Stress relaxation behaviour of high acyl gellan gels

Stress relaxation behaviour of high acyl gellan gels has been investigated, and data were fitted successfully by a seven elements empirical model and a four term modified Maxwell model with three fixed relaxation times (20, 200 and 2000 s). In addition, the effect of testing parameters on stress relaxation characteristics and the relationship between those characteristics and intrinsic gel properties were studied. High acyl gellan gels were tested in stress relaxation with different cross‐head speeds (0.1–10 mm s^?1) to applied strains (3–30%). The results showed that the cross‐head speed had little effect on stress relaxation behaviour of gels. With increasing the applied strain, the initial stress and the equilibrium stress increased. The equilibrium stresses from relaxation measurement were positively related to the hardness from TPA tests (R² = 0.991). Relaxation appeared to be associated with the shifting of cross‐links in the gel matrix.     

Stress Relaxation of Fruit Gels. Evaluation of Models and Effects of Composition

Stress relaxation behavior of kappa carrageenan (1%) gels (C) and kappa carrageenan (0.5%) plus locust bean gum (0.5%) gels (G) was studied. Effects of addition of sugar (18.3, 36.6 and 55° Brix) and fruit pulp (15%) on relaxation response were analyzed. Curves were fitted to Maxwell (M), Peleg (P) and simplified Maxwell (SM) equations. All three equations were valid to quantify relaxation behavior of fruit gels and to evaluate the effects of changes in composition, with P and SM equations being simpler. Locust bean gum increased initial stress decay rate and decreased proportion of unrelaxed force. Sucrose produced the opposite effect. Fruit pulp effects were weak and dependent on gelling system.     

Rheological properties of cereal starch gels and Mesona Blumes gum mixtures

The effect of Mesona Blumes gum (MBG) was examined on steady and dynamic shear of MBG/rice starch and MBG/wheat starch gels. In addition, stress relaxation and creep tests were performed for two types of cereal starch gels. The flow curves of both MBG/starch gels exhibited pseudoplastic behavior at shear rates between 0.01 and 10 s⁻¹, and the data were fitted into the power law model (R² = 0.91–0.98). Dynamic mechanical spectrum showed that all gels were strong gels in frequency between 0.1 and 10 Hz. Stress relaxation data at different strains indicated a strain‐softening phenomenon for both gels. Data were fitted into Maxwell model (R² = 0.91–0.98). Creep curves were conducted at the shear stress 6.4 Pa within linear viscoelastic region of both MBG/starch gels. Data were fitted into Burgers model (R² = 0.91–0.98). Apparent viscosity η, storage moduli G′, equilibrium stress relaxation modulus G_e and zero apparent viscosity η₀ of MBG/rice starch gels decreased in the following order: 6/0>6/0.5>6/0.35>6/0.1 (starch/gum w/w). Whereas η, G′, G_e, and η₀ of MBG/wheat starch gels increased gradually along side the increase of MBG contents. The stress relaxation time λ of MBG/rice starch gels increased in the following order: 6/0<6/0.5<6/0.35<6/0.1 (starch/gum w/w) while λ of MBG/wheat starch gels decreased gradually with the increase of MBG level. The influence of MBG on two examined cereal starch is totally opposite.     

Mechanical strength and rheological behaviour of barley kernels

Basic mechanical and rheological properties (stress relaxation behaviour) of single barley kernels under uni-axial compression were studied. The mechanical properties studied were the modulus of elasticity, yield and ultimate compressive strength, and the modulus of toughness as influenced by the moisture content. The stress relaxation experiments also included the effect of deformation rates and strain levels.
Results indicated that with an increase in moisture content, the strength of the barley kernels decreased while the energy required to cause rupture in the kernel increased initially and then decreased. The stress relaxation behaviour of barley kernels was described by a threeterm Maxwell model with a Maximum Relative Difference (MRD) ≤5%, when compared with one- and two-term Maxwell models. The elastic component (E¹) and relaxation time constant (τ¹) of the first term of the three-term Maxwell model decreased linearly with increase in moisture content. Of the different variables studied, moisture content had a significant influence on both of these parameters.     

Viscoelastic Characterization of Surimi Gel: Effects of Setting and Starch

Rheological properties of Alaska pollock surimi gels were affected by setting and addition of corn starch. Setting increased (p < 0.05) both elastic and loss moduli of gel in the absence of starch. However, in the presence of corn starch, setting decreased (p < 0.05) the elastic and loss moduli of the gels. In stress relaxation, the residual forces increased from 37.1 Newton (N) (nonsetting) to 51.8N (setting) in the absence of starch. However, in the presence of starch, the residual force decreased from 49.1N (nonsetting) to 39.0N (setting) which indicated an antagonistic effect between corn starch and setting. A Maxwell model was useful to quantify relaxation behavior of surimi gel and to evaluate the effects of changes in processing conditions.     

UNIAXIAL COMPRESSION AND STRESS RELAXATION TESTS ON ALGINATE GELS

The rheological behavior of alginate gels was analyzed using uniaxial compression and stress relaxation tests. The engineering compressive stress (σ_E)-deformation (ɛ_E curves were found to be concave upward and independent of the crosshead speed (V_t for V_t≥ 120 mm/min. By considering all the stress (σ_E)-time (t) data for t greater than 10 times the average loading period, the viscoelastic response of these gels was described by means of a 3-element Maxwell body characterized by a relaxation time of 300 s. Such gels exhibited an apparent linear viscoelastic solid behavior for ɛ_E≤ 8% and a nonlinear one for larger deformations. However, for ɛ_E>32% their solid viscoelastic behavior tended to a liquid one. Empirical models allowed the viscoelastic properties, as well permanent deformation, of these gels to be quantitatively described as functions of the deformation applied and/or loading rate.     

Interpretation of viscoelastic behaviour of sweet cherries using rheological models

Viscoelastic behaviour of some commercial sweet cherry cultivars of Iran has been studied in current research. For this purpose, stress relaxation test was conducted on five cultivars of sweet cherry including Siah Mashhad, Abarde, Victoria, Dovom Ras and Tak Dane. Two common models (Generalised Maxwell model and Peleg model) were fitted to the experimental data. Both models could describe stress relaxation behaviour of cherries (R² = 0.99), but Generalised Maxwell model had lower root mean square error (RMSE) than Peleg model. Based on analysis of stress relaxation data using models constants, Tak Dane exhibited more solid properties, while Victoria showed less elastic behaviour than other cultivars. The results revealed that relaxation ratio (R%) and the area under the stress relaxation curve could be effective alternative to models with easier mathematical procedure.     

猪鞋面革应力松弛时间谱的研究(Ⅰ)——猪鞋面革应力松弛行为的模拟和应力松弛谱的求解

研究了猪鞋面革的应力松弛模量,采用1～8个Maxwell单元组成的广义Maxwell模型,模拟了其应力松弛行为。结果表明:采用5个或5个以上Maxwell单元组成的广义Maxwell模型,可以较好地模拟猪鞋面革的应力松弛行为。利用模型模拟获得的应力松弛表达式和Schwarzl松弛时间谱二阶近似求解方法求解了松弛时间谱,得到了猪鞋面革的应力松弛谱图。     

酱牛肉应力松弛特性实验

以5个品牌酱牛肉为试样,从压缩试验及应力松弛试验研究中获得力学-流变学性质参数,利用SPSS软件进行非线性回归拟合,建立了广义Maxwell流变模型。结果表明:酱牛肉具有良好的压缩黏弹性力学性质,属黏弹性材料,五元件广义Maxwell模型适用于模拟酱牛肉的应力松弛现象。对应力松弛参数做主成分分析,应力松弛试验的7个参数可以分为两个主成分,归纳为2类,即黏弹特性和松弛时间。     

Textural and Rheological Properties of Cooked Potatoes

Cooked potato samples of different cultivars and specific gravities were analyzed for textural and rheological properties by sensory evaluation, texture profile analysis (TPA) and stress relaxation tests. Mealiness was correlated with the product of cohesiveness and adhesiveness, but not with other TPA parameters. Stress relaxation tests of cooked potatoes yielded a Maxwell model consisting of two viscous and three elastic elements. One of the elastic moduli was correlated with mealiness. Hardness by sensory evaluation correlated well with hardness and fracturability by TPA, and with the three elastic moduli in the relaxation model.     

Linear Viscoelastic Properties of Regular- and Reduced-Fat Pasteurized Process Cheese During Heating and Cooling

The rheology of process cheese during heating and cooling was examined by measuring the transient and dynamic linear viscoelastic properties of regular fat, lower moisture and an 80% reduced-fat, higher moisture pasteurized process cheese from 10 to 50°C. The dynamic (stress and frequency sweep) and transient (creep and recovery) rheological properties of the reduced-fat process cheese were found to be higher than that of regular-fat process cheese, indicating that fat content changed rheological properties more than moisture content. The temperature-dependent frequency dispersions of storage and loss moduli (dynamic mechanical spectra) were fitted with a power-law model, and master curves (at a reference temperature of 30°C) and shift factors were obtained by shifting the temperature-dependent frequency dispersion of dynamic mechanical spectra. The relaxation spectra (moduli, viscosities and relaxation times) of both cheeses were obtained from the master curves using the generalized Maxwell model and nonlinear regression. The viscosity distribution of corresponding Maxwell model elements were higher for the reduced-fat cheese by a factor of 1.6–4.7 compared to the regular-fat cheese, indicating that the higher moisture content in the reduced-fat process cheese did not loosen the protein matrix or soften the cheese even though higher moisture is recommended to cheese manufacturers in order to compensate for some textural defects in reduced-fat cheeses.     

CHARACTERIZATION OF GELLAN GELS BY UNIAXIAL COMPRESSION,STRESS RELAXATION AND CREEP 1

The rheological properties of 0.5–2.5% gellan gels were evaluated by uniaxial compression, stress relaxation and creep tests. The gel's strength was in the range of 0.1–1 kg.cm^-2 and their deformability modulus 1–6 kg.cm^-2. The asymptotic modulus determined in relaxation tests at two strains and the asymptotic creep compliance determined under two loads indicated that gellan gels have a yielding structure. The strains sustained in creep were considerably higher than the failure strain in uniaxial compression. Weight loss due to syneresis was on the order of 4–38% depending on the gum concentration, the deformation level in relaxation or the load in the creep tests.     

Rheological behavior of functional sugar-free guava preserves: Effect of the addition of salts

The addition of salts to carrageenan and locust bean gum gels functions to improve the characteristics of texture, thereby increasing gel strength. This effect is widely studied in gels of model systems but is studied to a lesser extent in complex systems, such as fruit preserves. The objective of this study was to evaluate the effect of adding salts on the rheological behavior of functional sugar-free guava preserves, as well as to correlate the rheological parameters. To this end, three types of texture properties were analyzed (texture profile, stress relaxation and uniaxial compression) in functional sugar-free guava preserves prepared with different concentrations of KCl and CaCl₂ salt. The analyses were performed with a texturometer (Stable Micro Systems, Model TA - XT2i), and the parameters were analyzed using a Scott-Knott test at 5% probability, principal components analysis and Pearson correlation. CaCl₂ was more effective for improving the characteristics of texture, especially gel strength (concentration near the F3: 0.33%), whereas KCl addition degraded gel strength. In the analysis of test relaxation, the Maxwell model parameters provided better discrimination between samples than the Peleg model parameters. Positive and negative correlations were observed, and the parameters of hardness, adhesiveness and elastic modulus ideal (E₁) were the most correlated with other rheological parameters.     

Stress relaxation characteristics of low-fat chicken sausages made in Argentina

Low-fat sausages were prepared with fresh chicken breast meat and formulated with different levels of added fat, whey protein concentrate, and hydrocolloids (xanthan and guar gums) to study the effect of composition on the stress relaxation behavior of the products. Stress relaxation experiments were conducted on precooked sausages at 25°C. Generalized Maxwell and empirical Peleg models were used to predict the stress relaxation behavior of the material. A model with seven maxwellian elements in parallel with a pure elastic element showed a very good agreement with experimental data. Results show that the proposed model satisfactorily fits the experimental data better than Peleg's model or Maxwell models with less elements. The relaxation time distribution functions were obtained. The characteristic relaxation time was shorter (2500s) for the formulations with no added fat which produced a less elastic product while the sausages with added fat showed longer characteristic relaxation time (5000s). The stress relaxation experiment differentiated the viscoelastic nature of different formulations due to reduction of fat content.     

Relaxation Time Spectrum of Hydrogels by CONTIN Analysis

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

Phenomenological viscoelasticity of some rice starch gels

The applicability of a powerful but still easy to use technique, based on a phenomenological theory of viscoelasticity, for processing and analyzing dynamic mechanical data of some rice gels was investigated. Based on this theory a continuous relaxation spectra was generated by application of Tikhonov regularization procedure on continuous Maxwell model. Interpretation of relaxation spectra in terms of number of peaks, its peak intensity H(λ) and appearance of its main distribution peak and magnitude of equilibrium elasticity modulus (G_e) of continuous Maxwell model was found to appropriately reflect main peculiarities of the viscoelastic behavior of rice starch gels. An increase in number of peaks in the relaxation spectra was observed for starch gels having higher amylose content indicating the creation of more heterogeneous structure. H(λ) and G_e values also increased with increase in amylose content demonstrating a transition of the system to more stable state like a gel.     

Viscoelastic behavior of persimmons dried at constant air temperature

The viscoelastic behavior of dried persimmons at different air-drying temperatures and velocities was evaluated. Air temperatures and velocities were varied according to a second-order central composite design, with temperature ranging from 40°C to 70°C and air velocity from 0.8 to 2.0 m/s. After drying, persimmons were equilibrated at four different water activities: 0.432, 0.576, 0.625 and 0.751. The rheological behavior of dried and conditioned persimmons was studied under uniaxial compression–relaxation tests. Three different rheological models were fitted to the experimental relaxation curves: Maxwell, Generalized Maxwell and Peleg and Normand. Based on the root mean square of residuals, the Generalized Maxwell model showed the best fit and a regression analysis was applied to obtain response surfaces for the model parameters. The dependence of the rheological properties on water activity was also analysed. Results showed that only the linear effect of air temperature was significant at a 5% level on the equilibrium stress and relaxation times. In a general way, these parameters increased with increasing air temperature and decreasing water activity.     

Stress relaxation behaviour of moth bean flour dough: Product characteristics and suitability of model

Stress relaxation characteristics of moth bean (raw and roasted) flour doughs having different moisture contents were determined at two different strain levels (0.05 and 0.50). The decaying relaxation curves for raw flour showed an initial stress between 2 and 22 kPa when the moisture content varied from 28% to 34% at a strain of 0.05. Roasted flour doughs exhibited initial stresses between 0.06 and 2 kPa at moisture contents between 40% and 46%. Dough formation was not possible below 40% moisture content for roasted samples. An increase in moisture content markedly decreased initial and residual stresses. It is desirable that dough should possess a low value of residual stress when used for developing flattened/sheeted products. Models containing two or three parameters were found suitable for large-strain stress relaxation characteristics while low-strain data can be modelled by a 4-element Maxwell model with residual stress.     

Influence of Poultry Species, Muscle Groups, and NaCl Level on Strength, Deformability, and Water Retention in Heat-Set Muscle Gels

The effects of poultry species (chicken and turkey), muscle groups (breast, thigh, and drum) and NaCl concentration (1% and 2%) on a low fat gel system were compared using measurements of shear stress (strength), shear strain (deformability), and two water retention measurements. Results showed doubling NaCl tended to double stress for all species and muscle groups and increased strain by 30–50%. Cross species differences showed higher stress in turkey gels than in chicken gels with 2% NaCl. For both species, stress, strain and water retention were lowest in breast gels with both NaCl concentrations, and strain was greater in thigh gels than in drum gels with 2% NaCl.