Transcript milk composition: proteins - Iowa State University: Animal Science

MILK COMPOSITION:
PROTEINS
Leo Timms
Iowa State University
MILK PROTEINS
• Synthesized in the mammary gland from
blood amino acids ( except serum albumin
and most immunoglobulins)
• CASEIN: major class of milk proteins
• whey proteins
• non protein nitrogen / others
• Species variation: 1% (primates) - >15%
• Limited variation within species/ animal
Protein Composition*
Protein
cow
human
sow
Total %
3.2-4.2
.88-1
6
Casein %
79
35
58
Whey protein %
21
65
42
Alpha-lactalbumin 3.5
17
15
Beta lactoglobulin
9
0
2
Lactoferrin
Immunoglobulin
Serum albumin
<1
3
1
17
11
6
1
7
2
CASEINS
• Major milk proteins (~ 80%)
• Excellent amino acid composition
• Highly digestible
• Secreted as a micelle (colloid + P + Ca)
• Acid precipitation at pH 4.6
• (normal milk pH 6.6-6.9)
• Micelle hydrolyzed by rennin (curd)
• Variants: alpha, beta, gamma, kappa
CASEINS *
Casein
alpha
beta
gamma
grams/ liter 13.7
6.2
phos. (P) % 1.05
.6
.15
.3
mol. wt.
24K
20K
19K
5
1
1
24K
Pi/ molecule 8-10
Carbohydrate %
1.2
kappa
3.7
5
NON - CASEIN PROTEINS
Protein
function
Conc. mg/l
alpha lactalbumin lactose syn.
700
beta - lactoglobulin*
???
3000
albumin **
like blood
300
Immunoglobulin ** immune prot.
600?
Lactoferrin
iron binding
18
Ceruloplasmin
Cu carrier
low
Protein hormones
Prl / PL
~(blood)
Other enzymes / growth factors/ etc.
low
* not in humans
** mainly blood derived
NON - PROTEIN NITROGEN
Compound
Total protein
Total NPN
Urea - N **
mg / l
30,000
~ 300
~ 150
** Mimics blood urea - excellent
diagnostic tool esp. for ruminants
The value of milk protein
Nomenclature and relative amounts
Fraction
as-1 - Casein
as-2 - Casein
b - Casein
k - Casein
b - Lactoglobulin
a - Lactalbumin
Blood serum albumin
Lactotransferrin
Immunoglobulins
% of protein
34-40
11-15
25-35
8-15
7-12
2-4
.5-2
trace
trace
AMINO ACIDS
• What do the blood amino acids look like
that get to the mammary gland?
• diet = mammary gland (non-ruminants)
• diet is different than mammary gland in
ruminants
• Rumen degradeable protein - Ammonia bacterial protein (CHO) or blood urea
• Rumen by-pass protein/AA = mammary
gland!
LIPIDS OR FATS
AMINO ACID (nutrient)UPTAKE
•
•
•
•
Arterial concentration of amino acids
Rate of mammary blood flow
Extraction rate of amino acids by gland
Measure venous concentration of AA
• ** Arterio venous difference (AVD)
*** AVD X blood flow rate = total nutrient
extracted
AMINO ACID (nutrient)UPTAKE
PROTEIN SYNTHESIS
• DNA
transcription
messenger
• RNA
ribosomal
transfer
translation
• PROTEINS
PROTEIN SYNTHESIS
ATP
AMP + P + P
PROTEIN SYNTHESIS
Attach + move AA on chain
2 GTP
2 GDP + 2 P
PROTEIN SYNTHESIS
GOLGI APPARATUS
casein + ATP
casein-P + ADP
(casein kinase)
Assembled into micelles
Casein-P + Ca
140 nanometers
secretory vesicles
CASEIN MICELLES
90% of Ca in micelle
MILK COMPOSITION
Casein micelles
Basement
membrane
Apical membrane
The value of milk protein
Amino acids
 20 possible amino acids – Connected via peptide
bond to form protein
+
+
+
 Casein is a phospho protein
(phosphorus is linked to a serine
amino acid)
+
+
P
P
P
Casein micelle structure –
calcium phosphate
interactions
Casein phosphate
centers act as nucleation
sights and several
peptide chains inhibit the
growth of calcium
phosphate microgranules
Proposed that 66 calcium
and 66 phosphates (1/3
organic) ions along with
water form calcium
phosphate microgranules
Adapted from Heat Induced Changes in Milk – 1995 Holt 105-133
Casein micelle structure – proposed model
10,000 polypeptide chains
of the four caseins
micro-granules of
calcium phosphate
glyco-macro peptide
portion on k-casein is
concentrated on the
surface
rennet coagulation remove hairs
Adapted from Adv. Prot. Chem – 1992, Holt 43:63-151
The value of milk protein
Casein vs whey protein
The value of milk protein
Casein vs whey protein
 Separated during cheese making
Cheese Structure – at set
Adapted from Kiely et al 1992
 The protein matrix contains embedded fat and moisture
 Protein matrix is cross-linked by calcium and phosphate
Cutting and removal of whey:
10 lbs milk
1 lb
cheese
9 lb
whey
The value of milk protein
Milk components
Protein is considered to be the most valuable
World demand for dairy protein currently
exceeds the world supply – will continue for
the next decade
Water Lactose
Fat
Protein Minerals
Whole Milk 87.6%
4.8%
3.7%
3.2%
.70%
Whole Milk
(dry basis)
---
38.7%
29.8%
25.8%
5.6%
Skim Milk
90.9%
4.98%
.05%
3.32%
.73%
Skim Milk
(dry basis)
---
54.8%
.55%
36.6%
8.0%
Fractionation of milk protein
Whey utilization
1970
Today