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used to monitor the structural changes, which revealedthat, both above and below the gel tempera ture , a highdegree of anisotropy was induced by shear which can relaxabove the gel-forming temperature bu t not below it.

Emulsions behave not too dissimilarly from dispersionswith regard to viscosity dependence on the aboveparameters. Typically, the viscosity of an emulsion wouldbe somewhat lower than the viscosity of the suspensionwith the all parameters in the above equation being equal.Thus whereas the Einstein equat ion predicts a s lope of2.5 for ideal suspensions at very low > the Taylor equationfor emulsions predic ts tha t the slope would depend onthe ratio of viscosities of the two phases and can be aslow as one when dispersed phase viscosity is negl igiblecompared to the continuous phase viscosity). This is dueto the interfacial mobility or fluidity. Particularly at smallmean droplet size and with strong adsorption layers of

emulsifiers, however, the effects of droplet distortion andinterfacial mobility are small and the above equation fordispersions would apply quite well to emulsions.

Emulsion rheology often does not ge t as much atten tionfrom researchers as suspension rheology has traditionallyreceived. For a review albeit a dated one) of emulsionrheology literature see Sherman [8]. Dickinson andGolding [9--] recent ly publ ished a paper for which theycarried ou t a detailed study of sodium-caseinate-stabilizedemulsions at varying levels of the caseinate. Theydelineated the regions of bridging flocculation at low

caseinate concentration when the emulsifier was not insufficient quantity to saturate the interface; a Newtonianbehavior was present at intermediate concentrations wherethe emulsifier provided a full surface coverage bu t verylittle excess emulsifier. Finally, a depletion flocculationregion appeared at high case inate concentra tion, tha t is,when it was present in a large excess beyond what wasneeded to saturate the interface. This paper describesthe rheological differences arising out of the two differentmodes of flocculation commonly encountered in foodemulsions.

In another paper, Dickinson and Yamamoto [10] haveused rheological measurements to invest igate the effec tof added lecithin on heat se t whey-protein-stabilizedemulsion gels. The order of addi tion of the ingredientswhey protein and lecithin) was shown to be crucial andthe effects were expla ined through the interactions oflecithin and whey proteins at the interface.

On e particularly striking way tha t the emulsions do differfrom dispersions is in not having a well defined closepacking limit. Emulsions can pack beyond the closepacking l imit of solid dispersions because the dropletscan and do distort. Thus for example, mayonnaise isan oil in water ofw emulsion with a dispersed phasevolume fraction well in excess of 0.8, and the droplets aredistorted from being a spherical shape in the product [11-].

Rheology of emulsions o m s and gels Borwankar and Case 8

A concentrated emulsion, is one in which droplets aredistorted because the volume fraction is above the closedpacking limit. As this resembles a foam more than it doesan ordinary emulsion , we will t reat it in the next section.Another important way that the emulsions can differ fromthe solid suspensions is in the fact tha t the droplets cancoalesce to form larger droplets. This does not pose anyproblem because coalescence basically results in a changein the psd.

heology of fo ms nd concentr tedemulsionsTypically, foams are dispersions of gas in a liquid where thevolume fraction of the gas is very high. As such, similar tothe concentrated emulsions, the foam bubbles are highlydeformed. In fact phase volume fractions of 0.95 or even0.99 are not uncommon in foams. Concentrated emulsionsdo not go nearly as high because the emulsion usually

inverts before that. Nevertheless, concentrated emulsionswith distor ted droplets are not uncommon. When flow isinduced in such a system, the droplets or bubbles mustdeform further and slide past one another. During thecourse of the deformation the droplets store energy and,hence, such systems exhibit elasticity. Such systems also.possess a yield stress which must be exceeded before flowcan occur. The scale of the shear modulus as well as theyield stress in this situation is se t by the Laplace pressureof the droplets 2yfR) where y is the interfacial tension andR is the radius of the bubble or the drop.

Microrheological models of foams have been receiving increased attention recently, especially with the availabilityof tremendous computer power. The pioneering work inthe area was begun by Princen [12-14] who was interestedin both foams and concentrated emulsions. Princen [12,13]and Princen and Kiss [14] used two dimensional modelscombined with experimental study to extend thei r workto three dimensional foams, and determined expressionsfor the yield stress and static shear modulus for foamsand concentrated emulsions. Subsequently, many attemptshave been made to extend these models to real threedimensional foams with their three dimensional structure.Very recently, Kraynik and Reinelt [15-] published a paperin which they computed bulk modulus and shear modulifor Kelvin, Williams and Weaire-Phelan foams using theSurface Evolver software. This software is fast becomingkey in such computations. This sof tware was also usedby Lacasse e t l [16-], who studied deformation of smallcompressed droplets.

Although such works are theore tical ly chal lenging andintellectually stimulating, in our opinion they have limitedpractical value, especially for food foams. This is especiallyso because many food foams are not dry enough for thesemodels to apply. or example , ice creams contain onlyabout 50 air or less) and whipped toppings contain about75 air. Also, in the food system, polydispersity playsa very important role. None of these models have been

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86 Food col loids emuls ions gels and foams

aplied to a food system, with th e exception of Chakrabartiet 0 (S Chakrabanti, J Guth, A Thomas, Society ofRheology Annual Meeting, 1991, unpublished data) whopresented an applicat ion of the functional form of thePrincen and Kiss model to mayonnaise, bu t fitted a newse t of constants. Recently, Coughlin et [17] publishedan experimental procedure called the punch indentationtes t to measure th e static shear modulus of foams. Theyused shaving foams in their measurement an d their resultsare in good general agreement with th e above theories.

h ology of g lsPhysical testing methods have been used to characterizegels for decades. The original methods were empirical,including, texture profile analysis [18], the cone penetrometer [19] and the Bloom Gelometer [20], to name onlya few. The values obtained from such empirical testsare useful only in reference to the specific method. Asequipment has become more affordable with user friendlysoftware, th e empirical type tes ting has been replacedalmost entirely, by rheological , methods as a means tostudy gel properties, resulting in a more fundament alunderstanding of th e mechanisms of gelation.

The area of gel rheology incorporates both small straintype tests, as well as large strain fracture tests. Th eformer have been used primarily to follow sol to geltransformation and structural properties of th e final gel,whereas the latter describes th e breakdown of the gelstructure. The results of these two types of measures are

not necessarily related. Mitchell [21] has noted that gels trength ident if ied by a single point rupture tes t will notalways rank a series of gels in th e same order as tests whichinvolve small deformations without rupture. This paperincludes recent examples of research of both small andlarge strain tests.

With new food formulation requirements, due to th e increased interests in fat-like properties in fat-free systems,the need to maintain specific functionality in light ofcontinual advances in processing and simply th e desire fornew unique food properties, probing and understandingof mixed gel systems have been qui te popular in theliterature over th e past several decades. Some of th e morerecent work includes th e paper by Nishinari et 0 [ ]

-This work identified synergistic interactions betweenpurified gellan and purified k-carragecnan gums as aresult of loose interchain interaction or a phase separationin th e double helical structure. This was ident if ied byfollowing th e impact of mixture ratios and temperatureon rheological properties. These were compared withpredicted values based on additivity of separated viscosityof both polymer solutions. Also from Nishinari s laboratoryare two recent papers s tudying the interaction of konj acglucomannan (KM) with corn starch [23] and with gellangum with and without salts [24]. The former paper useda combination of differential scanning calorimetry (DSC)with both small st rain and large stra in tests to determine

th e effect of KM on th e gelatinization and retrogradationof corn starch. The authors anticipated that th e additionof KM would stabi lize retrogradation of th e starch overtime. They actually found an increase in modulus andfracture s trength with the addition of KM over shorttimes. Retrogradation, however, was retarded slightly overlonger storage times (> 5 days) . In th e latter paper, gellangum and a medium molecular weight fraction of KMin th e presence of sodium chloride showed synergy th esuggested mechanism was interaction of KM molecules onth e surface of large aggregates of gellan helices) resultingin increased G and G with increased salt concentration.

The list of recently published rheological studies of mixedgel systems is long, inc luding systems of malto-dextr inand caseinate [25], casein-micelle-earrageenan mixtures[26] and high pressure treated gelatin-whey-protein gels[27] . Finally, a paper by Zasypkin ef 0 [28] beginswith a thorough overview of th e different classifications ofmulticomponent gels. This is followed by a review of workusing gelatin mixed with a variety of other componentsto demonstrate the various types of mixed gel behavior.Although the rheology is not described in great detail inthis review article, it gives a feel forthe impact rheologyhas on understanding th e mixed gel systems.

Although this work is necessary and useful, many ofth e mixed gel papers wri tten are rather uninteres ting inthat it is th e same method different gum syndrome.There have been, however, a few particularly interesting

studies describing gelation behavior of single componentgel systems using newer perspectives in identifying sol-geltransitions. Rather than identifying th e gel point as th eG and G crossover, as has been used traditionally,these researchers have adopted a more definitive analysisfrom th e polymer science area developed by Winter andChambon [29]. This procedure suggests that at the gelpoint G and G exhibi t a power law dependence on th eoscillation frequency. At th e gel point, G and G haveth e same power law exponent. The power law exponentof th e critical gel can lead to an under standing of thephysical mechanism of th e gelation process. The paperby Peyrelasse a 0 [30] is a nice treatment of th esol-gel transition of gelatin (a very well defined gellingsystem). Rigorous determinations of G and G conductedat th e gel po in t found th e power law exponent, n, tobe equal to 0.62 at concentrations between 0.17 and0.40 g/cm 3 They also noted that in plotting versus ro, inth e vicinity o f th e gel point, th e Martin-Adolf-Wilcoxonrelaxation ,function of G(t) provides a better fit withexperimental results than other models. They stated thatabove the gelation temperature, the molecules of gelatinin solution do not appear to be entangled as they lacka rubbery region on th e time-temperature superpositionmaster curve. In a simil ar paper, Labropoulos and Hsu[31] determined tha t whey protein isolate (WPI) gelshad a characteristic power law dependence of storageand loss moduli on frequency at the gel point. Different

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treatments of the WPI dispersion resulted in the samevalue of th e power law exponents n = 0.62-0.69 eventhough each WPI solution had a different gel time, rangingfrom 12min to 164min. Although the re has always beena core group of food and biopolymer scientists whoseapproach has been more from this perspective, interest hasbeen increasing and the gap which has existed betweenthe more theoretical approach of the syn thet ic polymerscientists and the work being done on biopolymer rheologyis ever shrinking. The greatest hurdle to overcome in thisarea is in working with th e polydispersiry and complexityof th e chemical and structura l aspects of th e biologicaldispersions.

A noted limitation in the traditional uniaxial compressiontest m et hod has bee n th e effect of the changing strainrate as the test progressed. Rohm et al [32] compared thestandard met hod of testing u nde r a constant sp eed to a

relatively ne w method of decreasing th e crosshead speedin order to maintain constant strain rate conditions. Theseresearchers, studying the impact of methodology on elasticfood gels, identified significantly lower values for apparentfracture strain under constant strain rate tes ting for gelswith failure Hencky deformation >0 5 0 6 A decrease infracture stress was also noted under constant strain rate.The impact of these findings on sensory correlations wasdiscussed.

In a series of three papers, Keetals et al [33 35]reported on the gelation and retrogradation of potatoand wheat starch. This work used th e standard smallstrain oscillatory tes ting to follow gelation. Large strainuniaxial compression tests, applying a constant crossheadspeed, were utilized to deter mi ne fracture properties.Combining small and large strain test ing, a hypothesis ofth e morphology of these gelled systems was developed.

dvances in rheological measurementtechniquesChakrabart i [36] has recently reviewed some of th e newdevelopments in the area of food rheology measurement.Part icularly interes ting in the context of emuls ions andfoams are the extensional viscosity measu rements andmeasurements of wall slip. A novel rheometer, based onthe concept of filament stretching, has been developed byPlucinski et al J Pluckinski, RK Gupta, S Chakarbarti ,personal communication and was applied to measu reextensional properties of mayonnaise. The extensionalviscosity of mayonnaise was found to be higher than itsshear viscosity.

Slip is an ever present complication in dispersed systemsand occurs through an apparent slip mechanism becauseof th e formation of a thin stratum of a lower viscosityliquid near the wall, which can be considered to be th e

suspending l iquid free of the dispersed phase. Indeed, atwall stresses below the yield stress of th e dispersion, flowcan occur because of the formation of th e slip layer. Heller

Rheology of emulsions foams and gels Borwankar and Case 87

and Kunrarnukkula [37] have reviewed th e foam rheologyliterature and concluded that in much, if not most, ofthis literature, the results are influenced by th e thicknessof the film of liquid formed along the walls, and do nottruly represent the rheological character of foams. Ignoringthese slip layer effects leads to geometr ic dependence ofrheological properties, and the authors warn that whereasapparent viscosities can give order of magnitude estimates,their dependence on uncontrolled variables can lead toinconsi stent results in practice. Food rheologists mus tbecome aware of the slip phenomena in dispersed systemsand make sure that their procedures eliminate or factor ou tth e effects of slip.

In a yet novel and evolving area of research, Israelachviliand co-workers [38 0 0 ] are investigating rheology of emulsions and dispersions in thin layers using thei r surfaceforces apparatus and relating it to th e structure and wetting

characteristics of the products.

onclusionsRheology represents a field that has tremendous potential in fur thering our understanding of dispersions,emulsions, foams and gels, and potentially impacting thetechnological advances in the food industry. Rheologicalmeasurements can provide increased understanding offood component interactions, and can lead to a controlof such interactions and food microstructure for ourbenefit. This can be achieved especia lly when we fullyunderstand th e relationships between rheology and foodtexture. Inte rpretat ion of rheology data- in terms ofingredient interactions and/or structural changes necessitates th e use of other physicochemical measurementsin conjunction with rheological measurements, and alsothe use of more than one rheological measurement.Indeed, interpretation of rheological data in structural orinterac tion terms is merely a specula tion unless backedby other physicochemical measurements. In this regard,[70 9 0 1 ] are noteworthy because th e authors all usedother physicochemical measurements in conjunction withrheological measurements, and often they used more thanone rheological measurement.

Ne w rheological measurement techniques are evolvingand being refined, yet considerable work needs to be donein establish ing the practical significance of rheologicalproperties in the context of food texture, engineeringprocess design and product structure and ingredientinteractions to leverage these in developing food products.

References and recommended readingPapers of particular interest published within the annual period of reviewhave been highlighted as:

o of special interest of outstanding interest

1. Borwankar RP; Food texture and rheology: a tutor ia l overview.In Rheology of oods Edited by Borwankar R ShoemakerCF.New York:ElsevierApplied Science; 1992:1-16.

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58 8 Food colloids emulsions gels a nd fo ams

2. Jeffrey OJ,Acrivos A: The rheolog ical p ropert ies of suspensionsof rigid particles. AIChE J 1976 22:417 432.

3. Metzner AB: Rheology of suspensions in polymeric l iquids .J Rheo/1985 29:739 775.

4. Hunter RJ: Rheology of colloidal dispersions. In Foundations ofColloid Science vol II. New York:Clarendon Press; 1989:992-1052.

5. Hiemenz PC, Rajgopalan R: The rheology of d ispers ions . In Principles of Colloid and Surface Chemistry edn 3. New York:

Marcel Dekker, 1997:145 192.This chapter presents n excellent overview of the rheology of dispersions,with particular emphasison the effect of interactions. Electroviscous effectsarepresented in special detail. The effect of phase volume fraction, solvationand shapes is also discussed. This chapter, together with reviews listedabove [2 3] provide an excellent starting point for the study of rheology ofdispersions and emulsions.

6. Greenwood R, Luckham P Gregory T: The effect of diameter ratio and volume ratio on the viscosity of bimodal suspensions

of polymer lattices. J Colloid Interface Sc i 1997 191 :11 21.A study of the rheology of bimodal suspensions. Experimentsrevealed thatminimum viscosities are attained for a volume ratio of 0.25 of small particlesto large particles. This is close to the theoretically predicted value of 0.27.Diameter ratios where viscosity of bimodal suspensions is minimized aredetermined. Dataare interpreted in terms of n increasein the effective close

packing limit of the suspension.7. Verduin H, de Gans BJ, Dhont JKG: Shear induced structu ral changes in gel forming suspensions studied by light scattering

and rheology. Langmuir 1996 12:2947 2955.Rheological measurements are combined with light scallering measurements to define shear-induced structural changes. Constant stress nd constant shear rheology are used.

8. Sherman P: Rheological p rop er ti es of emulsions. InEncyclopedia of Emulsion Technology vol 1. Edited by Becher P.New York: Marcel Dekker; 1983:405 437.

9. Dickinson E, G olding M: Rheology of sodium caseinate stabilized oil-in-water emulsions. J Colloid Interface Sc i 1997

191:166 176.This paper is n excellent study of emulsionrheology where three situationscorresponding to bridoing flocculation, no flocculation nd depletion flocculation are examined.Stress sweeps, small deformation dynamic {oscillatory}measurements nd time-dependent apparent viscosity are presented. Dataare interpreted in combination with the authors earlier data on interfacialcoverage nd creaming stability for the samesystems. Time-dependent rheological measurementshave been shown to be useful in characterizing slowdevelopment nd restructuring of floc networks in concentrated emulsions,and to give kinetic information conceming aggregation.

10. Dickinson E, Yamamoto, Y: Viscoelastic properties of heat-setwhey protein-stabilized emulsion gels with added lecithin.J Food Sc i 1996 61:811 816.

11. Ford LD, Borwankar R, Martin RW, Holcomb ON: Dressingsand sauces. In Food Emulsions edn 3. Edited by Friberg SE,Larsson K. New York:Marcel Dekker. 1997:361 412.

The chapter reviews dressing nd sauce emulsions nd contains a sectionon rheology with several references. Viscoelastic measurements are presented for emulsion gels nd for whey protein gels nd data interpreted interms of lecithin-whey protein interaction nd whey protein surface coverage. Order of addition effects are highlighted.

12. Princen HM: Rheology of foams and highly concentratedemulsions I. Elastic properties and yield stress of a cylindricalmodel system. J Colloid Interface Sc i 1983 91:160 175.

13. Princen HM: Rheology of foams and highly concentratedemulsions II. Experimental study of yield stress and walleffects for concentrated oil-water emulsions. J Colloid InterfaceSc i 1985 105:150 171.

14. Princen HM, Kiss AD: Rheology of foams and highlyconcentrated emulsions III. Static shear modulus. J ColloidInterface Sc i 1986.112:427 437.

15. Kraynik AM, Reinelt DA: Linear elastic behavior of dry soapo foams. J Colloid Interface Sc i 1996 181:511 520.Shear moduli computed for Kelvin, Williams and Weaire-Phelan foams using the Surface Evolver computer program This paper is one of severalpapers published by the authors jointly and by Kraynik alone on foams. Themain thrust of these papers has been the structure nd rheology of foams,

Reading these along with the papers by Princen including the ones listedabove) is a must for anyone interested in the field.

16. Lacasse M-D, Grest GS, Levine 0 : Deformation of smallo compressed droplets. Phys Re v E 1996 54:5436 5446.

Another use of the Surface Evolver program, this paper begins with theresponse of individual droplet to deformation nd builds it into a model forshear modulus.

17. Coughlin MF,Ingenito EP,Stamenovic 0 : Static shear modulusof gas-liquid foam determined by the punch Indentation testJ Colloid Interface Sc i 1996 181:661 666.

This paper provides direct measurementson shaving foams using the punchindentation test. The paper reports on several other measurements and ultimately reports good agreement with the static shear modulus relationshipsreported in references [13 14] above, which n be fortuitous consideringerrors in individual measurements.The authors,to their credit, provide a goodreview of the assumptions, experiments and artifacts nd their implicationon the conclusions. We noted some numerical discrepancies between thenumerical constants from references [13 14] cited in this paper versus theactual values reported by the original authors.

18. Friedman HH, Whitney JE,Szczesniak AS: The texturometer: ane w instrument fo r objective measurement. J Food Sc i 196328:390 396.

19. Haighton AJ: The measurement of hardness of margarineand fats with cone penetrometers. J Am Oil Chem So c 195936:345 348.

20. Mohsenin NM: Rheology and texture of food materials. InPhysical Properties of Plant and Animal Materials. edn 2. NewYork: Gorden nd Breach Science publishers; 1986:395 400.

21. Mitchell JR: The rheolog y o f gels. J Texture Studies 198011:315 337.

22. Nishinari K, Watase M, Rinaudo M, Milas M: Characterizationand properties of gellan k carrageenan mixed gels. FoodHydrocolloids 1996 10:277 283.

A combination of differential scanning calorimetry.and rheology was used todetermine the mechanism of interaction between gellan nd k-carrageenan nd understand their gel properties. This paper studied both a purified gummodel system to enable understanding of the observed behavior of dilutemixtures in solution, as well as commercial samples for the higher concentration gels to better address actual applications. This work identified themixedgel system of gellan and k-carrageenanas a phase-separated system.

23. Yoshimura M, TomohisaT,Nishinari K: Effects of konjacglucomannan on the gelatinization and retrogradation of cornstarch as determined by rheology and differential scanningcalorimetry. J Agric Food Chem 1996 44:2970 2976.

The food industry is always trying to control texture nd extend shelf life offoods. This paper looks for one more way to achieve that end through usingkonjac glucomannan in combination with com starch.

24. Miyoshi E, TomohisaT, Williams PA, Nishinari K: Effects ofsodium chloride and calcium chloride on th e interactionbetween gellan gu m and konjac glucomannan. J Agric FoodChem 1996 44:2486 2495.

25. Manoj P Kasapis S, Chronakis IS: Gelation and phaseseparation in maltodextrin-caseinate systems. FoodHydrocolloids 1996 10:407420.

A thorough examinationof the phase separation phenomena in polymer mixtures using maltodextrin nd caseinate as the subject of study. This workcombines calorimetry with a wide range of rheological tests on the singlecomponent nd the mixed biopolymer gel.

26. Langendorff V,Cuvelier G, LaunayB, ParkerA: Gelation andflocculation of casein micelle/carrageenan mixtures. FoodHydrocolloids 1997 11 :35 40.

This research was designed to better understand the observed undesirable) phenomena of sedimentation of casein micelles in the presence of carrageenanabove the gelation temperature.A combination of phase diagrams nd rheological testing was used to elucidate the mechanism involved ininteractions between casein micelles nd carrageenan chains.

27. Walkenstrom P,Hermansson AM: High-pressure treated mixedgels of gelatin and whey proteins. Food Hydrocolloids 199711 :195 208.

This paper begins with a thorough review of previous research on high pressure studies of various gelling systems.Sma/I nd large strain deformationrheological tests, in combination with light nd transmission electron microscopy, were used to develop a complete understanding of structure-function relationships of gels set via heat treatment or high pressure treatment.

28. Zasypkin DV, Braudo EE,Tolstoguzov VB: Mutlicomponent biopolymer gels. Food Hydrocolloids 1997 11:159 170.A nice tutorial from the research lab one of the leaders in this area Thepaper reviews the classifications of multicomponent gels systemscontainingtwo biopolymers.This is then followed by examplesof each type using gelatinmixed with a variety of other components. Rheology is used throughout thepaper to describe the interactions. Although the rheology is not described

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in great detail in this review the original papers can be accessed to obtaina more complete understanding of the rheology used to study these mixedsystems.

29. Winter HH Chambon F: Analysis of linear viscoelasticity of acrosslinking polymer at the gel point J Rheology 1986 30:367382.

30. PeyrelasseJ LamarqueM Habas JP Bounia N: Rheology of

gelatin solutions at the sol-gel transition. Phys ev E 99653:61266133.A thorough examination of the dynamic rheological properties of gelatin solutions as a function of temperature.Nice adaptation of methods developedin the synthetic polymer field to the study of biopolymer gelation.

31. LabropoulosAE Hsu SH: Viscoelastic behavior of whey proteinIsolates at the sol gel transition point J Food Sc i 1996 61:6568.

An easy to understand application of the power law dependence of thestorage and loss moduli and its relation to the physical mechanism of thegelation process of whey protein isolate.The gelation of whey protein isolatecould be described by a percolation mechanism.

32. Rohm H Jaros D Benedikt J:Constant strain rate compressionof biopolymer gels. J Texture Studies 1995 26:665-674.

33. KeetelsCJAM vanVliet T Walstra P: Gelation andretrogradation of concentrated starch systems: 1. Gelation.Food Hydrocolloids 1996 10:343353.

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34. Keetets CJAM vanVliet T Walstra P: Gelation andretrogradation of concentrated starch systems: -Retrogradation. Food Hydrocolloids 1996 10:355362.

35. Keetels CJAM vanVliet T Walstra P: Gelation andretrogradation of concentrated starch systems: 3. Effect ofconcentration and heating temperature. Food Hydrocolloids1996 10:363358.

36 . ChakrabartiS : Some recen t advances In food rheology.In Characterization Food Emerging methods Edited byGaonkar AG. Amsterdam: Elsevier 1995 :277308.

37. Heller P KuntamukkulaMS: Critical review of foam rheologyliterature. Ind Eng h em Res 1987 26:318 325.

38. Giasson S IsraelachviliJ YoshizawaH: Thin film morphology and tribology of food emulsions: a study of three mayonnaise

samples. J Food Sc i 1997 in press.This paper hypothesizesthattribological properties of foods areimportantinthe perception of food texture and presents measurementsof such properties using the surface forces apparatusequipped with optical interferometrictechnique. The tribological properties areexplained in terms of thin film rnorphology and other interfacial properties. It represents an emerging area ofresearch in food texture.