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 RHEOLOGICAL CONDITIONS ASSOCIATED WITH JUDGEMENT OF POURABILITY AND SPREADABILITY OF SALAD DRESSINGS

A panel of 25 students compared the pourability and spreadability of two commercial brand salad dressings (non-Newtonian) and a CMC-sugar solution (Newtonian). Correlation of their responses with visco-metric data for the 3 samples indicated that the shear stress and shear rate associated with the sensory evaluation of these two properties are 190–430 dyne cm⁻² and 0.9–6.3 s⁻¹, and < 190 dyne cm⁻² and < 0.9⁻¹, respectively.     

MEASUREMENT OF FLOW ABILITY OF BUTTER BY CAPILLARY RHEOMETRY

Capillary rheometry was used to study and compare the rheological behavior of butters made from supercritically fractionated high melting triglyceride (HMT) and anhydrous milk fat (AMF) at different temperatures. Application of shear rates in the range of 17.37 to 346.93 _S⁻¹ showed a strong dependency of flow behavior on the capillary diameter, indicating slippage at the walls. Shear rates were corrected for wall-slippage using capillaries with different diameters but the same length, and for non-Newtonian behavior using the Rabinowitsch procedure. Shear stresses were corrected for entrance effect using capillaries of the same diameter but different lengths. Power law consistency indices (K), determined from loglog plots of corrected shear stresses versus shear rates, were found to have a negative correlation with temperature. Higher K values were observed for HMT butter depicting its more solid-like behavior due to greater concentration of high melting triglycerides. A slight increase in the K values was observed on storage probably due to rearrangement of fat crystals and formation of secondary bonds between them. Flow indices were found to be relatively insensitive to both time and temperature of storage. From the flow curves (apparent viscosity versus shear rates), master curves were generated using the time-temperature superposition principle.     

INFLUENCE OF GRANULE SIZE ON VISCOSITY OF CORNSTARCH SUSPENSION

The influence of granule size and size distribution in gelatinized suspensions of 2.6% cornstarch heated at fixed temperatures between 70–90C on the power law viscosity was measured by laser diffraction. In the early stages of gelatinization, where the granule size standard deviation was less than 12 μm, dilatant behavior was observed over the shear rate range 200–1100 _S ⁻¹. At constant starch concentration and without granule rupture, suspension consistency, K, increased exponentially with granule mean diameter while the flow behavior index, n, decreased linearly with increase in the extent of gelatinization and the standard deviation of the granule size. The variation of K with granule size and that of n with standard deviation were found to be independent of temperature over 70–90C. In the later stages of gelatinization, where the granules lose their integrity, suspension viscosity progressively decreased as granule rupture continued.     

Effects of fat on the properties of halloumi cheese

Halloumi cheese was produced from 11 bovine milks with fat contents of 1.61–4.04%, giving a range of 32–53% fat in dry matter (FDM) in the cheeses. Starter culture and/or microparticulated whey protein (Simplesse ^® 100(E)) was also added to selected batches of milk. Hardness decreased with increasing FDM, with increase in moisture and with lower pH. On sensory evaluation, there was an increase in preference score with FDM ( R ²   = 0.8). Inclusion of microparticulated whey protein may have had a fat mimetic effect, as preference scores otherwise decreased with increasing protein levels ( R ² = 0.75).     

RHEOLOGICAL INDICES OF FRUIT CONTENT IN JAMS: EFFECT OF FORMULATION ON FLOW PLASTICITY OF SHEARED STRAWBERRY AND PEACH JAMS

The effect of formulation factors on Casson yield values measured at low shear rates (0.08 ≤γ≤ 1.01s^-1) (σ₀₁) and at medium shear rates (2.58 ≤γ≤ 387.30s^-1) (σ₀₂) was analyzed in previously sheared strawberry and peach jams. Twenty three samples of each fruit jam were prepared according to a second order composite rotatable design. Composition ranges were: fruit content, 25–55%; soluble solids content, 60–70° Brix; added pectin in strawberry jams, 0.3–0.7% and in peach jams, 0.1–0.5%. Variation of σ₀₁ values in strawberry jams depended mainly on the interactions between fruit and soluble solids and between fruit and pectin, while in peach jams, it depended on fruit-soluble solids and soluble solids-pectin interactions. Patterns of change of σ₀₂ values with composition were similar to those observed for σ₀₁ in both strawberry and peach jams. Predictive power of σ₀₁ and σ₀₂ values for estimation of fruit content was low, but taken in conjunction with soluble solids content and total pectin values, 79.5% of the variability of fruit content in strawberry jams and 91.1% of same in peach jams could be explained.     

CHANGES IN EQUILIBRIUM MODULUS AND αsl-CASEIN BREAKDOWN DURING THE RIPENING OF PORT SALUT ARGENTINO CHEESE AS AFFECTED BY FROZEN STORAGE

The effect of the freezing, frozen storage and thawing on textural parameters and α_sl-casein breakdown during the ripening of Port Salut Argentino cheese was studied. Moisture content, salt concentration, casein profiles and asymptotic equilibrium modulus were monitored in control cheeses ripened at 5C and in cheeses, stored at -22C for 30 days, thawed and ripened at 5C, for different ripening times (1, 6, 13, 27 and 56 days) and two sampling zones (central and external). The freezing process significantly increased the rate of α_sl-casein and α_sl-I-casein hydrolysis. This process may affect the susceptibility of α_sl-casein to chymosin attack and also the availability of hydrolytic enzymes released by damaged microorganisms, which may contribute to the faster hydrolysis of α_sl-I-casein. The freezing process did not significantly affect the decay rates of asymptotic equilibrium modulus. First order kinetics constants for decay of the asymptotic equilibrium modulus were 3.71 10^-2day^-1 (control cheeses, central zone), 8.48 10^-2 day^-1 (control cheeses, external zone), 4.52 10^-2 day^-1 (frozen cheeses, central zone), and 11.43 10^-2 day^-1 (frozen cheeses, external zone). Significant differences in the decay rates of asymptotic equilibrium modulus were found between central and external zones in control and frozen cheeses primarily due to differences in moisture contents of the sampling zones.     

Carotenoid composition and vitamin A value of some native Brazilian green leafy vegetables

The composition of carotenoids in five native Brazilian leafy vegetables was determined. The ranges of total carotenoid contents of Amaranthus viridis, Lepidium pseudodidymum, Xanthosoma spp., Sonchus oleraceus , and Portulaca oleracea were 347–468, 237–280, 225–361, 149–334 and 71–109 μg g⁻¹, with lutein and β-carotene predominating. The mean β-carotene contents (μg⁻¹) and vitamin A values (retinol equivalents RE_g⁻¹) were 110 and 18.4, 84.6 and 14.1, 67.3 and 11.2, 62.9 and 10.5, 29.8 and 4.99, respectively. The leaves contained 54–61% of lutein plus violaxanthin, 24–34% of β-carotene, 10–14% of neoxanthin and traces of zeaxanthin and α-crypto-xanthin. The native leafy vegetables were richer sources of provitamin A than the cultivated leafy vegetables analysed previously, justifying their commercial production.     

Determination of chloride, nitrate and calcium ions in sugar with ion selective-electrodes

Chloride, nitrate, and calcium ion-selective electrodes were tested and found satisfactory for the determination of the corresponding ions in highly refined white sugar, molasses and other impure sugar samples at different stages of sugar manufacturing or refining. The interferences by Br⁻ and I⁻ ions in Cl⁻ determination were eliminated by oxidation with 3 M HNO₃ and passing air to remove the free Br₂ and I₂ formed. The Cl⁻ concentration was then determined with a chloride-selective electrode against a calibration curve. Chloride should not be determined by ashing sugar, since this causes high losses of Cl⁻. It was found that NO₃⁻ and free Ca²⁺ ions could be determined directly in the sugar solution against corresponding calibration curves. Total calcium was determined after ashing of sugar samples. The ash was dissolved in 0.1 M HCl and passed through an anion exchange resin to remove PO³⁻₄ and SiO₂₋₃ ions, and Ca²⁺ was determined as before. Bound calcium was obtained by subtracting values of free from total calcium. Determination of bound calcium was useful to monitor the process of liming. For comparison, the three ions were also determined with acceptable precision without removing interfering ions using the method of standard addition and Gran's plot. The errors of determination for both direct and standard addition techniques were 3% for Cl⁻ and NO⁻₃ and 1–5% for Ca²⁺.     

RHEOLOGY OF STIRRED YOGURTS

Rheological characteristics of 2 commercial brands of stirred yogurt were evaluated using a Haake Rotovisco Model RV 20 with an M-5-Osc measuring head and MV-1 rotor assembly from measured shear stress values under a programmed 3-cycle up- and down-shear rate at 100 s⁻¹/min from 0–500 s⁻¹ (5 min). The upward shear-rate flow behavior of the yogurt samples could be described by a Herschel-Bulkley model while the downward shear-rate curves were essentially linear. Both upward and downward shear rate curves demonstrated progressive structural degradation with repeated shearing. The dependency of rheological parameters on temperature in the range 10–25°C followed both Arrhenius and Turian models.     

Apparent shear viscosity of native egg white

Temperature and shear dependency of the apparent viscosity of the thick and thin portions of egg white from fresh hen eggs, in their native state was investigated. The viscosity of the egg white was determined in the temperature range of 5–30°C and the shear rate range 34–600 sec^-1. The viscosity of both the thick and thin portions of the egg white decreased with increase in temperature. The thin portion showed no decrease in viscosity with shearing time at constant shear rates while the viscosity of the thick portion decreased within the first 6 min of shearing and then remained constant. The rate of decrease in viscosity with shearing time was more at higher temperatures. At 20°C and a shear rate of 34.2 sec^-1, the equilibrium-apparent viscosity of the thick portion was 160 cp, approximately forty times greater than that of the thin portion (4cp). This difference in viscometric properties may be due to the presence, in the thick portion, of high molecular weight complexes which are disrupted under shearing.     

RHEOLOGICAL BEHAVIOUR OF SHEARED JAMS. RELATION WITH FRUIT CONTENT

Eight samples of jam were sheared to destroy gel structure and their flow behaviour was analysed in a concentric cylinder viscometer. Jams were prepared from 4 different fruits—strawberry, peach, plum, and apricot—at 50% and 30% fruit content approximately. Flow of sheared jam showed time dependence, which could be quantified by the Weltmann model and which was eliminated by shearing samples for 8 min at 300 rpm. Casson's yield stress values were obtained at two ranges of shear rates: 0.08–1.01 s⁻¹ (O₀₁) and 2.58–387.30 s⁻¹ (o₀₂). o₀₂ values were about double the o₀₁ values. Flow was adequately described by the Herschel and Bulkley model, introducing either of the yield stress values (o₀₁ or o₀₂). Significant differences were found for some of the rheological parameters studied (Weltmann A and B constants, o₀₁, o₀₂, K₁, and n₁) between samples containing 50% and 30% fruit. These results demonstrate the role played by fruit particles in the rheological behaviour of this type of product and suggest the possibility of using rheological parameters as indices of fruit content in jams.     

RHEOLOGY OF FRESH, AGED AND GAMMA-IRRADIATED EGG WHITE

SUMMARY– Rheology of unmixed egg white at 2°C was studied with a narrow-gapped rotational viscometer over a 20-fold range of shear rates. For a constant shear rate, egg white consistency decreases with time and approaches an equilibrium value in a few minutes. Flow behavior is pseudoplastic at 2°C between shear rates of 8.1–147 sec^-1 and the shear stress-shear rate relation is accurately described by the power law and Casson models. The effects of gamma irradiation dose on Haugh unit score and equilibrium shear stress are discussed.     

The rheological properties of ketchup as a function of different hydrocolloids and temperature

The flow properties of ketchup were assessed upon addition of commonly used food thickeners: guar, xanthan and CMC gum at three different concentrations (0.5%, 0.75% and 1%) and four temperatures (25, 35, 45 and 55 °C). The ketchup without supplementation served as a control. All ketchup formulations exhibited non-Newtonian, pseudoplastic behaviour at all temperatures and hydrocolloid levels. The Power-law and Herschel-Buckley model were successfully applied to fit the shear stress versus shear rate data. The flow behaviour indices, n and n' , varied in the range of 0.189–0.228 and 0.216–0.263, respectively. The consistency coefficients, k and k' , were in the range of 8.42–27.22 and 6.56–20.10 Pa s ⁿ , respectively. The addition of hydrocolloids increased the yield point (τ₀) and apparent viscosity of the ketchup in comparison to that of the control. The Arrhenius equation was successfully used to describe the effects of temperature on the apparent viscosity of the prepared formulations. The E _a value appeared in the range between 5492.6 and 21475.8 J mol⁻¹.     

THE PSYCHOPHYSICS OF POURING, SPREADING AND IN-MOUTH VISCOSITY

The sensory viscosity of low calorie viscoelastic syrups in the mouth, while pouring out of a bottle, and spreading over a flat surface was satisfactorily estimated from the fundamental physical properties of these fluids. Appropriate sensory perception mechanisms were identified and linked to the operating conditions of each sensory test through psychophysical models. Oral sensory viscosity correlated with the shear stress in the mouth (R²= 0.96) (Kokini et al. 1977); pouring sensory viscosity correlated with the cross sectional area filled by the fluid at the neck of the bottle (R²= 0.86); and spreading sensory viscosity correlated inversely with the radial growth of the spreading fluid puddle (R²= 0.96).     

COLORIMETRY OF FOODS. 3. Carrot Puree

SUMMARY– A series of 10 carrot purees was prepared with added FD&C Green No. 1 food color in increments of 0.015 ppm. The samples were evaluated visually for color differences under illumination of 7400°K and measured with a G.E. spectrophotometer with D&H tristimulus integrator and a Hun terlab D₂₅ colorimeter in cells varying from 2–8 mm in thickness against both a white and black background. The instrumental data were calculated in conventional and K/S ratios and correlated with visual and theoretical rankings. The panelists could reasonably well rank samples differing by 0.10–0.15 delta E units, calculated as [(delta L)²+ (delta a)²+ (delta b)² 1/2. Highest correlations with the theoretical order were obtained with a black background and sample thicknesses of 6 mm or more. Adequate rankings with either instrument required a multiple correlation with all 3 color parameters (X Y Z or L a b). Optimum separation of samples with the G.E. spectrophotometer was obtained with a white background and a sample thickness of 5 mm or more. The Hunterlab D₂₅ colorimeter was superior to the G.E. spectrophotometer for separation of samples. Optimum separation was with a 2-in.-diameter aperture and a cell 4 mm or more in thickness against a white background. Correlations of Hunter data with theoretical order were higher than those with visual vs. theoretical ranking. Individual K and S values may be useful in characterizing purees for colorimetry.     

Circulation cleaning of a plate heat exchanger fouled by tomato juice

The effect of a change in flow rate of a 2% w/w caustic soda solution on the circulation cleaning of a plate heat exchanger fouled with diluted tomato paste has been investigated at a temperature of 90°C and fixed circulation cleaning times of 5 and 10 min.
For both circulation cleaning times a transition point was noted in the range 2.75–2.86 1/min (Reynolds number range 6600–6300, shear stress range 0.77–0.82 N/m²). Below the transition point the effect of flow rate on cleaning efficiency was minimal, but at flow rates above the transition point there was a marked increase in cleaning efficiency as the flow rate (and hence shear stress at the surface) increased.     

Consumer preferences and fungal and mycotoxin contamination of dried cassava products from Ghana

Summary Members of one hundred and twenty five households from 19 villages producing dried cassava products were interviewed in Ghana. Kokonte was the most important cassava product in 19% of the households processing it. Most kokonte was produced between January and March. Mould growth during processing or storage was a problem during June and July, which is part of the rainy season. Most producers and market traders preferred non-mouldy kokonte, although many (59%) would consume a mouldy product. There was a price premium for non-mouldy kokonte. The most commonly isolated fungi were yeasts and Cladosporium spp. (44 out of 49 samples). Other fungi isolated included Aspergillus spp. (20 samples); Penicillium spp. (15 samples) and Fusarium spp. (30 samples). Sterigmatocystin was detected in 10 samples at 0.17–1.67 mg kg^–1; patulin in 4 samples at 0.55–0.85 mg kg^–1; cyclopiazonic acid in 4 samples at 0.08–0.72 mg kg^–1; penicillic acid in 5 samples at 0.06–0.23 mg kg^–1 and tenuazonic acid in 3 samples at 0.02–0.34 mg kg^–1. Mycotoxin contamination of mouldy kokonte was a potential problem; there is therefore the need to improve kokonte processing to avoid mould growth.     

VISCOELASTIC CHARACTERIZATION OF RICE GEL

Rice flour dispersions with 15–25% solids were subjected to microwave heating for 0–75 s and samples were subjected to oscillatory frequency sweep between 0.01–10 Hz. The initial rapid increase in storage modulus (G¹) with frequency decreased to a plateau at higher frequencies. An increase in G¹ and complex viscosity (η*) with an increase in the time of treatment or concentration of solid indicate the extent of sol-gel transformation. The sensory data on hardness, elasticity and stickiness also followed similar trend. Principal component analysis (PCA) of the sensory results indicate that a minimum treatment time of 45 s is needed to achieve a rice gel with acceptable sensory characteristics. The G' values were modeled (0.936 ≤ r ≤ 0.988, P ≤ .01) by the 4-element spring-dashpot model.     

PREDICTING STEADY AND OSCILLATORY SHEAR RHEOLOGICAL PROPERTIES OF CMC/GUAR BLENDS USING THE BIRD-CARREAU CONSTITUTIVE MODEL

The Bird-Carreau constitutive model, a five parameter semi-empirical set of equations, was used to predict the steady shear viscosity η, the dynamic viscosity η', and the out-of-phase component of the complex viscosity divided by frequency η"/ω, for aqueous blends of sodium carboxymethylcellulose (CMC) and guar gum. The five Bird-Carreau constants were derived from equations which correlate the constants to concentration and molecular weight of the CMC/guar blends. Blends of ratios 3:1, 2:1, 1:1, 1:2, and 1:3 by weight were studied in concentrations of 0.5, 0.75, 1.0, and 1.5% by weight. For most blends the model accurately predicted the experimental η and η' data in the frequency/shear rate range of 0.1–100 s^-1 and the experimental η"/ω data in the frequency/shear rate range of 1.0–100 s⁻¹.