Physical properties of milk Flashcards
physical structures of milk
colloidal system
oil in water emulsion
solution
why is milk a colloidal system
casein micelles and globular protein
why is milk an oil-in-water solution
milk fat
why is milk a solution
sugar
proteins
minerals
vitamins
what is the Tyndall effect
the ability of colloids to disperse and scatter light
-when light is shined through milk you can see the path
why is milk fat yellow
beta carotene in the animal diet
why is milk white
scattering of light by colloidal particles and fat globules
why does whey have a greenish tinge
B2-riboflavin
why is trim milk slightly blue
caseins scatter blue wavelength slightly more than other colors
pH of raw milk
6.5-6.7
why is milk a buffer
diffusable salts (phosphates) and proteins
what is an indicator of mastitis in milk
slightly higher pH
why are somatic cell counts (SCC) counted in milk
to test for mastitis
-SCC are higher in milk when the cow has mastitis
what indicates a positive mastitis test
gelling of the milk and RMT solution
what does the electrical conductivity of milk assess
the total ionic content (Na+, K+, Cl-)
at 25 degrees what is the normal conductivity of milk
0.005 mho cm-1
why do whey and skim milk have higher conductivity than whole milk
fat reduces the specific conductance
How can conductivity be used to screen for mastitis
higher Na+ and Cl- in the milk of cows with mastitis
what else can electrical conductivity test for
residual cleaning agents in milk
index of milk fermentation
ohmic (electrical) heat processing
what happens to CaP during fermentation
fermentation dissolves CaP groups of casein which increases the amount of free ions increasing conductivity
what is the typical Eh (redox potential) of milk at 30C
+0.2-+0.3 volt
what does having a positive Eh mean
it wants to become negative so it’s prone to spoilage
what kind of Eh do heated/powdered milks have
negative
what does Eh depend on (5)
dissolved oxygen, ascorbic acid, riboflavin, cysteine-cysteine content, and pH
what two tests used to check the microbial quality of milk rely on redox potential
methylene blue
resazurin
what happens to milk without bacteria when methylene blue is added
the milk turns blue because the oxygen in milk keeps the methylene blue in the oxidised form
what colour is the oxidised form of methylene blue
blue
What happens to methylene blue when it is added to milk that has bacteria
the bacteria use the oxygen and the methylene blue turns into the reduced colourless form
what does the speed at which the methylene blue turn colorless relate to
the amount of bacteria present in the milk
difference between resazurin and methylene blue test
resazurin is more sensitive and gives an indication of pH
what color is resazurin at pH 3.8
orange/yellow
what color shows the best quality of milk when tested with resazurin
slate blue (no color change)
what colour indicates a pH above 6.5 (resazurin test)
purple
what is responsible for the surface tension of the milk
beta-casein + protease peptones (proteolytic products)
what is surface tension
work required to increase the surface area
lactose and salt ______ ST while fat ______ it
increases
decreases
surface-active properties of milk are relevant for… (4)
creaming
fat globule membrane function
foaming
emulsifier use in dairy products
what processing methods tend to increase surface tension
heating
sterilisation
homogenization
shear (rubbing/missing things through milk)
what is the specific heat
amount of heat required to raise 1g of milk by 1C
what happens to milk as it becomes more concentrated
thermal conductivity decreases
what is thermal processing used for
extend shelf life
increase WHC
reduce synereisis
why is milk heat stable
protein and salt content
How does foaming affect the heat efficacy of heat transfer
decreases it
what is a rheological property
how materials behave when heat is applied to them
why is milk considered a Newtonian fluid
no matter how much shear force is applied the viscosity remains the constant
what does the viscosity of milk depend on
-concentration and degree of hydration of casein micelles
-temperature
-concentration and size of fat globules
what are colligative properties
physical changes that can result from adding solutes to a solvent
examples of colligative properties
vapour pressure lowering
boiling point elevation
freeezing point depression
osmotic pressure
examples of colligative properties
vapor pressure lowering
boiling point elevation
freezing point depression
osmotic pressure
which colligative properties does milk have
boiling point elevation
freezing point depression
osmotic pressure
what does osmotic pressure depend on
osmotic concentration
-total concentration of all solutes in a solution
the freezing point of milk can be used to test ?
if it has been diluted
which has a lower freezing point yoghurt or milk
yoghurt because the lactose has been converted to lactic acid which increases the osmotic concentration
which has a lower freezing point yoghurt or milk
yoghurt because the lactose has been converted to lactic acid which increases the osmotic concentration
what is specific gravity
ratio of mass of a solution/substance to the mass of a similar volume of water
list ice-cream, whole milk and skim milk in order of increasing specific gravity
skim>whole>ice cream
what is specific gravity used for
testing if milk has been diluted
assess total solids
what instrument is used to measure specific gravity
lactometer
