Composition and Biosynthesis

Milk

Normal

Chemical composition

of milk

Milk is formed of:

1) Water: form 87%.2) Solids: form 13%. A- Organic constituents of milk.

1. Protein.2. Lipid.3. Carbohydrate.

B- Inorganic constituents of milk:4. Minerals.5. Vitamins.

A- Organic Constituents of Milk

1- Protein: Milk protein less in human than in cow's milk. All milk protein synthesized in the mammary gland. Milk protein of high coefficient digestibility (85 –

95%)

X 100Amount of N2 absorbed

Total N2 in the protein

Characterized by:1- Protein of high biological value as:

It contains all essential amino acids.Easily digested.Easily absorbed.Easily metabolized.

2- Contain moderate amount of non essential amino acids to decrease stress on body cells.

3- Essential to keep positive nitrogen balance (nitrogen intake more than nitrogen output).

5- Essential to maintain growth of newborn.

Types of Milk Proteins are:1. Casein.2. Lactalbumin.3. Lactglobulin.4. Milk enzymes.

1) CaseinIt is the main and most dominant milk

protein.It represents 25% in human's milk and 83%

in cow's milk.It is a compound protein (Phospho-protein)

of high biological value.The high phosphate content of casein allows

it to associate with calcium and form calcium phosphate salts.– So, at normal PH of fresh milk (6.6 PH)

casein present as insoluble Ca. caseinate phosphate complex.

B- Lactalbumin: Represent 87% of whey protein. PPT by full saturation with ammonium

sulphate. Rich in cystein and cystin so give +ve result

with sulpher test. Simple protein. Soluble protein. Easily digested.

Consists of two fractions:– α Lactalbumin 32% of whey protein.– β lactglobulin 55% of whey protein.N.B:

β lactglobulin called globulin because it need small amount of NaCl to be dissolved but it is not PPT by half saturation as albumin.

3- Lactglobulin: Represent 13% of whey protein. PPT by half saturation with ammonium sulphate

solution. Rich in cystein and cystin so give +ve result

with sulpher test. Simple protein. Soluble protein. Easily digested.

Consists of two fractions:True globulin (Euglobulin) 50%.False globulin (Pseudoglobulin) 50%.

They carry antibodies causing immunity so called immunoglobulins.

They present in higher concentration in colostrum.

4- Milk enzymes:1. Catalase.2. Peroxidase.3. Xanthin oxidase.4. Alkaline phophatase.5. Amylase.6. Lipase.7. Aldehyde oxidase.

2- Lipid:

Human's and cow's milk contain the same amount 3.5 gm/dl but buffalo's milk is a little higher 7 gm/dl.

Easily separated on standing. Responsible for white color of milk It consists mainly of triacylglycerol

distributed as coarse emulsion which contains oleic, myristic, palmitic and stearic fatty acids.

Also contain small amounts of:phospholipids 0.1%.

–Milk phospholipids are lecithin, cephalin, sphingomyelin (9:5:1).

–Phospholipids in cow's milk twice that of human milk.

Cholesterol 0.01%.Cow's milk contains higher proportion (mainly free

form) than human milk mainly (ester form) Milk contain fat soluble substance: e.g:

Fat soluble vitamins (vitamin A) which is higher in human milk than cow's milk.

Milk fat is formed mainly in lactating mammary gland from:

–Active acetate.–Glycerol: derived from glucose

(dihydroxyaceton phosphate). Milk fat may be derived from blood triacylglycerol but

to a limited extent.

3- Carbohydrates: Lactose (milk sugar) is the only carbohydrate of

milk. It is a reducing disaccharide consists of glucose

and galactose. Human's milk contains 7% lactose while cow's

milk contains 5% lactose. Lactose may be excreted in urine during last

third of pregnancy physiologically so it should be differentiated from glucose by osazon test.

Importance of lactose:1. It is less sweet than sucrose so allow the baby to take

large amount of milk without causing nausea.2. It is non fermentable carbohydrate so it doesn't

produce CO2 in GIT and the baby doesn't suffer from abdominal colic or distention.

3. Lactose help growth of lactic acid producing bacteria so help in absorption of Ca, P, Fe, Cu which prefer acidic medium for their absorption.

4. Lactose inhibits growth of putrefactive bacteria which cause abdominal distention by increasing the acidity of the intestine.

B-Inorganic constituents of milk

1- Minerals: Human milk contain less mineral elements

(0.4%) than cow's milk (0.8%). Milk rich in Ca and P which are present in their

proper ratio for absorption (2:1) in human milk while in cow's milk (1:2) which is not suitable for their maximum absorption .

Ca and P are essential for:Growth of bone and teeth.Stability of casein.

Milk is deficient in Fe and Cu which are supplied by their storage in liver during prenatal life (this store is sufficient till weaning time).

N.B: Milk is deficient in Iron but it is more in human

milk than cow's milk Thus anaemia in breast feeding is less common.

Milk contain adequate amount of Na, K, Mg. Human milk contains Na:K (1:2) which is

suitable for the optimal growth of newborn.

2- Vitamins: Milk is deficient in:

Vitamin C. Vitamin D. Vitamin K.

Milk contain adequate amount of vitamin B complex which are sufficient for first week of life

e.g: Pantothenic acid. Riboflavin (gives the whey the greenish tint in

sunlight).

N.B:Vitamin C must be supplied to the growing baby

in the form of fruit juices to withstand infection.Fortified vitamin D milk is used in order to

supply the baby with vitamin D requirement which is added from cod liver oil.

Exposure to sunlight in the early morning or before sunset help in formation of active vitamin D from cholesterol.

Basal membrane ER membrane Luminal membrane

LPL

FASACC

de novo FA synthesis (C4 - C16 )

TAG

synthesis

Glucose

SFA (C16 -C18 )

Synthesis Secretion

Acetate

ßHBA

TAG

Glucose

Circulation Translocation

UFA

FABPNEFA

+Glycerol

MFGM

GlycerolGlycerol-P

Milk Fat Synthesis

Epithelial Cell

MitochondriaMitochondria

E.R.

Blood Vessel

Nucleus

Fat Droplet

Fat Droplet migrating

Golgi Body Lysosome

Lumen of alveolus

Epithelial Cell

MitochondriaMitochondria

E.R.

Blood Vessel

Nucleus

Fat Droplet

Fat Droplet migrating

Golgi Body Lysosome

Lumen of alveolus

Epithelial Cell

MitochondriaMitochondria

E.R.

Blood Vessel

Nucleus

Fat Droplet

Fat Droplet migrating

Golgi Body Lysosome

Lumen of alveolus

Cell pinches off

Epithelial Cell

MitochondriaMitochondria

E.R.

Blood Vessel

Nucleus

Fat Droplet

Fat Droplet migrating

Golgi Body Lysosome

Lumen of alveolus

Cell pinches off

Major Fatty Acids (wt%) of Bovine Milk

Saturated Fatty Acids4:06:08:010:012:014:016:018:0

Butyric acidHexanoic acidOctanoic acidDecanoic acidLauric acidMyristic acidPalmitic acidStearic acid

4.52.31.32.73.010.628.212.6

Major Fatty Acids (wt%) of Bovine Milk

Monounsaturated Fatty Acids16:118:1

Palmitoleic acidOleic acid

1.621.4

Major Fatty Acids (wt%) of Bovine Milk

Polyunsaturated Fatty Acids18:218:3

Linoleic acidLinolenic acid

2.90.3

Effect of hydrogenation by rumen microorganisms on dietary lipids

Wt% of Fatty Acids

Fatty acid Diet Abomasal digesta

16:018:018:218:3

263

1731

294546

Milk Fat Concentration

Most variable component of milk Increased during periods of energy deficit Decreased when fed diets containing

unsaturated fatty acids (most grains) and low effective fiber (change rumen conditions)– Milk fat depression (<2.8% fat)

Fatty acid changes also occur

C a se ins(8 0 % )

b e ta -la c to g lob u lin(1 0 % )

a lp ha -la cta lbu m in(4 % )

S e ru m a lbu m in ,Im m u n og lo bu lins

(5 % )

N o n -ca se in (W h e y)(2 0 % )

M ilk P ro te ins

Milk Protein Concentration

Not too variableChanges related to dietary energy

intake, not dietary protein– little effect of dietary fat on milk protein

concentration unless perturb normal rumen function

– fermentable carbohydrate intake is the key! Maximize VFA and microbial protein production

Lactose synthesis:Lactose is synthesized in mammary gland from α

D glucose and β D galactose.– Galactose converted to galactose -1-

phosphate by galactokinase enzyme.– Galactose -1- phosphate and UDP glucose are

converted to UDP galactose and glucose -1- phosphate by uridyle tranferase enzyme.

– UDP galactose bounded to glucose by lactose synthase enzyme forming lactose.

Glucose-6-phosphatase

Fructose-1,6-bisphosphatase

glucose Gluconeogenesis Pi

H2O glucose-6-phosphate Phosphoglucose Isomerase

fructose-6-phosphate Pi H2O fructose-1,6-bisphosphate Aldolase

glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate Triosephosphate Isomerase (continued)