Rheological characterization of fresh and cooked potato tissues (cv.?Monalisa)

 The rheological properties of fresh and cooked (15 min in boiling water) potato tissue which had been deformed to a lessor or greater extent by uniaxial compression, shear, uniaxial tension, successive cycles of stress relaxation, creep compliance and texture profile analysis were evaluated. Structural failure of fresh tissue under compression always occurred along a single plane of maximum shear stress, while fresh tensile specimens failed under tension. Tensile tests proved better methods by which to determine the failure parameters of cooked specimens since it was possible to observe two differents modes of failure with this technique. Equivalent modes of failure (causing damage to the cell wall or cell separation) and changes in structural components caused by cooking proved easier to idetify with tensile tests. The six element Burger's model incorporating two discrete Voigt-Kelvin units was the most suitable for defining tissue creep behaviour. The instantaneous elastic modulus could be related to the internal cell pressure, and gelatinization of starch and viscoelastic units appeared to reflect the viscoelastic properties of pectic sustances and hemicelluloses, respectively. Received: 1 December 1997     

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.     

Mechanical properties of tomato exocarp,mesocarp and locular gel tissues

In order to accurately predict the internal stress distribution and damage region of tomato fruit subjected to an external force using finite element method, the tomato fruit can be regarded as a multibody system which consists of exocarp, mesocarp and locular gel tissues. In the paper, these tissues’ mechanical properties of two tomato varieties were determined using different types of test. Results showed the tension failure stress and elastic modulus of exocarp tissue varied from 0.421 to 0.582 MPa and 4.601 to 9.59 MPa respectively; the compression failure stress and elastic modulus of mesocarp tissue varied from 0.122 to 0.229 MPa and 0.726 to 0.846 MPa respectively; the compression failure stress and elastic modulus of locular gel tissue varied from 0.012 to 0.016 MPa and 0.048 to 0.124 MPa respectively; the failure stress and elastic modulus values of outer layer tissue of tomato fruit were higher than those of inner layer tissue.     

Viscoelastic characterization of glutenins in wheat kernels measured by creep tests

The effect of high and low molecular weight glutenin subunits on viscoelastic properties of wheat kernels was investigated using creep tests with generalized Kelvin–Voigt model (r² ≈ 0.94; P < 0.0001). Glu-A1, Glu-B1 and Glu-D1 affected the creep coefficients in Glu-1 loci. Regarding LMW-GS, the locus Glu-A3 had highest influence on creep followed by Glu-B3; Glu-D3 did not showed differences. In general, the modulus of elasticity E₀ ≈ 242 MPa and viscosity was μ₀ ≈ 1.6 × 10⁷ MPa·s. Higher elastic moduli and viscosity were found in HMW-GS and LMW-GS of good quality compared to the poor quality performance genotypic groups. Samples with subunit Glu-A1 null presented lower elastic modulus and viscosity compared to Glu-A1 1 and 2^∗. Glu-B1 13 + 16 presented higher elastic modulus and viscosity, which is associated with good quality performance. The elastic moduli, especially E₀ were positively correlated with SDS-volume and rheological properties. This methodology can be applied to few kernels is easy, rapid, and nondestructive.     

ASSESSMENT OF COHESION IN GRUYERE-TYPE CHEESE BY RHEOLOGICAL METHODS

The quantitative assessment of cohesion in unripened hard cheese was studied using static methods (uniaxial compression and tension, 3-point bending, and cutting) and transient methods (stress relaxation and creep). The two levels of cheese cohesion (denoted as weak and strong, according to the judgement of the cheesemaker) were obtained by two different manufacturing procedures. Statistical analysis showed that all static methods can be used to discriminate various levels of cohesion. Uniaxial tension proved to be the most powerful test to assess quantitatively the cohesive properties of hard cheese since the values of all parameters (fracture stress, fracture strain, work to fracture, modulus of deformability, and apparent flow modulus) depend significantly (α < 0.01) on the level of cohesion. Stress relaxation and creep tests were not as powerful as the rapid static methods in distinguishing the two levels of cheese cohesion.     

Modeling changes in rheological properties of potatoes during storage under constant and variable conditions

The changes in rheological properties of potatoes stored at 5, 15, 25 °C and variable (fluctuating) temperature for 16 or 26 weeks were evaluated in terms of elasticity and viscosity parameters using axial compression and creep tests. Cylindrical test specimens (15 mm diameter and 30 mm long) were used. A third-degree polynomial best fitted the force-deformation curves in axial compression test (R²=0.98-0.99) whereas a four-element (Burgers) mechanical model adequately described the creep response of potatoes (R²=0.95-0.99). The tangent modulus of elasticity in axial compression and elasticity and viscosity parameters in creep tests in general decreased significantly (P<0.05) with increase in storage time both under constant and variable storage conditions. The changes in rheological properties of potatoes stored under constant storage condition were sufficiently described by a modified exponential model (R²=0.89-0.96) except for the viscosity parameter of the Maxwell component of the four-element model. The logarithm of degradation rate constant, k, and the constant, n, were linearly related to storage temperature. For the variable storage condition, a bulk mean temperature (T_bm) was calculated to account for a series combination of storage time and temperature to which the potatoes were subjected. The changes in rheological properties due to variable storage temperature were then described as a function of T_bm and storage time, t_s, using stepwise multiple regression. The result indicated that except the viscosity parameter of the Maxwell component of the four-element model, it was possible to describe the changes in rheological properties as a function of T_bm and t_s (R²=0.84-0.99).     

THE EFFECT OF TEMPERATURE ON THE RHEOLOGY OF BUTTER, A SPREADABLE BLEND AND SPREADS

The texture of lipid-based food materials is an important topic for investigation. In this study, the rheological properties of five edible fats were determined. A puncture test was performed to investigate the rheological properties of the food materials at 5C and at 19C using a texture analyzer. The force-displacement measurements were converted to stress–strain by assuming incompressibility of edible fat food materials in a linear viscoelastic region (LVR). Young's modulus of each edible fat was calculated using stress–stain curves in a LVR. Shear elastic moduli of edible fats in a LVR were obtained using a rheometer. Further to that the effect of temperature on storage modulus and loss modulus and creep test were obtained using a rheometer. The tests showed good correlation between Young's modulus and shear elastic modulus for each material. The mechanical properties correlated well with the structural properties of each of the materials.

PRACTICAL APPLICATIONS

The texture of butter, blends and spreads is determined by temperature, rheological properties, processing conditions and the composition of the material. Our research investigated the effect of temperature on the rheological properties of these foods. The results may be used to model the relationship between the microstructure and mechanical properties of the fat crystal network.     

Assessment of physical and mechanical properties of sodium caseinate and stearic acid based film-forming emulsions and edible films

Edible films were prepared using sodium caseinate (6–8 g/100 g) and stearic acid (0–2 g/100 g). Effects of the ratio of stearic acid and sodium caseinate to water on the water vapor permeability (WVP) and mechanical properties of the prepared films were evaluated. Film-forming emulsions were also tested for rheological properties and surface tension. Changes in the ratios of sodium caseinate and stearic acid to water had significant effects on WVP (p < 0.05) and surface tension (p < 0.01). Higher values of consistency coefficient and elastic modulus were obtained in the presence of higher stearic acid. In addition, increase in stearic acid content increased the rate of water loss and gain of elastic modulus at the early stage of drying and resulted in production of less flexible film. The resultant edible film prepared with 6 g/100 g sodium caseinate and 2 g/100 g stearic acid showed the lowest WVP of 1.368 (g mm/m² h kPa).     

Changes in rheological and viscoelastic properties and protein breakdown during the ripening of 'Port Salut Argentino' cheese

Changes in the rheological behaviour and viscoelastic properties during ripening at 10°C of a soft cheese (Port Salut Argentino) packaged in a plastic film (EVA-EVA) were analysed. Casein degradation was measured by electrophoresis slab gels; α_s1 casein degradation was rapid and striking compared with that of β casein. Rheological parameters obtained from uniaxial compression tests changed during ripening: hardness decreased, adhesiveness and cohesiveness increased.
An exponential decay equation with two maxwellian elements and one elastic in parallel was fitted to characterize stress relaxation curves. Viscoelastic parameters (elastic moduli and relaxation times) were obtained using non-linear regression analysis. The elastic equilibrium modulus decreased 80% during the ripening period and the viscosity of the element with the highest relaxation time decreased 20%. These parameters represented the changes observed in cheese hardness and elasticity during ageing time and were related to the extent of casein breakdown.     

Influence of the particle size on the rheological behaviour of chestnut flour doughs

The rheological properties of chestnut flour (CF) doughs with different particle size were studied using a controlled stress rheometer. The mixing curves and heating–cooling cycle responses were previously obtained from the Mixolab® apparatus. Shear measurements (0.1–10 s⁻¹), oscillation measurements (1–100 rad s⁻¹ at 0.1% strain), temperature sweep (30–100 °C) to achieve the gelatinization temperatures and creep-recovery tests (loading of 50 Pa for 60 s) were conducted in the rheometer. Mixolab® showed that CF samples with smaller particle size needed more water absorption to reach a consistency of 1.1 Nm. Shear viscosities of CF doughs exhibited a Newtonian plateau at low shear rate and shear thinning behaviour at higher shear rate. Apparent viscosity increased with increasing average particle size and steady-flow curves were fitted using Cross model. Oscillatory measurements of CF dough showed that the storage modulus, G′, was greater than loss modulus, G″, for all samples in the tested angular frequency range. CF samples with smaller particle size presented lower G′ and G″ values. Creep-recovery tests of these flours showed that elasticity was limited and unrecoverable proportion was very high. Gelatinization temperatures measured using Mixolab® and rheometer were practically coincident.     

Interpretation of mechanical spectra of carob fibre and oat wholemeal-enriched wheat dough using non-linear regression models

Oscillation and creep shear tests were applied to test the effect of fibre-rich additions - carob fibre (0-5%) and black oat wholemeal (0-20%) - on the rheological properties of wheat flour dough. Interpretation of changes in the values of the storage modulus (G′) and loss modulus (G″) and the tangent of the phase angle (tan δ) in the function of oscillation frequency (f) was performed on the basis of proposed 3-parameter non-linear regression models. The results of creep measurements were analysed using the 6-parameter Burgers model. Increase in the share of the fibre-rich additions caused the G′(f) and G″(f) curves to shift towards higher values, and steeper slope and stronger bending of the shapes of the curves, while the tan δ(f) curve tended to shift towards lower values. The application of carob fibre and oat wholemeal at rates of 2% and 10%, respectively, caused that the ratio of viscous to elastic properties of wheat flour dough was the least dependent on the shear rate (f). The result of creep tests showed an increase in the instantaneous and retarded moduli of elasticity and the Newtonian viscosity of wheat flour dough with increase in the share of the additions.     

Effect of homogenisation and storage time on surface and rheology properties of whipping cream

The effects of homogenisation pressure and storage time on the droplet characteristics, surface protein concentration and rheological properties of whipping cream were investigated in this work. The droplet size distribution was in a range of 0.120-17.378 μm when homogenised once, and 0.0794-3.802 μm when homogenised twice. With the increase in homogenisation pressure, the mean droplet diameter shifted toward a smaller size and dispersion index. The homogenisation pressure change from 20 to 50 MPa resulted in a reduction of the surface protein concentration. As the time increased, the surface protein concentration declined, and a higher pressure led to its decline to a lesser extent. All the emulsions exhibited a shear-thinning and gel-like behaviour with the elastic modulus higher than the viscous modulus. When increasing the homogenisation pressure, the gel-like behaviour was more pronounced in both homogenisation systems. The Herschel-Bulkley model was able to describe well the rheological characteristics.     

Influence of green banana pulp on the rheological behaviour and chemical characteristics of emulsions (mayonnaises)

The influence of green banana pulp (GBP) on rheological behaviour and chemical characteristics of emulsions (mayonnaises) was investigated in this work. Five formulations were developed, using an experimental design for constrained surfaces and mixtures, with the following proportions (water/soybean oil/GBP): F1 (0.10/0.20/0.70), F2 (0.20/0.20/0.60), F3 (0.10/0.25/0.65), F4 (0.20/0.25/0.55) and F5 (0.15/0.225/0.625). Rheological properties of the emulsions were obtained with a rotational Haake Rheostress 600 rheometer and a cone and plate geometry sensor (60 mm diameter, 2° cone angle), with a gap distance of 1 mm. The samples presented high values for mineral salts and low calories. The emulsions showed pseudoplastic behaviour at 10 and 25 °C and rheological data were well described by the Herschel-Bulkley model. Formulations with high GBP content (F1 and F3) presented higher yield stress and apparent viscosity values. The results obtained with the response surface methodology, which were adjusted by the quadratic model, showed that GBP had a significant influence on the increase of yield stress and apparent viscosity. The consistency coefficient increased with an increase in soybean oil concentration and water contributed to high values of flow behaviour index in all emulsions samples.     

Non-linear creep-recovery measurements as a tool for evaluating the viscoelastic properties of wheat flour dough

Creep-recovery measurements were used to analyze the non-linear viscoelastic properties of wheat flour. First the effect of creep time, recovery time and shear stress was investigated on the non-linear viscoelastic properties of Bussard dough. The Burgers model was fitted to the creep and recovery curves. A linear increase of maximum creep compliance was observed with increasing creep time. On the other hand, maximum recovery compliance remained constant but an increase of the retardation time was observed which indicates a slower recovery. A recovery time of 10 min seemed to be sufficient to obtain most of the recovery. Maximum creep compliance increased proportionally with increasing shear stress until a plateau was reached. Maximum recovery was constant between 100 and 500 Pa but the speed of the recovery increased as shear stress increased. Finally, the optimised creep-recovery methodology was used to analyze the non-linear viscoelastic properties of 17 pure wheat cultivars. By applying principal component analysis, it was possible to identify three groups of wheat cultivars with similar rheological properties and bread volumes.     

Enrichment of stirred yogurt with soluble dietary fiber from Pachyrhizus erosus L. Urban: Effect on syneresis, microstructure and rheological properties

Yam soluble fiber (YSF) extracted from Pachyrhizus erosus was added (1 g per 100 mL) to a stirred yogurt (SY_YSF). Its syneresis and microstructure properties were evaluated and compared to those of a stirred yogurt (SY_C) without added YSF. The SY_C yogurt exhibited a more compact casein micelle aggregates network than that of the SY_YSF yogurt which was more open, relaxed and covered with fibrous structures attributed to the YSF components. The rheological analysis showed that the SY_YSF yogurt had lower storage modulus (G′) and loss modulus (G″) values in the linear viscoelastic region than the SY_C yogurt, but its flow behavior was characterized by a lower flow index (n), higher consistency index (k), and higher yield stress (τ₀) than the SY_C yogurt. Incorporation of the YSF reduced significantly the syneresis and produced a more acceptable mouthfeel in the SY_YSF yogurt in comparison to the SY_C yogurt, indicating the viability of the process to obtain a commercial product.     

RHEOLOGICAL BEHAVIOUR OF BUTTER AT LARGE DEFORMATIONS

Eleven traditionally produced butter samples were evaluated for rheological properties at large deformations by lubricated and nonlubricated uniaxial compression with varying sample geometries and compression rates, by stress relaxation in shear and creep experiments in compression at 5–20C each. Compression forces required to deform cylindrical samples in nonlubricated conditions were higher than in lubricated cases. Correction of shape changes in nonlubricated stress-strain curves resulted in good agreement with lubricated experiments at low compression rates up to stress maxima or yielding. As frictional forces were relatively smaller at high deformation rates, lower stresses were obtained in nonlubricated conditions. Samples with higher height/diameter ratios showed more pronounced stress maxima. A biaxial extensional viscosity, η be , was computed for nonbrittle, soft samples. Typical slopes in logarithmic plots of viscosity against strain rate were approximately -1. At strains of about 0.01 creep compliance depended on compression stress. The average ratios of recovered to initial instantaneous compliance were between 0.31 and 0.59, and increased with sample temperature. A calculated ηBE from creep compression showed fair agreement with ηBE from static compression experiments. At comparable strains, a conversion of creep compliance to the stress relaxation modulus by an approximate method showed a good fit of experimental results.     

Rheological behaviour of emulsions of avocado and watermelon oils during storage

Rheological properties of emulsions made out of avocado pulp and watermelon seed oils with whey protein concentrate were determined during different storage periods. The oils, as well as the emulsions behaved like non-Newtonian liquids, having shear-thinning characteristics. Both oils showed moderate shear-thinning characteristics as the flow behaviour indices were between 0.86 and 0.88. The shear-rate/shear-stress data could be adequately fitted (r = 0.997–0.999) to a common rheological equation, e.g. the power-law model. Avocado pulp oil was markedly more viscous than was watermelon seed oil which was also evident from the higher apparent viscosity and consistency index values.     

Using high pressure microfluidization to improve physical properties and lycopene content of ketchup type products

This study evaluates the effects of high pressure microfluidization of ketchup mixes on the physical properties and lycopene content. During the processing the pressure level ranged between 200 and 2000 bar, while the number of passes was kept constant. When the treatment pressure was increased from 200 to 1200 bar, the yield stress, elastic modulus, storage modulus, and Bostwick consistency values of the ketchup samples improved. However, over-processing was observed for the samples treated at 1600 and 2000 bar. Light and scanning electron microscope images were used as visual means to observe microfluidization induced structural changes. Colloidal stability measurement showed that increasing microfluidization pressure improved physical stability of the samples. Reducing the size and changing the nature of tomato solids through high pressure microfluidization also increased the detectable lycopene levels of the ketchup samples.     

Rheological properties of soy protein hydrolysates obtained from limited enzymatic hydrolysis

Soy protein products hexane-defatted soy flour, extruded-expelled soy flour, soy protein concentrate and soy protein isolate, were modified by using the enzyme bromelain to 2% and 4% degrees of hydrolysis (DH). Peptide profiles, water solubility, and rheological properties including dynamic shear, large deformation, and apparent viscosities of resulting hydrolysates were determined. Protein subunits profiles for the hydrolysed isolates and concentrates were extensively altered by the treatment while only minor changes were observed for the hydrolysed flours. Water solubility profiles of all hydrolysates in the pH range of 3.0-7.0 were enhanced by hydrolysis. For the unhydrolysed controls, the isolate had the highest storage modulus (G′), followed by the concentrate, the extruded-expelled flour and the hexane-defatted flour. The hydrolysates retained some of their gelling ability even though the losses in storage modulus (G′) were substantial. After heating step to 95 °C, the G′ values of all substrates at 25 °C decreased with increase in DH. Texture profile analyses of the soy protein gels were also lower in hardness after hydrolysis. The Power Law model provided excellent fit to hydrolysate dispersions flow (R²>0.99). Hydrolysis decreased the consistency coefficients of dispersion and increased flow behavior index resulting in thinner dispersions. These results suggest that limited protease hydrolysis of various soy protein meals with bromelain produce soy protein ingredients with modified rheological properties.