MICROSTRUCTURE AND RHEOLOGICAL BEHAVIOR OF WHIPPING CREAM

The mechanism of the formation of a rigid foam structure during whipping of reconstituted whipping cream was investigated. The rheological properties of whipped cream were estimated from an analysis of shear creep curves by a four element model. The values of each element increased exponentially during the process of whipping. From cryo-SEM observation, the surface of air bubbles consisted of a thin membrane which was penetrated by fat globules. Also, fat globules in the serum were joined together by free fat to form a framework structure.
The relationship between the rheological properties and microstructure of whipped cream was deduced from the rheological properties of two model systems. It is believed that whipped cream is a mixed matrix of a SOB O/W structure and a stiffer W/O structure.     

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 BEHAVIOR OF POMEGRANATE (PUNICA GRANATUM L.) JUICE AND CONCENTRATE

The rheological behavior of pomegranate juice (Punica granatum L.), prepared from fresh pomegranates, was studied as a function of solids concentration in the range 17.5–75°Brix at 10–55C, using a controlled stress rheometer. Concentration methods did not influence flow behavior. There were no significant differences (P > 0.05) between viscosity and activation energy values for the pomegranate juices concentrated by different methods. The juices exhibited Newtonian behavior regardless of the concentration method. The effect of temperature was described by an Arrhenius‐type equation with an activation energy in the range 5.34–32.2 kJ/mol depending on concentration. An exponential model described better the effect of the soluble solids on the viscosity and E_avalues. A simple equation was proposed for describing the combined effect of temperature and soluble solids content on the juice viscosity.     

FLOW BEHAVIOR AND RHEOLOGICAL MODELS OF RICE FLOUR PASTES

Rheological properties of a popular Chinese rice flour pastes were determined with a rotational viscometer at different levels of concentration and temperature. The rice flour had intermediate amylose content, medium gel consistency and high birefringence-endpoint temperature (BEPT). Cooked pastes exhibited shear-thinning behavior without yield stress at the concentration levels of 2～6% and temperatures of 25～80C and their flow properties were represented by a power law model. The effects of concentration and temperature on consistency index, flow behavior index and apparent viscosity were investigated.     

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).     

RHEOLOGICAL BEHAVIOR AND PHYSICOCHEMICAL CHARACTERISTICS OF GOLDENBERRY (PHYSALIS PERUVIANA) JUICE AS AFFECTED BY ENZYMATIC TREATMENT

Goldenberry (Physalis peruviana) juice was prepared and enzymatically treated with two dosages (300 and 600 ppm) of Pectinex Ultra SP‐L, then, the juice was concentrated to 30 and 40°Brix. Rheological properties of juices were studied at a wide range of temperatures (5 to 100C) and shear rate range from 0.3 to 100/s using Brookfield Digital Viscometer. The Herschel–Bulkley, Casson and Bingham rheological models were applied to describe the rheological properties of different juices. These properties are key parameters required to solve food industry problems in numerous areas such as quality control, evaluation of consumer acceptance and texture. The results indicated that juice concentrates behave as non‐Newtonian fluids (pseudoplastic) and have a definite yield stress. The Bingham and Casson, plastic viscosity, yield stress, consistency index and flow index decreased with increasing enzyme dosage and temperature. The effect of temperature on the viscosity described by means of an Arrhenius‐type equation. The activation energy for viscous flow depends on the total soluble solids. Moreover, sensory evaluation for juices was carried out. In consideration of industrial utilization, detailed knowledge on the rheological and physicochemical characteristics of cape gooseberry juice is of major importance.     

HTCA—丙酮溶液流变行为与可纺性关系的研究

用气压式毛细管流变仪研究了高温法二醋酸纤维素（ＨＴＣＡ）—丙酮纺丝溶液的非牛顿指数ｎ，结构粘度指数△η和粘液活化能△Ｅη与可纺性的关系．结果表明，ｎ值的增大和△η的减小有利于改善ＨＴＣＡ—丙酮溶液的可纺性，但难以作为可纺与不可纺的判据．而△Ｅη能较好地表征ＨＵＣＡ—丙酮溶液的可纺性，△Ｅη值越高．流动性和可纺性越差，实验表明△Ｅη高于８０ＫＪ／ｍｏｌ时．其溶液不可纺．在ＨＴＣＡ—丙酮溶液中加入１０—１５％左右的助溶剂Ｄ，可提高丙酮的溶解能力，使ｎ值增大，△η和△Ｅη降低，从而改善了可纺性。     

RHEOLOGICAL BEHAVIOR OF PROCESSED MUSTARD. II. STORAGE EFFECTS

Samples of processed yellow mustard with three different particle size distributions (slightly-coarse, standard, and fine) were prepared according to a standard formulation and pilot-plant replication of the commercial methodology and stored for 3 months at 5°C, 25°C, and 45°C. After 3-month storage at 45°C, all samples showed visible liquid separation (syneresis) and aggregation of colloidal particles, as indicated by increase in d _pop and decrease in % colloids. The fine milled samples exhibited syneresis and an increase in d _pop after 3-month storage even at 25°C. The bimodal distribution did not show significant changes as a result of aggregation. In general, increase in d _pop increased apparent viscosity (η _ap ), flow behavior index (n) from the Herschel-Bulkley model, and exponent n" from the loss modulus (G") power law equation. As d _pop increased, Bingham yield stress _o), plastic apparent viscosity (η _p ), shear stress constant (A) from the Weltman stress decay model, yield stress (_o) and consistency index (m) from the Herschel-Bulkley model, exponent n' from the storage modulus (G') power law equation, and minimum values of loss tangent (tan δ) decreased. In heterodisperse semi-solid foods such as processed mustard, the particle size distribution influences the structure (i.e., packing) and the rheological behavior. Therefore, time and temperature dependent instabilities such as syneresis may be minimized by controlling particle size distribution during processing.     

RHEOLOGICAL BEHAVIOR OF TOMATO PASTES IN STEADY AND DYNAMIC SHEAR

Tomato pastes (26–30 °Brix) at 40C were highly shear thinning fluids (n = 0.28) with large magnitudes of Casson yield stresses (78–212 Pa). Dynamic shear data revealed weak gel-like behavior of the pastes: magnitudes of G' were higher than those of G", and both increased with oscillatory frequencies. The Cox-Merz rule was not applicable to the steady and dynamic shear data on the pastes, but the two types of data conformed with the use of shift factor on frequency. Solids and liquid fractions in the pastes were satisfactory for correlating the dynamic properties, consistency index, and Casson yield stress.     

RHEOLOGICAL BEHAVIOR OF PROCESSED MUSTARD. I: EFFECT OF MILLING TREATMENT

The influence of milling level on the rheological behavior of processed mustard was investigated using three different particle sizes (slightly-coarse, standard, and fine) prepared according to a standard formulation and pilot-plant replication of the commercial methodology. All samples showed unimodal population-based particle size distributions which were described by the population-mode (d_pop). On a volume basis, all milling levels had well-defined bimodal distributions consisting of a smaller- and a larger-size group of particles described by their modes (d and D). The most important particle size index in relating milling level to rheological behavior was not the population mode (d_pop) but the larger-size volume mode (D) from the bimodal distribution. The coarser the milling, the higher the larger-size volume mode (D), and higher the values of: a) Apparent viscosity of nonmixed samples (η_p), b) Bingham yield stress (o⁻_o) and plastic apparent viscosity (σ_p), c) Shear stress constant (A) and coefficient of thixotropic breakdown (B) from the Weltman stress decay model, d) Yield stress (o⁻_o) and consistency index (K) from the Herschel-Bulkley model, and e) Storage (G¹) and loss (G^") moduli. As indicators of structure-texture relationship, the above rheological parameters can be profitably utilized to study the effect of manufacturing practices on long term storage stability of the product.     

RHEOLOGICAL PROPERTIES OF CONCENTRATED YOGHURT (LABNEH)

The effect of six techniques for the elevation of the total solids on the rheological properties of labneh were investigated using controlled-stress dynamic rheometer. All samples exhibited a weak viscoelastic gel structure with the storage modulus (G') higher than the loss modulus (G") over all of the measured range. None of the experimental materials produced the same overall gel strength as the control made by draining with traditional cloth-bags. The changes in the storage modulus (G') as a function of amplitude sweep were mirrored by changes in the loss modulus (G"). Considerable differences in the loss tangent (G"/G') values of the various materials were observed at higher stress amplitudes. Rheological differences in the overall gel strength at low amplitude and frequency suggest that, although the type of protein interactions in each case may be similar, there are differences in the degree of interactions. Subsequent breakdown at higher amplitudes and frequencies suggests that the overall domains of the treated proteins may have been reduced, and that different methods of manufacture may be producing materials that have different space occupancy in the gel.     

聚丙烯/改性聚酯共混体系流变行为研究

聚合物共混改性是利用现有高聚物采用性能组合方式开发新产品的重要途径之一。本论文主要利用哈克流变仪和毛细管流变仪研究聚丙烯/改性聚酯共混体系的流变行为。研究发现,改性聚酯的添加对共混体系的粘度有所影响,其影响规律与所施加的剪切力方式和大小有关。在一定的剪切力作用下,共混体系表观粘度随温度的变化符合阿累尼乌斯关系。     

RELATIONSHIP BETWEEN RHEOLOGICAL BEHAVIOR AND BOSTWICK MEASUREMENT DURING MANUFACTURE OF KETCHUP

ABSTRACT

The Bostwick consistometer remains an integral part of assessing the consistency of tomato products. For instance, the consistency of lots of tomato paste are designated at the factory by their Bostwick measurement; tomato ketchup is produced to meet Bostwick ranges given in U.S. Standards for Grades. This work addresses the blending of tomato pastes, at different Bostwick readings, for use in the remanufacture of ketchup. For both blends and ketchup, the Bostwick measurement was correlated to rheological parameters measured by two methods: a single‐point method and rotational viscometry. The correlations between the Bostwick measurement and the viscosity were compared with the theoretical prediction based on gravity current flow. The results illustrate that the relationship between the Bostwick measurement and (K/ρ)^?1/(2n+3) provides a means to predict the Bostwick length from at‐line or in‐line viscosity measurements.

PRACTICAL APPLICATIONS

The results of this work provide a means to predict the Bostwick length from at‐line or in‐line viscosity measurements. Specifically, the quantitative relationships presented in the article allow in‐line process control schemes to improve final product quality.

     

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.     

MODELING RHEOLOGICAL BEHAVIOR OF GELATINIZING STARCH SOLUTIONS USING MIXER VISCOMETRY DATA

A Brookfield RVTD mixer viscometer was used to gelatinize 5.5, 6.4, and 7.3% (d.b.) native corn starch solutions, and a 6% bean starch solution. Torque, the dependent variable, was used to estimate parameters in a generalized viscosity model. The independent variables were impeller speed, temperature (50–95°C), concentration, temperature-time history, and strain history. The starch dispersions thickened with decreasing temperature and had an Arrhenius activation energy between 6.4 and 12.7 kJ/mol. There was no evidence that retrogradation caused this effect. First-order reaction kinetics was accurate (9.8% standard deviation) in describing viscosity increase during gelatinization. After initial gelatinization, torque decayed exponentially with time. Predicted curves followed experimental pasting curves when interactions among the independent variables were accounted for in the model. The major variables controlling the pasting process were temperature-time history before peak torque, strain history between peak torque and cooling, and temperature during cooling.