FS3014 Section 4 (Viscoelastic) PDF

Section 4: Rheological characterisation of viscoelastic food materials FS3014 Macromolecules, Emulsions and Food Structure Dr Seamus O’Mahony 1  Classification of Rheology SOLIDS VISCOELASTIC LIQUIDS (Elastic)...

Section 4: Rheological characterisation of viscoelastic food materials FS3014 Macromolecules, Emulsions and Food Structure Dr Seamus O’Mahony 1  Classification of Rheology SOLIDS VISCOELASTIC LIQUIDS (Elastic) (Viscous)  Viscoelastic Rheological Behaviour Many food materials show an element of both viscous (liquid-like) and elastic (solid-like) character These types of materials are described as Viscoelastic Examples: mayonnaise, butter, margarine, chocolate, whipped cream, fruit purees, concentrated hydrocolloid solutions/dispersions Scenario 1: Mayonnaise will not easily squeeze out of a tube when moderate pressure is applied Scenario 2: Salad dressing will come gushing out of the bottle with only a slight pressure squeeze The fundamental common rheological parameter governing both of these situations is Yield Stress 3  Viscoelastic Rheological Behaviour The rheological properties of some different types of viscoelastic food materials are as follows: A minimum shear stress, known as the yield stress (to) must be applied to the food material before flow commences At an applied shear stress below the yield stress (τo) the food material behaves as a solid and strains without flow At an applied shear stress above the yield stress the food material behaves as a liquid and flows Therefore, for these food materials, elastic (solid) behaviour and liquid (flow) behaviour are displayed, but separately: solid below τo; liquid above τo For Plastic materials (i.e., material that exhibit a yield stress; e.g. butter, margarine, “weak gels” such as xanthan solutions) subsequent flow can then be Newtonian or non-Newtonian, depending on the nature of the material  Viscoelastic Rheological Behaviour Bingham Plastic Displays Newtonian flow above the “Yield Value” t = t0 + hplg t0 = Yield stress hpl = Plastic viscosity  Viscoelastic Rheological Behaviour Examples of 2 Bingham Plastic Fluids  Viscoelastic Rheological Behaviour Data Analysis for Bingham Plastic Materials Herschel-Bulkeley Model t = t0 + Kgn t = Shear stress t0 = Yield stress K = Consistency index (or viscosity index = hpl) n = Shear rate index i.e., based on the Power Law Model with extra term for Yield Stress  Viscoelastic Rheological Behaviour Data Analysis for Bingham Plastic Materials Casson Model (Equation) t1/2= tCA1/2 + (hCA g)1/2 hCA = Casson viscosity index tCA = Casson yield stress For graphing purposes: Plot g1/2 vs t1/2 If reasonably linear, slope = hCA1/2 and intercept = tCA1/2 Squaring the intercept gives the Casson value (tCA) of Yield Stress and squaring the slope gives Casson value of plastic viscosity (hCA) when the material is flowing  Viscoelastic Rheological Behaviour Data Analysis for Bingham Plastic Materials  Viscoelastic Rheological Behaviour Data Analysis for Bingham Plastic Materials........ based on graph on previous page  Viscoelastic Rheological Behaviour Not just Power Law, Herschel-Bulkeley Law and Casson Model!  Viscoelastic Rheological Behaviour Data Analysis for Bingham Plastic Materials  Typical Yield Stress Values 1 dyn cm-2 = 100 cP = 100 mPas 13  Yield Stress – Importance in Chocolate Enrobing with Chocolate Yield Stress: Enrobing of Chocolate https://www.youtube.com/watch?v=E3OEDvDGokA  Yield Stress – Importance in Chocolate Reducing Fat Content of Chocolate Product development involves trying to mimic the flow properties (Yield Stress and Plastic Viscosity) of full fat chocolate in reduced fat chocolate using emulsifiers (lecithing and PGPR) Chocolate Rheology: Low Fat Options?  Creep Testing of Viscoelastic Materials Creep Testing In a typical creep experiment, a fixed stress is applied to a material instantly and the strain is recorded over time (Creep/Retardation). The stress is then instantly removed and the strain is again recorded over time (Recovery/Relaxation)....... This approach is often referred to as Creep-Recovery testing Stress Relaxation Testing In a typical stress relaxation experiment, a stress is applied to deform the material to a set strain; the strain is maintained constant and force, stress or modulus is measured over time. Both Creep and Stress Relaxation experiments can be carried out either in compression or in shear....... most commonly done in shear format using a rheometer  Creep Testing of Ideal Solid Materials Apply a constant stress.... measure strain Apply Stress Remove Stress Creep Testing of Ideal Liquid Materials Apply a constant stress.... measure strain Apply Stress Remove Stress Creep Testing of Viscoelastic Materials Apply Stress Remove Stress Creep Testing of Viscoelastic Materials If a fixed stress is applied to a solid-like sample such as a biopolymer gel, the strain will gradually increase over time (creep compliance), and on release of the stress the sample will only partially recover its original geometry. This behaviour may be understood in terms of the gradual rearrangement of the network to accommodate the applied deformation by dissociation of pre-existing inter-chain junctions and their replacement by new interactions between different chain partners. In other words, the solid-like network is also capable of flowing like a liquid (i.e., Viscoelastic response). Creep Testing of Viscoelastic Materials The recoverable strain, which corresponds to the initial sharp increase when the stress is applied, gives a measure of the solid-like (elastic) character of the sample, and the irrecoverable strain, which corresponds to the gradual increase in strain (flow) during the initial creep period, gives a measure of liquid-like (viscous) character. Creep-Recovery Curves for Polysaccharides Polysaccharide gels (e.g., Carrageenan) show almost complete recovery when the stress is removed, with only a small irrecoverable strain due to network rearrangement (flow) during the creep period. Creep-Recovery Curves for Polysaccharides For solutions of entangled coils (e.g., Guar Gum) the response is dominated by liquid-like character, with little, if any, recovery (solid- like response) when the stress is removed. Creep-Recovery Curves for Polysaccharides “Weak gel” networks (e.g., Xanthan) although flowing appreciably during the creep period, still show substantial elastic recovery when the stress is removed.  Creep-Recovery Curves for Acid ‘Milk’ Gels Cow’s Milk Soy Milk Quinoa Milk Strongest gel as compliance changed most with increasing stress Creep-recovery curves recorded at 30oC for acidified bovine, soy and quinoa milks. The curves show the variation of compliance (J) in response to applied stress values ranging from 1.6 to 102.4 Pa (dashed line). All samples were viscoelastic in rheological properties Creep-Recovery Curves for Acid ‘Milk’ Gels  Typical Stress-Relaxation Curves Typical Stress-Relaxation curves for different types of materials