QUANTIFYING THIXOTROPY OF BÉCHAMEL SAUCE UNDER CONSTANT SHEAR STRESS BY PHENOMENOLOGICAL AND EMPIRICAL MODELS

Experimental results for Béchamel sauce are used to prove the validity of two rheological models for the study of thixotropic behaviour. The rheological-kinetic parameters are: structural parameter (Λ), rate of structural breakdown (kb2) and Herschel-Bulkley constants at zero and 40 min of shear stress i.e. yield stress (τλ), consistency index (K), flow index (n). The empirical parameters, obtained by multiple logarithmic regression, are a time consistency index (K'), a time flow index (n') and a thixotropic index (p'). For the two models the correlation coefficient were greater than 0.9. Parameters of both models suggest the milk protein-wheat starch interactions are mainly responsible for Béchamel consistency. The breakdown of this interaction can explain the thixotropy.     

Time-independent and time-dependent rheological characterization of vegetable-based infant purees

Flow behavior of vegetable-based infant purees was analyzed at different temperatures (5–65 °C) giving particular attention to their time-dependent properties in a shear rate range (5–200 s⁻¹). Power law model parameters describing flow behavior of samples depended on kind of infant puree, its water content and measurement temperature. Arrhenius model was used to explain temperature effect on apparent viscosity at 50 s⁻¹. Infant purees exhibited thixotropic behavior for all temperatures tested. For the same temperature, differences in hysteresis loop magnitudes were observed among purees, being more noticeable at lower temperatures. Two models were used to describe the time-dependent behavior, namely Weltman model, and second-order structural kinetic model. For all infant purees, the initial shear stress and the extent of thixotropy increased and decreased significantly with increases in shear rate and temperature. The breakdown rate of puree associations also accelerated at higher shear rates, but no trend was observed with temperature.     

Time-dependent flow properties of starch–milk–sugar pastes

The time-dependent flow properties of starch–milk–sugar (SMS) pastes have been studied. The flow properties were assessed from the measurement of the shear stress versus time of shearing at constant shear rate. Corn and wheat starches were used in this study, while the sugars were glucose, sucrose, and fructose. The Weltman model was used to evaluate the flow properties of SMS pastes prepared under different conditions. SMS pastes heated at 95 and 85 °C exhibited a thixotropic behavior, while pastes heated at 75 °C behaved like a rheopectic fluid. It was noted that the thixotropy occurred at high shear stress (above 50 Pa), and the rheopexy occurred at low shear stress (below 45 Pa). The degree of thixotropy, as assessed by the Weltman model parameters, increased significantly with starch concentration, and with less pronounced effect with sugar concentration. The effect of sugar type on the degree of thixotropy of SMS pastes heated at 95 °C decreased in the following order: fructose>sucrose>glucose. The type of starch played a role in the time-dependent flow properties of the SMS paste, with a general conclusion that wheat starch had a greater degree of thixotropy than corn starch.     

Flow Properties of Dulce de Leche,a Typical Argentine Dairy Product

Dulce de Leche (caramel jam) is a typical Argentine dairy product similar in composition to sweetened condensed milk. Its flow properties were measured for solids content between 55 and 70% and temperatures between 25 and 70° C. Thixotropicity of the product was quantified using Weltmann's model and decreased with solids content. The coefficient of thixotropic breakdown was correlated with temperature for each sample to allow prediction of thixotropicity. From shear stress-shear rate flow diagrams the Herschel-Bulkley model was adequate for Dulce de Leche. Except for the most concentrated samples at low temperatures, the flow index was nearly constant with an average value of .748, thus demonstrating Dulce de Leche to be a pseudoplastic substance. Yield stress and consistency index decreased with temperature increase and increased with solids content. A single expression to predict these parameters as a function of temperature and solids could not be found due to chemical changes that occur during the cooking process. An empirical exponential expression was used to correlate consistency index and yield stress versus temperature for each sample and a similar expression to correlate these parameters versus solids content at each temperature.     

Flow properties and thixotropy of selected hydrocolloids: Experimental and modeling studies

The flow curves and time-dependent rheological behavior of two traditional Iranian hydrocolloids obtained from Salep tubers (wild terrestrial orchids) and Balangu seeds (Lallemantia royleana) have been investigated. All samples exhibited thixotropic behavior at the concentrations used and both in forward and backward measurements were characterized by the power law model. At the constant shear rate, the apparent viscosity decreased rapidly with time of shearing within the first 100 s for Salep and 50 s for Balangu samples, and approached a constant value corresponding to a steady state after approximately 450 and 250 s, respectively. The breakdown rate of all samples accelerated at higher shear rates. Four time-dependent models namely the second order structural kinetic model, Weltman model, first-order stress decay models with zero and non-zero equilibrium stress values were used to describe the thixotropy behavior and different parameters of these models were analyzed. The results showed that the rate of the thixotropic breakdown increased with increasing shear rates for Salep, whereas, the extent of thixotropy increased with shear rate for Salep and decreased for Balangu. In this study, the selected hydrocolloids were adequately modeled by the first-order stress decay model with non-zero equilibrium stress value.     

Rheology and granule size distributions of corn starch dispersions from two genotypes and grown in four regions

Starch was extracted from corn of two genotypes (Clint and P3730) grown during the 1997 season in four regions in New Zealand and Australia. The granule size distributions in unheated and heated (80°C, 2–120 min) 2.6% starch dispersions (STDs) were different. Most of the heated STDs exhibited shear-thinning behavior, but dispersions of a Clint starch exhibited shear-thickening behavior at shear rates >300 s⁻¹. The power law consistency coefficient of the STDs increased, but their flow behavior index decreased with heating time. Values of the consistency coefficient of all the STDs were found to be related to the cube of the mean granule diameter. Frequency versus storage modulus (G′) data of a 5% Clint STD heated at 80°C, 30 min, showed increase in G′ with ω in a convex down manner, while the other STDs showed typical increase in a convex up manner.     

RHEOLOGICAL BEHAVIOR AND TIME-DEPENDENT CHARACTERIZATION OF ICE CREAM MIX WITH DIFFERENT SALEP CONTENT

The effect of salep concentration on the rheological characteristics of ice cream mixes (0.5–1.5% salep content), prepared from nonfat cow's milk and sugar, was studied using a controlled stress rheometer. The flow curves and time‐dependent flow properties of the ice cream mixes were assessed at different temperatures. The ice cream mixes’ samples showed slightly thixotropic behavior, which was reduced as the salep content decreased. The forward and backward measurements of the flow curves of ice cream mixes were modeled with the power law model. The ice cream mixes showed pseudoplastic flow behavior after destruction of the thixotropic structure. In mixes that were first presheared at a high shear rate flow properties could also be described by the power law model. The second‐order structural kinetic, first‐order stress decay and Weltman models were applied to model the time‐dependent flow properties of the ice cream mixes. Among these, the first‐order stress decay model was found to fit well the experimental data.     

蜂乳的触变性研究

针对蜂乳所表现出的流变性质,用旋转型粘度计,测定蜂乳在不同剪切速率下的剪切应力和表观粘度。通过回归分析,建立蜂乳在触变条件下的流变模型方程,探讨温度和剪切速率对蜂乳触变性的影响以及蜂乳的表观粘度随着剪切时间的变化规律。     

Rheological behavior of syrups containing sugar substitutes

The rheological properties of syrups prepared using bulk sweeteners such as sorbitol and bulking agents like maltodextrin and polydextrose along with aspartame were studied. The apparent viscosity, consistency index, yield stress, and flow behavior index were determined from the shear stress versus shear rate data. The Herschel–Bulkley model was found to adequately describe the flow behavior of the syrups. The activation energy for all the syrups at different concentrations was determined from the Arrhenius equation. The yield stress, flow behavior index, and consistency index were dependent on the temperature and concentration of the syrups. The apparent viscosity increased from 8.8 to 129 mPa·s for sugar and sorbitol syrups, respectively, over the concentration range from 35 to 65%. In general, the rheological characteristic of sorbitol syrup was similar to that of sugar syrup, while syrups made with polydextrose and its mixture with maltodextrin were significantly different from those of sugar syrup.     

RHEOLOGICAL CHARACTERIZATION OF ASEPTICALLY PACKAGED PUDDING

The flow behavior of commercially available, aseptically packaged vanilla pudding was measured using a Thermo Haake RheoStress rheometer (RS1, Thermo Electron Corporation, Waltham, MA) with a concentric cylinder system. Six different shear rates, ranging from 1 to 100/s, and three reasonable serving temperatures were investigated using constant shear rate tests. A time‐dependent, Herschel–Bulkley model characterized the thixotropic nature of pudding. The model's five parameters were related to temperature. The model was validated at three constant shear rates and at two different temperatures, and was found to predict viscosity well as a function of time, shear rate and temperature.     

Rheological behavior of concentrated yogurt (Labneh)

In this paper, the rheological behavior of concentrated yogurt (Labneh) is presented. The rheological behavior of Labneh was measured using a rotational viscometer in a temperature range of 5‐55°C. The tested Labneh exhibited shear‐thinning and thixotropic behaviors. The power law model expressed as τ = mγ ⁿ was found to be the most appropriate for the rheological model of Labneh. The effects of temperature on power law parameters were determined. The parameter (m) significantly decreased with increasing temperature, while the parameter (n) slightly increased with temperature. It was also found that the Weltmann's model described well the relationship between the apparent viscosity of Labneh and the shearing time. The activation energy of flow was nearly constant for all samples tested and not affected by the shear rate.     

Dynamic rheological characterization of salep glucomannan/galactomannan-based milk beverages

The steady flow and viscoelastic properties of glucomannan (salep) and galactomannans (locust bean gum, LBG and guar gum, GG) in milk beverages were investigated at 25 and 50 °C. The consistency index (K), flow behavior index (n), yield stress and thixotropic area were measured as functions of steady shear; elastic modulus (G′), loss modulus (G′′), tan δ and complex viscosity (η*) parameters were derived from oscillatory shear experiments. The steady flow behavior of mannan-based milk beverages was observed to be shear-thinning and thixotropic. Galactomannans exhibited greater shear-thinning and thixotropy than glucomannan in milk beverages. The synergistic effect was detected between salep and LBG with highest thixotropy. Increasing the temperature decreased Casson yield stresses of GG but not salep and LBG samples. Similar viscoelastic behavior was observed between salep and LBG, they could be classified as concentrated solutions but GG showed gel-like structure. At low frequencies, high tan δ values were observed for the salep and LBG samples indicating viscous character. On the other hand, GG sample had nearly the same tan δ values through the frequency sweep. Cox–Merz rule was tested to correlate the steady and dynamic viscosities of samples. It was found that Cox–Merz rule was applicable only to LBG–milk beverage among the studied samples.     

Evaluation of rheological properties of date syrup

Rheological behavior of date syrup is an important factor affecting the efficiency of sugar production and refining processes such as boiling, crystallization, separation and pumping. A rotational viscometer was used to characterize the flow behavior of date syrup solution at four different temperatures (20 °C, 40 °C, 60 °C and 80 °C) and four concentrations (17, 24, 31 and 39 °Brix). The samples were subjected to a programmed shear rate increasing from 10 to 100 s⁻¹ in 2 min, held constant at 100 s⁻¹ for 10 min and linearly decreasing to 10 during 2 min. The power law model was fitted to shear stress vs. shear rate data to obtain the consistency coefficient (m) and the flow behavior index (n). Both m and n were sensitive to changes in temperature and concentration. The apparent viscosity increases with increasing concentration of date syrup and a decrease in temperature.     

Effect of temperature on rheological characteristics of molasses: Modeling of apparent viscosity using Adaptive Neuro - Fuzzy Inference System (ANFIS)

In this study, the effect of temperature on the rheological characteristics of apricot and date molasses was studied separately. Rheological characteristics of both molasses were evaluated in the shear rate range of 0.1-100 s⁻¹ at different temperatures (10-40 °C). Power law model was used for the calculation of flow behavior index and consistency coefficients of molasses. Consistency coefficients of apricot and date molasses were in the range of 5.408-39.905 Pa sⁿ and 0.910-2.852 Pa sⁿ, respectively. Molasses samples showed a non-Newtonian flow behavior. The effect of temperature on apparent viscosity was described by Arrhenius equation and calculated activation energy at the shear rate of 54.2 s⁻¹ was 41.42 and 38.19 kJ/mol for apricot and date molasses, respectively. An efficient predictive model for apparent viscosity values of molasses was constructed using Adaptive Neuro - Fuzzy Inference System (ANFIS) and this model showed satisfactory prediction with high coefficient of determination (0.979-0.999) and low root mean squared error (0.12-0.46).     

THERMOPHYSICAL PROPERTIES OF PINK GUAVA JUICE AT 9 AND 11°BRIX

The effect of temperature on the conservation of pseudoplastic characteristics and on the physical properties of pink guava juice (Psidium guajava L.) variety Beaumont: B‐30 with two different total soluble solids (9°Brix and 11°Brix) was investigated. It was found that the juice exhibits pseudoplastic behavior in the range of shear rate between 40 to 160 s^?1 and within pasteurization temperature of 60–90C. While the flow behavior index (n) and density (p) increased, consistency coefficient (K), thermal conductivity (k) and specific heat capacity (C_p) decreased with increasing temperature. The linear regression equations or models for flow behavior index, consistency coefficient, density, thermal conductivity and specific heat capacity were determined with correlation coefficients ranging from 0.75 to 1.00.     

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.     

QUANTIFYING THIXOTROPY IN STARCH-THICKENED,STRAINED APRICOTS USING MIXER VISCOMETRY TECHNIQUES2

Starch-thickened, strained apricots were found to exhibit irreversible thixotropy. Shear stress and shear rate were estimated from mixer viscometer data. A structural parameter, yield stress, flow behavior index and a consistency coefficient were determined to quantify the rate of breakdown and equilibrium structure of the product. The techniques developed in this study are useful in describing the time-dependent behavior of suspensions because the slip at the wall is minimized.     

Rheokinetic model to characterize the maturation process of gelatin solutions under shear flow

The characterization of maturing gelatin solutions in shear flow is required for an appropriate formulation of food products. Under static conditions, the maturation process of gelatin solutions may be described through a basic structural parameter evolving to the percolation value (gel point). Within the rheological framework, two asymptotic viscosities are well identified for the maturing process of gelatin solutions at the limit zero shear rate. One involves the initial solution viscosity that may be associated with the null structural parameter (the microstructure is not formed yet). The other is the percolation zero shear rate viscosity (assuming an infinite value when approaching the gel time) and corresponds to the maximum value of the structural parameter. Under flow, thixotropic theories combined with the knowledge of suspension rheology allow one to convert directly experimental data obtained as shear stress versus time for a given shear rate into the time evolution of the structural parameter. Consequently rheometric experimental data available places the search for a rheokinetic model of the structural parameter. Here, different expressions for this model are investigated, mainly those involving both the rates of structure breakdown and buildup, where the average cluster size is affected by the shear rate. The rate equation thus obtained may be then applied to arbitrary shear rate histories. Numerical results of the rheokinetic model proposed in this work fit well experimental rheometric data obtained in shear flow for the maturing of different gelatin solutions. Experimental data acquired in this work are presented and discussed in relation to those reported previously in the literature.     

冷榨亚麻籽油的流变特性研究

通过低温压榨亚麻籽获得冷榨亚麻籽油（cold—pressed flaxseed oil,CFO）,并分析其主要理化指标,着重研究CFO的静态、动态流变特性,同时分别采用Casson、Herschel-Bulkley和Bingham模型对其流体行为进行拟合,并采用Arrhenius方程分析其粘度热动力学参数。研究结果表明：在剪切速率为0．1～200s“下,CFO由非牛顿流体逐渐转化为牛顿流体;当剪切速率大于10s^-1时,CFO呈牛顿流体;同时分析得出CFO的粘度活化能为3095．4cal／mol;CFO的粘度、剪切应力、损耗模量、塑性稠度系数、高剪切极限粘度和稠度系数随着温度升高而降低,但是温度变化对CFO的贮能模量影响不显著;另外通过比较3个流变模型得出Bingham模型适用于CFO。     

Ohmic Heating of Liquid Whole Egg: Rheological Behaviour and Fluid Dynamics

Although ohmic heating is used as an alternative heating method for liquid egg products commercially, there is a lack of information on the change of rheological properties and fluid dynamics characteristics of ohmically heated liquid whole egg in the literature. The change of rheological behaviour of the ohmically heated liquid whole egg, across a temperature range of 4–60 °C, was determined by using a concentric rotational viscometer. The ohmic heating was conducted by applying the voltage gradient (20 V/cm) at 50 Hz. The temperature dependency of the electrical conductivity of liquid egg was linear (R ² = 0.999). The rheological behaviour was found to be shear thinning since power law model had higher regression coefficient and lower χ ² and root mean square error values than Newtonian model. Ohmically heated liquid whole egg exhibited higher degree of thixotropic index indicating the occurrence of the protein denaturation at 60 °C. The flow behaviour of liquid whole egg in the continuous ohmic heating system was predicted as laminar (GRe range of 87.59–538.87) for the mass flow rate range of 0.0056–0.0166 kg/s. The friction factors and pressure losses in the system in those mass flow rates were also assessed. The result of this study will give necessary information on flow characteristics of liquid whole egg for the modelling, designing and the scaling up of the continuous ohmic heating systems for pasteurisation of liquid egg products.