food chemistry - viscosity

VISCOSITY

Viscosity• Consumer acceptability depends on the viscosity or consistency of the product• Liquid and Semisolid-type

food

• Viscosity of a solution relates to its resistance to flow under an applied force or shear stress• Ideal solution• The shear stress is directly proportional to the shear rate

• Proportional constant (Viscosity Coefficient)• Fluid that obey this expression are called Newtonian fluids

• Flow behavior of solutions is greatly influenced by solute type• High-molecular-weight

soluble polymers greatly increase viscosity even at very low temperature

Molecular Properties1. Size2. Shape3. Flexibility4. Hydration

• Solutions of randomly coiled macromolecules have greater viscosity compare to solutions of compact folded macromolecules

• Most macromolecular solutions do not display Newtonian behavior• Especially at high protein concentrations

• Viscosity coefficient (η) decreases as the Shear Rate ( increases• This behavior is known as Pseudoplastic or Shear thinning

Pseudoplastic • This behavior arises in protein solutions because of the

tendency of protein molecules to orient their major axis in the direction of the flow.• When shearing or flow is stopped, the viscosity may or

may not return to the original value• Solutions of • Fibrous proteins remains oriented and do not regain

their original viscosity• Globular proteins rapidly regains their viscosity

(Thixotropic)

• At high protein concentrations or in protein gels, where protein-protein unteractions are numerous and strong, proteins display plastic viscoelastic behavior• specific amount of force, known as “yield stress,” is

required to initiate flow

• Viscosity behavior of proteins is a manifestation of complex interactions among several variables• Size • Shape• Protein-solvent Interactions • Hydrodynamic volume• Molecular flexibility

•When dissolved in water proteins absorbs water and swell• Volume of the hydrated molecule (size and volume) is larger compare to their unhydrated size or volume• Protein–associated water induces long-range effects of the flow behavior• The greater the specific volume of the protein = greater flexibility

•The viscosity of dilute protein solutions is expressed in several ways• Relative viscosity• Ostwald-Fenske type capillary viscometer

• Specific viscosity• Reduced viscosity• Intrinsic viscosity