Milk chemistry and composition - Basics for Dairy processing
INTRODUCTION TO MILK
Definition, Composition, physical and chemical properties
Physical and chemical properties
National standards - India
Whole, fresh, clean, lacteal secretions, obtained by complete
milking of one or more healthy milch animals
Excluding that obtained 15 days before or 5 days after or
such periods as may be necessary to render the milk
practically colostrum free
Containing legally prescribed minimum percentage of fat and
Lacteal secretion, practically free from colostrum, obtained by
the complete milking of one or more healthy cows,
May be clarified and may be adjusted by separating part of
the fat there from; concentrated milk, reconstituted milk, and
dry whole milk.
Water, in a sufficient quantity to reconstitute concentrated and
dry forms, may be added
Complex chemical substance in which:
Fat is present in the form of an emulsion
Protein and some mineral matter in the colloidal state
Lactose with some minerals and soluble proteins in the form of
Description of Milk
Emulsion of milk fat globules which contain the milk lipids, fat
soluble vitamins, and the components of the FGM
Colloidal suspension of casein micelles (which contain casein
proteins, calcium, phosphate, citrate and water), globular
proteins, and lipoprotein particles
Solution of lactose, soluble proteins, minerals, vitamins, acids,
enzymes, and other components
All species of mammals secrete milk
To provide nutrients required for the optimum growth of the
Immunity, protecting it from some of the common diseases.
The development of the young one in all species of mammals is
not uniform as such
composition of the milk vary depending up on the nutritional
needs of the young one
Differences in the composition of milk
from various species
Buffalo and Sheep milk - maximum fat
Fat percent in goat milk is much similar to cow milk
Variation among the protein percent less
Higher percent of lactose and fat
Lesser percent of protein and ash content compared with
Energy supplied - highest in buffalo and sheep milk
Difference less between the milk from the remaining species
Legal standards for various classes of milk
First mammary gland fluid secreted by mammals during the
first 5-7 days after calving
Composition is similar to that of blood and differs
significantly from milk.
Nutrients (proteins, fats, lactose, essential fatty acids and
Non-nutrients (biologically active substances)
First food for neonates after the parturition that provides
them with all necessary nutrients
Contains more protein - serum protein
Insulin-like growth hormone (IGF) – IGF-I and IGF-II,
Transforming growth factor (TGF)
Epidermal growth factor (EGF)
Control the growth and development of gastrointestinal
For the functional maturation of the organism during the
first days after birth
Immune factors (Immunoglobulins, Ig, Lactoferrin)
For the passive immunization of the newborn
Antibacterial factors passes in the offspring
Supports their protection against infections during the first
days after birth
Colostrum composition and its biological value affected by:
Nutrition during the pregnancy
Health status of cows impact
Principal constituent in milk
79% to 90%, depending on the species
It encompasses all other constituents of milk (total solids) that
are either dissolved or suspended in it
Small amounts of water are hydrated or bound chemically to
lactose, salt, or protein
The water activity in milk is relatively high, 0.993
Removal of water increases shelf life – powdered milk
Regulations prohibit the addition of water to raw milk
In fat globules - protected by a membrane (FGM)
Fat globules range from 1 to 20 μm in diameter
Made up of app.
0.2% to 1% phospholipids
0.2% to 0.4% sterols
Phospholipids and proteins mostly associate with the fat
Contain traces of fatty acids; vitamins A, D, E, and K; and
More than 400 different fatty acids
Predominant fatty acids in bovine milk
Myristic acid (C14:0 )
Palmitic acid (C16:0 )
Stearic acid (C18:0 )
Oleic acid (C18:1)
Lipids, lipoproteins, cerebrosides, nucleic acids, enzymes, trace
elements (minerals), and some bound water molecules
stabilize and prevent the fat globules from coalescence
during milk processing and handling.
FGM prevents attack from lipases (lipolysis)
Or increase in the amount of diglycerides, monoglycerides,
and free fatty acids in milk
Free fatty acids are fairly water - soluble and are situated in
milk plasma and fat
Short free fatty acids situated in the milk plasma are ionized
and more water-soluble than long free fatty acids ( >C14)
found in fat and at the oil - water interface.
Minerals associated with the fat globule membrane are
copper (5-25%) and iron (30-60%)
Other minerals include cobalt, calcium, sodium, potassium,
magnesium, manganese, molybdenum, and zinc.
Compound lipids also occur in milk such as phospholipids and
phosphatides that are situated mainly in the fat globule
membranes but also in the milk plasma, lipoproteins, and milk
Phospholipids and phosphatides are highly surface active and
polar, and dissolve poorly in both water and oil.
Lipids can be crystallized, which affects the fat structure,
melting range, and rheological properties of milk.
Furthermore, autoxidation of the double fatty acid bonds or
residues can occur, leading to off flavors.
Whole milk contains 10 to 20mg/100g cholesterol (3.3% fat).
The amount of cholesterol is positively correlated with the
amount of fat in the product.
Cholesterol is located in the fat globule membrane, and
approximately 10% of the cholesterol is esterified.
Proteins are made up of amino acids with specific properties
that are determined by the side chains of the amino acids in
the polypeptide chain.
The conformation of the protein depends on the hydrogen
bonds, hydrophobic interactions, and salt bridges formed
between the peptide chains.
Regular arrangements include β-sheets and α-helices.
Temperature, ionic strength, and pH affect protein
Major classes – Casein, Whey or serum proteins
The proteins are synthesized in the mammary gland,
Protein content of milk
with a concentration range of 30 to 35g/kg.
influenced by the lactation stage of the cow.
Four main types of casein have genetic variants:
αS1-casein, αS2-casein, β- casein, and κ-casein.
They are phosphorylated and hydrophobic and associate
with themselves and each other.
They represent 38%, 10%, 36%, and 13% of whole casein
Overall, 21 variants of casein have been identified that occur
by genetic mutations.
Caseins have distinct disordered molecular structures that lack
Very heat stable, withstanding temperatures above 140 ° C
Their hydrophobicity is due to the
high ratios of apolar amino acids
including valine, leucine, isoleucine, phenylalanine, and
proline (between 35% and 45%).
Hydrophobicity - counteracted by the
high phosphate content
low concentrations of sulphur - containing amino acids such
as methionine and cysteine that allow the caseins to be
reasonably water soluble.
Their susceptibility to proteolysis is due to the
lack of secondary and tertiary structures (αS1-casein and β-
αS1-casein αS2 - casein
199 amino acids and is app.
highest charge of all the
Consists of at least eight
It has 17 proline residues that
ultimately disrupt the
formation of secondary
structures, such as α - helices
and β - sheets.
207 amino acids and is app
least hydrophobic casein
Several genetic variants,
contain between 10 and 13
Contains two cysteine
It exists as a dimer in milk.
β-casein consists of 209 amino acids and is approximately 24
It is the most hydrophobic casein molecule.
There are six known genetic variants that contain between zero
and five phosphoserine units.
Similar to αS1-casein, β-casein has few secondary structures
due to the presence of 35 proline residues.
γ-casein is derived by hydrolysis of β-casein by the enzyme
Three variants have been identified near the C - terminal end
of the β-casein molecule.
Κ - Casein
κ - casein consists of 169 amino acids and is approximately
19 kDa, and it contains both glycosylated and
It can exist as a dimer up to a decamer with the subunits held
together by disulfide linkages.
Unlike the other caseins, it is not sensitive to calcium and
surrounds the micelles, keeping them intact.
It usually contains one phosphoserine unit;
however, genetic variants containing two or three
phosphoserine units have been identified.
Further, nine variants have been identified that demonstrate
different degrees of glycosylation.
Casein micelles consist of a large portion (approximately
95%) of casein proteins that interact with each other and
They vary in size from 80 to 1,000 nm with an average
diameter of 150 nm in bovine milk.
The micelle sizes of sheep and goat milk are different, and the
caprine micelles are less heat stable than bovine milk micelles
Bovine casein micelles contain
water, protein (about 94%),
salts (about 6%), including calcium, phosphorus, magnesium,
citrate - colloidal calcium phosphate [CCP]
other traces of metals, enzymes (lipases, esterases,
proteases), and milk serum.
Casein micelles contain submicelles that range from 12 to 15
nm in diameter
Contain approximately 20 to 25 casein molecules and water
(2 to 5 g/protein)
with some submicelles containing K - casein.
Whey proteins are hydrophobic, globular, highly ordered
proteins that contain disulfide linkages.
Whey proteins have well - developed secondary, tertiary, and
Poorer heat stability
denature at temperatures greater than 75 ° C.
The two principal whey proteins in milk
α - lactalbumin and β - lactoglobulin
Synthesized in the mammary gland.
They constitute approximately 20% and 40% of total whey
protein in bovine milk
Other whey proteins are
α-lactalbumin is a spherical, glycosylated compactly folded
Consists of approximately 142 amino acids and is
approximately 14 kDa.
It is synthesized and secreted by the mammary gland
Contains four disulfide bonds and eight cysteine residues, and
is rich in tryptophan
Three genetic variants have been identified
It is the principal protein in human milk.
β-lactoglobulin consists of 178 amino acids with an
approximate molecular weight of 18 kDa.
It exists in both the monomeric and dimeric form at equilibrium
in bovine milk;
Its association depends on temperature, pH, protein
concentration, and ionic conditions.
The hydrophobic dimeric form linked by one to three disulfide
bonds is approximately 36 kDa.
Higher concentrations of β - lactoglobulin are present in
bovine milk when compared with human milk.
Better heat stability than α - lactalbumin due to the
presence of one free sulphohydryl unit.
It contains an open β - barrel enclosing a hydrophobic
cleft and a single three - turn α - helix.
It binds to several hydrophobic molecules including retinol and
fatty acids via the hydrophobic cleft, which in turn stimulates
Immunoglobulins are antibodies that are synthesized in
response to specific antigens.
They are large, heterogeneous molecules found in the blood.
The main immunoglobulins in milk are IgG, IgG2, IgA, and IgM.
They provide offspring with protection against pathogenic
microorganisms and their toxins, and the mammary gland
Approximately 0.7 to 1mg/ml is present in bovine milk.
The basic structural unit of the immunoglobulins is similar,
consisting of two heavy and two light chains joined together by
IgG is the main immunoglobulin in milk.
Bovine serum albumin
Bovine serum albumin consists of 582 amino acids
It is approximately 66 kDa
Predominantly composed of α - helices.
It makes up approximately 1% to 5% of total whey protein.
It is synthesized in the liver and enters the milk via secretory
Derived from the hydrolysis of β - casein.
Considered whey proteins
Because elute in the whey fraction when isolated from milk
Acid - soluble proteins
Mainly responsible for the foaming of skim milk
Have an immunological role
74 kDa and binds to iron (Fe) as it contains two metal binding sites
Bovine milk contains app. 20 to 200 mg/L
Human milk contains 2 g/L.
Other whey proteins
Other minor whey proteins includes
Vitamin - binding proteins
Folate, vitamin D, riboflavin, and vitamin B 12
Non - protein nitrogen compounds:
Urea, uric acid, creatine, creatinine, and hippuric acid
In trace amounts
Several carbohydrates in milk:
Glycoconjugates (oligosaccharides, glycoproteins, and
Main carbohydrate – lactose
4-5% of total milk content
Glucose, galactose, Oligosaccharides - app. 1 mg/ml
Disaccharide comprised of α / β-D – glucose and β - D -
galactose that are linked by a β 1-4 - O - glycosidic bond.
Depends on the milk yield and lactation stage of the cow
Amount of lactose decreases as the lactation stage advances
Lactose exists in three forms:
α - lactose monohydrate
β - lactose
Anhydrous α - lactose.
β - lactose form has the greatest solubility and is sweeter
than the α - lactose forms.
Major food source for bacteria during the fermentation of
The bacteria hydrolyse the milk into glucose and galactose
to produce lactic acid, which inhibits the growth of most
Minerals and Salts
Milk contains all minerals - essential for human nutrition
Including potassium (K), sodium (Na), calcium (Ca), magnesium
(Mg), chloride (Cl), and phosphate esters
Make up between 0.7% and 0.8% of total milk content.
Sodium, potassium, and chloride as free ions
Their concentrations are negatively correlated with lactose
Concentrations of calcium, magnesium, ionized phosphate, and
Depend on the casein content in the milk
Concentration of citrate varies depending on season and diet
of the cow
Affect the soluble calcium content and milk stability
Equilibrium between colloidal dispersion and salts
If the colloidal equilibrium is destabilized,
Concentration of minerals in milk may affect the processing
Require the addition of anions to bind to ionic calcium that
would restabilize the caseins against aggregation.
Salt is added as an additive in certain dairy products
The concentration of calcium in milk is relatively high
Milk - considered to be an important source of calcium.
The primary salts in milk are phosphates, citrates, chlorides,
sulphates, carbonates and bicarbonates of
sodium, potassium, calcium and magnesium.
Since milk contains organic and inorganic salts,
Level of salts is not equivalent to the level of mineral
Level of salts is by no means equivalent to the ash content.
Factors influencing salt composition:
Species, Breed of species
Stage of lactation and feed
In cow’s milk, ∼20 enzymes have been characterized
Another 40 enzymes - demonstrated via their activity.
Indigenous milk enzymes are found in, or associated with
Milk fat globule membrane
Milk serum or somatic cells
May originate from blood, somatic cells, the MFGM or the cell
Used as indices of
Thermal history of the milk
Functions in milk and dairy products:
Result in quality deterioration
Induce desirable changes
May also offer protective effects
Enzymes of known or potential technological significance
Predominant indigenous proteinase in milk.
Optimally active at pH ∼7.5 and ∼37◦C
Plasmin and plasminogen originate from the mammal’s
blood and are predominantly associated with the casein
micelle in milk
Cheese ripening and the stability of casein micelles in UHT
Optimal activity at pH 9.2 and 37◦C, relatively heat-labile
Initial digestion and absorption of milk lipids in the intestinal
tract and flavor development in certain cheeses from raw
Lipolysis - development of hydrolytic rancidity in milk
Optimum activity at pH 9.0–10.5 and ∼37◦ C
ALP is relatively heat-sensitive and its thermal stability is
only slightly higher than that of non-spore forming
Indicator of pasteurization efficiency
The enzyme's heat-stability profile closely follows that
necessary for adequate pasteurization
Catalyzes the oxidation of thiols and the formation of
disulfide bonds in proteins and peptides.
Sulfhydryl oxidase- treated UHT milk may have longer
flavor stability due to reduced lipid oxidation
relatively heat-stable milk enzyme; heating up to 80◦C
appears required to ensure thermal inactivation.
anti bacterial agent
pH optimum of ∼8.0, and exists primarily in the milk serum
N-acetyl-p-D-glucosaminidase - activity diagnosed for mastitis
Catalase – ass. with somatic cell membrane
Xanthine oxidase - contains all of the molybdenum in milk
Superoxide dismutase - Protective effect on lipid oxidation
γ- Glutamyltransferase - Transport of amino acids into
Lactose synthase – synthesis of Lactose