(6) Physical Analysis

Question 1

What is rheology

Answer 1

The study of deformation and flow of matter

Question 2

Define stress

Answer 2

Force per unit area (Pa)

- normal and shear stress leads to normal and shear strain

Question 3

Define strain

Answer 3

represents the relative deformation of a matter

• dimensionless
• normal and shear strain

Question 4

Define viscosity

Answer 4

internal resistance to flow; greater the friction, the greater the amount of force required to cause this movement (shear)

Apparent viscosity = shear stress/shear rate Pa.s

Question 5

what is normal stress? +example

Answer 5

force directly perpendicular to a surface

• kneading dough
• chewing

Question 6

what is shear stress? +example

Answer 6

force acts in parallel to sample surface

- spreading butter over toast

Question 7

why measure viscosity? (4)

Answer 7

To obtain:

1. Useful behavioural and predictive information for various products (prevent cheese from sliding off pizza)
2. knowledge of the effects of processing, formulation changes and ageing phenomena
3. quality control for raw materials that must be consistent from batch to batch
4. a high viscosity liquid requires more power to pump

Question 8

How to predict product behaviour? (5)

Answer 8

1. Collect rheological data
2. infer which components of the food product or ingredient make the viscosity or texture
3. test the effect of identified components
4. correlate rheological data with product behaviour to build mathematical model
5. the model can then be reversed and rheological data used to predict performance and behaviour

Question 9

newtonian fluids

Answer 9

viscosity independent of shear rate at any given temperature (water)
- plot of shear stress vs shear rate results in a straight line passing through the origin with gradient being viscosity

Question 10

viscosity for newtonian fluid?

Answer 10

Coefficient of viscosity/ newtonian viscosity

Question 11

non-newtonian fluids + types (3)

Answer 11

viscosity of fluid is a function shear rate (apparent viscosity)

1. Pseudoplastics (shear thinning) - viscosity decreases with increasing shear rate (ketchup)
2. Dilatant/shear thickening - viscosity increases with increasing shear rate (corn starch)
3. Plastics - behaves as a solid under static condition; threshold stress has to be applied before fluid flows (toothpaste)

Question 12

Mechanism of viscometer

Answer 12

1. dial viscometer rotates a sensing element in a fluid and measures the torque required to overcome the viscous resistance to the induced movement
2. this is accomplished by driving an immersed element (spindle) through a spring, the degree to which the spring is wound indicated by a pointer is proportional to the viscosity of the fluid
   * immerse at an angle to avoid formation of air bubbles which will affect viscosity

Question 13

Measurement considerations of a viscometer (4)

Answer 13

• Measure with laminar flow, not turbulence
• Consider effect of temperature
• Consider shear rates of pumps and pipes in the factory
• For high rates in a factory, extrapolate the data from a number of lower shear rates in a viscometer

Question 14

Rheological analysis for solids (texture)

Answer 14

TPA (texture profile analysis) analysed using a universal testing machine

1. controlled forces applied to a food sample
2. resistance and deformation properties of food measured

Question 15

TPA limitations

Answer 15

Only homogeneous (uniform in composition throughout) food allow reproducible data (processed cheese); not useful for heterogeneous food (jam)

Question 16

Factors that affect solubility (3)

Answer 16

Temperature, solvent and particle size

Question 17

Methods of particle analysis

Answer 17

1. Standard and electron microscopy
2. Laser diffraction
3. Dynamic light scattering

Question 18

basis of light interaction with particles

Answer 18

based on differentiating light waves that are scattered, diffracted or absorbed by the dispersed particles

SCATTERED:

• consists of reflected and refracted waves
• depends on the form, size and composition of the particles

ABSORBED:

• absorbed light waves are removed from the incident beam and converted to heat or electrical energy by interaction with the particles
• dependent on size and composition

DIFFRACTED

• arises from edge phenomena and is dependent only on the the geometric shadow created by each particle in the light beam
• independent of the composition of particles

Question 19

laser diffraction purpose

Answer 19

• measures particle size based on principle that small particles in the path of a light beam scatter the light in a characteristic, symmetrical pattern which can be viewed on a screen
• can deduce the distribution of particle sizes given a certain pattern of scattered light intensity as a function of angle to the axis of incident beam

Question 20

Texture qualities of solids (4)

Answer 20

• springiness
• fracturability
• gumminess
• chewiness

Question 21

Texture quality of solids: Springiness

Answer 21

• elasticity
• height that food recovers during the time that elapses between the end of the first bite and the start of the second
  (how well a product physically springs back after it has been deformed during the first compression)

Question 22

Texture quality of solids: Fracturability

Answer 22

• brittleness

- force at which the material fractures, the force at the 1st significant break

Question 23

Texture quality of solids: Gumminess

Answer 23

the quantity of force to simulate the energy required to disintegrate a semi-solid food product to a state ready for swallowing

Question 24

What does TPA do?

Answer 24

provides insight into how samples behave when chewed

- often called the “two bite test” because the texture analyser mimics the mouth’s biting action

Question 25

Texture quality of solids: chewiness

Answer 25

the quantity to simulate the energy required to masticate a solid food to a state ready for swallowing

Question 26

what is size analysis for?

Answer 26

for particles and droplets;

1. dry powders: flour, powdered milk
2. full fat milk

Question 27

why is particle size important?

Answer 27

particle size of fat droplets present in dairy and other food emulsions important in defining properties such as flavour release, mouth feel and emulsion stability

Question 28

Sample prep: laser diffraction

Answer 28

• requires that the particles are in a dispersed state (either in liquid (suspension) or in air (aerosol))
• liquid: “wet method” (LAS-W)
• dry: “dry method” (LAS-D)

Question 29

LAS-W

Answer 29

• better state of dispersion achieved in an appropriately selected liquid medium
• solution conditions can be modified by changing pH or adding chemical dispersing agents

Question 30

LAS-D

Answer 30

• stream of compressed air/vacuum is used to disperse the particles and transport them to the sensing zone
• works well for large, non colloidal phase spheroids (non homogenous), but otherwise limited in application

Question 31

large vs small droplet size

Answer 31

• large emulsion droplets lead to poor flavour release, greasy mouth feel and poor stability due to creaming (settling at the top or bottom)
• emulsification to a smaller droplet size tends to reduce creaming and improve taste of product
• balance is required when doing this bc decreasing the particle size increases available SA which can lead to flocculation (clumping) if the emulsifier conc is not controlled

Question 32

dynamic light scatering

Answer 32

aka photon correlation spectroscopy (PCS)

- used to characterize molecules and particles in solvent and dispersant environments