LONG-TERM STORAGE OF ASEPTICALLY PROCESSED AND PACKAGED DAIRY FLUIDS1

Aseptically processed and packaged dairy fluids were investigated for stability during long-term storage. Model solutions of whey and sodium caseinates, and combinations of them with milk fat were used. Sedimentation in whey protein solutions was extensive. No significant sedimentation was observed in casein solutions even after 500 days of storage. Addition of milk fat retarded sedimentation in whey protein solutions initially. Very little fat separation was observed in whey-fat solutions. Larger amounts of fat separation were observed in casein-fat or casein-whey-fat solutions. Casein-whey-fat sediments were in the form of a gel. Sporadic gelation was observed in casein solutions. Sedimentation rates from cheese whey alone are similar to gelation rates for milk for similar thermal treatments. Reactivated enzymes may be responsible for both phenomena.     

RHEOLOGICAL CHARACTERIZATION OF KETCHUP

Ketchup flow properties were investigated with a concentric cylinder viscometer in the temperature range from 30°C to 80°C. A partial characterization of the product viscoelastic behavior at 30°C was also made by “Strain test” and “Frequency test.” The Harper and El Sahrigi equation (η_a= acβ e^Ea/RT), as well as the equation that links the apparent viscosity of ketchups to the serum viscosity and to the pulp content (η_a=η_serum+A(%P)^B), do not describe adequately the ketchup flow behavior that we found to be highly affected by the production technology and by the raw materials used.     

RHEOLOGICAL CHARACTERIZATION OF APPLE CORTEX

The incremental creep and recovery theory was used to develop a constitutive relationship for apple cortex without assuming the nature and the characteristics of the material. Successive loading and unloading cycles of increasing duration were applied on the cylindrically shaped specimens of apple cortex. The elastic strain component was linear with stress. The viscoelastic and permanent strain components were time dependent and nonlinear functions of stress.
Three-step creep tests and stress-strain tests at various deformation rates were also conducted on cylindrically shaped specimens of apple cortex. Prediction of strains under these loading histories by using the constitutive equation showed a close agreement between the theoretical and experimental strains. An equivalence between the critical creep stress (found from the incremental creep and recovery tests) and the ultimate stress, obtained from the stress-strain tests at various deformation rates, were also established.     

RHEOLOGICAL CHARACTERIZATION OF COMMERCIALLY PROCESSED FLUID MILKS

Flow profiles of commercially processed whole, two-percent, one-percent and nonfat milks with added solids as well as skim milk without added solids were measured using a rotational steady shear viscometer. Also tested were mixtures of concentrated skim milk added to skim milk over a range of 9.7% to 20.2% total solids. A shear rate range of 121 to 486 s^-1 was used with a cone and plate geometry. In all cases, linear plots of shear stress versus shear rate with small nonzero intercepts were obtained.     

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.     

NONLINEAR RHEOLOGICAL BEHAVIOR CHARACTERIZATION OF SOLID FOODS

Nutting's general law of deformation was used to characterize the nonlinear rheological behaviour of bread. A simple experimental technique submits a sample of regular geometry to a sequence of compression runs in which the ultimate strain, or the ultimate load, was kept constant. In order to avoid sample failure as a consequence of mechanical fatigue an ultimate strain as low as 5% and an ultimate load of 150 g were chosen. An extremely good agreement (correlation coefficient 0.9997) between the model prediction of bread stress-strain behaviour and the experimental results was found up to 80% compression. This result appears quite promising since it provides a simple way to predict and compare material performances under very large deformations.     

Rheological behaviour and colour changes of ginger paste during storage

Ginger paste was prepared from fresh ginger by addition of 8% common salt and citric acid. The paste was thermally processed and packed in glass, polyethyleneterephthalate or high‐density‐polyethylene containers and stored at 5 ± 1 and 25 ± 1 °C for 120 days. The rheological characteristics of the paste were studied by using a computer controlled rotational viscometer over the temperature range of 20–80 °C. Samples were subjected to a programmed shear rate, increasing linearly from 0 to 200 s^?1 in 3 min, followed by a steady shear at 200 s^?1 for 3 min and finally decreasing linearly from 200 to 0 s^?1 in 3 min. Ginger paste exhibited pseudoplasticity with yield stress and flow adequately described by the Herschel–Bulkley model. The yield stress decreased exponentially with process temperature and ranged between 3.86 and 27.82 Pa. The flow behaviour index (n) varied between 0.66 and 0.82 over the temperature range. Both consistency index and apparent viscosity decreased with increase in temperature and the process activation energies were found to be in the range of 16.7 to 21.9 kJ mol^?1. The effect of temperature was significant (P < 0.05) on the Hunter colour combination value of the paste during storage; however it was not affected by type of packaging material (P > 0.05). It is recommended that ginger paste is stored at 5 ± 1 °C in polyethyleneterephthalate or glass containers.     

RHEOLOGICAL CHARACTERIZATION OF BUTTER USING LUBRICATED SQUEEZING FLOW

Rheological characteristics of butters made from anhydrous milk fat (AMF) and supercritically fractionated high melting triglyceride (HMT) fraction were compared. An Instron Universal Testing Machine was used to compress the samples between two lubricated plates at constant deformation rate (0.5 mm/min). Stress growth coefficients, computed from the ratios of stresses and biaxial strain rates, were plotted as function of time. HMT butter had higher stress growth coefficients than AMF butter, indicating its greater degree of firmness. Stress growth coefficients were negatively correlated to temperature due to lower crystallinity of fat at higher temperatures and increased on storage due to formation of weak van der Waal forces of attraction between crystalline fat chains.     

乙醇/水溶液中魔芋葡甘聚糖流变性能研究

初步研究了魔芋葡甘聚糖(KGM)在不同乙醇浓度的乙醇/水溶液中的流变性能,运用粘度法分析了KGM溶胶的溶胀速率、峰值粘度及其稳定性与乙醇浓度的相关性.结果表明,KGM的流变性能与乙醇浓度存在明显的依赖关系,当乙醇浓度=11%时产生突变.KGM溶胶的溶胀时间与稳定性随乙醇浓度增加而逐渐延长;其最大溶胀速率和峰值粘度随乙醇浓度的增加而减小.这些规律均为KGM在流体中的实际应用提供了参考依据.     

APPLICATION OF AN IN-LINE RHEOLOGICAL CHARACTERIZATION METHOD TO CHEMICALLY MODIFIED AND NATIVE CORN STARCH

The application of an in‐line ultrasonics‐based rheological characterization method for measuring the rheological properties of 6% (w/v) acid‐thinned and native corn starch suspensions and gels was studied. The measurements were performed in steady, laminar pipe flow using a 5 MHz frequency transducer to determine the radial shear rate distribution. Two pressure transducers were used to measure the pressure drop allowing the shear stress distribution to be calculated through the conservation of linear momentum. It was possible to obtain shear viscosity over a range of shear rates, 1–50 1/s, from a single velocity profile. A comparison of the shear viscosity function at different flow rates showed that this method could be a valid method of process rheometry during the manufacturing of starch‐based products. Both ultrasonics and rotational rheometry results showed that acid‐thinned and native corn starch suspensions exhibited Newtonian behavior before heat treatment and non‐Newtonian behavior after heating. Power law consistency index and flow behavior index of the acid‐thinned corn starch gel at 39°C were 0.60 Pa.s ^0.68 and 0.68, respectively; whereas those of the native corn starch were 5.90 Pa.s ^0.37 and 0.37 at 38C. Their consistency index increased and flow behavior index decreased with cooling and a thermoreversible change in the flow behavior index was observed in the native corn starch after storage.     

ON-LINE RHEOLOGICAL CHARACTERIZATION OF PIZZA SAUCE USING TUBE VISCOMETRY

The performance of a tube process viscometer was evaluated forits ability to characterize the rheological properties of pizza sauce. Tube flow was described using common rheological models (Power Law, Herschel‐Bulkley, Casson and Bingham plastic) and model parameters were compared to laboratory rheometer data. Derived model parameters (tube) and measured pressure drop data were both correlated with other off‐line instruments, namely the Bostwick Consistometer and Brookfield viscometer. Bingham Plastic model parameters of viscosity and yield stress as generated from the tube viscometer were found to be comparable to the rheometer generated equivalents. Measured pressure drop readings using linear and Power law regression showed good predictive capability (R² >0.99 & 0.93) of Brookfield and Bostwick Consistometer readings respectively. It can be concluded that tube viscometry would result in effective real time rheological characterization of tomato‐based products. Limitations of the Bostwick Consistometer are highlighted.     

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.     

RHEOLOGICAL CHARACTERIZATION OF RICOTTA CHEESES BY IMPERFECT SQUEEZING FLOW VISCOMETRY 1

Samples of three commercial ricotta cheeses made from whole milk, part skim milk and fat free, were compressed in a 140 mm diameter shallow Teflon® container with a wide Teflon® plate, of 64 or 100 mm in diameter, to induce imperfect lubricated squeezing flow. The log force versus log specimen height relationship had a linear region from which the region of a dominant squeezing flow regime could be clearly identified. Its slope was on the order of ?1.0 to ?0.8 in the whole milk ricotta, ?1.6 to ?0.9 in the part skim milk and ?1.7 to ?1.2 in the fat free cheese. The apparent stresses at two selected heights (1 and 2 mm) and at two given times after relaxation (60 and 120 s) were sensitive measures of consistency and could be used to distinguish between the ricottas of the three types. The coefficient of variation of these measures was 10% or less, while the differences between the ricottas or the different brands were 20 to 100%. The apparent stresses, before and after relaxation, had a modest dependence on the upper plate diameter, but the latter had no effect on their sensitivity as measures of consistency. Doubling the compression rate, from 0.1 to 0.2 mm. s^?1, had a relatively small effect on the magnitude of the apparent stresses, probably a consequence of the high yield stress.