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 <1 17 1 Immunoglobulin 3 11 7 Serum albumin 1 6 2
CASEINS• Major milk proteins (~ 80% cows)• 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 kappa
grams/ liter 13.7 6.2 1.2 3.7
phos. (P) % 1.05 .6 .15 .3
mol. wt. 24K 24K 20K 19K
Pi/ molecule 8-10 5 1 1
Carbohydrate % 5
WHEY PROTEINS (non-casein)Protein function Conc. mg/l
alpha lactalbumin lactose syn. 700beta - lactoglobulin* ??? 3000albumin ** like blood 300Immunoglobulin ** immune prot. 600?Lactoferrin iron binding 18Ceruloplasmin Cu carrier lowProtein hormones Prl / PL ~(blood)Other enzymes / growth factors/ etc. low
* not in humans ** mainly blood derived
NON - PROTEIN NITROGEN
Compound mg / l Total protein 30,000 Total NPN ~ 300 Urea - N ** ~ 150
** Mimics blood urea - excellentdiagnostic tool esp. for ruminants
Nomenclature and relative amounts
traceImmunoglobulins
traceLactotransferrin
.5-2Blood serum albumin
2-4a - Lactalbumin
7-12b - Lactoglobulin
8-15k - Casein
25-35b - Casein
11-15as-2 - Casein
34-40as-1 - Casein
% of proteinFraction
The value of milk protein
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• RNA
messenger ribosomal transfer
translation• PROTEINS
PROTEIN SYNTHESIS
ATP AMP + P + P
PROTEIN SYNTHESISAttach + move AA on chain2 GTP 2 GDP + 2 P
PROTEIN SYNTHESISGOLGI APPARATUS
casein + ATP casein-P + ADP(casein kinase)
Assembled into micellesCasein-P + Ca
140 nanometerssecretory vesicles
CASEIN MICELLES
90% of Ca in micelle
MILK COMPOSITION
Apical membrane
Basementmembrane
Casein micelles
¨ 20 possible amino acids – Connected via peptide bond to form protein
The value of milk proteinAmino acids
+ + + + +
¨ Casein is a phospho protein (phosphorus is linked to a serine amino acid)
PPP
¨ 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
Casein micelle structure – calcium phosphate interactions
Adapted from Heat Induced Changes in Milk – 1995 Holt 105-133
¨ 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
Casein micelle structure – proposed model
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
Adapted from Kiely et al 1992
Cheese Structure – at set
¨ 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 proteinMilk 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%
Whey utilization
Fractionation of milk protein
1970 Today