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