Week 2Sept. 16 All About Milk Walt Hurley

The Components of Milk

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Lactation cycle for a dairy cow

1 yr

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1st 2nd 3rd 4th 5th 6th

Lactation number

Udder-to-Cell

udder

cross-section

lobules

alveoli

alveolusepithelial

cells

organelles

Alveolus

Fat globules

Epithelial cells

Alveolar lumen

Alveolus

Fat globules

Epithelial cells

Alveolar lumen Alveolus

Alveolus

Epith

elial

cells

Epithelial cells

Alveolar lumen

Alveolar lumen

d d d

Alveolar lumen

Basement membrane

Apical membrane

Epithelial cell

capillaryglucose

nucleus

Milk

SV

Golgi

RERSER

amino acids fatty acids

What are the main components that you would expect to be in milk?

• Water• Lactose• Lipid• Proteins• Minerals• Vitamins• Others

1 liter m

ilk

870 mL w

ater

49 gr lacto

se

35 gr milk

fat (oil)

32 gr protein

Cow milk composition

Physicochemical Properties of Milk

Lower magnification

Milk viewed through a microscopeMilk is:

an emulsion of fat globules floating in the plasma phase of milk (skim milk),

Diluted whole milk

Milk total solids:

(whole milk) – (water) = total solids

milk fat, protein, lactose, minerals, vitamins, and any other non-water component

Cow milk is about 12.5% solids

Includes:

Therefore, water is the major component of milk

87.5% water

Physicochemical Properties of Milk

Lower magnification Higher magnification

Milk viewed through a microscopeMilk is:

an emulsion of fat globules floating in the plasma phase of milk (skim milk),

which in turn is a suspension of casein micelles suspended in the milk serum phase (whey),

which in turn is composed of all the water soluble components (lactose, whey proteins, etc)

What happens when you let raw milk stand?

1.030 g/mL

1.009 g/mL

1.033 g/mLMilk density (at 20 C)

~0.916 g/mLMilk fat

Cream

Skim

Pellet Somatic cells

Proteins, lactose, water

Milk fat, some protein, some leukocytes

Composition of fractions

What are the main components that you would expect to be in milk?

• Water• Lactose• Lipid• Proteins• Minerals• Vitamins• Others

Lactose is:

glucosegalactoseD-Gal-(ß1-4)-D-Glc

Lactose

glucose glucoseD-Glc-(ß1-4)-D-Glc

Maltose

glucose fructoseD-Glc-(a1-2)-D-Fru

Sucrose

The major milk sugar (carbohydrate)

The is least variable organic component in milk

A disaccharide

d d d

Alveolar lumen

Basement membrane

Apical membrane

Epithelial cell

capillaryglucose

nucleus

Milk

SV

Golgi

RERSER

amino acids fatty acids

GolgiApparatus

SecretoryVesicles

Casein micelles

Golgi Apparatus

b-Galactosyltransferase

a-Lactalbumin

Lactose

Glucose

Galactose

Water

Synthesis of Lactose

Cytoplasm

Glucose & galactose (monosaccharides) can cross the Golgi membrane

a-Lactalbumin interacts with b-Galactosyltransferase to synthesize lactose

Lactose (a disaccharide) cannot diffuse across the Golgi membrane

As a result, water is drawn into the Golgi and secretory vesicle

Lactose is the major milk osmole

What are the main components that you would expect to be in milk?

• Water• Lactose• Lipid• Proteins• Minerals• Vitamins• Others

Milkfat

Milk lipid is used for:

• Accumulation of body adipose soon after birth

• A source of lipids for growth and development

• A source of energy Newborn Harp Seal

A few days old

Mom and baby seal

Milk is ~ 54% fat

Physicochemical Properties of Milk

• Milk is an emulsion of fat globules• Emulsion = a liquid dispersed in an immiscible liquid

(incapable of mixing to homogeneity; oil in water)

• Milk is a partially stable emulsion• Cream slowly rises• Homogenization - stabilizes the fat emulsion

Cream vs Milk Fat

Cream separator(centrifuge)

Cream out

Skim milk out

Milk inMilk fat Oil

Remove the moisture and non-fat solids from butter

AnhydrousMilk fat/Butter oil

Butter

C16:0 = palmitic acid

C16:0 = palmitic acid

C12:0 = lauric acid

C14:0 = myristic acid

Glycerol

Fatty acid

Triacylglycerol

Milk fat is 98% triacylglyceride(triglyceride)

d d d

Alveolar lumen

Basement membrane

Apical membrane

Epithelial cell

capillaryglucose

nucleus

Milk

SV

Golgi

RERSER

amino acids fatty acids

FA synthesis

Milk fat secretionLipid droplet(~1-10 microns dia;mean = 2 microns)

Milk fatGlobules

Note the membrane around the globule of triglyceride.

1 micron = 1 millionth of a meter

micron = micrometer

Diluted whole milk

Cow milk fat globuleshave a density of 0.92g/ml

Total surface area of fat globules in a liter of milk is ~80 m2

range from 0.1 to 15 micrometers in diameter

Variability of Milk Composition

• Composition will vary depending on:• Species• Strain or breed of animal• Stage of lactation

• Fat is most variable organic component in milk• Lactose is least variable component

Formation of butter from cream

ButterCream

Butter

Buttermilk

Cream to butter -

Break up the membrane

Hydrophobic lipid coalesces

Churning milk or cream

600 pounds of butter

part of the Illinois fair for more than 90 years

recycled and reused for up to 10 years

While sculpting, butter is kept outside the cooler so it is soft and pliable when needed.

A properly refrigerated butter sculpture can stay in place for four to six weeks.

Once in the cooler, the butter sets in about 20 minutes.2019

What are the main components that you would expect to be in milk?

• Water• Lactose• Lipid• Proteins• Minerals• Vitamins• Others

d d d

Alveolar lumen

Basement membrane

Apical membrane

Epithelial cell

capillaryglucose

nucleus

Milk

SV

Golgi

RERSER

amino acids fatty acids

Amino acids

+H3N-CH-C-O-

OII

IR1

+H3N-CH-C-O-

OII

IR2

+

H2O

+H3N-CH-C-N-CH-C-O-

OII

IR1

IR2

IH

Peptide bond

OII

Partial amino acid sequence of bovine k-casein:

-CYS-GLU-LYS-ASP-GLU-ARG-PHE-PHE-SER-ASP-LYS-ILE-ALA-LYS-

secretory vesicles

Proteins Amino Acids to Proteins

rough endoplasmic reticulum Golgi apparatus

Casein micelles

Most milk proteins

Lactose

Water

Minerals

Water-soluble vitamins

Caseins

Whey proteins

Secretory Vesicles

as1-Casein

as2-Caseinb-Casein

k-Casein

β-Lactoglobulin

α-LactalbuminSerum albumin

ImmunoglobulinsOther Proteins

80% casein

20% whey protein Cow milk proteins

Physicochemical Properties of Milk• Casein micelles are suspended in aqueous fraction

• Suspension - a solid dispersed in a liquid

• Disruption of micelle structure -- formation of curds• Basis for cheese industry

>1000 varieties of cheese

Started >8000 years ago

Scanning electron micrograph of a casein micelle

2018 World Record Milk Production Cow

Selz-Pralle Aftershock 3918

Selz-Pralle Dairy, Humbird, Wis.

365-day lactation

78,170 lb of milk

3,094 lb of butterfat

2,393 lb of protein

rolling herd average of 30,917 lb/year14,023 kg/year

35,457 kg

1,403 kg

1,085 kg

214 lb of milk

Average daily production

8.5 lb of butterfat

6.55 lb of protein2.97 kg

8.6 lb per gallon = 25 gallons

2.4 kg of casein

Caseins (CN)The major milk protein in most species

Caseins:

Secreted as a casein micelle: Caseins, calcium, phosphorous

as1-CN , as2-CN , ß-CN , k-CN

Some caseins are phosphorylated:

-GLU-SER-ILE-SER-SER-SER-GLU-

PO3- PO3

- PO3-

PO3-

PO3-

PO3-

PO3-

PO3-Ca++

Ca++

Casein proteins

Calcium phosphate forms an amorphous ground substance that helps hold the casein micelle together

amorphous calcium phosphate

insoluble

aS1-caseinaS2-caseinb-casein

k-casein

Calcium phosphate nanoclusters

~10,000 peptide chains

~800 nanoclusters Sphere radius = 10-300 nanometers

Thermodynamically stable complexes of casein molecules and nanoclusters of

amorphous calcium phosphate

Casein Micelles

Density = 1.11 g/ml

8 g calcium/100 g casein

0.2 µm

Physicochemical Properties of Milk

Lower magnification Higher magnification

Milk viewed through a microscopeMilk is:

an emulsion of fat globules floating in the plasma phase of milk (skim milk),

which in turn is a suspension of casein micelles suspended in the milk serum phase (whey),

which in turn is composed of all the water soluble components (lactose, whey proteins, etc)

Precipitation of casein with acidStart with skim milk

Whey

Curds

Slowly bring down the pH to 4.6 with acid, with continual stirringPrecipitate is primarily casein =

the curdsResidual liquid is acid whey

acid+ -

1 2 4 5 6 7 83pH

solu

bilit

y Casein solubilityin 0.1 M NaCl

pH 4.6

k-Casein

- rennin cleaves between Phe 105 and Met 106

Rennin = chymosin

Rennet = isolated from stomach of calves, lambs or kids (goats)

~90% chymosin

Casein micelle

Micellestructure

k-casein

renn

in

Where are the salts?

Ca Mg K Na PO4 Cl Cit

20%

40%

60%

80%

100%Serum phase

Colloidal phase

Ca++PO4

-

Ca++

PO4-

Ca++

PO4-

Casein

Milk MineralsAsh 0.2% - 2.0%Cow milk = 0.75%

CalciumPotassium SodiumMagnesium

CitrateTotal Phosphate ChlorideCarbonateSulfate

~1100 mg/L~1450 mg/L

~475 mg/L~125 mg/L

~1700 mg/L~1000 mg/L~1000 mg/L

~200 mg/L~100 mg/L

Macrominerals Microminerals

FeZnCuMnSeIFlMo, Co, Cr, Ni

Al, As, Pd, Hg

~100 mg/L in blood

Anio

ns (-

)Ca

tions

(+)

Milk Vitamins

Fat-soluble vitaminsCream

A D E K

Water-soluble vitaminsSkim

B Especially B12, thiamin, riboflavinC

Vitamin A

Low fat milk defined as greater than 0.5% but less than 2% milk fat

Vitamin A palmitate

Normal growth, vision, reproduction, differentiation of epithelial cells

Form found naturally in animal sources and produced synthetically

Naturally present in milk Most is associated with milk fat

What about low-fat milks?

Over-fortification of D or A can cause toxic effects, soft tissue damage, kidney failure

Vit. D over 800 IU/quart

Vit. A over 6000 IU/quartPotential public health threat

Skim milk defined as milk fat less than 0.5%

Vit. A should be added in an amount necessary to replace the amount of vitamin lost in removal of fat

(Retinyl palmitate)

Vitamin D

Vitamin D-fortified milk contains 400 IU (10 µg) of vitamin D per quart.

Added as synthetic vitamin D3 (cholecalciferol)

Calcium absorption and mineralization of bone

Since the 1930s, the standard is to fortify all milk to provide 30 percent of daily value of vitamin D in every 8-ounce glass

To help combat rickets

Early 1900’s – rickets was common in children.Osteomalacia in adults

Olmsted County, Minnesota

Rickets is not only an ancient disease

Photooxidation of vitamin D and A may contribute to off-flavors of milk

Exposure to light at greater than 500 nm wavelength causes destruction of light-sensitive vitamins

Vitamin A Riboflavin Vitamin C

Typical high temperature short time pasteurization does not affect vitamin content

Ultra-high temperature pasteurization does cause loss of water-soluble vitamins in milk

7 - 9 % loss