COMPARISON OF STEADY SHEAR RHEOLOGICAL PROPERTIES AND SMALL AMPLITUDE DYNAMIC VISCOELASTIC PROPERTIES OF FLUID FOOD MATERIALS

Small amplitude dynamic viscoelastic properties of apple butter, mustard, tub margarine, and mayonnaise are compared with their respective properties in steady shear flow in the range of shear rates and frequencies of 0.1-100 sec⁻¹ using a Rheometrics Mechanical Spectrometer. Comparison of dynamic and steady viscosities shows that dynamic viscosities (η*) are much larger than steady viscosities (η). Consequently the Cox-Merz rule is not obeyed. Moreover the obvious nonlinearity suggests that shift-factor type relationships are not obeyed either. The in-phase (η') component of the dynamic viscosity shows similar behavior when compared to the steady viscosity (η).
Comparison of G'/ω² and Ψ₁ also suggests that any of the results predicted by classical theories of viscoelasticity are not obeyed. G'/ω² is always much larger than Ψ₁ and the relationship between these two material functions is nonlinear.     

FAT CRYSTALS AND THE FLOW RHEOLOGY OF BUTTER AND MARGARINE

Abstract The viscosities of fat crystals extracted by 1 % Aerosol O.T. from butter and margarine have been studied using a Weissenberg Rheogoniometer. By applying the Brodnyan (1959) equation for non-spherical particles to the viscosity data at high rates of shear, after correcting for secondary flow phenomena, it was deduced that the axial ratios of the margarine fat crystals were between 3.05 and 3.94, with the fat crystals from butter lying midway between the two extremes. By applying viscosity equations of the exponential type to such data, it should be possible to calculate the viscosity of butter and margarine at very high rates of shear when the viscosity of the liquid oil in the fat phase and the volume concentration of water drops are known. This information could be useful for predicting the spreadability characteristics of butter and margarine.     

EFFECT OF MECHANICAL TREATMENT ON THE HARDNESS OF MARGARINE AND BUTTER

The work-softening of margarine and butter resulting from passage through perforated plates was determined. Various plates were used differing in size and number of perforations, but identical in the total area of perforations. Greater work-softening was obtained with plates having many small perforations, but the back-pressure in the equipment was higher. Passing the product through two or three plates slightly increased work softening. Additional mechanical action was provided by a propeller rotating immediately behind the perforated plates. Only small additional work-softening was obtained by this action and little remained after 8 days storage. Increasing the speed of the propeller up to 100 r.p.m. increased work softening. Increasing it to 300 r.p.m. did not further increase work softening. It appears that excessive mechanical action has little permanent softening effect on plastic fats.     

EVALUATION OF A SLIDING PIN CONSISTOMETER FOR MEASUREMENT OF THE HARDNESS AND SPREADABILITY OF BUTTER AND MARGARINE

A sliding pin consistometer (SPC) has been shown to be particularly suitable for the assessment of hardness and spreadability of butter and margarine. It appears advantageous over other methods in that it is portable, self-compensating for wear, and is convenient and rapid to use on undamaged samples in their original packaging. Four products, hard and soft butter, and hard and soft margarine, with product temperatures in the range 0.8 to 20.0deg;C, were used. The hardness and spreadability of these products at these temperatures were also scored by a panel of 21 adults using a 9-point scoring scale. SPC measurements explained 96% of the variation in panel scores for both hardness and spreadability. The maximum panel score for spreadability was at an SPC value of 5.5 × 10⁻²N for all four products. The SPC should prove useful to manufacturers of butter and margarine in assessing consumer preference for their products.     

APPLICATION OF STRESS-CONTROLLED ANALYSIS TO THE DEVELOPMENT OF LOW FAT SPREADS

Creep testing was carried out on low fat commercial spreads at 3% (Promise) and 22.5% (Delight) fat content. A 'full fat' product (Flora margarine) and gelatin gels were also analysed to provide background understanding of the viscoelastic behaviour in the low fat samples. Increasing levels of applied stress deform both the Flora margarine and the gelatin gels which creep and eventually achieve steady flow. The linear creep compliance-time dependency for a given stress/level of initial deformation was imitated by a mechanical model combining a Maxwell element in series with one or more Voigt elements, which exposed the elastic nature of gelatin associations as opposed to the 'plasticity' of a fat lattice. Deviations from linearity in gelatin gels and Flora margarine lead to a catastrophic disruption of networks and the termination of experiments due to excessively high angular velocities on the rheometer. The nonlinear regime in low fat spreads, however, produces a three part deformation-time profile comprising creep with steady shear flow, accelerated rate of deformation, and a second level of flow ('terminal'). The accelerated rate of deformation signifies the catastrophic disruption of the biopolymer matrix and results in a viscoelastic solution with suspended fat aggregates. The viscosity of this system ('terminal') is much lower than the preceding steady shear viscosity before the yield point. Eventually, the fat aggregates break down leading to the sequence of viscosity collapse, excessive angular velocity, and experiment termination. The understanding achieved on phase behaviour and structural properties was used to develop economically water continuous spreads (5%–7.5% fat) without the undue gel-like behaviour of Promise imparted by the gelatin component. The prototype spreads possess a stress-strain profile under compression similar to that of Flora margarine at refrigerator temperature.     

PROPERTIES OF BIXIN AND NORBIXIN AND THE COMPOSITION OF ANNATTO EXTRACTS

SUMMARY— The melting point and ₁ ^1% _cm-values of a-bixin, β-bixin, and a-norbixin were determined and the stability of a-bixin examined under different conditions. The total pigment content of butter colors was determined. a-Bixin was the principal pigment; in addition, at least eight different pigments were present. The content of a- and β-bixin in butter colors was also determined. Because of complex formation of bixin and norbixin with a yellow pigment, the simple chromatographic method became rather laborious. This complex formation did not occur in the annatto suspensions in oil and fat. These preparations were more concentrated, while the amount of decomposition products of a-bixin was relatively slight. The stability of the pigments in butter colors appeared to be great. Investigation of annatto cheese colors showed that these extracts contain at least seven color components. The total amount of red pigment and the principal pigment a-norbixin were determined.     

MEASURING RHEOLOGICAL CHARACTERISTICS AND SPREADABILITY OF SOFT FOODS USING A MODIFIED SQUEEZE-FLOW APPARATUS

The rheological characteristics of soft foods such as peanut butter, mayonnaise and margarine were evaluated by the UW Meltmeter, a modified squeezing-flow device, applying a constant load under constant volume and changing area. Sample thickness profiles were measured as a function of time and the yield stress was calculated via a Herschel–Bulkley model as a measure of spreadability. Peanut butter exhibited the highest yield stress value (1.31–1.45 kPa) followed by margarine (0.80 kPa) and mayonnaise (0.31 kPa). Calculated values of biaxial extensional viscosity and consistency coefficient values calculated from the UW Meltmeter data followed the expected trends of relative spreadability of peanut butter, margarine, and mayonnaise. These results show that the UW Meltmeter is a relatively simple and reliable device for measuring yield stress and rheological properties of soft foods.

PRACTICAL APPLICATIONS

A new method has been proposed to measure yield stress of spreadable foods.     

USE OF THE BIRD-LEIDER EQUATION IN FOOD RHEOLOGY

Shear stress development at inception of a constant shear rate was studied for marshmallow cream, peanut butter, squeeze margarine, tub margarine, whipped butter, whipped cream cheese and whipped dessert topping using the cone and plate geometry of the Rheometrics Mechanical Spectrometer. For most foods studied transient shear stresses showed increasing shear stress oversoots with increasing shear rate. At the highest shear rate of 100 s^?1, transient stresses were as large as 425% of the steady state shear stresses, with the actual magnitude of overshoot depending on the particular food investigated. The Bird-Leider equation was chosen to model this time dependent flow behavior by incorporating both steady viscous and elastic properties of the foods. By assuming that the viscosity function and primary normal stress coefficient followed power-law behavior, and constructing a time constant from the calculated parameters, the Bird-Leider model provided a good prediction of maximum and steady state stresses as well as the time at which they occur, but only a crude prediction of shear stress decay. The model supplied two more empirical constants “a” and “b” where “a” can be regarded as a pseudo-elastic modulus. Variation in “a” with shear rate shows that all materials studied are nonlinear viscoelastic.     

EXAMINATION OF THE MICROBIAL STATUS OF SELECTED INDIGENOUS FERMENTED FOODS IN NIGERIA

Four Nigerian traditionally fermented foods (wara, nono, ogi and kununzaki) were evaluated for the presence of some microorganisms of public health concern. Among the dairy foods , Staphylococcus aureus and Klebsiella sp. were isolated from wara while Escherichia coli, Salmonella sp. and Klebsiella sp. were isolated from nono. The cereal-based fermented foods (ogi and kunu-zaki) contained Bacillus subtilis, E. coli, S. aureus, Klebsiella sp. and Enterococcus faecalis. The mesophilic aerobic counts were: 5 × 10⁵ for wara; nono, 1.53 × 10⁷; ogi, 3.6 × 10⁶ and kunu-zaki, 2.6 × 10⁶ cfu/mL. The enterobacteriaceae counts on nono, wara, ogi and kunu-zaki were 1.79 × 10⁷, 4.5 × 10⁵, 4.0 × 10⁵ and 1.2 × 10⁶ cfu/mL, respectively. No Vibrio count (detection limit: <10 cfu/mL) was recorded in all the food samples considered. The yeast and mold counts ranged from 1.0 × 10⁵– 3.31 × 10⁷ among the food products. The antimicrobial susceptibility patterns of the organisms isolated from dairy products (nono and wara) revealed that they were resistant to ampicillin (100%) and sensitive to gentamicin (100%) and nalidixic acid (100%). Most isolates from cereal based products (ogi and kunu-zaki) were 100% resistant to penicillin, ampicillin and chloramphenicol. This work highlights the need to maintain hygienic standards in the preparation of our locally fermented cereal and dairy foods.     

SOY AND MEAT PROTEINS AS FOOD EMULSION STABILIZERS 2. INFLUENCE OF EMULSIFICATION TEMPERATURE, NaCl AND METHANOL ON THE VISCOELASTIC PROPERTIES OF CORN OIL-IN-WATER EMULSIONS INCORPORATING ACID PRECIPITATED SOY PROTEIN

The rheological properties of concentrated corn oil-in-water emulsions stabilised by acid precipitated soy protein have been studied as a function of emulsification temperature, methanol and salt concentration. Viscoelasticity parameters were derived from the creep compliance-time response of the emulsions at a constant stress of 47.8 dyne cm⁻². All of the emulsions exhibited higher parameter values, both immediately after preparation and during storage, than comparable emulsions prepared at ambient temperature in the absence of methanol or NaCl. This was attributed to the increased unfolding of adsorbed protein molecules which permits greater interlinking, by hydrophobic bonds, of molecular segments projecting from the surfaces of adjacent oil drops.     

MODELING THERMAL SOFTENING KINETICS OF POTATOES USING FRACTIONAL CONVERSION OF RHEOLOGICAL PARAMETERS

Texture degradation in potatoes was investigated during heating of cylindrical test samples, 15 mm diameter and 30 mm long, in water at 60, 70, 80 and 90C by mechanical testing. The rheological parameters obtained from the axial and radial compression, creep and stress relaxation tests decreased exponentially with an increase in heating duration and temperature approaching steady levels corresponding to the remaining texture after prolonged heating. A single first-order kinetic model described the changes in various rheological parameters due to thermal softening of potatoes using fractional conversion technique indicating very high values of the coefficient of determination (R² > 0.97). In general, the visco-elastic test parameters were relatively more sensitive to texture degradation in potatoes in comparison with the quasi-static test parameters. Overall comparison showed that elasticity and viscosity parameters in Kelvin component of the 4-element model (E₂ and η₂) from creep tests and elasticity moduli from axial and radial compression tests (E_a and E_r) best presented the texture degradation kinetics in potatoes with R² ranging from 0.98 to 0.99.     

THE EFFECT OF TEMPERATURE ON SOME RHEOLOGICAL PROPERTIES OF WHEAT FLOUR DOUGHS

Abstract. Doughs from three flours were sheared between a cone and plate at constant rates in the range 6 times 10^-4-2 times 10^-2 s^-1.
At temperatures between 25 and 40C, the apparent viscosity decreased with increasing temperature and with increasing rate of shear. The effects of the temperature and of the rate of shear were independent one of another, and can be described by an Arrhenius type equation and a power equation, respectively. At temperatures between 45 and 60C, the apparent viscosity increased rapidly with increasing temperature; this is ascribed to starch gelatinization. In this temperature range, the apparent viscosity decreased more rapidly with increasing rate of shear than at lower temperatures.
At temperatures between 25 and 45C, the shear modulus decreased with increasing temperature and slightly increased with increasing rate of shear. From the former fact it is concluded that the elasticity of dough has an origin different from rubber elasticity. In this temperature range, the shear modulus can be described by an equation similar to that for the apparent viscosity. At temperatures between 45 and 65C, the modulus increased with increasing temperature, though to a lesser extent than the apparent viscosity.
Changes in the rate of stress relaxation are in accordance with the effects of temperature and rate of shear on the apparent viscosity and the modulus.     

ORAL PERCEPTION OF VISCOSITY IN FLUID FOODS AND MODEL SYSTEMS

The perceived in-mouth thickness ( T ) of a range of fluid foods and model systems was assessed by a trained sensory panel, using a ratio scaling technique, and correlated with objective measurements of viscosity (η_N). For Newtonian samples a simple linear correlation (r²= 0.98) is observed between log T and log η with exponent η= 0.22. The linear relationship between log T and log η for Newtonian materials was used to calculate the equivalent Newtonian viscosity (η_N) from subjective panel scores for the non-Newtonian samples studied. The shear rate at which observed viscosity is equal to η_N decreases with η_N, as reported previously by Shama and Sherman, but also decreases with increasing shear-rate dependence of viscosity. This implies oral perception of viscosity over a range of shear rates. Comparison of flow curves for samples assigned similar thickness scores, but showing very different shear thinning behaviour, suggests that the perceived thickness of extremely shear-thinning materials is dominated by their high viscosity at low rates of shear. As a simple practical index, viscosity at 10 s⁻¹ shows a better correlation (r²= 0.95) with the perceived thickness of the samples studied than values calculated by the methods suggested by Wood, or by Shama and Sherman (r²= 0.90 in both cases). The close agreement between panel scores for perceived thickness and perceived stickiness previously observed for concentrated solutions of random coil polysaccharides does not apply for several of the food systems studied, or for very concentrated solutions of xanthan.     

THE VISCOSITY OF SUPERCOOLED FRUCTOSE AND ITS GLASS TRANSITION TEMPERATURE

The viscosities of supercooled liquid fructose and glucose are measured between a lower viscosity limit corresponding to 95°C and an upper limit of about 10⁶ Pa.s. The glass transition temperature of fructose is estimated from the viscosity data using different extrapolation procedures. The Vogel-Tamman-Fulcher and power-law equations gave estimates of 283K and 289K, respectively and the Williams-Landel-Ferry equation 286K. These values are in agreement with recent calorimetric determinations of the glass transition temperature.     

A COMPUTERIZED METHOD TO ANALYZE THE CREEP BEHAVIOR OF VISCOELASTIC FOODS

A computer-aided method was developed to simplify the analysis of creep behavior and to determine the elasticity, the Newtonian viscosity and individual viscosities of viscoelastic foods. The method enabled the inclusion of all creep compliance data in the analysis, and reduced the analysis time from hours to minutes. The method was tested in the calculation of rheological properties of wheat flour dough.     

FOOD TEXTURE MEASUREMENTS WITH THE PENETRATION METHOD

Following the work of Bourne, penetration measurements were performed with two sets of punches. The first set had a constant area and various perimeters. The second set had a constant perimeter and various areas. At constant perimeter, the relationship between punch area and force was a straight line with process cheese, butter and margarine, the latter two going through the origin. At constant area, the relationship between punch perimeter and force was a straight line for process cheese, but no definite relation between punch perimeter and force was obtained with butter and margarine. The relationship between the nature of the curves and the physical structure of the food is discussed. It was concluded that penetration tests of cellular products such as fruits studied by Bourne may involve shear and compression factors. In products with strongly bound network structures, e.g. process cheese, the main factors are shear and flow. In products with weakly bound network structures, such as plastic fats, flow is the only major factor involved.     

NMR VELOCIMETRY STUDY OF THE TEMPERATURE DEPENDENT RHEOLOGY OF BUTTER, SEMISOFT BUTTER AND MARGARINE

A new rheological characterization technique is described in which nuclear magnetic resonance (NMR) velocimetry is used to measure velocity profiles across the annular gap of a Couette cell, for materials that exhibit yield stress and power law shear thinning properties. The materials studied were butter, semi-soft butter and margarine over a temperature range from 25C to 42C. The method enables the observation of heterogeneous behavior across the different stress regimes that are present in the Couette gap and flow visualization makes it possible to clearly identify the existence or absence of slip effects at the wall. The results show that the power law exponents are strongly temperature dependent for the edible fat materials investigated.     

EFFECTS OF VISCOSITY OF LIQUID FOODS ON PALATAL PRESSURE

The deglutition of non-Newtonian liquids introduced into the mouth was studied dynamically by measuring palatal pressure (P) with pressure transducers set at three locations on the palate. The value of P and the swallowing pressure (S) changed only from 100 to 200 g/cm² over the viscosity range 10⁻² to 10¹ Pa.s. The retaining time (T) and work (W), required for swallowing after the liquid entered the mouth, remained almost constant up to a critical value of 1.0 Pa.s. above which both T and W increased markedly. When the viscosity was low, all of the liquid was swallowed in one deglutition, up to 15 mL volume. Therefore, T was almost constant but S increased with the volume. When the viscosity was high, the liquid was swallowed in several smaller portions. When the volume was high, T increased and S was either constant or it decreased.     

RHEOLOGICAL BEHAVIOR OF COCOA DISPERSIONS WITH COCOA BUTTER REPLACERS

The steady shear viscosity and yield stress of cocoa powder dispersed in three cocoa butter replacers (CBRs) are measured over a wide range of powder concentration and temperature. The flow activation energies of the three CBRs are nearly the same, and close to that of cocoa butter. Values of intrinsic viscosity of cocoa powder dispersed in corn oil and Socolate 36–38 are in the range of 4.0–5.0, (similar to that in cocoa butter), suggesting that these two CBRs are good cocoa replacers. A lower value of intrinsic viscosity is found for the powder dispersed in Super YZ-2. Viscosity measurements indicate that all three CBR dispersions exhibit Newtonian behavior at low volume fraction, but non-Newtonian shear thinning characteristics when the volume fraction φ > 0.3 at 60C. The non-Newtonian behavior can be described by the Casson model when φ exceeds a critical value, φ mo , the maximum packing fraction at zero shear. The yield stress can be related to the powder volume fraction using a four-parameter equation originally proposed by Zhou et al. (1995).     

COMPARISON OF MECHANICAL TESTS FOR EVALUATING TEXTURAL CHANGES IN POTATOES DURING THERMAL SOFTENING

The changes in the texture of cylindrical samples of potato tissues immersed in water at 60, 70, 80 and 90C for up to 80 min were monitored at each temperature in terms of tangent modulus of elasticity in axial and radial compression tests, and elasticity and viscosity parameters in creep and stress relaxation tests. The magnitude of all mechanical test parameters decreased with an increase in heating time and temperature. The creep and stress relaxation responses of individual potato samples were adequately represented by respective mechanical models (R²= 0.94 to 0.99). The mechanical test parameters followed apparent first-order degradation kinetics due to the effect of thermal softening, and the rate constant was used as an index of the sensitivity of a mechanical test. The radial compression test was relatively more sensitive than the axial test. Based on an overall comparison, the parameters from creep and stress relaxation tests were found to be the most sensitive in describing the textural changes during thermal softening of potatoes.