MEASUREMENT OF FLOW PROPERTIES OF FLUID FOODS - DEVELOPMENTS, LIMITATIONS, AND INTERPRETATION OF PHENOMENA

Rheological behavior of fluid foods is described with examples under each type of behavior. Benefits of fundamental rheological data are illustrated for engineering studies, sensory evaluation, and detection of compositional changes. Some problems likely to be encountered due to sampling are discussed along with possible remedies. Theoretical bases for the proper use of capillary, concentric, and plate and cone visco- meters are outlined as well as the effects of violating the theoretical requirements. Xeld stress and rheological equations employed to describe foods possessing yield stress are reviewed. Use of mixers as viscometers for studying flow properties of foods is covered.     

Effect of damaged starch on the rheological properties of wheat starch suspensions

The influence of damaged starch content on particle size distribution and rheological properties of unheated starch suspensions and pasting properties were investigated. Four samples containing different amounts of damaged starch were studied. Particle size distribution curves shifted toward higher diameters, and a greater overlap of both populations of particles (A- and B-type granules) and a decrease of both peak heights were produced. The flow curves of unheated starch suspensions were fitted using the power law model. The flow behavior indexes were higher than the unit. The consistency coefficient increased with the increment of damaged starch content. Unheated starch suspensions showed time-dependent rheological behavior and were described by the Weltman model. The unheated suspensions exhibited a thixotropic behavior. With regard to the pasting process, the increment of damaged starch content produced gradual reductions in peak viscosity, final paste viscosity, breakdown and setback. Results demonstrated the importance of the presence of physically damaged granules in wheat starch rheological characteristics.     

COLLOIDAL ASPECTS OF YEAST FLOCCULATION: A REVIEW

In this review, areas of colloid science including the DLVO theory, flocculation kinetics and suspension rheology are outlined and their applicability to the study of yeast flocculation discussed. Specifically, fundamental methods of predicting cell-cell interaction energies, orthokinetic flocculation rates and rheological flow properties of flocculent suspensions are detailed. While the application of these theories to brewing systems is somewhat difficult, they may aid our understanding of brewing yeast flocculation. The limited information available on the colloidal nature and properties of brewing yeast cells is also summarized.     

Squeezing flow viscometry for nonelastic semiliquid foods--theory and applications

In most conventional rheometers, notably the coaxial cylinders and capillary viscometers, the food specimen is pressed into a narrow gap and its structure is altered by uncontrolled shear. Also, most semiliquid foods exhibit slip, and consequently the measurements do not always reflect their true rheological properties. A feasible solution to these two problems is squeezing flow viscometry where the specimen, practically intact and with or without suspended particles, is squeezed between parallel plates. The outward flow pattern mainly depends on the friction between the fluid and plates or its absence ("lubricated squeezing flow"). Among the possible test geometries, the one of constant area and changing volume is the most practical for foods. The test can be performed at a constant displacement rate using common Universal Testing Machines or under constant loads (creep array). The tests output is in the form of a force-height, force-time, or height-time relationship, from which several rheological parameters can be derived. With the current state of the art, the method can only be applied at small displacement rates. Despite the method's crudeness, its results are remarkably reproducible and sensitive to textural differences among semiliquid food products. The flow patterns observed in foods do not always follow the predictions of rheological models originally developed for polymer melts because of the foods' unique microstructures. The implications of these discrepancies and the role that artifacts may play are evaluated in light of theoretical and practical considerations. The use of squeezing flow viscometry to quantify rheological changes that occur during a product's handling and to determine whether they are perceived sensorily is suggested.     

吞咽障碍者增稠流体食品流变学研究进展

吞咽障碍已经成为当今社会面临的一项严重的健康问题。增稠流体能够有效降低吞咽障碍病人吸入性肺炎 的风险,在吞咽障碍病人的治疗和护理过程中应用广泛。增稠流体的生产和使用与其流变特性密切相关,因而流变 学成为了吞咽障碍食品领域一个十分重要的研究方向。本文根据近期国际上在该方向的最新研究成果,详细介绍了 利用流变特性评判增稠流体黏稠程度的原则以及测量方法,深入分析了各种因素对于增稠流体流变特性的影响,并 对影像学造影剂、增稠流体食品流变特性的差异和匹配、口腔对于流变特性变化的分辨能力等问题进行了探讨。目 的是为我国增稠流体食品研究提供理论指导和技术参考。     

Rheology of liquid foods under shear flow conditions: Recently used models

Proper modeling of flow or viscosity curves as a function of shear rate is a useful tool in any engineering activity. The rheology of foods depends on the composition, processing to which they have been subjected and the state of dispersion in which they are found. Liquid foods are complex biosystems, that show non-Newtonian behavior under flow conditions. This review presents models used in recent decades to describe the experimental rheological behavior of various liquid foods, ranging from Newtonian fluids to the most complex. Some non-Newtonian parameters such as those of the Ostwald-de Waele, Bingham, Herschel–Bulkley, Casson, Cross, and Carreau models are summarized. Examples of thixotropic behavior described by the Weltman and Abu-Jdayil models are also presented. In each model, explanations based on the composition and dispersion state of the food are made. This is useful in innovative processing technologies and for scientists new to the field of food rheology. An attempt is made to exemplify and group the expected behavior for most fluid foods, including some for a dysphagia diet, depending on their composition or the dispersed system formed, which will be useful for professionals who wish to compare reported rheological parameters with those obtained experimentally.     

透明质酸多糖增稠适用于吞咽困难的肠内营养制剂及其流变学性质

胶体增稠的食物流体黏度是吞咽困难饮食护理中的关键参数。这类流体需有较高的黏度和较强的非牛顿流体行为。本研究采用流变学方法表征了透明质酸多糖对含豌豆蛋白和大豆油的营养制剂的增稠效果,研究了增稠营养制剂的流动性质、黏弹性和黏度的剪切时间依赖性。结果表明：未经增稠的营养制剂是表观黏度很低（仅约为0.010 Pa·s）的牛顿流体,不能满足安全吞咽的要求;而透明质酸具有良好的增稠效果,当透明质酸质量分数为0.5%时,营养制剂的初始黏度可达8 Pa·s,增稠后营养制剂具有方便可调节的表观黏度、剪切变稀性质以及良好的黏弹性,流变学性质能满足安全吞咽的要求。     

Clustering of instrumental methods to characterize the texture and the rheology of slimy okra (Abelmoschus esculentus) suspensions

Okra (Abelmoschus esculentus) is one of the ingredients widely used in African gastronomy because of the unique slimy texture it gives to sauces. However, processing and formulation can affect the textural and rheological properties of these sauces, leading to unacceptable quality for the African consumer. The aim of this study was to select the instrumental measurements best enabling (a) characterization of the rheology and texture of slimy sauces prepared from okra and (b) monitoring its evolution during the preservation process. Thirty-seven slimy suspensions (sauces and purées) were measured with 16 rheological and textural parameters. A principal component analysis revealed that flow consistency index K and flow behavior index n were well correlated with visco-elastic, adhesive, and shear thinning properties, and that stringiness was well correlated with elongational, cohesive, and ductile properties. These two sets of measurement methods are sufficient to characterize their rheological and textural properties, and necessary to discriminate them according to their process and formulation.     

κ-Carrageenan-sodium caseinate microgel production by atomization: Critical analysis of the experimental procedure

The influence of atomization process to produce κ-carrageenan and κ-carrageenan/sodium caseinate microgels was studied experimentally (aspect ratio and particle size distribution) and theoretically (dimensionless parameters). Moreover, rheological behavior of microgel suspensions was evaluated to examine their potential application in food products. Experimental results demonstrated that the size of microgels was influenced by feed flow rate, compressed air flow rate and composition of solutions, while their shape depended on the viscosity and surface tension of biopolymer solutions. Regarding the dimensionless numbers, higher values of Reynolds number of liquid layer (Re_λl) and Weber number (We_l) led to smaller particles, while the decrease of Ohnesorge number (Oh) was related to lower sphericity of microgels. Rheological behavior of suspensions depended on not only the morphology and size of microgels, but also their composition. Incompatibility between κ-carrageenan and sodium caseinate in mixed microgels led to suspensions with more complex rheological behavior at determined biopolymer concentrations.     

Relation Between Oxygen-17 NMR and Rheological Characteristics of Wheat Flour Suspensions

Water binding has a significant influence on the rheological properties of wheat flour suspensions. Wheat flour-water suspensions at concentrations from 60 - 95% wet basis were subjected to rheological and NMR analyses in order to ascertain the relation between the two methods. The rheological data obtained by the Haake RV-3 Rotoviscometer followed the Power Law, thus comparisons were based on consistency coefficient (a) and flow behavior (b) values. The NMR spectrometer measured oxygen-17 relaxation rates (34 MHz in water and in deuterium oxide) as an indication of water mobility. The data showed a general inverse relationship between consistency coefficient and water mobility. However, the data indicated three distinct regions, each showing a linear relation with water mobility decreasing as apparent viscosity increased.     

Rheological Characterization of Crosslinked Waxy Maize Starch Solutions under Low Acid Aseptic Processing Conditions using Tube Viscometry Techniques

A rheological characterization of 1.82 and 2.72% (g dry starch/100g water) waxy maize starch solutions was performed at 121.1, 132.2, and 143.3°C using tube viscometry techniques. Dilatancy was observed in 22 out of 23 experiments and the flow behavior index increased with concentration and decreased with temperature. Rheological behavior was explained in terms of the rigidity and volume fraction of the swollen granules combined with small shear stresses in the fluid due to the high temperatures and low shear rates. Parameter correlation analysis showed a near correlation between rheological parameters with the flow behavior index being the dominant parameter. The effect of dilatancy on residence time and heat transfer in the aseptic processing of particulate and nonparticulate foods is discussed.     

WHITE GUAVA FRUIT AND PUREES: TEXTURAL AND RHEOLOGICAL PROPERTIES AND EFFECT OF THE TEMPERATURE

This study focuses on the effect of temperature on the rheological properties of white guava puree. The results of the textural analysis on the whole guava as well as the rheological analysis on the puree made from it showed different behaviors depending on whether the fruit was unripened or ripened. A relationship was observed between the textural properties of the whole guava and the rheological parameters of its puree. The behavior of white guava puree was thixotropic and pseudoplastic, and its rheological properties were dependent of the temperature. Its consistency coefficient decreased and its flow behavior index increased with an increase in temperature. Experimental data were fitted by the power law, and the activation energy of the process was calculated from the Arrhenius equation. Oscillation test data showed that the viscoelasticity for the guava puree was also affected by temperature.

PRACTICAL APPLICATIONS

Knowledge of the rheological properties of foods is useful for quality control, and design and evaluation of process equipment. In this study, it has demonstrated the relationship between textural properties of whole white guava fruit and rheological properties of the purees obtained from these fruits.     

Rheological and structural characterization of tomato paste and its influence on the quality of ketchup

Three hot break tomato pastes were investigated to determine the effect of their characteristics on the properties of tomato ketchup, processed in an industrial-scale facility (i.e. diluted, heated, homogenized and cooled). Pastes and ketchups were characterized by particle size distribution, volume fraction, and rheological behavior in steady and dynamic shear. The ketchups were also subjected to sensory assessment. The processing of pastes into ketchups induced large structural changes, which were reflected in all parameters studied. The volume fraction of solids (φ) accurately reflected the changes that the paste suspensions underwent during processing and it appeared to be a good predictor of the flow behavior of both the pastes and the ketchups. The corresponding flow curves were found to be well described by the Carreau model in a large range of shear rates and concentrations. However, the rheological characteristics of the commercial pastes studied did not directly correlate to those of the corresponding ketchups. Instead, our results suggest that the change in structure induced by processing might be governed by other properties of the paste, such as the fraction of small and large particles and their sensitivity to breakage, together with the viscosity of the aqueous phase.     

CONTACT AND FRACTURE ELEMENTS AS COMPONENTS OF THE RHEOLOGICAL MEMORY OF SOLID FOODS1

Most solid foods are rheologically nonlinear and their mechanical response may depend on their deformation history. Such behavior can be represented by modifying the conventional rheological models with discontinuous components. It is demonstrated that linear elastic and viscous elements combined with contact and fracture elements can form consistent models applicable under a wide range of deformation histories. This provides an explanation for the memory phenomena based on the physical aspects of the deformation mechanism.     

The rheology of starch dispersions at high temperatures and high shear rates: a review

Engineering design of continuous processes for liquid and semi-liquid foods involves a complex heat transfer and fluid flow coupling, which strongly depends on the rheological behaviour of fluid foods. Most of these processes are performed at temperatures above 95°C. Lack of rheological data is one major problem encountered during numerical simulation and equipment design of liquid food processing at high temperatures. Insufficient rheological studies in the literature have been conducted with food products under these conditions. This paper presents a literature review of experimental devices and methods used to obtain rheological data for starch dispersions at temperatures below and above 95°C. Results and gelatinization models from the literature are analysed and discussed.     

Effect of shear thinning on aspiration – Toward making solutions for judging the risk of aspiration

Polysaccharide solutions containing a contrast medium were prepared so that their viscosity at 50 s⁻¹, which was reported as a shear rate in the oral cavity, is the same but different at other shear rates. Preparation procedure of such solutions based on heat treatment using widely used polysaccharides in foods is described, and the rheological properties of these solutions are discussed in relation with flow behaviours in the swallowing process. Videofluorographic observation was performed for healthy persons and patients and compared with rheological characteristics of these solutions to understand the mechanism of aspiration. It was shown that the degree of aspiration depended on the degree of shear thinning of polysaccharide solutions. This study showed the possibility to find what kind of fluid induces the aspiration and to reduce the risk of aspiration in dysphagic patients by controlling the food rheology.     

不同品种花生蛋白流变特性的研究

对8种花生蛋白的流变特性进行了探讨。静态流变特性测定结果表明,8种花生蛋白的流变模型为幂率模型,均为非牛顿流体。在同样的流动方式下,忠毅9616的黏度最大,花育16次之,其他几种变化无明显差别。动态流变特性测定结果表明,随着温度的升高,不同品种花生蛋白的贮藏模量、损失模量以及损失系数均呈现出不同的变化趋势。其中,忠毅9616品种蛋白的贮藏模量最大,红珍珠次之,花育16最小;红珍珠、奇山208、的损失模量高于其他几种,花育16最低;忠毅9616的损失系数最小,奇山208最大。     

Computational Fluid Dynamics Approaches in Quality and Hygienic Production of Semisolid Low‐Moisture Foods: A Review of Critical Factors

Low‐moisture foods have been responsible for a number of salmonellosis outbreaks worldwide over the last few decades, with cross contamination from contaminated equipment being the most predominant source. To date, actions have been focused on stringent hygienic practices prior to production, namely periodical sanitization of the processing equipment and lines. Not only does optimum sanitization require in‐depth knowledge on the type and source of contaminants, but also the heat resistance of microorganisms is unique and often dependent on the heat transfer characteristics of the low‐moisture foods. Rheological properties, including viscosity, degree of turbulence, and flow characteristics (for example, Newtonian or non‐Newtonian) of both liquid and semisolid foods are critical factors impacting the flow behavior that consequently interferes heat transfer and related control elements. The demand for progressively more accurate prediction of complex fluid phenomena has called for the employment of computational fluid dynamics (CFD) to model mass and heat transfer during processing of various food products, ranging from drying to baking. With the aim of improving the quality and safety of low‐moisture foods, this article critically reviewed the published literature concerning microbial survival in semisolid low‐moisture foods, including chocolate, honey, and peanut butter. Critical rheological properties and state‐of‐the‐art CFD application relevant to quality production of those products were also addressed. It is anticipated that adequate prediction of specific transport properties during optimum sanitization through CFD could be used to solve current and future food safety challenges.     

Rheology of Concentrated Tomato-Derived Suspensions: Effects of Particle Characteristics

In the present work, the effect of particle properties on the rheological behaviour of tomato-derived suspensions was investigated systematically. Hereto, a range of relatively monodisperse suspensions, containing either cell fragments or single cells with varying average particle size (～148, 267, 303 and 393 μm) and pulp content (from 25 to 60 wt.%), was prepared by the reconstitution of tomato tissue-based particles in water. The effect of the presence of a serum phase on the rheological properties of tomato-derived suspensions was investigated by the comparison of the rheology of reconstituted tomato purées in water with those in serum. All the tomato-derived suspensions were non-Newtonian liquids exhibiting a yield stress. The flow behaviour could be described well by the Herschel–Bulkley model. The undisrupted network structure of all suspensions could be classified as a weak gel with a rather low critical strain. Particle concentration, size and morphology (surface/shape) turned out to be key structural properties controlling the rheological parameters of these tomato-derived suspensions. Increase in yield stress and storage modulus with particle concentration could be fitted to a power law model. The ratio of static yield stress to dynamic yield stress turned out to be larger for particles with a more irregular, less intact surface, showing the enhanced tendency of the latter particles to build up structure in rest conditions. A unique linear relation was found between the static yield stress and the maximal elastic stress in oscillatory tests, independent of the particle properties. Finally, replacing the serum phase by water led to a substantial decrease in network strength, especially in quiescent conditions.     

Texture and texture assessment of thickened fluids and texture‐modified food for dysphagia management

Thickened fluids and texture‐modified foods are commonly used in the medical management of individuals who suffer from swallowing difficulty (known as dysphagia). However, how to reliably assess texture properties of such food systems is still a big challenge both to industry and to academic researchers. This article aims to identify key physical parameters that are important for objective assessment of such properties by reviewing the significance of rheological or textural properties of thickened fluids and texture‐modified foods for swallowing. Literature reviews have identified that dominating textural properties in relation to swallowing could be very different for thickened fluids and for texture‐modified foods. Important parameters of thickened fluids are generally related with the flow of the bolus in the pharyngeal stage, while important parameters of texture‐modified foods are generally related with the bolus preparation in the oral stage as well as the bolus flow in the pharyngeal stage. This review helps to identify key textural parameters of thickened fluids and texture‐modified foods in relation to eating and swallowing and to develop objective measuring techniques for quality control of thickened fluids and texture‐modified foods for dysphagia management.