; stoichiometric Ca²⁺; 0–10 wt% gelatin) formed in the presence or absence of 1 M NaCl have been analysed using a single adjustable parameter, p, to characterise partition of solvent. The analysis of samples incorporating 1 M NaCl assumed complete segregation of calcium pectinate into dispersed particles in a continuous gelatin matrix, with p defined as the ratio of water/polymer in the gelatin phase divided by the corresponding ratio for the pectin phase. Relative phase volumes at each trial value of p were used to determine the polymer concentration in each phase, and the corresponding moduli were obtained from standard calibration curves. For solvent distributions where the calculated modulus of the continuous gelatin phase was higher than that of the dispersed calcium pectinate phase, co-gel moduli were derived using the Takayanagi isostrain model, and the isostress model was used for the converse situation. The p factors required to give perfect agreement with the moduli observed experimentally were tightly grouped around a single value (p=1.21) for all concentrations of gelatin studied, indicating that the assumption of complete segregation is reasonably valid. Calculated moduli for the gelatin phase were in good agreement with experimental values obtained by melting the gelatin network, centrifuging to sediment the dispersed calcium pectinate particles, and re-gelling the gelatin supernatant. The same p factor (1.21) was used to derive calculated moduli for co-gels formed in the absence of NaCl, where the mixed solutions remain monophasic, by application of the relationship proposed by Davies for bicontinuous composites. The modulus of the calcium pectinate gel, which is already present when the gelatin network forms, was calculated (i) on the assumption of dynamic cross-linking (i.e. using the concentration-dependence of G′ for calcium pectinate alone), and (ii) for permanent cross-linking (by application of deswelling theory). The experimental moduli moved from close agreement with the former model to close agreement with the latter as the gelatin concentration increased from 0 to 10 wt%, consistent with a progressive increase in the extent of rearrangement of the calcium pectinate network required to accommodate the compression introduced by gelation of gelatin.     

High pressure calorimetry at sub-zero temperature: evaluation of the latent heat and frozen water ratio of gelatin gels

An isothermal differential high-pressure calorimeter was specifically designed and described in this study. After calibration, the apparatus was validated by measuring the latent heat of ice melting. A good agreement was observed between the experimental results and literature data. The latent heat of gelatin gels at two different dry matter contents (2 and 10% w/w) was measured at sub-zero temperatures and high pressure. The main result showed that the latent heat values were influenced by both the melting temperature and the dry matter content. A decrease of the latent heat of melting of the gels with increasing melting pressure (decreasing of the melting temperature) probably indicated a modification in the ratio of frozenable water in these gels under pressure.