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.     

REMOVAL KINETICS OF BACILLUS CEREUS SPORES FROM STAINLESS STEEL PIPES UNDER CIP PROCEDURE: INFLUENCE OF SOILING AND CLEANING CONDITIONS

Cleaning efficiency is of prime importance for food industries to ensure both the quality and safety of the products. The removal kinetics of Bacillus cereus spores adhering to unheated stainless steel pipes was studied under turbulent flow conditions (Reynolds’number of 77500 and 116300) in order to be close to those encountered in industrial practice. The experimental data was fitted using a hyperbolic tangent model. Variance analysis was then performed to underline any potential effects on the kinetics of the processing parameters, such as soiling conditions, soiling media and mean walls shear stress during cleaning. A significant influence of the adhesion medium (milk or saline) is shown at the level of spore removal (P < 0.001). This trend could probably be explained by the change in the surface properties of spores and stainless steel surfaces when covered by milk macromolecules. After milk soiling in turbulent flow conditions, removal efficiency was enhanced by a factor of 2.5 to remove 50% of the initial spore contamination and by a factor of 2 for the remaining spores after 30 min of cleaning. No effect of the two mean wall shear stresses (9.4 and 19.1 Pa) has been identified. The removal kinetic model proposed here could now permit the effect on the cleaning efficiency of a wide range of CIP conditions to be tested.     

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.     

LINEAR AND NONLINEAR VISCOELASTIC BEHAVIOR OF OIL-IN-WATER EMULSIONS STABILIZED WITH POLYSACCHARIDES

The rheological behavior and stability of oil-in-water emulsions stabilized by different thickening agents were analyzed. Food emulsions were prepared with commercial sunflower oil (40% w/w oil-in-water) and stabilized with 1% emulsifier. The tested thickeners were: (1) 1% w/w xanthan gum (XG), (2) 5% w/w potato starch (PS), (3) 5% PS + 0.5% XG, (4) 1% w/w guar gum (GG), and (5) 0.5% XG + 0.5% GG. Mean droplet size and droplet size distribution (DSD) of emulsions were determined by static light scattering. Steady flow (viscosity versus shear rate), transient flow (viscosity versus time) and oscillatory shear tests (linear viscoelasticity) were performed. The addition of thickening agents improved the stability of the emulsions, the effect was less marked in systems containing only GG. DSD was not significantly modified in emulsions containing starch or hydrocolloids. Microscopic observations showed that all the tested emulsions were flocculated due to the presence of hydrocolloids. The observed shear thinning behavior was attributed to the molecular structure of the polysaccharides and to the flocculation/deflocculation process; viscosity data were satisfactorily fitted to the Cross model. Frequency sweeps showed that emulsions with PS or XG have a weak gel structural network (G' > G); those with GG correspond to a polymeric solution where G' and G" curves intersect within the range of tested frequencies. The viscoelastic linear behavior was described according to the Maxwell generalized model. The discrete relaxation spectrum and relaxation times were estimated from the experimental values of G' and G" for emulsions with PS, PS + XG, and XG. Nonlinear viscoelasticity was also studied from stress relaxation curves at different shear strains. The damping function was calculated and the Soskey-Winter parameters were determined. Transient flow viscosities at different shear rates were comparable to the values estimated from stress relaxation measurements.     

STUDIES ON FLOWDYNAMICS OF FALLING FILMS OF WATER AND ITS EFFECT ON CLEANING OF STAINLESS STEEL

SUMMARY— Factors such as inclination angle of the test surface, flow rate and temperature and composition of cleaning solutions were examined for their influence on cleaning an 18-in.-wide sheet of No. 4 finish stainless steel by gravity flow of falling films of water and detergent solutions. A suspension of B. subtilis spores was used to contaminate the test surface. After a cleaning treatment, the percentage of spores remaining as measured by a direct surface agar plate method IDSAPI was used to calculate cleaning efficiency. Flows of water and cleaning solutions ranged from 50 to 250 lb/min. After detergent solutions, a post-rinse with water at 100°F for 5 min was used. Another setup with a smaller plate and a constant flow rate of 45 lb/min was used to study the effect on cleaning of varying plate angle between the limits of 60 degrees either side of vertical. Washing time was standardized at 10 min for the large plate and 15 min for the smaller one. Results indicated that cleaning improved with temperature, flow rate and with the presence of acid or alkaline cleaning agents. Cleaning was most effective when the plates were in the vertical position.     

RHEOLOGICAL CHARACTERIZATION OF STIRRED YOGHURT: VISCOMETRY

Rheological characteristics of stirred yoghurt, varying in dry matter content, fermentation temperature and composition of bacteria cultures, were evaluated using a Bohlin VOR Rheometer. Flow curves, covering the shear rate range 29–920 s⁻¹, generally indicated the presence of a yield point and pseudoplastic behaviour, but also Bingham plastic behaviour was observed for some of the specimens. Systematic variations in the profile of the flow curves were observed as a result of the applied manufacturing conditions. Flow curves exhibiting pseudoplastic behaviour with a yield stress could not, over this broad shear rate interval, be described satisfactorily by the often applied Casson or Herschel-Bulkley models, and consequently a new one, the QRS-model was introduced. This model contains three parameters and gave a significantly better fit, with R² values generally above 0.99.     

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.     

GRANULE SIZE DISTRIBUTION AND RHEOLOGICAL BEHAVIOR OF HEATED MODIFIED WAXY AND UNMODIFIED MAIZE STARCH DISPERSIONS

The effect of granule size distribution, supported by microscopic analysis, on the steady shear and dynamic shear rheological behavior of 5% gelatinized modified waxy maize starch (MWMS) dispersions heated at 80C for time periods from 0.5 to 30 min was examined and data obtained with 5% unmodified maize starch (UMS) dispersions was used to provide additional insight. The MWMS dispersions showed a faster and more extensive swelling than the UMS dispersions at 80C. They also exhibited rheopectic (anti-thixotropic) time-dependent and shear thinning with yield stress behavior. UMS dispersions showed shear thickening characteristics after short heating times that gradually turned to shear thinning as the heating time was increased. Dynamic tests on MWMS dispersions showed G' was about ten times higher than G". G' showed a small dependence on frequency over 0.6–63 rad/s while G", after an increase at low frequencies, became less sensitive. The gelatinized MWMS dispersions can therefore be classified as 'weak gels.' The flow behavior index of UMS dispersions was strongly     

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.     

棉籽糖纯化过程中电渗析脱盐研究

为了除去棉籽糖提取液中的盐分杂质,本实验采用电渗析法脱盐,考察了不同操作电压和循环流量下,棉籽糖提取液脱盐率变化和能耗变化。结果表明：电渗析对棉籽糖提取液中的盐分去除非常有效,在10～20 V的电压范围内,操作电压越大,脱盐率越高,继续升高电压,脱盐率保持不变;在20～60 L/h的流量范围内,循环流量越小,溶液脱盐率越高。在操作电压为25 V,循环流量为20 L/h,脱盐120 min后,脱盐率可达到91.2%,棉籽糖回收率达94.5%。若达到相同的脱盐率,在20～60 L/h的流量范围内,流速越低,能耗越小;在20～30 V电压范围内,电压越高,能耗越大。利用电渗析法对棉籽糖提取液脱盐具有一定的应用价值。     

Rheology of scleroglucan dispersions

The rheological properties of a scleroglucan, industrially produced by Sclerotium rolfsii, either in aqueous or dimethylsulfoxide (DMSO) dispersions at concentrations (c) ranging from 0.01% to 1.5% (w/w) were determined by using Cannon–Fenske capillary viscometers and a dynamic stress rheometer with plate-and-cone geometry under steady shear flow, small amplitude oscillatory shear and creep. The intrinsic viscosity [η] in water and DMSO was found to be equal to 12.1±0.3 and 4.6±0.3 dl/g, respectively. Despite the creep and oscillatory tests pointed out the presence of more or less strong molecular entanglements resulting in a three-dimensional network for c0.35% (w/w), the steady shear flow ones using a stress increasing exponentially with time showed a Newtonian behaviour at low enough shear rates, a pseudo-plastic one in the middle shear rate range and a second Newtonian plateau at very high shear rates. The Cross equation was capable of describing the rheological behaviour of the aqueous scleroglucan dispersions both in the sol and gel domain for shear rates and scleroglucan concentrations in the ranges 5×10⁻⁵–3.5×10³ s⁻¹ and 0.15–1.5% (w/w), respectively; while the Ostwäld-de-Waele model well fitted the apparent viscosity of the scleroglucan dispersion in DMSO at c=1% (w/v) and ranging from 2.5 to 180 s⁻¹.     

ROLE OF GRANULE SIZE AND SIZE DISTRIBUTION IN THE VISCOSITY OF COWPEA STARCH DISPERSIONS HEATED IN EXCESS WATER

The role of granule size and size distribution, measured by laser diffraction, in affecting the flow behavior and yield stress of cowpea starch dispersions (2.6% w/w) heated for various time intervals at 67, 70, 75 and 80C was studied. Flow data on the dispersions at 20C were fitted to the power law flow equation. The standard deviation of the granules described the transition of flow behavior from shear thickening (dilatant) in the early stages of gelatinization to shear thinning (pseudoplastic) in the latter stages. It was an important variable for also correlating the critical shear rate, λ˙c, for the onset of shear thickening in starch dispersions. The granules swelled to a maximum of about 3.5 X raw starch granule mean diameter and a granule mass fraction of 65%. The consistency index of the dispersions increased with granule mean diameter.     

FLOW BEHAVIOR AND FUNCTIONAL PROPERTIES OF MALT EXTRACT CONCENTRATES

Flow characteristics and functional properties of malt extract concentrates were investigated. Flow behavior was studied using a coaxial viscometer over shear rates of 0.6–145.8 s^-1. The shear rate-shear stress data followed the power law, Casson, Herschel-Bulkley and Bingham plastic models. Herschel-Bulkley values for flow behavior index which ranged from 0.73–0.94, and presence of yield stress indicated the Bingham plastic nature of malt extract concentrates. The activation energy of flow for various concentrates was in the range of 14.8–17.7 kcal/gmole. Magnitudes of Reynold numbers (0.0–49.9) for all concentrates were lower than 2100. Temperature, yield stress and plastic viscosity considerably influenced the flow of concentrates in straight pipes. The pressure drop and power requirements for flow of concentrates in pipes at 50°–60°C were lower than those at 30°C, but negligibly different from those at 80°C. Coefficients of correlation between the Bingham plastic values for pressure drop and yield stress (r = 0.86, P < 0.01), and plastic viscosity (r = 0.97, P < 0.01) were highly significant. Hulled barley and corn concentrates containing high amounts of reducing sugars, showed low viscosities and yield stresses, and required desirably low power for flow in pipes at 50°–60°C. The high protein concentrates of wheat and triticale malts showed high whip volume and good foam stability. Wheat concentrate was superior in fermentability. Hulless barley and finger millet malts produced concentrates with high viscosities which might facilitate their utilization in gums, candies and other such products.     

Kinetics of Thermomechanical Destruction of Thiamin During Extrusion Cooking

Maize grits were extrusion cooked in a CM45-F conical, counter-rotating twin-screw extruder, at different barrel temperatures (140–200°C), feed moistures (11.8–14.2% w.b.), feed rates (37–51 kg/h), screw speeds (65–81 rpm), and initial thiamin concentrations (9–93 mg/kg). Residence time distribution was measured by a dye tracer technique. A first-order rate equation was used to model the reaction kinetics, which allowed the calculation of the destruction rate constant using residence time distribution curves. The k-values were most dependent on barrel temperature, feed moisture, and screw speed. The destruction rate constant of thiamin during extrusion cooking was a function of product temperature and shear stress.     

Rheological properties of ultraviolet-irradiated and thermally pasteurized Yankee pineapple juice

The rheological behaviour of Yankee pineapple juice was examined for the effect of ultraviolet (UV) irradiation (53.42 mJ/cm²) and compared with untreated juice and a thermally pasteurized (80 °C for 10 min) juice. A rheological test was performed on all types of juice in the temperature range 5 °C to 25 °C using a concentric cylinder rheometer at a shear rate range of 10–290 s⁻¹. The comparative analysis found that the best flow curves were described by the Bingham model with an initial shear stress. The entangled pulps in the juices prevented free flow at zero shear rate. There was no significant variation between the plastic viscosities of the untreated and UV-irradiated juice at all temperatures. The activation energy (E_a) of the untreated, UV-irradiated and thermally pasteurized juice was 6.80, 8.19 and 8.50 kJ/mol respectively.     

AN APPROACH to STUDY and MODEL the HYDRODYNAMIC CLEANING EFFECT

A sensor has been developed for continuous measurements of the removal of deposit. the sensor, which was based on heat transfer measurements, was also able to measure the wall shear stress. The device was used to study the mechanical effect during cleaning. It was found that the removal rate of deposits, at a surface, could be scaled by the mean wall shear stress. Measurements were performed under both laminar and turbulent flow conditions. the turbulent fluctuations did not influence the removal rate.     

Shear Induced Starch Conversion During Extrusion

Using a capillary rheometer and a singlerscrew extruder, waxy corn starch was extruded at a relatively low temperature range. Notable conversions induced by shear energy were obtained in very short residence times (from several sec to 1 min). Conversions up to > 70% were also achieved. Kinetic studies showed that conversion caused by shear energy was more efficient than that by thermal energy. The shear rate constant, k, was an exponential function of shear stress and a linear function of shear rate. The minimum shear stress required to cause starch conversion was from 10⁴ to 10⁶ N/m² for 100°C to 21°C. In such “shear” experiments, the maximum temperature rise due to viscous thermal dissipation, measured experimentally and calculated theoretically, was too low to cause any significant thermal cooking of starch. The shear converted starch extrudates were unpuffed and had shapes of traditional pasta.     

SENSORY AND MECHANICAL ATTRIBUTES OF GEL TEXTURE

Eighteen experienced judges evaluated the texture of gels varying in gelatin concentration (22-45 g/L) in terms of firmness by oral and manual shear and compression, cohesiveness, and extent of breakdown in the mouth. Manual compression and biting with the front teeth discriminated well across gel concentrations. All sensory measures except extent of breakdown increased with gelatin concentration. Instron (IUTM) measurements showed that increasing gelatin concentration resulted in an increase in maximum force and force/deformation, but had little effect on deformation at yield and rupture or in elasticity and cohesiveness. Results from mechanical measurements varied with the type of force applied (compression, shear or puncture), the loading rate (50 or 200 mm/min), and the extent of deformation attained (40–90%). The highest discrimination across gel concentrations was achieved with shear force at a rate of 200 mm/min and at greater deformations. Sensory responses correlated most highly with the following IUTM measurements: (1) Compression forces at yield and at deformations of 70 and 85% at the higher crosshead speed; (2) Compression forces below the yield point at the lower crosshead speed; and (3) Shear forces measured at maximum deformation (90%) at 200 mm/min.     

Characterization of rheological properties of systems containing sugar substitutes and carrageenan

The effects of addition of κ-carrageenan to solutions containing 40% (w/w) sugar substitute on the rheological properties of these systems were investigated at 25 °C using a rotational viscometer at rotational speeds of 6, 12, 30, and 60 rev min⁻¹. The flow behaviour of the solutions was adequately described by the power law model with or without yield stress. The consistency index ( K ) and the flow behaviour index ( n ) were determined from shear stress vs. shear rate data. The flow parameters of the solutions depended on the carrageenan concentration. The apparent viscosities of the systems decreased with increasing shear rate, indicating pseudoplastic behaviour. The final carrageenan concentration was found to be an effective factor controlling the degree of sliminess of the model food systems studied.     

Steady shear flow properties of wild sage (Salvia macrosiphon) seed gum as a function of concentration and temperature

The steady shear flow properties of dispersions of a new potential hydrocolloid, sage seed gum (SSG), were determined as a function of concentration (0.5–2% w/w), and temperature (20–50 °C). SSG dispersions exhibited strong shear-thinning behavior at all conditions tested, which was even more pronounced than commercial hydrocolloids like xanthan, guar gum and locust bean gum. Different time-independent rheological models were used to fit the experimental data, although the Herschel–Bulkley model (H–B) was found the best model to describe steady shear flow behavior of SSG. An increase in gum concentration led to a large increase in yield stress and consistency coefficient values, whereas there was no definite trend with an increase in temperature. On the other hand, the above-mentioned increases in concentration and temperature did not yield a clear evolution of the shear-thinning characteristics of SSG dispersions. An Arrhenius-type model was also used to describe the effect of temperature. The activation energy (E_a) appeared in the range of 3949–16384 J/mol, as concentration increased from 0.5 to 2%, at a shear rate of 100 s⁻¹. The yield stress values estimated by viscoplastic rheological models were much higher than the data determined by stress ramp method. Apparent viscosity of SSG surpassed many commercial hydrocolloids such as guar gum, locust bean gum, Tara gum, fenugreek gum and konjac gum at the same conditions, which suggest it as a very good stabilizer in food formulations.