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Food Chemistry 1 > P3. Milk proteins > Flashcards

P3. Milk proteins Flashcards

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1
Q

7 common sources of proteins?

A
  1. milk
  2. meat
  3. eggs
  4. fish
  5. cereals
  6. soybean
  7. pulses (pea, lentil, chickpea)
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2
Q

each protein system is _________ and its properties also depend on what? (2)

A
  • unique
  • processes used to extract or isolate it
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3
Q

fluid milk contains __% solids, of which ___% is protein

A

12% solids
- 3.5% proteins

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4
Q

milk proteins have high _________ value (what does it mean?), surpassed only by ____ _________ and are a good source of ________ (an _________ aa)

A
  • nutritive value (good aa balance)
  • by egg albumin
  • lysine (essential aa)
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5
Q

2 groups of milk proteins?

A
  • casein
  • whey proteins
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6
Q

_________ is a general term for milk proteins and have an isoelectric point at pH _____ –> isolated by adjusting pH of milk to _____ using _______
- proteins left in solution are termed ______ proteins

A
  • casein
  • 4.5
  • 4.5 using acid
  • whey
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7
Q

% of total milk protein:
- caseins
- whey proteins

A
  • casein: 83%
  • whey proteins: 17%
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8
Q

what are the main types of casein in order of highest to lowest percentage of caseins?

A
  1. a(s1) = 44%
  2. b = 25%
  3. k (kappa) = 14%
  4. a(s2) = 11%
  5. y (gamma) = 5%
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9
Q

main types of “whey proteins” in order of highest to lowest percentage?

A
  1. beta-lactoglobulin = 58%
  2. a - lactalbumin = 13%
  3. immunoglobulin = 12%
  4. minor proteins = 12%
  5. serum albumin
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10
Q

which whey protein are 7% of population allergic to?

A

beta-lactoglobulin

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11
Q

in 1877, O Hammarsten distriguished (how many) proteins in milk: name them

A
  • 3!
  • casein + lactalbumin +lactoglobulin
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12
Q

O Hammarsten outlined the following procedure for the isolation of casein: (2 steps ish)

A
  1. skim milk is diluted, then acidified with acetic acid
  2. casein flocculates, while whey proteins stay in solution
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13
Q

specific property of casein: it is __________ in ______ _______ media

A

insoluble in weakly acidic media

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14
Q

casein was subsequently established to be a ___________ with _______ being attached to the aa ________

A
  • phosphoserine
  • phosphate being attached to serine
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15
Q

in 1939, casein was shown by _________ to consist of 3 fractions: ?

A
  • electrophoresis
  • alpha, beta and kappa
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16
Q

how are different types of casein differentiated? (3 ish)

A
  • isoelectric point
  • molecular weight (different aa sequence)
  • phosphorus content
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17
Q

North American cows have which variants?

A

A and B (vs C and D are more Australian)

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18
Q

y-casein are the products of proteolysis of __________

A

beta-casein

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19
Q

in milk, most of casein proteins are present in _________ particles called ________ __________

A
  • colloidal particles
  • casein micelles
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20
Q

casein micelles are responsible for the ________ ______________ of milk, an optical effect due to ______ ___________

A
  • opaque whiteness of milk
  • due to light scattering
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21
Q

micelles are very complex structures: ____________ assemblies made up of the different _______ _________

A
  • supramolecular assemblies
  • different casein fractions
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22
Q

casein micelles __________ if Ca2+ is dialyzed from solution –> will reform if Ca2+ is ??

A
  • dissociate
  • if Ca2+ is added back
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23
Q

what is a submicelle? vs a micelle?

A
  • submicelle: 1 b + 1 a(s1) + 1 a(s2) + 1 k –> 10 nm
  • micelles = lots of submicelles together –> 20-300 nm
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24
Q

alpha (s1) - casein:
- how many aa?
- which aa distributed all along peptide chain –> hindering formation of ?
- contain ___ phosphoserine residues
- how many phosphoserine residues localized btw positions ____ and _____ –> this portion also contains 12 ? = very _________
- aa 100-199 are _______ = responsible for ?

A
  • 186-199 aa
  • proline –> hinder formation of ordered structures
  • 8 phosphoserine residues
  • 7 localized between 43 and 80 –> also contains 12 carboxyl groups (glutamic and aspartic residues) = extremely polar
  • apolar = responsible for strong self-association tendencies of a(s1)-casein/hydrophobic interactions (despite repulsive forces btw phosphate groups and lack of cysteine residues)
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25
Q

where can Ca2+ aggregate in a(s1) casein?

A

in the polar portion of the peptide chain (btw 43-80)

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26
Q

in the presence of Ca2+ ions at levels found in milk, a(s1) casein forms ?

A

insoluble Ca-salt

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27
Q

a(s2) casein
- how many aa?
- ordered or not?
- ___ phosphoserine and ___ cysteine residues
- precipitates with Ca2+ more/less easily than a(s1) casein

A
  • 207
  • more ordered structure than a(s1), more alpha helices
  • 11 phosphoserine + 2 cysteine
  • more easily!
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28
Q

b-casein:
- most ___________ casein
- has (2) thus resembling _____-like molecules
- ___ phosphoserine residues are localized btw positions __ and ____ of the peptide chain
- precipitates in presence of Ca2+ are what levels?

A
  • hydrophobic
  • polar head + apolar tail –> soap-like molecule
  • 5 phosphoserine btw 1 and 40
  • at levels found in milk
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29
Q

y-casein:
- _________ products of __-casein formed by milk _______
- obtained by cleavage of residues __-___
- if you have y-casein = not good ?

A
  • degradation products of b-casein
  • residues 1-28
  • pasteurization: proteases not deactivated enough
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30
Q

kappa-casein:
- how many aa residues
- ____ prolines + ___ cysteines + ___ phosphoserine
- normally occurs as a ______ or higher _________
- self-association involves formation of ______ ______ btw the _______ residues
- what molecules (3 examples) are bound to peptide chain through ________ residues (at position _____, _____ and ____ in variant __)

A
  • 169 aa
  • 20 prolintes, 2 cysteine, 1 phosphoserine
  • trimer or higher oligomer
  • disulfite bonds btw cysteine residues
  • carbohydrates (galactose, galactosamine, N-acetylneuramic acid) –> through threonine residues (131, 133, 135 of variant A)
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31
Q

numb of carb molecules bound to k-casein peptide chain is ________
- ie for N-acetylneuramic acid, galactose, galactosamine

A
  • variable:
  • N-acetylneuramic acid: 0-3 moles
  • galactose: 0-4 moles
  • galactosamine: 0-3 moles
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32
Q

kappa-casein is the only constituent of casein that remains ______ in the presence of ___ ions at concentrations found in milk

A

soluble
- Ca2+ ions

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33
Q

3D model for k-casein –> described as ? model (3 parts)
- this model suggests that kappa-casein contains approx __% a-helix and __% b-structure

A
  • horse and rider
  • N-terminal = horse
  • C-terminal = rider
  • 2 legs = beta sheets
  • 16% a-helix + 27% b-structure
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34
Q

which sections of k-casein are very hydrophobic? –> role?

A
  • leg sections
  • postulated to be area of interaction with other caseins
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35
Q

only up to __% of the total casein present in milk is in the form of _________, usually designated as _______ caseins

A
  • 10%
  • monomers
  • serum caseins
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36
Q

the majority of casein proteins __________ to form casein complexes and micelles. the extent of __________ is dependant on ____ concentration and ___

A
  • aggregate
  • aggregation
  • Ca2+ concentration and on pH
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37
Q

which constituent of casein remains soluble in present of Ca2+ ions at concentrations found in milk?

A

k-casein

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38
Q

aggregation of (2 types of casein) with __-casein prevents/accelerates their coagulation in present of Ca2+ ions

A
  • a(s1) and b-caseins with k-casein
  • prevents (the formation of really really big micelles that could hurt the cow when milking)
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39
Q

what is the utmost important property for formation of stable casein complexes and casein micelles as occurs in milk?

A

the property of k-casein to prevent coagulation in presence of calcium ions

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40
Q

3 forms of casein structure (NOT a, b, k and y)
________ –> ________ –> ________
5 properties that will increase formation on 1 side vs the other

A
  • monomers (soluble caseins) –> casein complex –> micella
  • to form casein complex and micella:
    1. lower pH (increase H+)
    2. increase Ca2+
    3. remove citrate
    4. remove phosphate
    5. increase temperature
    (inverse of these 5 steps to dissociate casein complex)
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41
Q

dialysis of casein complexes against a _______ agent (ex?) would shift equilibrium towards __________ vs against a high Ca2+ concentration, shift would be to _______ ________

A
  • chelating agent (EDTA)
  • monomers
  • high ca2+ concentration –> large micelles
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42
Q

diameter of micelles ranges from __ to ____ nm, with a mean of _____, corresponding to volume of ____ x 10^__ nm^3

A
  • 10-300 nm
  • 150 nm
  • 1.6 x 10^6 nm^3
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43
Q

average of how many monomers per micelle?

A

400 - 40 000 monomers per micelle

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44
Q

micelles are highly _______ and hence are _____

A
  • highly solvated
  • are porous
45
Q

casein micelles are kept together/casein stability:
(3 interactions)

A
  1. hydrophobic interactions: minimal at temperature below 5°C
  2. electrostatic interactions, mostly as calcium and calcium bridges between phosphoserine and glutamic acid residues
  3. hydrogen bond
46
Q

what disrupts the electrostatic interactions between calcium & PO4 and calcium & COO-? how?

A

additions of acetic acid! (decrease in pH)
- neutralizes negative charges –> adding acid will protonate the side chains

46
Q

what disrupts the electrostatic interactions between calcium & PO4 and calcium & COO-? how?

A

additions of acetic acid! (decrease in pH)
- neutralizes negative charges –> adding acid will protonate the side chains

47
Q

what are the 2 types of submicelles that exist?

A
  • one contains k-casein
  • the other does not
48
Q

characteristics of submicelles that contains k-casein (2)
- aggregation of submicelles proceeds until what?
- k-casein coverage is around __ nm thick = provide _________

A

1) k- casein are arranges on surface of submicelles
2) hydrophilic c-terminal domains protrude like hairs from surface, preventing further aggregation
- until entire surface of forming micelle is covered with k-casein
- 5nm –> provide stability

49
Q

are k-caseins also found inside the micelle?

A

yes

50
Q

cleavage of k-casein causes casein proteins to _______ = ______ formation
- this is the underlying principle of use of ________ in cheese making

A

coagulate
- curd formation
- rennet

51
Q

rennet is a complex of _______ produced in _____ stomach of cow that contains what?

A
  • enzymes
  • fourth stomach
  • contains precursor of enzyme chymosin (rennin)
52
Q

what is chymosin?

A

chymosin = acid protease having specificity for k-casein
(cleaves k-casein from micelle and casein complex)

53
Q

chymosin specifically recognizes sequence in k-casein chain from _____-___ to ___-_____ and selectively cleaves the peptide bond between _____-____ and ____-____

A
  • His-98 to Lys-111
  • cleaves Phe-105 and Met-106
54
Q

k-casein cleaved into which 2 fragments?

A
  • para-k-casein
  • glycomacropeptide
55
Q

2 fragments of k-casein: which is soluble and which one precipitates?

A
  • glycomacropeptide is soluble
  • para-k-casein precipitates in presence of Ca2+ ions
56
Q

2 ways to precipitate casein:
1. how? produces ________ = first step in ?
2. how? produces __________ –> for use as ?

A
  1. chymosin reaction –> produces paracasein –> first step in cheese making
  2. addition of acetic acid. produces acid casein or isoelectric casein
    - for use as functional ingredient
57
Q

one similar characteristics for acid/isoelectric casein and paracasein?

A
  • similar physically
  • not temp sensitive
  • can boil them bc very little 2° structure
58
Q

which types of whey proteins are mainly used in food science? (2) what are the other 3 types?

A
  • b-lactoglobulin
  • a-lactalbumin
    NOT used:
  • bovine serum albumin
  • immunoglobulin
  • minor proteins
59
Q

percentage of total milk protein, isoelectric point and molecular weight of b-lactoglobulin and a-lactalbumin

A

b-lactoglobulin:
- 10% of total milk protein
- 5.34 isoelectric point
- 18.3 kDa
a-lactalbumin:
- 2% of total milk protein
- 4.8 isoelectric point
- 14.1 kDa

60
Q

which milk protein is also sold as a nutraceutical/medical claims?

A

alpha-lactalbumin

61
Q

beta-lactoglobulin:
- ____% of whey protein fraction
- how many aa? molecular weight?
- what differs between genetic variant A and B?
- ___ disulfite bond + ___ free sulfhydryl group + ____ phosphorus

A
  • 58%
  • 162 aa –> 18.3 kDa
  • glycine in variant B replaced by aspartic acid in variant A
  • 2 disulfite bond + 1 free sulfhydryl group + 0 phosphorus
62
Q

describe the 2 disulfite bonds of b-lactoglobulin

A

one is fixed between Cys-66 and Cys 160
- the second one is dynamic: either Cys-106 with Cys-119 (+ free sufhydryl Cys 121) OR Cys-106 with Cys 121 (+ free sufhydryl Cys 119)

63
Q

the 2° structure of beta-lactoglobulin is homologous to that of __________-binding proteins

A

retinol

64
Q

beta-lactoglobulin:
- ___ strands of beta-structure –> ___ of them arranged to form a ________ and a single ________ that resembles a _____ on the ___________
- the center of the _________ is hydrophobic/hydrophilic and can be involved in binding of what?

A

9 strands of beta structure
- 8 of them form a beta-barrel
- single alpha helix resembles a lid on the beta-barrel
- center of the barrel is hydrophobic + involved in binding of hydrophobic molecules (pharma tried to hide drugs in the barrel)

65
Q

although b-lactoglobulin is manufactures specifically in _________ ______ for inclusion in _______, its biological role is ___________

A
  • mammary gland for inclusion in milk
  • unknown
66
Q

b-lactoglobulin’s __________ pocket is quite effective in binding __________ and it has been speculated that the binding of ________ may have a regulatory role in __________ gland

A
  • hydrophobic
  • retinol
  • binding of vit. A
  • mammary
67
Q

because of its prevalence in _______ milk, to a large extent, the properties of whey protein ________ and whey protein ________ employed in the food industry are in effect, the properties of ____________

A
  • bovine
  • concentrates
  • isolates
  • beta-lactoglobulin
68
Q

alpha-lactalbumin:
- ___% of total milk protein vs ____% of total whey protein
- ____ aa + MW of ?
- ___ disulfite linkages and _____ phosphate groups

A
  • 2% of total milk VS 13% of total whey protein
  • 123 aa + 14.16 kDa
  • 4 disulfite links and no phosphate groups
69
Q

alpha-lactalbumin variant A vs variant B?

A

variant A: glutamine (Gln)
variant B: arginine (Arg)

70
Q

disulfide bonds of alpha-lactalbumin are between which cys residues?

A
  • 6 and 120
  • 28 and 111
  • 61 and 77
  • 73 and 91
    (not important though)
71
Q

role of alpha-lactalbumin?
- what happens if absence of a-lactalbumin?

A

modify activity of enzyme galactosyl transferase (a-lactalbumin is a cofactor ish)
- galactosyl transferase adds UDP-galactose to N-acetylglucosamine groups that are attached to proteins

72
Q

enzyme galactosyl transferase can transfer _______ to _______, but the Km for glucose is _____ mM and thus the reaction proceeds slowly/fast
- alpha-lactalbumin serves to increase/lower the Km for _________ to __ mM and the enzyme complex will not add ________ to ________ to produce ________ and UDP

A
  • UDP-galactose to glucose
  • 1400 mM
  • slowly, if at all
  • lower the Km for glucose to 5 mM
  • add UDP-galactose to glucose to produce lactose and UDP
73
Q

milk of all mammals that contains lactose also contains ___________

A

alpha-lactalbumin

74
Q

40% of residues btw a-lactalbumin and _________ are the same, including all _______ residues
- another ___% have similar ___________

A
  • lysozyme
  • cysteine
  • 20% have similar structures
75
Q

similar structure btw a-lactalbumin and lysozyme coupled with the fact that a-lactalbumin helps what? suggests that the molecules are closely related

A

that a-lactalbumin helps to synthesize same linkage that lysozyme cleaves
- glycosyl linkage

76
Q

do a-lactalbumin and lysozyme work on same substrates and are related antigenetically?

A

no and no

77
Q

a-lactalbumin is unusual in that the molecule is more/less stable to heat in presence of ________ whereas proteins show increased/decreased heat sensitivity in the presence of _________

A
  • more stable in presence of calcium
    vs
    increased heat sensitivity in presence of calcium (ie b-lactoglobulin will precipitate with calcium and heat)
78
Q

increased sensitivity to heat of most proteins is probably due to ability of calcium to promote what? with most proteins. these __________ hold the molecules in proximity and increase/decrease the likelihood of aggregation upon heating
VS a-lactalbumin uses calcium to form what?

A
  • promote formation of ionic intermolecular cross links with most proteins
  • these crosslinks
  • increase
  • to form intramolecular ionic bonds that tend to make molecule resistant to thermal unfolding
79
Q

under favorable conditions of ______ and ____, a-lactalbumin can remain soluble after exposure to temperatures up to ______°C

A
  • calcium and pH
  • 100°C
80
Q

a-lactalbumin has a lot of _________ for 2° structure

A

helical structures

81
Q

bovine serum albumin (BSA) isolated from milk is identical to BSA isolated from _______ _______

A

blood serum

82
Q

how does BSA enter the milk?

A
  • BSA not synthesized in mammary gland but rather enters milk through bloodstream (diffusion)
83
Q

BSA:
- MW?
- ___ phosphorus + ___ disulfides + __ free sylhydryl group

A
  • 69 kDA
  • no phosphorus + 17 disulfides + 1 free sylhydryl group
84
Q

function of albumin in blood plasma?
- molecule has specific binding sites for ______ molecules and may bin them in _____ as well

A
  • carrier of free fatty acids
  • hydrophobic
  • milk
85
Q

milk contains a great number of ________. the extent of heat activation/inactivation of enzymes indicates type and extent of heating to which milk has been subjected
- ex.:

A
  • enzymes
  • inactivation
  • lipase
86
Q

casein and caseinates are widely used in food processing for (2)

A
  • protein enrichment and/or
  • achieve stabilization of certain physical properties of processed meats, baked products, candies, cereal products, ice creams, whipping creams and coffee whiteners (and cheese)
87
Q

what are the non-food uses of casein and caseinates? (4)

A
  • manufacture of paper as sizing/coating to improve adhesion of printer’s ink to surface of paper
  • glue manufacturing as a type of waterproof glue
  • textile industry to make fabrics water-repellent
  • production of certain plastics
88
Q

for the recovery of casein from milk, coagulation is achieved by _________ or __________ __________

A
  • acidification
  • enzymatic treatment
89
Q

how is acid coagulation achieved (to recover casein from milk)?
- results in what type of casein?

A
  • achieved by lactic acid fermentation
  • OR by direct addition of acids such as HCl, H2SO4, lactic acid or H3PO4 to milk at 35-50°C to attain pH of 4.2-4.6, at which casein precipitates out as coarse grains
  • acid casein
90
Q

what is the process of enzymatic treatment of milk? (2 steps)
- results in what type of casein?

A
  • addition of protease enzymes such as chymosin and pepsin to the milk at 45°C
  • in cheese production, the enzyme causes coagulation of casein
  • to terminate the process, milk is heated to 65°C to inactivate enzyme (pepsin)
  • rennet casein
91
Q

what happens after coagulation of casein?

A
  • casein is separated form milk in a sedimentation centrifuge, washed and dried (whirlwind drier)
92
Q

to produce a casein product that is soluble or readily dispersible: casein is dispersed in _____ and treated with ______ (5 ex.) at ___-____°C and pH ___-___.
- the solubilized product (2) is then ________.

A
  • water (20-25% w/w)
  • treated with alkali (NaOH, Ca(OH)2, alkali or alkaline-earth carbonates or citrates
  • at 89-90°C
  • pH 6.2-6.7
  • sodium caseinate or calcium caseinate
  • spray dried
93
Q

why accumulates in considerable amounts in production of ________. until recently, whey was considered a __________ by-product of __________ production, in part bc whey is too high in ______ to be consumed directly in large amounts and was dumped as _____________

A
  • cheese
  • valueless by product of cheese
  • too high in salt
  • wastewater
94
Q

whey byproducts of cheese-making:
although _______ methods were available, costs were too high/low in relation to value of product. however, in recent years tighter __________ regulations have required expensive waste treatment in most jurisdictions

A
  • desalting
  • too high
  • environmental
95
Q

what allowed recovering proteins from whey?
- take advantage of (2) of whey protein
- what are spray-dried whey and whey produces used in? (7)

A
  • improved technological processes
  • high nutritional quality + excellent functional properties of whey protein
  • infant formula, baked goods, confectionery, candies, beverages, animal feed and pet food
96
Q

whey’s solids composition is especially high in ________ –> making them super _______
- what has higher protein and lower lactose?
- what is mostly protein and no lactose?

A
  • lactose –> super cheap
  • whey protein concentrates (1-2x ultrafiltration fo lower lactose)
  • whey protein isolates
97
Q

3 steps for drying whey
- processing costs may be reduced by preliminary concentration of why to __-___% dry matter by what?

A
  1. concentration of whey to 50-55% dry matter in falling-film evaporator (thermal or mechanical vapour compression)
  2. spray drying (1 or 2 step)
  3. subsequent drying on a fluidized bed drier
    - 21-25% dry matter by reverse osmosis using a perm-selective membrane that holds back the protein, lactose and salts
98
Q

what is reverse osmosis? (other word for it?)
- goal?

A
  • water is forced through membrane by applying pressure exceeding osmotic pressure (which would normally water back across the membrane)
  • hyperfiltration
  • to force lactose out! but keep protein
99
Q

how are whey protein concentrates produced?
2 methods
- downfall of the 2nd method?

A

more porous membrane employed to hold back proteins but allows lactose and salts to pass through with water
VS
less gentle method: heating whey (95°C for 3-4min) by direct steam injection followed by precipitation of denatured proteins at pH 4.5 + separation in a sedimentation centrifuge and drying
BUT highly detrimental to functional properties of whey proteins

100
Q

how is lactose recovered from whey protein concentrates?

A
  • filtrate obtained in ultrafiltration step can be concentrated by reverse osmosis using a membrane that selectively holds back lactose while allowing salts/smaller molecules to pass through
101
Q

once whey protein concentrates and lactose are isolated, what is done to them?

A
  • whey protein concentrates: seeded and cooled slowly to induce sugar crystallization
  • lactose may be recrystallized to yield a raffinade (pharmaceutical-grade lactose), which is used as a filler in tablets
102
Q

Lactose (isolated from whey protein) is used in manufacturing of (5)

A
  • dietary food products
  • baked products dehydrated food
  • cocoa products
  • beverages
  • ice creams
103
Q

whey protein isolates require extensive processing (3) to obtain highly purified protein

A
  • ultrafiltration
  • microdialysis
  • and/or ion exchange
104
Q

whey protein isolates are relatively __________ to produce + are used primarily in high value-added production of _____ ________ ___________

A
  • expansive
  • whey protein hydrolysates
105
Q

how are whey protein hydrolysates produces?

A
  • enzymatic treatment of whey protein isolates to break proteins down into smaller peptides that are more easily digested while nutritional quality of protein remains very high
  • cut down b-lactoglobulin into smaller fragments
106
Q

why are whey protein hydrolysates often used as protein source in infant formulas?

A

bc believed to be less likely to trigger allergic reactions than non-hydrolyzed whey protein
- bc beta lactoglobulin is cut down

107
Q

do dairy proteins have a lot of properties as ingredients in food processing?

A

yes!
emulsification, stabilization, aeration, film formation, opacity, water/fat binding, texturization, thickening, heat stability, gelation, acid solubility, flavor development, browning

108
Q

typical formulation of imitation cheese? (6 ingredients)

A
  • water (51.1%)
  • Ca/Na caseinate (26%)
  • vegetable oil (18%) to give texture
  • glucono-delta-lactone (emulsifier and flavor) (2.8%)
  • salt (2%)
  • color and aroma substances

Food Chemistry 1 (22 decks)

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