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.     

Film forming solutions based on gelatin and poly(vinyl alcohol) blends: Thermal and rheological characterizations

The objective of this work was to study the rheological and thermal properties of film forming solutions (FFS) based on blends of gelatin and poly(vinyl alcohol) (PVA). The effect of the PVA concentration and plasticizer presence on the flow behavior, and viscoelastic and thermal properties of FFS was studied by steady-shear flow and oscillatory experiments, and also, by microcalorimetry. The FFS presented Newtonian behavior at 30 °C, and the viscosity was not affected neither by the PVA concentration nor by the plasticizer. All FFS presented a phase transition during tests applying temperature scanning. It was verified that the PVA affected the viscoelastic properties of FFS by dilution of gelatin. This behavior was confirmed by microcalorimetric analysis. The behaviors of the storage (G′) and loss (G″) moduli as a function of frequency of FFS obtained at 5 °C were typical of physical gels; with the G′ higher than the G″. The strength of the gels was affected by the PVA concentration.     

高酰基结冷胶的溶胶和凝胶特性研究及结构初步分析

采用黏度计和质构仪对高酰基结冷胶的溶胶和凝胶性质进行研究。结果表明：高酰基结冷胶在质量浓度为 0.05～0.1g/100mL 时,表观黏度随质量浓度增加而增加,并呈现假塑性流体特性;在质量浓度为0.2～0.5g/100mL时形成凝胶,凝胶强度受胶体及 Ca2+ 质量浓度的影响明显,同时高酰基结冷胶比例增加,高低复配胶体的凝胶强度减小。并利用红外光谱扫描和核磁共振技术对高酰基结冷胶结构进行初步分析。     

Role of cosolutes in gelation of high-methoxy pectin. Part 2. Anomalous behaviour of fructose: calorimetric evidence of site-binding

Gels of high-methoxy pectin (DE 70; 1.0 wt%; pH 3.0) in the presence of fructose at concentrations of 50, 55, 60 and 65 wt% showed an intense endotherm followed immediately by an intense exotherm on heating. These transitions occurred over approximately the same temperature-range as initial gelation on cooling (characterised by low-amplitude oscillatory measurements of G′ and G″) and increased in magnitude with increasing concentration of fructose. The displacement of both transitions, and particularly the exotherm, to progressively higher temperature as the rate of heating was increased was much greater than anticipated from simple thermal lag, indicating that the underlying structural changes are slow. The proposed interpretation is that fructose is capable of site-binding to pectin in both the ordered (threefold helix) and disordered state; the endotherm is attributed to helix melting and displacement of fructose; the subsequent exotherm is attributed to re-attachment of fructose to disordered chains, and the slow kinetics of this process to the conformational mobility of disordered pectin. On cooling over the same temperature range, a single exotherm was observed; the absence of detectable splitting is attributed to rapid re-attachment of fructose to conformationally-rigid helices. The magnitude of this endotherm (ΔH≈20 J/g) is close to the value found for cosolutes that show no evidence of site-binding, and to the net change in enthalpy for the endothermic and exothermic processes observed on heating, suggesting that the values of ΔH for displacement and re-attachment of fructose are essentially equal and opposite, with the net change coming from formation or melting of threefold helices. A smaller thermal process at higher temperature (endothermic on heating and exothermic on cooling) is attributed to hydrophobic association, which was also seen as an increase in G′ and G″ on heating in two consecutive cycles of temperature change.     

Rheological characterization and activation energy values of binary mixtures of gellan

The present study determined the flow behavior and activation energy of high (HA) and low (LA) acyl gellan dispersions (0.2%) and their mixtures as a function of preparation temperature (25 and 90 °C) and of the presence or absence of Ca²⁺ (30 mM). Heated gellan mixtures containing calcium were acidified with δ-gluconolactone to obtain gels and determine linear viscoelasticity using the Kelvin–Voigt model. The studied dispersions showed non-Newtonian shear-thinning behavior. HA dispersions (with and without Ca²⁺) showed the highest activation energy values, 88.60 and 51.18 kJ/mol. Whereas, LA dispersions showed the lowest activation energy values, 3.73 and 9.19 kJ/mol. With respect to the rheological studies, it was observed that the relationships between HA and LA gellan did not affect the recovery percentages because similar values were obtained (86.90–90.00%), and this behavior along with the mean viscosity values obtained in the gel mixtures could indicate that the hydrogen bond formation between both gellan helix (HA, LA) is possible. These results can contribute to possible industrial applications of gellans in the development of new alimentary products.     

Shear-induced structuring of particulate whey protein gels

The effects of steady shear on particulate whey protein isolate (WPI) gels, at pH 5.4, have been investigated by light microscopy (LM) and dynamic oscillatory measurements. The steady shear was performed on suspensions at constant rates between 0.5 and 126/s. The gel point under static conditions (T_g) was around 78 °C and the shearing was performed during heating from 20 to 76 or to 82 °C. The gel point was postponed by the shear up to 82 °C. Steady shear up to 76 °C, at rates less than 6/s, resulted in a weaker storage modulus (G′), less frequency dependence and a higher stress at fracture compared to the unsheared gel. Steady shear up to 82 °C, at rates below 6/s, resulted in the formation of two different types of network structure. One structure was similar in appearance to the unsheared network, showing pores in the range of 50 μm. The other structure was dense, composed of smaller particles than the unsheared network and with pores in the range of 10 μm. The gels composed of two structures showed a lower G′ and stress at fracture compared to the unsheared gel. A shear rate above 24/s up to 76 °C resulted in irregular networks, which were composed of two different types of structures. One was loose and open, similar in appearance to the unsheared network structure. The other structure was dense and compact, and was present as individual aggregates. These gels also showed a weaker G′ than the unsheared gel. A shearing up to 82 °C at rates above 24/s resulted in a coarse, inhomogeneous network structure. The gels showed a weak G′, indicating aggregate break-up during the steady shear.     

Gelling properties of kefiran, a food-grade polysaccharide obtained from kefir grain

The physicochemical and gelling properties of kefiran, a water-soluble glucogalactan with probed health-promoting properties, were investigated. Gel permeation chromatograms revealed a single distribution of molecular weight corresponding to 10⁷ Da. Intrinsic viscosity of kefiran determined using Huggins extrapolations was 6.0 dl/g and using Kramer approximations was 5.95 dl/g.Kefiran has a Newtonian behaviour in diluted solutions, which becomes pseudoplastic at higher concentrations. Rheological behaviour of the solution before and after freeze drying was evaluated by small deformation oscillatory rheological measurements. The mechanical spectrum of solution corresponded to an entangled network behaviour. After freeze–thaw treatment of the solution, a rheological behaviour transition from a liquid-like system to a gel was observed. The storage modulus (G′) in cryogels was 35 times higher than the value obtained for the solution. Rheological characteristics of the cryogel were influenced by kefiran concentration. As the polymer concentration increased, higher number of interactions was evident for the increment in both moduli (G′ and G″). The behaviour of kefiran cryogels about 37 °C determines its ability to melt at mouth temperature. These results suggest that kefiran cryogels could be an interesting alternative for its application in food formulations.     

Characterization of sol-gel transitions of food hydrocolloids with near infra-red spectroscopy

Near infra-red spectroscopy (NIR, 600-1100 nm) was used to characterize sol-gel transitions of food hydrocolloids including whey protein (WP), high acyl (HA) and low acyl (LA) gellan gums. Whey protein (20% w/w), LA and HA gellan (1% w/v) aqueous solutions with selected calcium concentrations (0-40 mM) were used. Principal components analysis (PCA) and principal component regression (PCR) were performed to analyze spectral data of gel dispersions at a series of controlled temperatures (20-95 °C). Gelling temperatures of gellan dispersions were determined directly based upon the PCA results without requiring for reference values. The gelling temperatures determined with the NIR method were similar to those obtained by dynamic rheological tests and dielectric property test. The PCR models showed a good predictability for temperatures of gellan dispersions with selected calcium concentrations during cooling or heating process. This study indicates that the NIR method has a great potential to detect structural change, and to monitor time and temperature dependant behaviour of food hydrocolloids during sol-gel transition period.     

Effect of food matrix and heat treatment on the rheological properties of salmon-based baby food

Shelf stable baby foods from Alaskan salmon were developed from red and pink salmon with and without bones. The effect of salmon type, presence of bones and thermal treatment (121 °C for 55 min) on the dynamic (viscoelastic) and flow models were evaluated. Rheological behaviors of all samples were also tested over a temperature range of 25–55 °C. All samples had a higher viscoelastic behavior with consistently higher storage modulus (G′) than loss modulus (G″) over the entire frequency range used (0.5–100 rads/s at 25 °C). Thermal treatment had a significant effect (p < 0.0001) on viscoelastic behavior of baby foods when the exponential model (G′ or G″ = A(ω)^b) was used. A values were higher for the processed food for G′ and G″, and b values. The Casson model was found to be the best for the shear rate – shear stress for all types of tested samples. Retorted samples exhibited lower yield stress than their unretorted counterparts. Retorted samples were susceptible to temperature change more than unretorted samples as shown by energy of activation values (E_a) when the effect of temperature (25–55 °C) was studied with an Arrhenius type model. Red salmon without bone had higher E_a values among all samples and with pink salmon with bone recording a lowest among all of them.     

Role of cosolutes in gelation of high-methoxy pectin. Part 1. Comparison of sugars and polyols

Mixtures of high-methoxy pectin (DE 70; 1.0 wt%; pH 3.0) with ethan-1,2-diol, glycerol, xylitol, sorbitol, glucose, fructose or sucrose at concentrations of 50, 55, 60 and 65 wt% were prepared at 95 °C and changes in storage modulus (G′) and loss modulus (G″) during cooling to 5 °C, heating to 90 °C and re-cooling to 5 °C (at 1 °C/min) were measured at 1 rad s⁻¹ and 0.5% strain. In all cases, the onset temperature for gelation during cooling and the moduli recorded at 5 °C increased with increasing concentration of cosolute. Both values, however, were substantially lower for the liquid cosolutes (ethan-1,2-diol and glycerol) than for mixtures incorporating the same concentrations of the solid cosolutes. The difference is attributed to inhibition of pectin–pectin interactions by pectin–cosolute interactions, which in turn are inhibited by cosolute–cosolute interactions, these being weaker for the liquid cosolutes than for the solids (as indicated by lower melting points). On heating, there was an initial reduction in modulus, with the same temperature-course as the increase on cooling; for the solid cosolutes, this was followed by an increase attributable to hydrophobic association of methyl ester substituents. No such increase was seen with the liquid cosolutes, but differential scanning calorimetry studies showed two (reversible) thermal transitions in all cases, one over the temperature-range of the initial gelation process on cooling and the other coincident with the increase in modulus on heating in the presence of the solid cosolutes. The absence of any detectable increase in modulus on heating with the liquid cosolutes is attributed to accumulation of cosolute around the polymer chains (i.e. pectin–cosolute interactions) promoting hydrophobic association between methyl ester groups on the same chain, or within small clusters of chains, with, therefore, no contribution to network structure. At high concentrations of the solid cosolutes, the increase in modulus on heating was followed by a decrease at higher temperature; this is attributed to excessive aggregation, and was reflected in lower moduli on subsequent re-cooling to 5 °C, in contrast to the enhancement in gel strength after heating and cooling that was observed at lower concentrations of the same cosolutes.     

On solid-like rheological behaviors of globular protein solutions

Dynamic viscoelastic and steady flow properties of β-lactoglobulin, bovine serum albumin, ovalbumin, and α-lactalbumin aqueous solutions were investigated at 20°C. When a sinusoidal strain in the linear viscoelastic region was applied, the solutions of the globular proteins except for α-lactalbumin showed typical solid-like rheological behavior: the storage modulus G′ was always larger than the loss modulus G″ in the entire frequency range examined (0.1–100 rad/s). Under a steady shear flow, strong shear thinning behavior was observed with increasing shear rate from 0.001 to 800 s⁻¹, for the globular proteins except for α-lactalbumin. The values of the steady shear viscosity η were lower than those of the dynamic shear viscosity η* at a comparable time scale of observation, violating the Cox–Merz rule, and thus suggesting that a solid-like structure in a globular protein solution was susceptible to a steady shear strain. During isothermal gelation of the protein colloids at 70°C, no crossover between G′ and G″ was observed so that the gelation point was judged by an abrupt increase in the modulus or a sudden decrease in tanδ.     

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.     

Rheology and granule size distributions of corn starch dispersions from two genotypes and grown in four regions

Starch was extracted from corn of two genotypes (Clint and P3730) grown during the 1997 season in four regions in New Zealand and Australia. The granule size distributions in unheated and heated (80°C, 2–120 min) 2.6% starch dispersions (STDs) were different. Most of the heated STDs exhibited shear-thinning behavior, but dispersions of a Clint starch exhibited shear-thickening behavior at shear rates >300 s⁻¹. The power law consistency coefficient of the STDs increased, but their flow behavior index decreased with heating time. Values of the consistency coefficient of all the STDs were found to be related to the cube of the mean granule diameter. Frequency versus storage modulus (G′) data of a 5% Clint STD heated at 80°C, 30 min, showed increase in G′ with ω in a convex down manner, while the other STDs showed typical increase in a convex up manner.     

Structural properties of single and mixed milk/soya protein systems

Single-component gels were prepared by cold-setting aqueous preparations of thermally processed milk and soya proteins. Small deformation mechanical measurements on soya protein samples showed a strong elastic response (G′) even at the hydration temperature (50°C). Both proteins produced an initial monotonie increase in G′ on cooling, followed by a relatively constant modulus during a subsequent time sweep at the setting temperature (5°C). Networks were fully reversible on heating; the milk protein gels melting out completely at temperatures >60°C, whereas the soya protein gels maintained significant structure even at the highest accessible temperature (95°C). The lack of thermal hysteresis or of sharp, cooperative melting was also confirmed by differential scanning calorimetry. Further investigation of the macromolecular properties of the gels, comprising G′ dependence as a function of frequency of oscillation and creep experiments, suggests that gels remain stable within the time scale of the measurements (90 min). Finally, under increasing amplitude of oscillation, networks withstood structural breakdown up to strain levels of ~70%; behaviour anticipated for biopolymer gels. Mixed gels were studied using a fixed amount of milk protein (10% w/w) with soya protein concentrations from 6 (minimum gelling requirement) to 16% w/w (solubility limit). Comparison of melting profiles (G′ vs. T) for the phase separated systems with those obtained for the individual components indicated phase-inversion from a milk protein continuous network to a soya continuous system at a soya protein concentration of ~11%. Analysis of solvent partition between the constituent phases utilized classical theory of network deswelling for polymer combinations below the phase inversion point and phase equilibria treatment for the soya continuous network with milk protein inclusions. In the case of equilibrium separation of the two components, results were expressed in terms of a single adjustable parameter, p (the ratio of solvent to polymer in one phase divided by the corresponding ratio in the other phase), indicating a soya hydrophilicity of ~1.25 times that of milk protein.     

Rheological behavior of binary and ternary mixtures of polysaccharides in aqueous medium

The rheological behavior of mixtures of xanthan with different galactomannans is examined to evaluate the influence of the structure of galactomannan and that of the mixture composition on the physical properties; the larger synergy is observed for locust bean gum in the presence of xanthan. It is also shown that pH has only a slight influence on the rheology down to pH = 3.59; at lower pH, the G′ modulus decreases significantly.Then, the behavior of xanthan–methylcellulose mixtures is studied, paying particular attention to the storage modulus (G′) of the system, often equated to “gel strength”. The modulus values for direct dissolution of the two polysaccharides in 0.1 M NaCl show that xanthan and methylcellulose are incompatible. The rheological behavior observed is in agreement with DSC results which indicate that no specific interaction between the two polymers exists. However, upon increasing temperature, the modulus of methylcellulose increases substantially between 65 and 70 °C because of physical gelation and dominates the rheology of the mixtures. When temperature is decreased to 37 °C, the clear methylcellulose gel formed remains stable, still giving the main contribution to the overall rheology of the system. Finally, ternary systems are studied when xanthan is mixed with galactomannan and methylcellulose. In this case H-bonds involved in galactomannan–xanthan interaction break when temperature increases causing a decrease in rheological moduli which is then compensated by the gelation of methylcellulose giving an original large increase in moduli for the ternary systems. This can be interpreted as the presence of two independent but interpenetrating networks. The role of pH for these ternary systems is interesting: it is still dominated by methylcellulose and nearly independent of acidic pH (down to pH = 1.8).     

低酰基结冷胶酸性凝胶的凝胶特性

以葡萄糖酸-δ-内酯(glucono-δ-lactone,GDL)作为酸诱导剂,制备低酰基结冷胶(low acyl gellan gum,LA)酸性凝胶,考察基体质量浓度、GDL/LA复配比例以及酸液浸泡对酸性凝胶凝胶特性的影响。研究结果表明,GDL酸化为缓慢酸化,GDL/LA复配比例越高、体系的pH值越低,酸化速率越快。基体质量浓度和GDL/LA复配比例对酸性凝胶结构影响显著,断裂应力和保水性随着GDL/LA复配比例的增大先升高后降低。基体质量浓度越高,断裂应力和不透明性越大。GDL/LA复配比例增大,断裂应变减小,不透明性增大。当酸液pH值为1时,酸液浸泡对GDL/LA复配比例为2∶1和4∶1的酸性凝胶强度无影响,但GDL/LA复配比例为1∶4、1∶2和1∶1时,凝胶强度随浸泡时间的增加而增强,酸液浸泡可以促使酸性凝胶进行结构重建。     

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。     

Influence of complexing carboxymethylcellulose on the thermostability and gelation of α-lactalbumin and β-lactoglobulin

Thermostability and gelation of the main proteins of whey, α-lactalbumin (α-lac) and β-lactoglobulin (β-lg) recovered by selective complexation with carboxymethylcellulose (CMC) was studied to evaluate its functionality in food systems. Their behavior was compared to the non-complexed proteins. Both complexes showed a maximum stability at pH 4, that is close to the pH of obtention of β-lg/CMC coacervate (pH 4) and α-lac/CMC coacervate (pH 3.2). Protein complexation increased the thermostability of β-lg by approximately 6–8 °C and that of α-lac by approximately 26 °C due to immobilization of protein molecules in a complex, mainly by electrostatic interactions and because of different amounts of bound polysaccharide. The denaturation enthalpy of complexed proteins markedly decreased as compared to free proteins. Storage modulus (G′) and loss modulus (G″) were recorded to reflect the structure development during heating β-lg/CMC and α-lac/CMC complexes at different pH values. β-lg/CMC complex at 20 wt% was a viscoelastic liquid at pH values within 2 and 8 but upon heating turned to a particulate viscoelastic gel. However, α-lac/CMC complex formed before heating opaque, large visible white particulate aggregates that sticked together to give a solid viscoelastic structure that was not further modified by thermal processing.     

Rheology-structure properties of waxy maize starch–gellan mixtures

Dynamic oscillatory shear and confocal laser scanning microscopy were used, respectively, to study the viscoelastic properties and ultrastructure of mixtures of unmodified waxy maize starch (3%) and gellan (0.005–0.05%), pasted at 75 and 90 °C. The two temperatures resulted in different rheological properties and ultrastructure. At 75 °C swollen and partially disrupted granules were observed, while at 90 °C the dominant feature was the presence of granule remnants. Addition of gellan produced mixtures with different elastic properties depending on the extent of granular disruption and gellan concentration in the mixture. Below 0.02% gellan, swollen and disrupted starch granules were surrounded by compact, yet slightly interconnected, gellan networks resulting in enhanced blends at both temperatures. Above such gellan concentration no enhancement was observed and gellan dominated the viscoelastic behavior of the mixtures because of the existence of more evenly distributed networks with swollen or disrupted starch granules exerting a weakening effect on the resulting structure.     

Rheological and thermal properties of milk gels formed with κ-carrageenan. I. Sodium caseinate

Mixed gels, formed by κ-carrageenan, and sodium caseinate were studied by differential scanning calorimetry (DSC) and rheometry. DSC showed that during gelation (i.e. cooling) the thermal behaviour of κ-carrageenan was almost uninfluenced by the presence of sodium caseinate. Thus the interaction of κ-carrageenan with sodium caseinate has little (or no) effect on the carrageenan's coil-to-helix transition. In contrast, during melting, added sodium caseinate strongly modified the thermal behaviour. The DSC peak became progressively broader with addition of sodium caseinate, indicating that the junction zones are highly heterogeneous in the mixed gel. Rheometry showed that sodium caseinate strongly influences the storage modulus (G′). In experiments in which the concentration of sodium caseinate was fixed and that of κ-carrageenan varied, plots of G′ vs. concentration of κ-carrageenan were biphasic, with an abrupt change in slope at a concentration that increased linearly with the concentration of sodium caseinate. When the concentration of κ-carrageenan was constant and that of sodium caseinate varied, G′ as a function of concentration of sodium caseinate passed through a minimum. This behaviour could be modelled quantitatively, by assuming that: (a) the sodium caseinate adsorbs κ-carrageenan, but with a limited adsorptive capacity; (b) sodium caseinate aggregates (sub-micelles) with adsorbed κ-carrageenan can associate via interaction between free ends of adsorbed κ-carrageenan chains and form a gel network; and (c) the contributions to G′ from the sodium caseinate–κ-carrageenan network and the network formed by κ-carrageenan alone are additive. At low κ-carrageenan to sodium caseinate ratios, the sodium caseinate and κ-carrageenan combine to form a mixed gel. As the ratio of κ-carrageenan to sodium caseinate increases, the sodium caseinate becomes saturated and no further association with κ-carrageenan can occur—the increase in G′, as further κ-carrageenan is added, comes from a gel network formed by κ-carrageenan alone.