Fracture Analysis of Alginate Gels

The fracture properties of alginate gels were investigated using torsion and compression. The gel fracture stress correlated with Ca²⁺ and alginate concentration, whereas the fracture strain was insensitive to composition. Considering the relationship of fracture stress with gel network crosslink density and the energy to break covalent and noncovalent bonds, the fracture of alginate gels is hypothesized to result from the disruption of junction zones. Consequently, the fracture stress was the stress required to overcome electrostatic forces that formed junction zones. The fracture stress‐strain relationship for alginate gels can be described by the Blatz, Sharda, adn Tschoegl (BST) equation, suggesting that for a given gel, the fracture strain can be predicted based on fracture stress, small‐strain shear modulus, and a fitted parameter describing nonlinearity of the gel. In addition, the fracture properties were affected by deformation rate. The influence of deformation rate on fracture was ascribed to structural changes among the alginate junction zones.     

FRACTURE PROPERTIES OF STARCH GELS AND THEIR RATE DEPENDENCY

Fracture properties of potato starch gels, consisting of swollen granules, were studied by bending, uniaxial compression, tension and cutting experiments at various strain rates. The effects of starch granule size and retrogradation were also investigated. Starch gels are very notch-sensitive. Fracture starts from inherent defects in the gel with a size comparable to that of the swollen granules. Fracture parameters like stress and strain at fracture and fracture energy depend on the rate of deformation, in contrast with the rate independent elastic behaviour observed at small deformations. This rate dependency can be different for the different parameters. A possible explanation for the rate dependent fracture is discussed. It implies that the granular structure of the gels is essential for fracture behaviour.     

RHEOLOGICAL PROPERTIES OF RENNET-INDUCED GELS DURING THE COAGULATION AND CUTTING PROCESS: IMPACT OF PROCESSING CONDITIONS

The effects of gelation temperature (GT), pH, milk solids nonfat (MSNF) content and aging time on the small and large deformation rheological properties of rennet‐induced skim milk gels were studied. Small amplitude oscillatory rheometry (SAOR) was used to study gel formation. A constant shear rate was applied to gels of various ages to try to simulate the cutting process used in cheese vats. Second‐order polynomial models successfully predicted (R² ≥ 0.83) the relationship between processing parameters and rheological properties of gels. The processing parameters – gelation pH, GT and MSNF – had a significant effect on the rheological properties of rennet‐induced gels. The type and the nature of bonds in these networks and the time scale of applied deformation affected the rheological properties of rennet gels. As time after rennet addition increased, storage modulus, loss modulus and yield stress values increased. This resulted from an increase in the number and strength of bonds with time. The yield strain decreased with time probably because of rearrangements in the network making the gel shorter/brittle in texture. When the impact of the time scale of the applied deformation was compared between the small (storage modulus as a function of frequency) and large (yield stress as a function of constant shear rate) deformation properties of rennet‐induced gels, similar power law exponents were obtained. This similarity presumably reflects the type and relaxation behavior of bonds in this casein network. These results identify the impact of several important processing variables on both the small and the large deformation rheological properties of rennet‐induced gels, which could be useful in identifying gelation properties that improve cheese yield.     

ELECTROSTATIC EFFECTS ON PHYSICAL PROPERTIES OF PARTICULATE WHEY PROTEIN ISOLATE GELS

Physical properties of particulate whey protein isolate gels formed under varying electrostatic conditions were investigated using large strain rheological and microstructural techniques. The two treatment ranges evaluated were adjusting pH (5.2‐5.8) with no added NaCl and adjusting the NaCl (0.2‐0.6 M) at pH 7. Gels (10% protein w/v) were formed by heating at 80C for 30 min. The large strain properties of fracture strain (γ_f), fracture stress (σ_f), and a measure of strain hardening (R_0.3) were determined using a torsion method. Gel microstructure was evaluated using scanning electron microscopy (SEM) and gel permeability (B_gel). Overlaying σ_f and γ_f curves for pH and NaCl treatments demonstrated an overlap where gels of equal σ_f and γ_f could be formed by adjusting pH or NaCl concentration. The high fracture stress (σ_f～ 23 kPa and γ_f～ 1.86) pair conditions were pH 5.47 and 0.25 M NaCl, pH 7.0. The low fracture stress (σ_f～ 13 kPa and γ_f～ 1.90) pair conditions were pH 5.68 and 0.6 M NaCl, pH 7.0. The 0.25 M NaCl, pH 7 treatment demonstrated higher R_0.3 values than the pH 5.47 treatment. When the sulfhydryl blocker n‐ethylmaleimide was added at 2 mM to the 0.25 M NaCl, pH 7 gel treatment, its rheological behavior was NSD (p>0.05) to the pH 5.47 gel treatment, indicating disulfide bond formation regulated strain hardening. Altering surface charge or counterions, and disulfide bonding, was required to produce gels with similar large strain rheological properties. An increase in gel permeability coincided with an increase in pore size as observed by SEM, independent of rheological properties. This demonstrated that at the length scales investigated, microstructure was not linked to changes in large strain rheological properties.     

STRESS RELAXATION BEHAVIOR OF MICROWAVE-VACUUM-DRIED ALGINATE GELS

Swelling of alginate polymer matrix in water involves a build up of network pressure due to an elastic extension of the polymeric matrix. When this network pressure undergoes relaxation by means of dehydration, shrinkage may take place. Three different types of wet alginate gels were prepared and dried using microwave‐vacuum‐drying technique. Dried alginate gel solids had a porous structure. To understand the stress relaxation behavior of alginate gel‐based porous solid structures, uniaxial compressive relaxation studies were performed at selected strain rates, preloads and relaxation times Experimental relaxation curves were normalized and fitted to an empirical relationship, and relaxation behavior was explained. Stress relaxation data were also fitted to another empirical model. All three types of gels had similar elastic components. At lower strain rate, all samples had more resistance to elastic deformation. Stress relaxation information of the dried gel was related to its microstructure. Type 2 gel had more stiffness than type 1 and type 3 gels. The mechanism involved in stress relaxation was entanglement coupling of larger polymer chains in covalently cross‐linked alginate gels.     

Deformation and fracture of emulsion-filled gels: Effect of oil content and deformation speed

The large deformation properties of gelatine, κ-carrageenan and whey protein isolate (WPI) gels filled with bound and unbound oil droplets were studied as a function of compression speed. The rheological properties of the gel matrices controlled the compression speed-dependency of the gels containing oil droplets. Polymer gels (gelatine and κ-carrageenan gels) showed a predominantly elastic behaviour. Their Young's modulus was not affected by the compression speed. The increase of fracture stress and strain observed with increasing compression speed was related to friction between the structural elements of the gels and, for gelatine, to the unzipping of physical bonds. Particle gels (WPI gels) showed a more viscoelastic behavior. Their Young's modulus and fracture stress increased with compression speed. This was attributed to the viscous flow of the matrix and friction phenomena between structural elements of the gel. The effect of an increase in the oil volume fraction (φ) on the Young's modulus was for all gels according to the Van der Poel theory. In addition, oil droplets embedded in the gel matrix acted as stress concentration nuclei and increased friction. The relative impact of these two effects was related to the viscoelastic properties of the gels and to droplet–matrix interaction. For polymer gels and gels with bound droplets, stress concentration phenomena played a relatively larger role. For particle gels and gels with unbound droplets, friction phenomena were relatively more important, increasing the viscoelastic character of the gels. As a result, an increase in φ resulted in a decrease of both fracture stress and fracture strain for polymer gels and in an increase of the fracture stress and a decrease of fracture strain for particle gels.     

CONSTANT STRAIN RATE COMPRESSION OF BIOPOLYMER GELS

Usually, uniaxial compression of food materials is performed at constant speed, which causes an increase of the true strain rate during experiments. By using a method described previously (Jaros and Rohm 1994) highly elastic biopolymer gels were compressed at standard conditions and with decreasing crosshead speed in order to maintain constant strain rate conditions during test. In the case of gels showing failure at a Hencky deformation of > 0.5-0.6, significantly lower values for apparent fracture strain were found in constant strain rate compression. Differences in stress at apparent fracture showed good agreement with respect to this critical value. Implications and consequences on interrelation between sensory and instrumental measurements as well as on relationships between rheological and fracture properties are discussed.     

COMPARATIVE STUDY OF LARGE STRAIN METHODS FOR ASSESSING FAILURE CHARACTERISTICS OF SELECTED FOOD GELS

Vane rheometry was compared with uniaxial compression and torsion in evaluating the effects of strain rate on failure shear stress and deformation of soybean protein (tofu) and gellan gum gels. A Haake VT 550 viscotester was used for torsion and vane tests, and compression was performed with an Instron/MTS universal testing machine. Strain or angular deformation at failure was independent of strain rate in the three testing modes. In vane rheometry, failure shear stress increased with increasing low shear rates (< 0.100 s ⁻¹) and was rate independent at higher rates. This strain rate dependency was also evident in compression, varying with the material. For torsion, fracture stress appeared to be rate independent. Shear fracture stresses measured in torsion and compression were in good agreement at strain rates above 0.025 s ⁻¹ and 0.100 s ⁻¹ for tofu and gellan gels, respectively. Shear stresses from the vane method were lower than shear stresses of torsion and compression. Similar texture maps of the food gels studied were generated by plotting stress and strain or angular deformation values of the three testing methods. The findings validate the vane technique as an alternative to torsion and compression for rapid textural characterization of viscoelastic foods.     

TEMPERATURE-TOLERANT FISH PROTEIN GELS USING KONJAC FLOUR

Shear stress and strain values of surimi gels made from Alaska pollock and Pacific whiting were measured upon cooling, reheating, and freeze/thawing. Konjac flour was introduced to investigate its ability to maintain fracture properties of surimi gels against various temperatures. Gel colors (CIE L*,a*,b*) were also measured as affected by various levels of konjac flour. Konjac flour (5%) showed its ability to reinforce shear stress of gels 8-10 times in both whiting and pollock surimi. Gels with 4% konjac flour were the most heat-tolerant in both surimi. Surimi gels with konjac flour exhibited an ability to maintain consistent shear strain values against repeated freeze/thaw abuse. Konjac flour, up to 2%, increased lightness of gels, while yellow hue increased gradually up to 5%.     

COLD GELATION OF WHEY PROTEIN EMULSIONS

Stable and homogeneous emulsion‐filled gels were prepared by cold gelation of whey protein isolate (WPI) emulsions. A suspension of heat‐denatured WPI (soluble WPI aggregates) was mixed with a 40% (w/w) oil‐in‐water emulsion to obtain gels with varying concentrations of WPI aggregates and oil. For emulsions stabilized with native WPI, creaming was observed upon mixing of the emulsion with a suspension of WPI aggregates, likely as a result of depletion flocculation induced by the differences in size between the droplets and aggregates. For emulsions stabilized with soluble WPI aggregates, the obtained filled suspension was stable against creaming, and homogeneous emulsion‐filled gels with varying protein and oil concentrations were obtained. Large deformation properties of the emulsion‐filled cold‐set WPI gels were determined by uniaxial compression. With increasing oil concentration, the fracture stress increases slightly, whereas the fracture strain decreases slightly. Small deformation properties were determined by oscillatory rheology. The storage modulus after 16 h of acidification was taken as a measure of the gel stiffness. Experimental results were in good agreement with predictions according to van der Poel's theory for the effect of oil concentration on the stiffness of filled gels. Especially, the influence of the modulus of the matrix on the effect of the oil droplets was in good agreement with van der Poel's theory.     

Combined microscopic and dynamic rheological methods for studying the structural breakdown properties of whey protein gels and emulsion filled gels

The microstructural and large deformation rheological properties of model food gels were studied by performing notch propagation tensile testing on the gels using a tensile stage and observing changes in the microstructure of the gels during tensile testing using confocal laser scanning microscopy (CLSM). Heat-set whey protein (WP) gels containing either added sodium caseinate (NaCN) or sunflower oil droplets emulsified with WP or NaCN as the emulsifier protein were prepared in 0 or 50 mM NaCl. The WP gel structure strengthened in the presence of added NaCl and NaCN. The rheological properties of WP gels containing sunflower oil droplets emulsified with WP or NaCN were influenced by the NaCl concentration, oil concentration and extent of oil droplet aggregation in the gel or by the type of emulsifier protein used. During tensile testing, the notch length in all gels increased above a certain critical stress, leading to fracture of the gels through the notch. Also, the microstructural changes in the oil phase of emulsion filled gels subjected to tensile testing were influenced by the structural properties of the WP gel matrix and the proximity of the oil droplet to the fracture path.     

UNIAXIAL COMPRESSION OF GELS MADE FROM PROTEIN AND K-CARRAGEENAN

Investigations of gels (18% total solids) made from pea protein isolates (PPI) or soy protein isolate (SPI) with differing amounts of _K-carrageenan added showed that the gel strength increased with the concentration of _K-carrageenan. When the concentration of_K-carrageenan exceeded 0.4%, gels made with PPI were stronger and more stiff than equivalent gels made with SPI. Addition of _K-carrageenan stabilized gels made with PPI towards variations in brittleness (indicated by strain at fracture) with pH. This was not the case when SPI was used. Preheating (75C, 2 min) suspensions containing protein isolates and _K-carrageenan before gel formation increased the strength and stiffness of the final gels, most pronounced when SPI was used. Addition of NaCl (0.5–2%) reduced strength and stiffness of gels, whereas CaCl₂ had no influence on gel properties. Mixtures of PPI and SPI proved to be weaker and more brittle than gels made from only SPI of PPI. Results indicate that using proteins of different origin can cause differences in gel structure.     

MEASUREMENT OF THE TENSILE FAILURE OF GELS

Various methods of probing the tensile failure characteristics of both extensible and brittle gels using the Instron machine were evaluated. The true strain of the gels at failure determined from high speed video camera recording and the corresponding true stress were used as the reference. When the ratio of the test length to the length of the end-pieces of a dumbbell shaped specimen exceeds 10:1, the effect of the end-pieces was negligible and the measured stress and strain at failure by the Instron machine represent the true values in the test sector of the gel. Experimental data from tensile testing using the above method suggest that both extensive and brittle gels were weaker in tension than in compression.     

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.     

MECHANICAL PROPERTIES OF TWO-PHASE DISPERSE AGAR/GELATIN MIXED GELS

Agar/gelatin mixed gels with the same composition but with a different two-phase disperse structure were prepared and their mechanical properties compared. The agar/gelatin mixture was first kept at temperature above the gelling temperature of gelatin but below that of agar and stirred for the selected period, before cooling it below the gelling temperature of gelatin. For the low rupture stress system the agar concentration was 0.7% (w/w), while the gelatin concentration was 4.5% (w/w) to achieve the same rupture stress as the agar gel. The mixing temperatures selected were 20 and 37C. For the high rupture stress system, the agar and gelatin concentration was 2.8 and 10.4% (w/w), respectively, to achieve the same rupture stress. The mixing temperatures selected were 37 and 40C. The both mixed gels prepared by this method consisted of a dispersed phase of agar and a continuous phase of gelatin. The rupture stress of the mixed gels decreased as the content of the dispersed phase increased. The rupture stress had a tendency to be lower as the size of the dispersed particles increased. These results suggest that the interface between the dispersed phase and the continuous phase plays an important role as Griffith's crack, with the rupture of mixed gels occurring from that place.     

右旋糖酐/蚕豆蛋白复合凝胶的流变特性

为研究乳酸菌右旋糖酐对蚕豆蛋白食品相关性质的影响，采用哈克流变仪和质构仪等测定了添加不同浓度右旋糖酐时GDL诱导的酸致蚕豆蛋白复合凝胶质构和流变特性等指标。结果表明：添加右旋糖酐能显著增加蚕豆蛋白凝胶保水性，空白组蚕豆蛋白凝胶保水性为60.38%，1%右旋糖酐与蚕豆蛋白形成的复合凝胶保水性为70.08%（p<0.05）；右旋糖酐浓度在0.25%~1%之间，对复合蛋白凝胶色泽有一定影响；0.5%右旋糖酐与蚕豆蛋白形成的复合凝胶强度为0.27 N，与空白组蚕豆蛋白凝胶（0.37 N）差异显著，可软化含高蚕豆蛋白食品质构特性；右旋糖酐/蚕豆蛋白复合凝胶流动曲线符合carreau模型（R2>0.99），具有假塑性流体的特性；应变扫描的弹性模量G''均高于黏性模量G''，说明右旋糖酐/蚕豆蛋白复合凝胶的弹性占主导；频率扫描结果显示添加右旋糖酐可软化蚕豆蛋白凝胶，使凝胶G''、G''降低，更易于加工。在蚕豆食品中添加右旋糖酐可改善蚕豆蛋白的质构和流变特性，为拓展其应用范围提供参考。