15visc - III.C.ii.-1 Viscosity Definition • Fluid foods...

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Unformatted text preview: III.C.ii.-1 Viscosity Definition • Fluid foods flow in response to a force (called shear) – Viscosity (η) represents the resistance to flow knife peanut butter bread Fluid dissipates energy through friction between fluid layers — depends on fluid viscosity III.C.ii.-2 Viscosity • For simple Newtonian fluids, viscosity is a constant – e.g. water, oil, sugar solution (honey, syrup), milk – Non-Newtonian foods often shear-thin • Viscosity affected by temperature – Exponential dependence η = A eB/ kT (less viscous w/ greater shear) – As T ↑, interactions between molecules ↓, viscosity ↓ • Addition of solutes or particles increases viscosity – e.g. add 20% sucrose to water, η doubles – e.g. honey (80% sugar), very viscous III.C.ii.-3 Viscosity of Honey 160 140 120 100 80 60 40 20 0 14 16 18 20 22 viscosity (g/(cm s)) water content (wt%) g Viscosity of water = 0.01 Poise = 0.01 cm s III.C.ii.-4 Effect of Dispersed Material • Addition of dispersed materials increases friction in fluid – emulsions, suspensions, foams – e.g. peanut butter (solid fat in oil) – Viscosity depends on volume occupied by particles III.C.ii.-5 Dispersed Phase Effects 6 5 4 3 2 1 0 0 10 20 30 40 50 volume percent of particles Milk Cream Consumer can detect small changes in viscosity Impact of particles ↑ as concentration ↑ III.C.ii.-6 Macromolecular Solutes • Macromolecules occupy large volume per molecule – Physically entrap water → Big effect on η per molecule – Molecular entanglements Consider effects of 1 wt% carbohydrate solutions: Solute Fructose Sucrose Carrageenan Alginate η relative to ηwater 1.03 1.03 54 214 “effective” volume of macromolecule ...
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15visc - III.C.ii.-1 Viscosity Definition • Fluid foods...

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