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 % Casein % Whey protein % Alpha-lactalbumin Beta lactoglobulin Lactoferrin < Immunoglobulin 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 ) Micelle hydrolyzed by rennin (curd) Variants: alpha, beta, gamma, kappa

CASEINS * Casein alpha beta gamma kappa grams/ liter phos. (P) % mol. wt. 24K 24K 20K 19K P i / molecule Carbohydrate % 5

WHEY PROTEINS (non-casein) 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 mg / l Total protein 30,000 Total NPN ~ 300 Urea - N ** ~ 150 ** Mimics blood urea - excellent diagnostic tool esp. for ruminants

Nomenclature and relative amounts traceImmunoglobulins traceLactotransferrin.5-2Blood serum albumin 2-4  - Lactalbumin 7-12  - Lactoglobulin 8-15  - Casein  - Casein  s-2 - Casein  s-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 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 Apical membrane Basement membrane Casein micelles

 20 possible amino acids – Connected via peptide bond to form protein The value of milk protein Amino acids  Casein is a phospho protein (phosphorus is linked to a serine amino acid) P P P

 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

 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 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 WaterLactoseFatProteinMinerals Whole Milk87.6%4.8%3.7%3.2%.70% Whole Milk (dry basis) %29.8%25.8%5.6% Skim Milk90.9%4.98%.05%3.32%.73% Skim Milk (dry basis) %.55%36.6%8.0%

Whey utilization Fractionation of milk protein 1970Today