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.     

Physicochemical properties of pectins from okra (Abelmoschus esculentus (L.) Moench)

Okra pectin obtained by hot buffer extraction (HBSS) consists of an unusual pectic rhamnogalacturonan I structure in which acetyl groups and alpha galactose residues are substituted on rhamnose residues within the backbone. The okra Chelating agent Soluble Solids (CHSS) pectin consists of slightly different structures since relatively more homogalacturonan is present within the macromolecule and the rhamnogalacturonan I segments carry slightly longer side chains. The rheological properties of both okra pectins were examined under various conditions in order to understand the unusual slimy behaviour of okra pectins. The viscosity of the okra HBSS pectin was 5–8 times higher than the viscosity of the okra CHSS pectin. The okra HBSS pectin showed an elastic behaviour (G′ > G″) over a wide range of frequencies (10⁻¹–10 Hz), at a strain of 10%, while okra CHSS and saponified okra HBSS/CHSS pectin showed predominantly viscous responses (G′ < G″) over the same frequency range. The results suggest that the structural variation within the okra pectins greatly affect their rheological behaviour and it is suggested that acetylation of the pectin plays an important role through hydrophobic associations. Dynamic light scattering was used to study the association behaviour of both okra pectins at low concentration (0.001–0.1% w/w). Results showed that the saponified okra pectins did not exhibit a tendency to aggregate in the concentration range studied, whereas both non saponified samples showed a substantial degree of association. These results suggest that the unusual slimy behaviour of the non saponified samples may be related to the tendency of these pectins to associate, driven by hydrophobic interactions.     

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.     

Rheological behaviour of uncross-linked and cross-linked gelatinised waxy maize starch with pectin gels

Rheological characterisation of uncross-linked (UPS) and cross-linked (CPS) waxy maize starch with pectin was conducted to determine the influence of pectin on the properties of the starch. The viscoelastic behaviour of 5% (w/v) gel systems containing UPS and CPS polysaccharides at 25 °C was evaluated by small angle deformation oscillation rheometry. Viscoelasticity measurements of the cross-linked polysaccharides indicated that the elastic component increased after cross-linking. Among all gels studied, the properties of the CPS mixtures (ratios 2:3 and 3:2) showed quite high storage (G′) and loss (G″) moduli (compared with gels of other ratios), indicating that gels of these two particular ratios had the greatest degree of elasticity and were very well structured. The results suggest that cross-linking between starch and pectin molecules can give rise to novel rheological properties.     

Viscoelastic properties of xanthan gum hydrogels annealed in the sol state

Viscoelastic properties of xanthan gum aqueous solutions and hydrogels were investigated using a cone-plate type rheometer. The change of viscoelasticity during annealing the solution and cooling to gelation temperature was examined as functions of annealing time, temperature and frequencies. In the annealing process, the storage modulus G′ increases with increasing annealing time. In the subsequent cooling process, G′ of the annealed solution increased, whereas the G′ of non-annealed solution remained almost constant. G′ of hydrogels increased with the increase of annealing temperature and concentration. Based on the experimental results obtained, the structural change of the solution in the annealing process and the structure of gels were investigated.     

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.     

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.     

Gel—sol transition in gellan gum solutions. II. DSC studies on the effects of salts

The thermal properties of sodium form gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were studied by differential scanning calorimetry (DSC). The DSC cooling or heating curves for 1% gellan gum solutions without salt showed a single exothermic or endothermic peak at ~30°C. DSC cooling curves showed a single exothermic peak, with the setting temperature (Ts) shifting to progressively higher temperatures with increasing concentration of the added NaCl or KCl. At low concentrations of NaCl or KCl, DSC heating curves showed a single endothermic peak; however with more addition of salt the endothermic peak gradually developed a bimodal character and eventually split into more than two distinct peaks. The onset of detectable splitting occurred at a high salt concentration which coincided with that at which elastic gels are formed at even a higher temperature as was observed by viscoelastic measurements. With a sufficient addition of monovalent cations the endothermic curve became again a single peak shifting to higher temperatures. In the presence of divalent cations, although Ts shifted to higher temperatures with increasing concentration of added CaCl₂ or MgCl₂, the melting temperature (Tm) in heating DSC curves shifted to higher temperatures (up to a certain temperature) and then shifted to lower temperatures with increasing concentration of salt. With increasing concentration of CaCl₂ or MgCl₂, the exothermic and endothermic enthalpies estimated for a main peak increased up to a certain salt concentration and then decreased; however many other peaks were observed at higher temperatures. The endothermic peaks for gels with excessive divalent cations were too broad to be resolved from the baseline; in contrast the exothermic peaks were much sharper and readily recognized. In comparing thermal properties with rheological properties, gellan gum solutions with excessive divalent cations form firm gels on cooling to below the setting temperature, and then it was difficult to remelt them. This was quite different from the behaviour of thermoreversible gels formed in the presence of monovalent cations. It seems that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.     

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.     

Impact of cosolvents on formation and properties of biopolymer nanoparticles formed by heat treatment of β-lactoglobulin–Pectin complexes

The purpose of this study was to determine the influence of neutral cosolvents on the formation and properties of biopolymer nanoparticles formed by thermal treatment of protein–polysaccharide electrostatic complexes. Biopolymer particles were formed by heating (85 °C, 20 min) an aqueous solution containing a globular protein (β-lactoglobulin) and an anionic polysaccharide (beet pectin) above the thermal denaturation temperature (T_m) of the protein under pH conditions where the biopolymers formed electrostatic complexes (pH 5). The impact of two neutral cosolvents (glycerol and sorbitol) on the self-association of β-lactoglobulin and on the formation of β-lactoglobulin–pectin complexes was examined as a function of solution pH (3–7) and temperature (30–95 °C). Glycerol had little impact on the pH-induced self-association or aggregation of the biopolymers, but it did increase the thermal aggregation temperature (T_a) of the protein–polysaccharide complexes, which was attributed to its ability to increase aqueous phase viscosity. Sorbitol decreased the pH where insoluble protein–polysaccharide complexes were formed, and greatly increased their T_a, which was attributed to its ability to increase T_m, alter biopolymer–biopolymer interactions, and increase aqueous phase viscosity. This study shows that neutral cosolvents can be used to modulate the properties of biopolymer nanoparticles prepared by thermal treatment of protein–polysaccharide electrostatic complexes.     

Studies of ovalbumin gelation in the presence of carrageenans and after manothermosonication treatments

Ovalbumin is a key ingredient in many food products due to its ability to form a three-dimensional network after heat denaturation, that is, to form gels. We have explored two ways of modifying gels based on ovalbumin: (1) the addition of carrageenans, and (2) manothermosonication (MTS) treatments of ovalbumin suspensions prior to gelation. Ovalbumin gels showed elastic modulus (G′) values of 1216 Pa after a heating and a cooling period. The addition of kappa, lambda, and iota carrageenan enhanced G′ values by 6, 1.7, and 1.7 fold, respectively. Kappa carrageenan was not only the most effective carrageenan in the improvement of ovalbumin gelation but changed also the gelation process at the end of the cooling phase. MTS treatments reduced G′ values of ovalbumin gels by about 50% measured immediately after MTS treatments but only by ca. 25% after 24 h, which reveals that ovalbumin modifications introduced by ultrasound, whatever they are, are reversible. This reduction does not preclude the use of manothermosonicated liquid egg as gelling agent.     

Thermal Characterization of the Gelatinization of Corn Starch Suspensions with Added Sodium Hydroxide or Urea as a Main Component of Corrugating Adhesives

The effects of sodium hydroxide and urea on the gelatinization of corn starch suspensions, a main component of corrugating starch adhesives, were studied using differential scanning calorimetry (DSC). Sodium hydroxide and urea decreased the gelatinization starting temperature (T_s1), peak temperature, conclusion temperature, enthalpy of gelatinization (DSCΔH), apparent activation energy (ApEa), and van’t Hoff enthalpy change (ΔH_vHº). DSC analysis showed the gelatinization endotherm, the melting endotherm of starch−lipid complexes, and an exotherm attributable to the alkali-decomposition of starch. This exotherm was only observed in the first scan for sodium hydroxide additions of more than 3.8 mol%. The existence of the starch−lipid complexes was also suggested by the reappearance of the corresponding endotherm in the second DSC scan. The increases in the gelatinization rate constants caused by the two additives were deduced from the Arrhenius equation using ApEa. The relationship between the temperature corresponding to the half gelatinization transition (T_m) and DSCΔH, and that between T_mand ApEa quantitatively agreed for the two additives, suggesting that gelatinization might be stimulated by a similar mechanism involving breaking intermolecular hydrogen bonds in starch. Only sodium hydroxide caused a significant decrease of T_s1 and broadening of the gelatinization peak. Therefore, ΔH_vHºand the size of the gelatinizing cooperative unit estimated from the ΔH_vHº/DSCΔH ratio were decreased more by sodium hydroxide than by urea, indicating that sodium hydroxide facilitated the mobility of starch molecules more effectively during gelatinization.     

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.     

Effect of inulin on the rheological properties of silken tofu coagulated with glucono-δ-lactone

This study investigated the rheological properties of inulin-containing silken tofu coagulated with glucono-δ-lactone (GDL) upon heating. Inulin (Raftiline^® HP-gel) was added to a soy protein isolate-enriched cooked soymilk at 0%, 1%, 2%, 3% and 4% (w/v) levels along with 0.4% (w/v) GDL to prepare acid-induced silken tofu. Gelation was induced by heating the soymilk mixture from 20 to 90 °C at a constant rate (1 °C/min) or isothermally at 90 °C for 30 min. The gelling properties were measured with dynamic small-deformation mechanical analysis and static large-deformation compression tests. The rheological changes in soymilk during gelation were dependent upon both the pH decline (hydrolysis of GDL) and the specific temperature of heating. Control samples heated to 50 °C, with the pH lowered to 5.95, started to gel, showing a rapid increase in storage (G′) and loss (G″) moduli afterwards. The addition of 2% inulin lowered the on-set gelling temperature by 2.8 °C and improved (P < 0.05) both rheological parameters of the tofu gel as well as hardness and rupture force (textural profile analysis) of the formed silken tofu. The results indicated that inulin enhances the viscoelastic properties of GDL-coagulated silken tofu, and the textural effect of inulin is an added benefit to its current application mainly as a prebiotical ingredient in food.     

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.     

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.     

Whey protein concentrate/λ-carrageenan systems: Effect of processing parameters on the dynamics of gelation and gel properties

The thermal characteristics, dynamics of gelation and gel properties of commercial whey protein concentrate (WPC), WPC/λ-carrageenan (λ-C) mixtures (M) and WPC/λ-C spray-dried mixtures (DM) have been characterized. In a second stage, the effect of the gelling variables (T, pH, total solid content) on gelation and textural properties of DM was evaluated through a Doehlert uniform shell design.The presence of λ-C either in mixtures (M) or in DM promoted the WPC gelation at lower concentration (8%). M showed higher rates of formation and better gel properties (higher hardness, adhesiveness, springiness and cohesiveness) than DM.Nevertheless, when the effects of pH (6.0–7.0), heating temperature (75–90 °C) and total solid content (12–20 wt%) on gelation dynamics and gel properties of DM were studied, gels with a wide range of rheological and textural properties were obtained. While pH did not affect the gelation dynamics, it had some effect on rheological and textural properties. Total solid content and heating temperature were the most important variables for the dynamics of gelation (gelation rate (1/t_gel), gelation temperature (T_gel), rate constant of gel structure development (k_G), elastic modulus after cooling (G′_c) and textural parameters (hardness, springiness and cohesiveness).     

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.     

Analysis of cooling curve to determine the end point of freezing

The cooling curve method used for the measurement of freezing point of food is further analyzed to explore whether it can be used to identify the end point of freezing or glass transition. In this method, slope of the cooling curve is determined and plotted as a function of time to identify the end point of freezing (T′_m). Initially, the slope is decreased and then reached a minimum value, which is identified as the nucleation of ice. Then the slope is increased until the end point of freezing. The end point of freezing is identified when the slope starts to decrease from its highest value or plateau. Sucrose solutions and starch gels were used to measure its T′_m in identifying validity of the proposed method. The measured values of T′_m by the proposed method is very close to the literature values.     

Effect of ultrasonication on the properties of skim milk used in the formation of acid gels

Skim milk was ultrasonicated for times up to 30 min either with or without temperature control. Ultrasonication (US) without temperature control resulted in the generation of considerable heat, with the milk reaching  95 °C within 15 min of treatment. The whey proteins were denatured. Changes to the casein micelle size were observed, with decreases during the early stages of US and increases (because of aggregation) on prolonged treatment. Significant κ-casein dissociated from the micelles. Acid gels prepared from these ultrasonicated samples increased in firmness (final G′) up to a maximum final G′ after  15 min of US, followed by a decrease from this maximum on prolonged treatment. US with temperature control demonstrated that the denaturation of the whey proteins was entirely due to the heat generated during US, although the casein micelle size was still reduced. Acid gels prepared from ultrasonicated skim milk in which the temperature remained below the denaturation temperature of the whey proteins had low final G′, although a small increase was observed with increasing US time. Acid gels prepared from the samples that were ultrasonicated at temperatures above the denaturation temperature of the whey proteins had higher final G′, which could reach values similar to those obtained by the conventional heating of milk. The results of this study indicate that, in skim milk, most of the effect of US can be related to the heat generated from the treatment, with US itself having only a small effect on the milk when the temperatures are controlled.

Industrial relevance

The control and the manipulation of the firmness of acid skim milk gels are important in many dairy food applications such as yoghurts and some types of cheese. US is an emerging technology that could be used to process skim milk for use in acid gelled products. This study has demonstrated that acid gel firmness can be substantially manipulated when skim milk is ultrasonically treated before acidification; however, most of the effect is due to the heat generated during US treatment. As the effects of US are similar to those obtained through conventional heating processes, and as US can control spoilage microorganisms, using US under controlled temperature conditions could be an alternative to conventional heating to give desired functional properties and storage stability to milk products. However, the temperature/denaturation/aggregation would need to be carefully controlled to minimize the detrimental effects of excessive heating.