摘要:

Wheat starch granules dispersed in water and heated to 70°C swell extensively to form dispersions of irregularly shaped, deformable particles. Because the granules are deformable under shear, the rheological properties of the dispersions can be examined at volume fractions up to unity, which corresponds to a starch concentration of 14%. At concentrations of 10–13%, yield stresses are observed in couette flow at 23°. At concentrations of 14% and above, the dispersions show essentially solidlike behavior in shear. Shear stress‐shear strain diagrams for these gels were obtained in a parallel plate rheometer. The major experimental problem in this testing mode is slip between the gel and the parallel plates. This problem was resolved by using cyanoacrylate ester adhesive to attach the gel to the parallel plates. With slip eliminated, the shear stress‐shear strain diagrams show an initial linear region from which a small deformation modulus is obtained. As shear strain increases, a yield point is observed where the stress‐strain curve shows an inflection point. At higher strains, the curve rises again, and forces normal to the direction of shear are generated. Failure finally occurs at strains of 25–50%.

ISSN:0148-6055    

DOI:10.1122/1.549717

出版商:The Society of Rheology    

年代:1983

数据来源: AIP

摘要:

When wheat flour/water (100:69) mixtures were tested in a parallel plate configuration in the Rheometrics mechanical spectrometer atε<0.1without protecting sample edges from dehydration and 30 min after sample preparation, an interesting rheological disequilibration occurred at approximately 1–5 Hz in the frequency spectrum with a maximum at about 4–5 Hz. It manifested itself as a pronounced dip in theG′andG″values, the magnitude of which increased with increasing starch/protein ratio in the flour. The dip decreased materially when the dough hydration time was increased. It was eliminated by addition of shortening, but not by addition of other common dough ingredients. It was also eliminated by increasing the strain amplitude or by addition of urea. The dip was not exhibited by wheat gluten and was reintroduced by adding back a critical level of starch. The dip was eliminated in flour/water mixtures by coating sample edges with silicone oil. It is postulated that the dip reflects the shear breakdown of the moisture‐deprived starch granules system, although the possibility that it is related to gluten rheology, as affected by starch, cannot be discarded.

ISSN:0148-6055    

DOI:10.1122/1.549718

出版商:The Society of Rheology    

年代:1983

数据来源: AIP

摘要:

Fish pastes were prepared from comminuted fish muscles after washing with water to remove water‐soluble proteins. The pastes were made with 2% salt and adjusted to a 78% moisture content. Shear moduli and energy damping were monitored continuously during heating of the raw paste from 5 to 90°C using a special temperature‐controlled shear fixture (TSRM) in a universal testing machine. The shear modulus‐temperature relationship had peaks at approximately 13 and 37°C and began a permanent increase at about 47°C. Rigidity was also heating rate dependent. Apparent energy loss, (hysteresis area)/(work of deformation), increased slightly from 10 to 25°C reaching about 38% after which the loss decreased rapidly to 50°C and then more slowly to 70°C reaching approximately 8%. Structural failure of the cooked product (gel) was evaluated using torsion and uniaxial compression tests. True shear stresses and true shear strains at failure were not significantly different comparing torsion and uniaxial compression although the gels failed in shear when uniaxially compressed and in a tension‐shear combination when tested in torsion. The methodology developed is useful in studying thermal processing effects on heat‐induced protein gels and in studying basic structural failure characteristics related to food texture.

ISSN:0148-6055    

DOI:10.1122/1.549730

出版商:The Society of Rheology    

年代:1983

数据来源: AIP

摘要:

In this work, a new model for viscosity decay at constant shear rate is tested and the thixotropic behavior of representative food products is experimentally analyzed. The equilibrium viscosity (or steady‐state viscosity) of some food products, obtained after a sufficiently long time of shear at a constant shear rate, is found to be well represented by the Herschel‐Bulkley model and by an exponential model in which a maximum of two terms of an infinite series are required. The model for viscosity decay, that is, the decrease in viscosity with time at constant shear rate, assumesnthorder kinetics for the decay of a structural parameter λ. The rate constantk, for the decay of λ, is found to be a power law function of the shear rate. The equation for structure decay is combined with a scalar constitutive equation for the shear stress and the resulting model represents adequately the data for viscosity decay of foodstuffs in the range of shear rates50<γ̇<5420 s−1.Data for suspensions such as tomato juice are observed not to follow the expected structural breakdown behavior. Experimental hysteresis curves show that no consistent pattern between shear stress and maximum rate(γ̇0)or time(t0)to reachγ̇0was found, making it impossible to use the information from the viscosity decay experiments to predict the results of the hysteresis experiments.

ISSN:0148-6055    

DOI:10.1122/1.549719

出版商:The Society of Rheology    

年代:1983

数据来源: AIP

摘要:

Small‐amplitude dynamic viscoelastic properties of ten fluid and semisolid food products such as butter, canned frosting, ketchup, and whipped cream cheese have been compared with their respective properties in steady shear. The complex viscosity (η*) and dynamic viscosity (η′) were always found to be larger than steady viscosities (η), resulting in relationships of the form:η*(ω)=C⋅η(γ̇)α,η′(ω)=C′⋅η(γ̇)α′whereCandC′are constants(Pa1−α s1−α, Pa1−α′ s1−α′),α and α′ are constants (dimensionless), ω is the frequency(s−1),and γ̇ is the shear rate(s−1).The primary normal stress coefficient(ψ1)was found to correlate well withG′/ω2which is larger thanψ1in all cases, andG′/ω2can be related to the primary normal stress coefficient by a power relationship of the form:G′/ω2=C*⋅ψ1(γ̇)α*,whereC*is a constant(Pa1−α* s2(1−α*))and α*is a constant (dimensionless). This makes it possible to relate dynamic viscoelastic properties to steady properties for the fluid and semisolid foods studied.

ISSN:0148-6055    

DOI:10.1122/1.549732

出版商:The Society of Rheology    

年代:1983

数据来源: AIP

摘要:

The European community has been concerned with problems that arise in the dissemination and utilization of rheological data on foodstuffs. In an effort to resolve some of these problems an EEC (European Economic Community) committee has been assessing the reliability of data obtained on food materials in different laboratories. As a result of the work of this committee it was felt worthwhile to reemphasize that the reliability of data depends on both the precision and the accuracy of the measurements. Precision relates only to the comparison of measurements of like with like and may be further subdivided into repeatability and reproducibility. These mainly involve the technique of handling the measuring instrument and the sample. Accuracy refers to an absolute standard and concerns the approach of the parameter measured to its true value, independent of the method of measurement. It is mainly concerned with calibration and a proper understanding of the behavior of the sample in the instrument. Accuracy and precision are not necessarily concomitant, though it is unlikely that the former will be achieved without the latter. Some shortcomings in reliability are illustrated by means of simple examples of the viscometry of some fluid food‐stuffs.

ISSN:0148-6055    

DOI:10.1122/1.549731

出版商:The Society of Rheology    

年代:1983

数据来源: AIP

摘要:

Rheological constitutive relations for the apple cortex tissue have been developed by studying the creep behavior under both shear and hydrostatic loading conditions. Creep behavior in shear was studied by subjecting cylindrical samples to various magnitudes of steady torque and the measurement of the unit angle of twist with time using a specially developed multiaxial creep apparatus. Based upon the results, a nonlinear constitutive relation in shear involving two kernel functions has been obtained. Dilatational creep behavior was also studied by subjecting tissue samples to various magnitudes of hydrostatic stress and the measurement of the volume change with time. A rheological constitutive relation has been developed that agrees well with the dilatational creep test data. Finally, general rheological constitutive relations have been obtained by combining the constitutive relations for shear and dilatational deformations. In addition, creep behavior under uniaxial compressive loading has been predicted from the developed constitutive relations and compared with the experimental data. The predicted values have been found to agree with the experimental results.

ISSN:0148-6055    

DOI:10.1122/1.549720

出版商:The Society of Rheology    

年代:1983

数据来源: AIP