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1、Relevance of Rheological Properties in Food Process Engineering 19 Relevance of Rheological Properties in Food Process EngineeringJ. Vlez-RuzCONTENTSAbstract19.1 Introduction to Food Rheology19.2 Measuring Techniques19.3 Rheological Studies in Food Process Engineering19.3.1 Momentum Transfer Operati
2、ons19.3.2 Heat Transfer Operations19.3.3 Mass Transfer Operations19.3.4 Structural Characteristics and Physical Changes19.4 Final RemarksReferencesAbstractBasic aspects related to the rheology of foods are introduced, including the definitionof rheology, stress and strain concepts, as well as the cl
3、assification of food materials.Physical relationships between force, deformation, and material properties or rheolog-ical properties, as well as the most known rheological models, are mentioned. Morecommonly used rheological techniques are briefly commented on regarding their mainfeatures: rotationa
4、l rheometry, tube rheometry, back extrusion rheometry, extensionalviscometry, and squeezing flow rheometry, as well as mixer viscometry. Mainly, flowproperties and their relation to food process operations are discussed; studies in foodprocess engineering, momentum transfer operations, heat transfer
5、 processes, masstransfer unit operations, and physical-structural changes are presented. Momentumtransfer operations and their relation to flow properties of liquid materials are estab-lished; and the relationships between rheological parameters and food transport sys-tems, mechanical separations, m
6、ixing, and pumping are emphasized. For heat transferoperations, the influence of rheological properties such as apparent viscosity, consis-tency coefficient, and flow behavior index are related to the heat transfer coefficient,? 2002 by CRC Press LLC presenting four representative equations develope
7、d and proposed by recognizedauthors. Similarly, in mass transfer operations, exemplified by spray drying and fer-mentation, the role of rheological behavior and its influence on process performanceare mentioned. Finally, the relationship of structural changes and/or physical changesto the rheology o
8、f food products and components are briefly discussed.19.1 INTRODUCTION TO FOOD RHEOLOGYThe goal of this chapter is to present a general overview of the rheology applied tofood materials, with emphasis on the relationship between rheological behavior andfood process engineering.Rheology, defined as t
9、he science of flow and deformation of materials, is afundamental interdisciplinary science that has been gaining importance in the fieldof foods. According to Rao,1 Steffe,2 Holdsworth,3 Vlez-Ruiz and Barbosa-Cno-vas,4 Bhattacharya et al.,5 and Vlez-Ruiz,6 among others, there are numerous topicsof i
10、nterest to the food industry related to rheology, such as? process engineering applications involved in equipment and processdesign? physical characterization of liquid and solid foods? development of new products or reformulation? quality control of intermediate and final products? correlation with
11、 sensorial evaluation? understanding of food structureRheology can be used to characterize not only flow behavior of biological andinorganic materials, but also structural characteristics. Flow properties, such asviscosity, yield stress, thickness, pourability, softness, spreadability, and firmness,
12、contribute substantially to facilitate transport and commercial processing as well asto promote consumer acceptance. Insight into structural arrangement helps to predictbehavior or stability of a given material with storage, change in humidity andtemperature, and handling.7 Consequently, basic rheol
13、ogical information on materi-als is important not only to engineers but also to food scientists, processors, andothers who might utilize this knowledge and find new applications.Although food exists in a variety of forms, solids and liquids are of primaryimportance in food rheology. Many foods are n
14、either solid nor liquid but exist in anintermediate state of aggregation known as semi-solid. As a consequence of thecomplex nature and lack of a precise boundary among solids and semi-solids, manyfoods may exhibit more than one rheological behavior, depending on their specificcharacteristics and th
15、e measuring conditions used during physical characterization.Deformation may be conservative or dissipative, depending on whether it is relatedto solids or liquids. Flow is a time-dependent form of deformation and, consequently,is more related to fluids.8 Two mechanical parameters (stress and strain
16、) are the basis for material classi-? 2002 by CRC Press LLCfication, from a rheological viewpoint, into three recognized groups: elastic, plastic, and viscous. Mohsenin,9 based on the physical response of biological material to agiven stress or strain, expressed a paramount visualization of rheology
17、 using threefundamental parameters: force, deformation, and time. This general classification isoutlined in Figure 19.1. Accordingly, the specific relationship developed betweenan applied stress and the resulting deformation of the material is known as arheological property.10A wide range of models
18、are available for the rheological characterization of foods.According to Holdsworth,3 the rheological models may be divided into three maingroups: time-independent models, including Bingham, Power Law, and Her-schelBulkley; time-dependent models, such as Carreau, Hahn, PowellEyring, andWeltmann; and
19、 viscoelastic models, with KelvinVoigt element and Maxwell bodybeing the best known. Certainly, there exist other, less popular models in food prod-ucts, and others specifically applied to fit the effect of concentration and temperature.? 2002 by CRC Press LLCFigure 19.1 Rheological classification o
20、f food materials. In rheology, strain and stress are two relevant physical variables that need to beconsidered when a material deforms in response to applied forces. Strain representsa relation of a change in length with respect to the original dimension. This parameteris essentially a relative disp
21、lacement, and there are many definitions associated withthis concept. For instance, the engineering strain is expressed by the followingequation:2,11,12(19.1)where c = engineering strain, also called Cauchy strain?L = change in lengthL = final length after deformation of the material L0 = original l
22、ength before deformationThis definition is expressed in terms of a simple shear. Strain is determined bydisplacement gradients, and strain rate by velocity gradients. Strain and stress aretensor quantities and are represented by nine components.10,13Stress that relates the magnitude of the force ove
23、r the surface of application canbe compressive, tensile, or shear, depending on how the force is applied. Nineseparate components are required to adequately describe the state of stress in amaterial.2,10,14 The stress at any point in a body may be represented by the followingmatrix:(19.2)where ij is
24、 the stress tensor; the first subscript indicates the orientation of the faceupon which the force is acting, and the second subscript refers to the direction ofthe force. This matrix may be simplified, depending on each specific physical system.For instance, in steady-simple shear flow, also known a
25、s viscometric flow, the matrixis reduced to only five components.2,10A deformed body, as a result of an applied force, will develop internal stressesand strains that may be of various types.15 The relationship shown by any foodmaterial between applied stress and resulting strain defines the rheologi
26、cal propertiesof the material. These relationships can be expressed either empirically or in termsof a rheological equation of state.10 Figure 19.2 illustrates the correspondencebetween the physical forces and stress (), as well as the functional relation betweendeformation and strain () and shear r
27、ate (strain/time).There exists a particular approach in which the rheological behavior of a materialis analyzed on a simplified deformation called single shear or uniaxial deformation.This approach is the basis for many rheological measurement techniques and permitsc?LL0-L L0L0-= =ij11 12 1321 22 23
28、31 32 33=? 2002 by CRC Press LLCthe characterization of many food materials.10,16 19.2 MEASURING TECHNIQUESMeasuring the rheological properties and identifying the rheological behavior offood materials is necessary when employing any of the available commercial instru-ments that allow objective char
29、acterizations. The accuracy of these rheometers hasbeen improved due to the incorporation of microcomputer technology and friction-less devices.Based on their elemental geometry, the existing common instruments are dividedinto two major groups (rotational class and tube class2,7), but there are othe
30、r rheo-metric approaches that have not been completely developed and therefore are notcommercially available. These include back extrusion flow,17,18 extensional viscom-etry,19 mixing devices of various kinds,20 and squeezing flow.21 Each group ofrheometers has advantages and disadvantages that must
31、 be considered whenever arheological characterization of food systems is carried out. The selection of a rhe-ometer for a particular task is a matter of great importance.In food rheology, the rotational type is more commonly used than tube fixtures.22These rheometers are most suited for low-viscosit
32、y fluids,23 and they have threemain performance features:1. A uniform shear rate may be applied.2. The effect of time on the flow behavior can be followed.3. Different geometries may be utilized: concentric cylinders, cone and plate,Figure 19.2 Physical relationships between force, deformation, and
33、material properties.? 2002 by CRC Press LLCand parallel plates. Most oscillatory or dynamic experiments are carried out in these rheometers. Onthe other hand, tube viscometers have been broadly used in flow characterization ofdifferent viscous foods such as fruit purees and juices, vegetable concent
34、rates, gumsolutions, and food dispersions. Tube viscometers that apply higher shear rates thanrotational instruments can be easily manufactured following basic recommendations;therefore, they are simple and low-cost equipment, with several limitations.2428In back extrusion, the technique is simple a
35、nd requires a texture meter, such asthe Instron Universal Testing Machine or a similar instrument. Back extrusionrequires graduated cylinders to hold the samples and plungers. Osorio and Steffe18developed mathematical relationships for the rheological characterization of New-tonian and power law foo
36、d fluids. This approach is also known as annular pumping,18or the compression-extrusion test,29 and it offers a good measuring alternative.In many industrial processes (e.g., blow molding, dough sheeting, extrudateexpansion, fiber spinning, flow through porous beds, mouth feel and swallowing ofbever
37、ages, food spreading, and vacuum forming), it has been recognized that elon-gational deformation is the most important type of deformation.23,30 Therefore, elon-gational or extensional deformation has been developed as a proper technique forrheological characterization of some food materials.30 Data
38、 and applications ofextensional deformation in foods are scarce. This technique is adequate for foodswith proteins and polysaccharides and requires a generation of controlled extensionalflows for rheological measurements.19,23,30 Squeezing flow viscometry, as a variation of the extensional flow meas
39、uring, isbased on compression of a fluid material between two parallel plates. This type ofmeasurement has been mathematically analyzed by Campanella and Peleg,31 whogenerated the respective equations for lubricated squeezing flow. Fluids with highviscosity, such as cheese,32 peanut butter,31 mustar
40、d, mayonnaise, and tomatoketchup,33 have been tested to evaluate their rheological properties. The application of mixing impellers for the estimation of rheological parametershas been proposed by various authors.3440 Mixers have often been applied in rheo-logical studies for fermentation processes.
41、Particularly, the helical ribbon impellerhas been very useful for complex fluids and suspensions, generating stable processviscosity values and avoiding phase separation,20 a very common flow problem infood dispersions. Unfortunately, the adoption of mixer viscometry techniques hasbeen limited becau
42、se of the development of complex flow and the high costs of theequipment.Rao41 established that, due to the complex composition and particular structureof foods, approaches to characterizing a foods rheological behavior may be classi-fied into five methods: empirical, phenomenological (including fun
43、damental andimitative), linear viscoelastic, nonlinear viscoelastic, and microrheological. In thecase of foods, empirical and phenomenological approaches have been predominantin those studies associated with flow properties.41,42 With respect to viscoelasticnature, the approaches mainly include fund
44、amental and linear viscoelasticity; mostfood systems are considered to be linear viscoelastic materials at small strains.Nonlinear viscoelastic materials exhibit physical properties that are a function not? 2002 by CRC Press LLConly of time but also of the magnitude of applied stress.43On analyzing
45、the published studies, one may observe that there are four maindirections in rheological research.23 1. Description of the macroscopic phenomena developed during the defor-mation of materials2. Explanation of the phenomena from a molecular point of 6view3. Experimental characterization of parameters
46、 and functional relationshipsdescribing these phenomena4. Practical application of the aforementioned directions19.3 RHEOLOGICAL STUDIES IN FOOD PROCESS ENGINEERINGAfter commenting on some aspects of food rheology, it is interesting and importantto identify some of the food process engineering opera
47、tions in which flow andviscoelastic properties play an outstanding role. Those unit operations may begrouped in four areas: 1. Pipeline transport, mixing, pumping, and mechanical separations2. Heat transfer operations such as heating, cooling, and evaporation3. Mass transfer processes such as air drying, fermentation, osmotic concen-tration, and membrane separa