Summary Food ingredient Functionality - Rheology and gels

Rheology
• Interactions between ingredients in
different foods determine final structure of
food and the structure determines the final
properties of the food
• Subjects rheology
1. Types of material behaviour:
- Purely elastic solid
- Purely viscous fluid
- Viscoelastic materials
2. Rheology of dispersed systems:
- Hard spheres in Newtonian fluid
- Spheres with steric stabilization
- Emulsions
• Rheology is the science that studies the relation between amount of applied force and
deformation of a system:
- Applied force is referred to as the stress
- Deformation can be seen as flow of the product
- Amount of deformation depends on whether the food is more liquid or more solid like
o Gels: elastic properties + elasticity of solids
▪ Do not really deform and have more elastic behaviour
▪ Example: gelatin gel or gels prepared from polysaccharide
▪ Relevant property: elasticity of the solids
o Solutions: viscous properties + viscosity of liquids
▪ More liquid foods deform very easily  more flow properties
▪ Viscous properties  relevant for solutions
▪ Example: Sugar solution/sugar syrup
▪ Relevant property: viscosity of liquids
• Elastic solids
- 2 different types of deformation upon application of a force
o Simple shear: solid is moved in horizontal direction
▪ Relative deformation that can be described by a certain angle alfa
▪ Relative deformation determined by the force applied/stress  force divided
by area
▪ Applied stress and deformation are related through a parameter that is called
the shear modulus (Pa)
o Elongation/compression: solid is moved in vertical direction
▪ Due to a compression in the z-direction, the length of the solid will increase
▪ Relative change in length is relevant deformation
▪ Stress related to the change in length via the elongational modulus
- Apply force in both directions  2 ways of changing solid




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,• Viscous liquids
- The force applied will deform the liquid different than in solids
- Deformation often applied with rotation  stirring motion
- Using cylinders: Liquid placed in cup and a cylinder  cup does not move + cylinder will
rotate difference in movement of the liquid
o Exert stress δ (force per unit area)Measure shear rate with viscometer Viscosity η
o Close to the cup (outer cylinder)  no movement
o On inside (close to inner cylinder)  movement of liquid will be high
o Deformation measured as shear rate
o Applied stress related to shear rate, via a viscosity denoted as eta
o 2 methods to measure: Applying shear rate of applying force




- Newtonian liquid
o Stress linearly related to shear rate  viscosity is constant  Newtonian liquid
▪ Viscosity independent on shear rate
o Viscosity determined as slope between stress and the shear rate
o Low viscous liquid
o In food: often not newtonian




- Non-Newtonian behaviour  Shear rate thinning and Shear rate thickening  stress and
shear rate effect not proportional  viscosity does depend on shear rate
o Shear thinning:
▪ Start shearing  become more liquid  lower force required
▪ Faster change in shear rate than the applied shear stress
o Shear thickening:
▪ Start shearing  becomes more solid  higher force required
▪ Faster increase in shear stress than shear rate
▪ Starch solutions and peanut butter




- Viscous liquids with yield stress
o Yield stress: minimum amount of force/stress needs to be applied before the product
starts to flow
o Stress versus shear rate does not start in origin
o Applied stress lower than yield stress  material behaves as elastic solid

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, o Applied stress higher than yield stress  material behaves as a viscous liquid and
materials starts to flow
o After the material starts to flow  2 different options
▪ Viscosity constant and independent of shear rate  Bingham fluid
(Newtonian)  does not exist in food materials
▪ Material behaves as non-Newtonian fluid when starts to flow  most often
shear thinning  referred to as general plastic flow shear rate and stress not
proportional (shear thickening/thinning)
➢ Behaviour can be modelled  power law model  extended by yield
stress
➢ A lot of products have a yield stress: in rest they behave as a solid and
there is no movement  a lot of force applied  material starts to flow




- Dense emulsion
o Network of oil droplets Force needed to break up network/clusters
o Droplets line out
o Non-Newtonian:
▪ Yield stress
▪ Shear thinning
- Examples of materials with a yield stress
o Plastic flow  products have a
structure that allows a certain stress
required for moving
o Products often contain a network and
form a gel  formation of oil droplet
network, casein network, polysaccharide network, combined network of particles and
polysaccharides
▪ Tomato ketchup
▪ Margarine
▪ Soft (cream) cheese
▪ Marmalade
▪ Mayonnaise
▪ Salad dressings with suspended particles  needs a yield stress  prevents
suspended particles from sinking to the bottom
- Thixotropic behaviour
o Time dependent viscosity  viscosity profile changes
o Breakdown and restoring of a
structure takes time
o At constant shear rate 
viscosity lower
o Structure broken  viscosity not
the same as before  time
required to repair the structure
o Hysteresis: amount of
thixotropic behaviour



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