MILK

Food Material Science 2010/11

Inneke Hantoro
Definition

 The normal secretion of the mammary

glands of all mammals (Potter &
Hotchiss, 1996).
 Milk is a complete food for the new
born.
 High density of nutritious components.
MILK COMPOSITION &
STRUCTURE
The average composition of milk

Source: Walstra et al. (2006)

Principal components
 Lactose or milk sugar is the distinctive
FATTY ACIDS
carbohydrate of milk. It is a disaccharide
composed of glucose and galactose.

 The fat is largely made up of triglycerides,

constituting a very complicated mixture. The
component fatty acids vary widely in chain
length (2 to 20 carbon atoms) and in saturation
(0 to 4 double bonds). Other lipids that are
present include phospholipids, cholesterol, free
TRIGLISERIDA
fatty acids, monoglycerides, and diglycerides.
Principal components
Protein
 About four fifths of the protein consists of
casein, actually a mixture of four proteins: αS1-,
αS2-, β-, and κ-casein. The caseins are typical
for milk.
 The remainder consists, for the most part, of
the milk serum proteins, the main one being β-
lactoglobulin.
 Moreover, milk contains numerous minor
proteins, including a wide range of enzymes.
Principal components
 The mineral substances — primarily K, Na,
Ca, Mg, Cl, and phosphate — are not
equivalent to the salts. Milk contains
numerous other elements in trace quantities.
The salts are only partly ionized.
 The organic acids occur largely as ions or
as salts; citrate is the principle one.
 Furthermore, milk has many
miscellaneous components, often in trace
amounts.
 Water Organic acids Proteins
Carbohydrates citrate casein
lactose formate -lactoglobuline
glucose acetate -lactalbumine

SERUM others
Minerals
lactate
oxalate
serum albumin
immunoglobulines
Ca, bound others proteose pepton
Ca, ions Gases NPN
Mg oxygen peptides
K nitrogen amino acids
Na Lipids urea
Cl glycerides ammonia
phosphate fatty acids Enzymes
sulfate phospholipids acid phosphatase
bicarbonate cerebrosides peroxidases
Trace elements sterols many others
Zn/Fe/Cu and Vitamins Phosphoric
many B vitamines esthers
others ascorbic acid Others
Compotition
and
Structure
(A) Uniform liquid. However, the liquid is turbid
and thus cannot be homogeneous.

(B) Spherical droplets, consisting of fat. These

globules float in a liquid (plasma).

(C) The plasma contains proteinaceous

particles, which are casein micelles. The
remaining liquid (serum) is still opalescent, so it
must contain other particles. The fat globules
have a thin outer layer (membrane) of different
constitution.
Fat Globules
 The surface layer or membrane of the fat globule is
not an adsorption layer of one single substance but
consists of many components; its structure is
complicated.
 The dry mass of the membrane is about 2.5% of
that of the fat.
 A small part of the lipids of milk is found outside the
fat globules.
 At temperatures below 35°C, part of the fat in the
globules can crystallize.
 Milk minus fat globules is called milk plasma, i.e.,
the liquid in which the fat globules float.
 Composition and structure of milk fat
MEMBRAN
Water
FAT
GLOBULE Protein
Glycerides Phospholipids
triglycerides Cerebrosides
diglycerides Glycerides
monoglycerides Fatty acids
Fatty acids Sterols
Sterols Other lipids
Carotenoids Enzymes
Vitamins A,D,E,K alkaline phosphatase
Water xanthine oxidase
many others
Cu and Fe
Casein Micelles
 Casein micelles consist of water, protein, and
salts. The protein is casein.
 Casein is present as a caseinate, which means
that it binds cations, primarily calcium and
magnesium.
 The other salts in the micelles occur as a
calcium phosphate, varying somewhat in
composition and also containing a small amount
of citrate. This is often called colloidal
phosphate. The whole may be called calcium-
caseinate/calcium-phosphate complex.
Casein Micelles
 The casein micelles are just ‘small particles.’
 The micelles have an open structure and,
accordingly, contain much water, a few
grams per gram of casein.
 Milk serum, i.e., the liquid in which the
micelles are dispersed, is milk minus fat
globules and casein micelles.
CASEIN
Protein
MICELLE
Casein
Proteose pepton
Salts
Ca
Phosphate
Citrate
K, Mg, Na
Water
Enzymes (lipase, plasmine)
Other Milk Constituents
 Serum proteins are largely present in milk
in molecular form or as very small
aggregates.
 Lipoprotein particles, sometimes called
milk microsomes, vary in quantity and
shape. Presumably, they consist of
remnants of mammary secretory cell
membranes. Few definitive data on
lipoprotein particles have been published.
Other Milk Constituents
 Cells, i.e., leukocytes, are always present in
milk. They account for about 0.01% of the
volume of milk of healthy cows. Of course,
the cells contain all cytoplasmic components
such as enzymes. They are rich in catalase.
Other milk constituents
LEUKOCYTE LIPOPROTEIN
PARTICLE

Many enzymes Lipids

e.g. katalase Protein
Nucleic acids Enzymes
Water Water
 Properties of the main structural elements
of milk
fat casein globular lipoprotein
micelles proteins paricles
Main component(s) Fat Casein, Serum Lipids,
water salts proteins proteins
To be considered as Emulsion Fine Colloidal Colloidal
dispersion solution dispersion
Content (% dry matter) 4 2.8 0.6 0.01
Volume fraction 0.04 0.1 0.006 10-4

Particle diameter 0.1 – 10 m 20 – 300 nm 3 – 6 nm 10 nm

Number per ml 1010 1014 1017 1014

Surface area (cm2/ml milk) 700 40.000 50.000 100

Density (20 0C; kg/m3) 900 1100 1300 1100

Diffusion rate (mm in 1 h) 0.0 0.1 – 0.3 0.6 0.4

Isoelectric pH ~3.8 ~4.6 4–5 ~4

MILK FORMATION
Digestion-1
 Milk components are for the most part formed in
the mammary gland (the udder) of a cow, from
precursors that are the results of digestion.
 In ruminants like the cow, considerable
predigestion occurs by means of microbial
fermentation, which occurs for the most part in
the first stomach or rumen.
 It contains numerous bacteria that can digest
cellulose, thereby breaking down plant cell
walls, providing energy and liberating the cell
contents.
Digestion-2
 From cellulose and other carbohydrates, acetic,
propionic, butyric and lactic acid are formed,
which are taken up in the blood. The
composition of the organic acid mixture
depends on the composition of the feed.
 Proteins are broken down into amino acids. The
rumen flora uses these to make proteins but
can also synthesize amino acids from low-
molar-mass nitrogenous components. Further
on in the digestive tract the microbes are
digested, liberating amino acids.
Digestion-3
 Also, food lipids are hydrolyzed in the rumen
and partly metabolized by the
microorganisms.
 All these precursors can reach the
mammary gland.
 Milk Synthesis-1
 The synthesis of
milk components
occurs for the
greater part in the
secretory cells of
the mammary
gland.
Milk Synthesis-2
 At the basal end precursors of milk components
are taken up from the blood, and at the apical
end milk components are secreted into the
lumen.
 Proteins are formed in the endoplasmic
reticulum and transported to the Golgi vesicles,
in which most of the soluble milk components
are collected.
 The vesicles grow in size while being
transported through the cell and then open up
to release their contents in the lumen.
Milk Synthesis-3
 Triglycerides are synthesized in the
cytoplasm, forming small globules, which
grow while they are transported to the apical
end of the cell.
 They become enrobed by the outer cell
membrane (or plasmalemma) while being
pinched off into the lumen.
 This type of secretion is called merocrine,
which means that the cell remains intact.
Excretion-1
 The glandular epithelium, consisting of layers of
secretory cells, form spherical bodies called
alveoli.
 Each of these has a central lumen into which
the freshly formed milk is secreted.
 From there, the milk can flow through small
ducts into larger and still larger ones until it
reaches a cavity called the cistern.
 From the cistern, the milk can be released via
the teat.
Excretion-2
 Excretion of the milk does not happen
spontaneously. The alveoli have to contract,
which can be achieved by the contraction of
muscle tissue around the alveoli.
 Contraction is induced by the hormone
oxytocin. This is released into the blood by
stimulation of the teats of the animal, be it by
the suckling young or by the milker.
 The udder is not fully emptied.
Lactation
 When a calf is born, lactation — i.e., the
formation and secretion of milk — starts.
The first secretion greatly differs in
composition from milk.
 Within a few days the milk has become
normal and milk yield increases for some
months, after which it declines.
 The yield greatly varies among cows and
with the amount and the quality of the feed
taken by the cow.
 Colostrum
 Colostrum is the secretion produced over the first few days
after parturition. The components of colostrum are
synthesised in the mammary gland over several days prior
to parturition.

 Colostrum is rich in special nutrients for the newborn.

 Colostrum contains more mineral salts and protein and

less ash than later milk. Ca, Na, Mg, P, and chloride are
higher in colostrum but K is lower.

 The most remarkable difference between colostrum and

milk is the high concentration of immunoglobulins (Ig’s) in
colostrum. Ig’s are related to passive immunity against gut
pathogens.
Colostrum
 Colostrum has a higher level of -carotene, imparting
an intense yellow colour, and a high level of somatic
cells.

 Recently there has been a lot of commercial interest

in colostrum because of its elevated levels of
bioactives, especially growth factors, and there is a
wide range of literature supporting the health
benefits of colostrum

 Colostrum is 10 times more expensive than milk

powder.
MILK ATTRIBUTES
Milk quality
 Factors that determine the quality of fresh milk
(standard indicators) are:
 Total solid contents, including protein (min.

2.7%), fat (min. 3%), solid non fat (min. 8%).

Raw milk is purchased by weight, but processed
milk is sold by volume.
 Freezing point

 Density
Milk quality
 Some factors can influence the quality of milk,
including:
 Feed

 Genetic

 Climate

 The health status of cattle

 Milking process and storage

 Post harvest handling

Fresh Milk Deterioration
 Milk can deteriorate fast since milk contains high
nutrient contents such as carbohydrate, fat and
protein which required by bacteria to grow.
 Moreover, pH of milk is close to neutral pH. This
is very suitable for the growth of microorganisms.
 Lastly, since most of microorganism (mesophilic
and psychotrophic bacteria) can grow very well
at room temperature, fresh milk stored in room
temperature is susceptible to microbial
deterioration.
Fresh Milk Deterioration

 Many of the psychrotrophic bacteria isolated from milk

produce extracellular enzymes that degrade milk fat
and protein (proteolysis and lypolysis).
 Bacterial lipase causes serious degradation of milk fat.
 Beside microbial degradation, fresh milk also
susceptible to enzymatic degradation. Raw milk has an
abundance of lipoprotein lipase, an enzyme that will
rapidly hydrolyse milk fat to free fatty acids (FFAs).
 Some of these FFAs have low organoleptic thresholds
and produce odors and flavors (rancid, bitter, soapy or
unclean).
UHT vs Pasteurized Milk

 Generally, there are two heat treatment given to

fresh milk, i.e. pasteurization and sterilization using
ultra high temperature (UHT).
 Pasteurization is done at 63oC for 30 min or 72-
75oC for 15-20 s (high temperature short time -
HTST). Pasteurization is used mostly to kill Gram-
negative psychrotrophs bacteria, but only has little
effect on extracellular degradative enzymes.
 While UHT is done at 135 - 140oC for a few
seconds. It can kill both pathogen and spoilage
microorganisms. The most heat resistant
pathogenic spore – C. botulinum and some
enzymes also can be inactivated.
UHT vs Pasteurized Milk

 UHT products are commonly stored at room

(ambient) temperature and good quality
products should be microbiologically stable.
 Nevertheless, chemical reactions and physical
changes will take place which will change the
quality of the product. These include oxidation
reactions, Maillard browning and chemical &
physical changes which may give rise to age-
thickening and gelation.
UHT vs Pasteurized Milk
 In pasteurization, thermoduric bacteria and spore
forming bacteria can survive. Bacillus cereus spores
are relevant here, being the main pathogen which will
survive pasteurization and grow at low temperature. It
will certainly cause spoilage in heat-treated milk.
 Enzymes in raw milk may give rise to problems in
pasteurized milk. For example, indigenous lipases
may give rise to soapy off-flavors. However, it is
unlikely that bacterial lipases and proteases, which
are very heat resistant, will cause problems in
pasteurized milks because of their relatively short
shelf-life and refrigerated storage conditions.
Milk & Dairy Products Adulteration

 Watering of milk
 Milk of different species
 Addition of non-dairy protein
 Altering the casein/whey protein ratio
 Addition of buttermilk or whey powder to milk
powder
 Addition of vegetable or animal fats to milk
fat
 Addition of reconstituted milk to fluid milk
 Non-authorized preservatives.
Milk Coagulation
 Desirable coagulation of milk can be seen in
dairy products processing such as cheese,
yoghurt, etc.
 Undesirable coagulation occur in liquid milk.
It can caused by lactic acid (produced by
bacteria) --- the reduction of pH or by
physical separation (due to density
difference) such as creaming, flocculation or
coalescence --- see emulsion chapter).
Milk Coagulation
 Milk protein, such as whey protein and casein
have important role in coagulation.
 The example of desirable coagulation:
 Acidification forms the basis of production of
all fermented milks. The gels of fermented
milks, such as yoghurt, are formed by
acidification of milk. As the pH is reduced, the
casein precipitates selectively. The first signs
of aggregation occur around pH 5 and once
the pH falls to 4.6 all the casein becomes
insoluble.
Milk Coagulation
 Some factors influence coagulation,
including:
 pH

 Temperature

 Heat treatment

 Casein concentration

 The presence of salt

Emulsion
 Milk proteins have excellent emulsifying
properties.
 Milk is categorized as o/w emulsion, since
the oil part is dispersed in the water.
 Milk proteins, both caseinates and whey
proteins, are surface active, they are
absorbed rapidly to the oil-water interface,
forming stable emulsions.
Emulsion
 The primary processes leading to emulsion instability
are:
 Creaming – refers to the gravitational separation

of emulsified droplets to form a densely packed

phase without change in droplet size.
 Flocculation – denotes the aggregation of droplets

via interactions between adsorbed proteins.

 Coalescence – an increase in droplet size,

gradually results in separation of the oil and

aqueous phases.
 2 layers
formation

Coalescence

Creaming

Flocculation

Kinetically stable
emulsion
Creaming
 Since the specific gravity of lipids and skim milk
is 0.9 and 1.036, respectively, the fat globules in
milk held under quiescent conditions will rise to
the surface under the influence of gravity, a
process referred to as creaming.

 The rapid rate of creaming is due to the strong

tendency of the fat globules to cluster due to the
effect of indigenous immunoglobulin M which
precipitates onto the fat globules when milk is
cooled (cryoglobulins).
Creaming
 Large globules rise faster than smaller ones, collide
with them and form aggregates. The clusters of
globules rise rapidly and therefore the creaming
process is accelerated as the globules rise and
clump.

 Creaming is inhibited by reduction of the fat globule

size by homogenisation. The milk fat globules are
reduced in size by pumping at very high pressure (up
to 400 bar) through a small slit. The size reduction
results in an increase in specific surface area .
Whipping & Foaming
 As milk proteins are surface active, they have
the ability to adsorb to the air-water interface
during foam formation.
 Foams are most commonly formed by
mechanically dispersing air into a solution
containing surface-active agents. A rapid
diffusion of the protein to the air-water interface
to reduce surface tension, followed by partial
unfolding of the protein is essential for the
formation of protein-based foams.
Whipping & Foaming
 Caseinates generally give higher foam overruns but
produce less stable foams than whey protein
concentrates (WPC).
 The foaming properties are influenced by many
factors, including:
 protein concentration,

 level of denaturation,

 ionic strength,

 preheat treatment and

 presence of lipids.
The Changes of Milk Flavor
 Deterioration of milk flavor can be caused by
degradation milk fat and protein.
 Rancidity is a common indicator of the forming of
undesirable flavor.
 Factors stimulating the off-flavor in fresh milk:
 Light
 Ion metals
 Transferred from cow to milk
 Microorganisms
 Enzymatic reactions
References
 Walstra, P., J.T.M. Wouters & T. J.
Geurts. 2006. Dairy Science and
Technology 2nd Edition. Taylor and
Francis Group. Boca Raton.
Thank You….