Chapter 10: Introduction to measurement of physical properties and biological effects of food

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Chapter 10Introduction to measurement of
physical properties and biological effects of food
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Examples of important physical properties

• Particle size
• Solubility
• Water binding / holding
• Viscosity
• Gel strength
• Food thermal analysis
• Emulsification
• Flour and baking quality

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Role of physical analysis

• Important in predicting and understanding the
  function of food ingredients in
• food processing behavior
• effect on final product
• food formulation as rapid predictor of consumer
  acceptability

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Particle size

• Sieving

1000 microns 1 mm
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• Static light scattering

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Solubility / Insolubility

• Dissolving in water
• Centrifuging
• Weighing insoluble residue to give solubility
• AOAC dietary fibre method (985.29) measures
  soluble and insoluble fibre under
  physiological conditions

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Water binding / water holding

• Soaking sample in water / buffer
• Centrifuging to pellet hydrated fibre
• Decanting off free water
• Measuring bound water by increase in weight of
  hydrated fibre compared to dry fibre
• Expressed as g water / g fiber

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Rheology and texture

• Rheology is the study of how materials respond
  to applied force (Nielsen 2003, p 505)
• force, deformation and flow
• considered a component of food texture
• homogeneity is important
• Rheology evaluates
• stress (force per area) and strain (deformation
  per length)
• normal stress tension or compression directly
  perpendicular to a surface
• shear stress acts in parallel to sample surface
  
• viscosity internal resistance to flow

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Rheology and texture (cont)

• With increased shear rate fluid viscosity changes
  (time independent)
• linear viscosity increase
• viscosity diminishes, shear thinning or
  pseudoplastic
• viscosity increases, shear thickening or dilatent
• Liquids that thin thicken with time are
  thixotropic anti-thixotropic respectively
• detected by monitoring viscosity at constant
  shear rate in relation to time

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• In general yield stress is required to make
  fluids flow
• minimum force, or stress required to initiate
  flow
• Viscoelasticity materials display solid like
  (elastic) and fluid like (viscous) behaviour

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Rheometry-Rotational viscometer, units RPM

• Comparative viscosity using a Brookfield
  Viscometer
• centipoise units represents the energy required
  by the viscometer to overcome the resistance to
  stirring of the sample
• study of viscosity is part of rheology set
  reading
• Test fixture (bob) in contact with sample rotates
  and shears the sample
• as the bob moves through the sample fluid the
  viscosity impedes free rotation this determines
  shear stress at the bob surface

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Rheometry-Rotational viscometer, units RPM (cont)

• Need to choose test fixture, concentric cylinder,
  cone or plate
• cylinder good for low viscosity fluids, large
  sample required
• cone or plate good for medium and high viscosity
  samples small sample required
• Shear rate at a constant temperature

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Role of physical properties in the technological
functionality of food

• Particle size of insoluble dietary fibres related
  to acceptability of high fibre products
• High solubility of whey protein powders required
  for powdered beverages and nutritional
  supplements
• Specific viscosities required for protein
  ingredients and starches in food formulations
  particularly beverages, sauces, toppings

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Role of physical properties in the physiological
effects of food

• Particle size of insoluble dietary fibres
  influences their effect of bowel transit time
  which is related to risk of bowel dysfunction
• Highly soluble fibre are often highly viscous in
  GI tract and highly fermentable in colon

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Rheometry-solids compression, extension and
torsion

• Strength
• Measured in gels made from protein, starch and
  gums by
• energy required to compress gel
• energy required to penetrate gel
• texture profile analysis

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Gel strength

• Gel strength important in product development to
  provide correct texture for high consumer
  acceptance
• processed meat product, desserts, confectionery
• Oil binding of vegetable protein, starch and
  fibre ingredients necessary in meat analogues

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Food thermal analysis

• Techniques that measure chemical or physical
  changes of a substance subjected to controlled
  temperature over time (Nielsen 2003, p 519)
• natural polymers such as amylose and amylopectin
  or actin and myosin
• total combustion of food to determine total
  mineral and caloric content
• Differential Scanning calorimetry (DSC) is used
  extensively in food thermal analysis
• involves measurement of heat absorbed or given
• endothermic and exothermic

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Dynamic thermal analysis Calorimetry - DSC

• Determination of heat absorbed (endothermic) or
  given (exothermic) when a definite amount of
  material undergoes a chemical or physical change
  (Neilson 2003, p520)
• If test and inert reference samples are heated or
  cooled concurrently under identical conditions
• test sample temperature will be either higher or
  lower than the reference

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Differential Scanning calorimetry

• This technique records difference in energy
  influx needed for zero temp. difference between
  sample reference material against time or temp.
• subject to identical heating or cooling regimes
• Measures temperature and enthalpy (?H) of
  transition
• sample size between 6-12mg
• slow rate of heating 1-10?C / min

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Differential Scanning calorimetry
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Differential Scanning calorimetry wheat starch
thermogram
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Analysis of food emulsions

• Emulsion, two immiscible liquids (oil water)
  with one liquid dispersed as small spherical
  droplets in the other
• oil in water milk, cream, mayonnaise, salad
  dressing
• water in oil margarine, butter spreads
• Appearance, texture and stability of these
  products depend on
• composition, microstructure and colloidal
  interactions

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Emulsion definitions

• Dispersed or internal phase
• substance within the droplets
• Continuos or external phase
• substances of surrounding liquid
• Process of converting water oil into an
  emulsion is called homogenisation
• may be mechanical, ultrasonic or a colloidal mill
• For emulsion kinetic stability - days, months
• use emulsifiers and / or thickening agents

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Emulsion stability
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Emulsifying Capacity-water soluble emulsifiers

• Defined as maximum amount of oil dispersed in
  aqueous solution containing specific amount of
  emulsifier with out the emulsion breaking down or
  inverting
• slowly add oil to aqueous suspension of protein
  whilst blending
• stable emulsion will form
• increase in viscosity
• no separation of oil and water phase
• endpoint is collapse of emulsion
• viscosity suddenly drops
• oil and water phases suddenly separate

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Emulsion stability index

• Centrifuge emulsion at given speed time to
  predict the stability of an emulsion to
• creaming by using low speed
• coalescence by using speeds high enough to
  rupture the interfacial membranes
• may not reflect emulsion instability under normal
  storage conditions
• does not take into account chemical biochemical
  reactions
• Quantitative method
• measure emulsion particle size distribution
• laser particle size analysis
• measured under similar conditions
• pH, ionic strength, composition, temperature

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• Measure emulsion surface tension
• emulsifier adsorption, packing of emulsifier
  molecules at interface, critical micelle
  concentrations surface pressure increase
• Surface tensions is measured by tensiometers

Nielsen 1999 pp 578
Coultate, 2002
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Flour quality

• Falling number
• measure of ?-amylase activity
• breaks down starch reduced viscosity of heated
  flour / water suspension
• suspension heated to 100?C, stirred for 60 sec.
• measure time for plunger to fall through
  suspension (250 sec. acceptable for bread)
• high levels of ?-amylase
• weaken bread structure, soft sticky crumb,
  difficult to slice, softens dough and reduces
  amount of water added during mixing

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• Colour test, indication of flour whiteness
• indicates colour of endosperm
• affects colour of final bread
• indicates amount of bran remaining
• The Flour Colour Grade (FCG) is produced by
• placing a flour paste in a glass cell
  reflecting / measuring light at 540nm.
• low FCG corresponds to whiter flour
• FCG affected by, variety, fungal contamination
  improper grinding and sieving

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Test baking

• Slow, expensive, needs highly trained staff
• 1 to 2 kg flour is mixed and baked via standard
  method
• loaves produced are compared to standard control
  flours
• Key loaf performance indicators
• volume (seed displacement)
• hight (ruler)
• visual assessment (under standard light)
• colour (trained expert score 1-10)
• texture (trained expert score 1-10)
• good texture score dense, fine bubbles with
  uniform size distribution

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Test backing
Coarse crumb
Crust too thick
Good texture
Side wall collapse
Loaf small volume
Coring
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• Small Volume
• increase yeast level optimise dough development
  increase dough weight increase proof time
• Crust too thick
• reduce gluten level optimise dough development
  reduce pan greasing agent increase humidity in
  final proof increase oven temperature
• Side wall collapse
• adjust level of bread improver avoid
  over-proofing increase baking temperature and /
  or time depan immediately once out of the oven

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• Coarse Crumb Texture
• use a suitable bread improver at a correct level
  Optimise dough development Adjust floor time /
  intermediate proof Check moulder setting and
  conditions
• Coring near crust
• adjust moulding technique Lower level of pan
  greasing agent Use cooler bread pans Avoid
  dough skinning during final proof Correct
  proofing conditions