Create Presentation
Download Presentation

Download Presentation
## Rheology of food materials

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**Rheology**of food materials**Overview**• Measuring systems for rotational and oscillatory rheometers • Flow and viscosity curves in a wide shear rate range • Examples: Water; Polymer solutions (polysaccharide); Emulsions; Binder solutions • - Special measuring systems: measuring with the ball measuring system • Examples: Marmalade, Bolognese sauce with meat chunks • - Yield point in flow curves (via rotational tests) • Examples: Creams; Ketchups • Yield point as the limiting value of the linear-elastic deformation range (via rotational tests) • Example: Ketchup • Structure at rest as G’ value (via oscillatory tests: amplitude and frequency sweeps) • Examples: 1) Butter; 2) Starch gels; 3) Pudding; 4) Milk drinks; 5) Emulsions • - Structure regeneration of coatings, leveling, sagging behavior and layer thickness • Step tests (oscillatory and rotational tests) • Example: Ketchup • - Temperature-dependent behavior during heating, softening, melting, cooling, solidification, • crystallization (using rotational and oscillatory tests) • Examples: Chocolate; Ice creams; Spreading cheese and melting cheese • Gel formation (using time-dependent and temperature-dependent rotational and oscillatory tests) • Examples: Corn starch; Gelatin**Typically used measuring systems Cone - plate and plate -**plate systems Usesandblastedorprofiledmeasuringsystemsforoilyandfattysubstances ! Gel-likesamples G‘ > G‘‘ andtemperaturetests Profiled geometries for mozarrella type of cheeses, sandblasted for cream cheese PP25 Viscoelastic, high viscous, caution to particles and structures sizes Paste like, sticky and almost not flowing CP25-1, CP25-2, CP25-2 Viscoelastic, medium viscosity (free flowing and significantly above 100 mPas (1000mPas) (larger particlesor super structures ) PP50 Viscoelastic, medium viscosity (free flowing and significantly above 100 mPas (1000mPas) Flowing liquid but larger super-structures (CP50-2) CP50-1 Low viscosity CP75-0,5**CPxx: Cone & PlateCone truncation**Cone with truncation: cone truncation = measuring gap Crash!! + Shear rate and shear strain constant + Easy to clean - Measurement of friction if particles are below the tip of the cone Standards: ISO 3219, DIN53019, DIN53018**Double Cone BI-C60-1°Applications: Food, Cosmetics, Pharma**• precise determination of melting and crystallization temperature • homogenous heating and cooling • low temperature measurements without condensation (inert, dry) • no evaporation of water or solvents Peltier-hood heating & cooling by convection and radiation N2 modular Bi-cone for inset-Peltier Peltier basis-control by conduction**PTD – Peltier Temperature DeviceExcellent temperature**control from the bottom to the top • unique combination of radiation, convection (frost protection) & conductive heating & cooling • homogenous heating and cooling • low temperature measurement without having • condensation (inert, dry) • frost formation optional evaporation blocker Peltier-Hood Heating / Cooling byConvection and Radiation Peltier Basis Temp.-ControlbyConduction**Sealing the Gap: PP & CP Applications: Food, cosmetics,**coatings • evaporation of solvent / water ? • skin formation ? 2 - guard ring oil(10 mPas Si-Oil)**Sealing the Gap: PP & CPApplications: Food, cosmetics,**coatings • evaporation of solvent / water ? • skin formation ? 1a - solvent trap solvent sample**Typically used measuring systems Concentric cylinders**systems • Above 1000mPas (100mPas) • CC27 • Above 10mPas and below 1000mPas • CC39 • Above 10mPas and below 100mPas with super-structures • DG27 (same dimensions like CC27), gap size = 1mm • Below 10mPas und homogenous, small structure • DG26.7, gap size = 0.4mm • Easy to prevent sample from drying-out (oil film on top of sample) • No trimming • Good solution for all kind of liquids in rotational mode • CC: not recommended for oscillation; DG: also recommended for oscillation • CC: Helical groove if phase separation or vertical profiling to prevent slippage Standards: ISO 3219, DIN53019, DIN53018**Flow Behavior: ideallyviscousbehavior**water 10 1000 mPa mPas 10 lg h 1 lg t constant viscosity 1 DG 42 (double gap MS) T = +20°C 0.1 0.01 0.1 1 10 s-1 100 lg Double-gapmeasuringsystemsarespecialsystemsdesigned forlow - viscosityliquids.**Natural Food ProductsMeasure natural product without**destroying the initial structure by cutting into the sample structure A special measuring system for: E.g. natural yoghurt • ST22-4V-40 measuring system • aluminum cup • or stainless steel cup • Advantages: • allows measurement of brittle, natural materials • excellent penetration characteristics • dimensions similar to standard CC27 • alternative: combination with flexible cup holder - >**Special GeometriesCC withSurface Treatment orVanes,**Stirrers, Propellers • Coarse disperse materials • Building materials • Slurries • Food (Yoghurt) • Better grip • No slip • More Stirrers on request: - User defined - Brookfield Spindels - Krebs Stormer Spindels - ...**Special Geometries (Relative Values)**Helix 1 Helix 2 Blade Anchor Ball Measuring System All these stirrers are relative measuring systems Stirrer for Building Materials Starch Stirrer**Rheometry with special GeometriesBall Measuring System (BMS)**for dispersions containing coarse-grained particles (showing a diameter up to 10mm) Example: Marmalade containing fruit pieces**Rheometrywithspecial GeometriesBall Measuring System (BMS)**Flow and Viscosity Curves of two Marmalade Preparations**Rheometrywithspecial GeometriesBall Measuring System (BMS)**Flow and Viscosity Curves of a Sauce Bolognese Spaghetti Sauce containing meat pieces (testing reproducibility)**Further measuring systems/ temperature control systems**• Starch (pressure) measuring cell • Tribology cell • Penetration measurements • Interfacial rheology (IRS) • Sentmanat extensional rheology(SER) • Flexible Toolholder • Rheo-Microscopy**Flow BehaviorRheo - Microscopy**dispersions water / oil emulsion Size and shapeof the dropletsare dependingon shear rateand “shear history”. lg**rest**high shear rates high viscosity low viscosity Shear-Thinning flow Behavior Suspension 1: Orientation of particles (needle shaped) Suspension 2: Agglomerated particles Break-up of agglomerates Emulsion: Deformation and break-up of droplets**Shear-Thickening flow Behavior**At low shear load: The rod inclines slowly. Low viscosity At high shear load: Solidification of the liquid due to shear thickening. High viscosity**Flow BehaviorShear-thickeningBehavior**dispersions Suspensions shear - thickening of suspensions at - high solid concentrations- high shear loads 1 f ... volume fraction of solid particles**Flow BehaviorYield Point**High stress…sample starts moving t2 The applied force is higher than the structural force Low stress…no movement t1 Flow Curves on a linearscale Yield Point as a limitingvalueoftheshear stress 2 Break ofthestructure - at - rest. Super - structureby a chemical- physicalnetworkvia interactiveforces. 1 ty 1 without a yield point 2 having a yield point y Examples: Pastes, concentrated dispersions, suspensions, ketchups, mayonnaises, chocolate melts, butter, gels**Pa**2500 2000 1500 Ketchup t 1000 t shear stress 500 0 0 200 400 600 s-1 1000 shear rate Flow behavior: yield point Flow curveshowing a yield point(on a linear scale) Yield point can hardly be read-off**Flow BehaviorYield Point, comparisonlin / log diagrams (2)**food 104 Flowcurveshowing a Yield Point(on a logarithmicscale) Pa 1000 lg t Ketchup 100 yield point y = 48 Pa 10 1 10 100 s-1 1000 lg**Flow behavior: yield point**Flow curves on a logarithmic scale ty ty Yield point analysis in the low-shear range, e.g. read offon the - axis Yield point analysis in the low-shear range, e.g. read offat = 0.01 s-1**Flow behavior: yield point**Mathematical curve fittingfor flow curves on a linear scale(approximation, "regression") examples: models according to Bingham:flow curve of a material with a yield stressand a constant viscosity (foodor cosmetics) B- “yield pointacc. toBingham“ B - “Bingham viscosity“ Windhab:chocolate and other cocoa products 0- yield point 1- linear yield point - “high-shearviscosity“ • other often used models: • - Cassonblood, food • Herschel / Bulkley • materials with a yield • stress and shear thinning or shear thickening behavior**Flow BehaviorYield Point**food Analysis using Approximation Functionsfor Flow Curveshere:accordingtoCasson (OICC 1973),andWindhab(IOCCC 2001 / ICA) Chocolate Melts(T = +40°C) Bitter White Whole Milk shearrate Analysis CassonWindhab0(Pa) 0 (Pa)Bitter15 18 White1925Whole Milk2123 Summary: Yield Points are not material constants, sincetheyaredepending on themeasuringmethodand on theanalysismethod.**Viscoelastic BehaviorYield Point (using a / -**Diagram) Yieldpointasthelimitingvalueoftheshear stress: The sample startstoflownot beforetheexternalforcesareexceedingthenetwork-of-forcesoftheinternalstructure. Belowtheyieldpoint thereiselasticdeformation. Testingwithcontrolledshear stress lg lg lg lg yieldpointyusingthe best fit straightline (“tangent“) in the linear-elasticdeformationrange yieldpointyusingthe „tangentcrossover“method**106**% without binderyield point 13.5 Pa 104 lgg with binder yield point 114 Pa 102 100 10-2 Pa 0.1 1 10 100 1000 shear stress lgt Viscoelastic BehaviorYield Point (using a / - Diagram) food Comparisonoftwo Ketchups deformation**IntroductionViscoelastic Behavior** viscous viscoelastic elastic with tand= G'' / G'**ApplicationShearModulus**Material StiffnessandShear Moduli Example: different typesofcheese 1 2 5 4**Viscoelastic BehaviorAmplitude Sweeps**food Gel Strength, Dependence on the Binder - Concentration 10,000 15 w-% Pa Starch Gel (in water) 10 w-% 1000 Summary: Gel strengthis dependent on the binder concentration lgG' 7.5 w-% 5% w-% 100 10 ω = 10 rad/sT = +23°C loss factor tan = G‘‘ / G‘ First check in the LVE range: tan < 1 for all samples ( = gel structure) ? Yes ! lg tand 1 0.1 0.1 10 % 100 1 strain lg g**Viscoelastic BehaviorAmplitude Sweeps**food TemperatureDependenceof Butter 10 MPa T = +10°C 1 Summary: cold butter shows brittle break,hence poor spreadability lgG' 0.1 lg G'' T = +23°C 0.01 ω = 10 rad/s 0.01 0.1 1 % 10 strain lg g**Amplitude Sweep /CSD /CSSMargarine as semi-solid material**withflowpoint CSD CSS**ApplicationSedimentation, Long-term Storage Stability**dispersions Stability of DispersionsExample: Salad Dressings in the beginning after 15min Behavior in the low-shear range or at rest, respectively**FrequencySweepStabilityofsuspensions**Time dependentstructuralstrength • G’ decreasing • - Long termbehavior= Fluid -like- StrengthofthestructureG’ decreases • Goodflowcharacteristics- Low stability • G’ constant, lightdecreasing- Long time structuralstrengthG‘ - Bad flowcharacteristics- High stability 2 1 1 t = 1 / omega G‘ G‘‘ 2 2 1 w = 1 / Time**Amplitude SweepSedimentation-Stability**Milk: Geometry DG26.7* • Pure milk • Chocolate Milk Plus • Chocolate Milk Budget • Caenriched Mill Mechanicalstoragestability**Amplitude SweepStructuralstrength G´ asfunctionof stress**-1 10 Pure Milk (noG‘ ) Pa CA Milk CHOC plus CHOC budget -2 10 G' -3 10 tLVE tLVE tLVE -4 10 Pa 0,0001 0,001 0,01 0,1 TAULVE = Yield stress = Externalforcetoovercomethestructureatrest Shear stress t *) Strain-Test, plottet asfunctionofstrain**FrequencySweepSedimentation Stability**Pure milk DG 26.7 G' Choc milk, plus DG 26.7 G' 0 10 Choc milk, Std. Pa DG 26.7 G' -1 10 CA-Milk DG 26.7 G' -2 10 G' -3 10 -4 10 1/s 0,1 1 10 100 w Measurement ofstructuralstrengthatrestormechanicalstabilityof milk**5**10 Pa G' 4 10 G'' 5°C 20°C 36°C 3 10 2 3 4 10 10 10 t Pa Amplitude SweepRepresentationasfunctionof stress todeterminetheflowpoints • Spreadcheese • Temperaturebehavior • Flow point= SpreadabilityascrossoverpointatG‘ = G“ Spreadcheese5°C G' G'' Spreadcheese20°C G' G'' Spreadcheese36°C G' G''**Penetration measurementsSoft cheese**• Presetting 0.3N contactpressure • Temperature60°C • Temperatureofcheesebeforetest ca. 25°C Start Depth End Time**Penetration measurementMargarine**• Presetting: Penetration velocity down/up= 2000µm/s • Alternatively: Normal forcecontrolledtesting down stop up Time**Flow BehaviorTemperature - dependentBehavior**softeningandmelting, orsolidificationandcrystallization preset: constantshearconditions (shear rate orshear stress)result: viscosity / temperaturecurvewithsteadilydecreasingorincreasingviscosityvalues, respectively T gelformationandcuring preset: constantshearconditions (shear rate orshear stress)result: viscosity / temperaturecurveshowing a viscosityminimum min T**Flow BehaviorTemperature - dependentBehavior**food 10 Coolingprocess: CrystallizationTemperature ofCocoa Butter Pas 8 ChocolateMelt 6 h 4 crystallization 2 0 20 25 30 35 °C 40 temperature T**Starch gelling**• Electrical heated cell • Watercooling • Fast heating and cooling rate • Stirrer acts against sedimentation of particles**Viscoelastic BehaviorTemperature - dependentBehavior**meltingorcrystallizationprocess preset: constantshearconditions (amplitudeandfrequency) (with an amplitude in the LVE-range, andmostlywithω = 10 rad/s) result: steep decrease or increase, resp.,in a narrow temperature range Tk ... crystallization temperature