(6) Physical Analysis Flashcards
Physical analysis, understanding product texture and physical measurement of ingredient qualities
What is rheology
The study of deformation and flow of matter
Define stress
Force per unit area (Pa)
- normal and shear stress leads to normal and shear strain
Define strain
represents the relative deformation of a matter
- dimensionless
- normal and shear strain
Define viscosity
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
what is normal stress? +example
force directly perpendicular to a surface
- kneading dough
- chewing
what is shear stress? +example
force acts in parallel to sample surface
- spreading butter over toast
why measure viscosity? (4)
To obtain:
- Useful behavioural and predictive information for various products (prevent cheese from sliding off pizza)
- knowledge of the effects of processing, formulation changes and ageing phenomena
- quality control for raw materials that must be consistent from batch to batch
- a high viscosity liquid requires more power to pump
How to predict product behaviour? (5)
- Collect rheological data
- infer which components of the food product or ingredient make the viscosity or texture
- test the effect of identified components
- correlate rheological data with product behaviour to build mathematical model
- the model can then be reversed and rheological data used to predict performance and behaviour
newtonian fluids
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
viscosity for newtonian fluid?
Coefficient of viscosity/ newtonian viscosity
non-newtonian fluids + types (3)
viscosity of fluid is a function shear rate (apparent viscosity)
- Pseudoplastics (shear thinning) - viscosity decreases with increasing shear rate (ketchup)
- Dilatant/shear thickening - viscosity increases with increasing shear rate (corn starch)
- Plastics - behaves as a solid under static condition; threshold stress has to be applied before fluid flows (toothpaste)
Mechanism of viscometer
- dial viscometer rotates a sensing element in a fluid and measures the torque required to overcome the viscous resistance to the induced movement
- 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
Measurement considerations of a viscometer (4)
- 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
Rheological analysis for solids (texture)
TPA (texture profile analysis) analysed using a universal testing machine
- controlled forces applied to a food sample
- resistance and deformation properties of food measured
TPA limitations
Only homogeneous (uniform in composition throughout) food allow reproducible data (processed cheese); not useful for heterogeneous food (jam)
Factors that affect solubility (3)
Temperature, solvent and particle size
Methods of particle analysis
- Standard and electron microscopy
- Laser diffraction
- Dynamic light scattering
basis of light interaction with particles
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
laser diffraction purpose
- 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
Texture qualities of solids (4)
- springiness
- fracturability
- gumminess
- chewiness
Texture quality of solids: Springiness
- 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)
Texture quality of solids: Fracturability
- brittleness
- force at which the material fractures, the force at the 1st significant break
Texture quality of solids: Gumminess
the quantity of force to simulate the energy required to disintegrate a semi-solid food product to a state ready for swallowing
What does TPA do?
provides insight into how samples behave when chewed
- often called the “two bite test” because the texture analyser mimics the mouth’s biting action
Texture quality of solids: chewiness
the quantity to simulate the energy required to masticate a solid food to a state ready for swallowing
what is size analysis for?
for particles and droplets;
- dry powders: flour, powdered milk
- full fat milk
why is particle size important?
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
Sample prep: laser diffraction
- 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)
LAS-W
- better state of dispersion achieved in an appropriately selected liquid medium
- solution conditions can be modified by changing pH or adding chemical dispersing agents
LAS-D
- 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
large vs small droplet size
- 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
dynamic light scatering
aka photon correlation spectroscopy (PCS)
- used to characterize molecules and particles in solvent and dispersant environments
