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FOSC202 > Physical properties of milk > Flashcards

Physical properties of milk Flashcards

1
Q

physical structures of milk

A

colloidal system
oil in water emulsion
solution

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2
Q

why is milk a colloidal system

A

casein micelles and globular protein

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3
Q

why is milk an oil-in-water solution

A

milk fat

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4
Q

why is milk a solution

A

sugar
proteins
minerals
vitamins

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5
Q

what is the Tyndall effect

A

the ability of colloids to disperse and scatter light
-when light is shined through milk you can see the path

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6
Q

why is milk fat yellow

A

beta carotene in the animal diet

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7
Q

why is milk white

A

scattering of light by colloidal particles and fat globules

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8
Q

why does whey have a greenish tinge

A

B2-riboflavin

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9
Q

why is trim milk slightly blue

A

caseins scatter blue wavelength slightly more than other colors

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10
Q

pH of raw milk

A

6.5-6.7

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11
Q

why is milk a buffer

A

diffusable salts (phosphates) and proteins

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12
Q

what is an indicator of mastitis in milk

A

slightly higher pH

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13
Q

why are somatic cell counts (SCC) counted in milk

A

to test for mastitis
-SCC are higher in milk when the cow has mastitis

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14
Q

what indicates a positive mastitis test

A

gelling of the milk and RMT solution

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15
Q

what does the electrical conductivity of milk assess

A

the total ionic content (Na+, K+, Cl-)

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16
Q

at 25 degrees what is the normal conductivity of milk

A

0.005 mho cm-1

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17
Q

why do whey and skim milk have higher conductivity than whole milk

A

fat reduces the specific conductance

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18
Q

How can conductivity be used to screen for mastitis

A

higher Na+ and Cl- in the milk of cows with mastitis

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19
Q

what else can electrical conductivity test for

A

residual cleaning agents in milk
index of milk fermentation
ohmic (electrical) heat processing

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20
Q

what happens to CaP during fermentation

A

fermentation dissolves CaP groups of casein which increases the amount of free ions increasing conductivity

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21
Q

what is the typical Eh (redox potential) of milk at 30C

A

+0.2-+0.3 volt

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22
Q

what does having a positive Eh mean

A

it wants to become negative so it’s prone to spoilage

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23
Q

what kind of Eh do heated/powdered milks have

A

negative

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24
Q

what does Eh depend on (5)

A

dissolved oxygen, ascorbic acid, riboflavin, cysteine-cysteine content, and pH

25
Q

what two tests used to check the microbial quality of milk rely on redox potential

A

methylene blue
resazurin

26
Q

what happens to milk without bacteria when methylene blue is added

A

the milk turns blue because the oxygen in milk keeps the methylene blue in the oxidised form

27
Q

what colour is the oxidised form of methylene blue

A

blue

28
Q

What happens to methylene blue when it is added to milk that has bacteria

A

the bacteria use the oxygen and the methylene blue turns into the reduced colourless form

29
Q

what does the speed at which the methylene blue turn colorless relate to

A

the amount of bacteria present in the milk

30
Q

difference between resazurin and methylene blue test

A

resazurin is more sensitive and gives an indication of pH

31
Q

what color is resazurin at pH 3.8

A

orange/yellow

32
Q

what color shows the best quality of milk when tested with resazurin

A

slate blue (no color change)

33
Q

what colour indicates a pH above 6.5 (resazurin test)

A

purple

34
Q

what is responsible for the surface tension of the milk

A

beta-casein + protease peptones (proteolytic products)

35
Q

what is surface tension

A

work required to increase the surface area

36
Q

lactose and salt ______ ST while fat ______ it

A

increases
decreases

37
Q

surface-active properties of milk are relevant for… (4)

A

creaming
fat globule membrane function
foaming
emulsifier use in dairy products

38
Q

what processing methods tend to increase surface tension

A

heating
sterilisation
homogenization
shear (rubbing/missing things through milk)

39
Q

what is the specific heat

A

amount of heat required to raise 1g of milk by 1C

40
Q

what happens to milk as it becomes more concentrated

A

thermal conductivity decreases

41
Q

what is thermal processing used for

A

extend shelf life
increase WHC
reduce synereisis

42
Q

why is milk heat stable

A

protein and salt content

43
Q

How does foaming affect the heat efficacy of heat transfer

A

decreases it

44
Q

what is a rheological property

A

how materials behave when heat is applied to them

45
Q

why is milk considered a Newtonian fluid

A

no matter how much shear force is applied the viscosity remains the constant

46
Q

what does the viscosity of milk depend on

A

-concentration and degree of hydration of casein micelles
-temperature
-concentration and size of fat globules

47
Q

what are colligative properties

A

physical changes that can result from adding solutes to a solvent

48
Q

examples of colligative properties

A

vapour pressure lowering
boiling point elevation
freeezing point depression
osmotic pressure

49
Q

examples of colligative properties

A

vapor pressure lowering
boiling point elevation
freezing point depression
osmotic pressure

50
Q

which colligative properties does milk have

A

boiling point elevation
freezing point depression
osmotic pressure

51
Q

what does osmotic pressure depend on

A

osmotic concentration
-total concentration of all solutes in a solution

52
Q

the freezing point of milk can be used to test ?

A

if it has been diluted

53
Q

which has a lower freezing point yoghurt or milk

A

yoghurt because the lactose has been converted to lactic acid which increases the osmotic concentration

54
Q

which has a lower freezing point yoghurt or milk

A

yoghurt because the lactose has been converted to lactic acid which increases the osmotic concentration

55
Q

what is specific gravity

A

ratio of mass of a solution/substance to the mass of a similar volume of water

56
Q

list ice-cream, whole milk and skim milk in order of increasing specific gravity

A

skim>whole>ice cream

57
Q

what is specific gravity used for

A

testing if milk has been diluted
assess total solids

58
Q

what instrument is used to measure specific gravity

A

lactometer

FOSC202 (28 decks)

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