Chemical composition of milk and

milk properties
Milk
• White colloidal suspension
– Complicated system with variable biological
value
– Up to 100 thousand various compounds

• Liquid secret of lactic gland of female

mammals
Raw material for dairy production
• The most used
– Cow’s milk
– Buffalo’s milk
– Ewe’s and goat’s milk

• Different chemical composition depending

on the needs of different animal species
Colostrum
• Yellow / sand colour
• Viscous sticky
consistency
• Immunity transfer from mother to calf
• Different composition from mature milk
– High dry matter
– High immunoglubulines
– High mineral compounds
Regulation (EU) No. 1308/2013
• Raw milk
– Heated up to 40 °C
• Whole milk
– Heat-treated milk with a fat content at least
3.5 %
• Semi-skimmed milk
– Heat-treated milk with a fat content 1.5–1.8 %
• Skimmed milk
– Heat-treated milk with a fat content up to
0.5 %
Composition of cow’s milk
• Main constituents
– Water 86 – 88 %
– Total solids 14 – 12 %
• Proteins 3.1 – 3.8 %
• Fat 3.5 – 4.5 %
• Lactose 4.5 – 5.0 %
• Minor constituents
– Minerals, vitamins, enzymes, hormones,
gases
Proteins
Two main groups of milk proteins
• Caseins
– 80 % of total proteins
– insoluble at pH 4.6
– phosphoproteins

• Whey proteins
– 20 % of total proteins
– soluble in pH 4.6
– globular proteins
Caseins
• αs1-casein
• αs2-casein
4 main casein fractions
• β-casein
• ĸ-casein

• Other casein fractions = derivatives

• γ-caseins
• λ-casein
Caseins
• Open structure of caseins
– High content of proline
– Limited creation of secondary structure
– Digestibility
– Technology aspect
aS1-casein
Caseins
• Hydrophilic segments of caseins due to
phosphoserine groups

• Casein can be used as emulsifier

– Hydrophilic and hydrophobic segments
– Native open structure
– Processed cheese
Caseins
Casein micelle
• Aggregation of casein fragments to
complexes (caseine micelles)
• Caseine micelles created from submicelles
– Shape of spin ellipsoid
– Hydrophobic (non-polar) parts directed
inwards
– Hydrophylic (polar) parts oriented on the
surface (phosphoserine groups and
saccharide part of ĸ-casein)
Casein micelle
• αs1-, αs2- and β-caseins
– Mainly concentrated in the middle of micelles
– Salts with calcium are insoluble in water

• ĸ-casein
– Creation of oligomers which cover the surface
of micelles
– Protecting and stabilization of other casein
fractions against Ca2+
– Salt with calcium is soluble in water
Casein
micelle
Destabilisation of casein micelles
• Change of pH
– decreasing to the isoelectric point
– Fermented/cultured products
– Acid cheese

• Hydrolysis of ĸ-casein by rennet (enzyme)

– majority of cheese production

• Additon of ethanol
– Alcohol test of milk
Precipitation by acid
• Drop of pH to 4.6, the isoelectric point of
casein
– Direct addition of acid (mascarpone, ricotta)
– Activity of LAB (fermented products, quark,
cottage)
– Undesirable effect of microbial contamination
(defects of drinking milk)
Defects of drinking milk
Precipitation by acid

H+

pH 6.6 pH 4.6
Negative charge of casein micelles Electroneutral molecules (no charge)
Repulsion Approaching
Homogenic suspension Precipitation / coagulation
Precipitation by enzymes
First phase
• Remove of hydrophilic macropeptides
• Enzyme (rennet) splits bond of ĸ-casein
between 105th and 106th amino acids
– Para-ĸ-casein (1 to105 amino acids) is
insoluble and remains in the curd with α- and
β-caseins
– Glycomacropeptide (106 to169 amino acids)
is soluble due to present of carbohydrate →
whey
Precipitation by enzymes
Precipitation by enzymes
Second phase – coagulation and syneresis
• Formation of curd due to creating links
between hydrophobic segments and
calcium bridges
• Formation of 3D structure of casein gel
• Syneresis – increasing the number of links
cause the water molecules leave the
structure – separating of whey
Precipitation by enzymes
Precipitation by enzymes
Precipitation by enzymes
• Syneresis is supported by
– Temperature increase
– pH reduction
– Processing of curd

cheese production
Precipitation by enzymes
• Tertiary phase
– Too long activity of rennet (more than 50 min)
– Lack of bonds between 105th and 106th
amino acid
– Cleaving of other bonds in protein chains
– Less yield
undesirable
– Bitterness of cheese

– Inactivation of rennet by temperature

– Selection of more suitable rennet
Whey proteins
• α-lactalbumin – ¼ of whey proteins
• β-lactoglobulin – ½ of whey proteins
• Content of thiol groups (-SH)
• Link by sulphur bridge on the casein micelles
(through ĸ-casein) after denaturation
• Worsening of rennet activity – necessary longer
time for renneting
• Effect on properties of curd
– Very difficult creation of gel
– Water bonding on β-lactoglobulin
– Very weak curd texture
Whey proteins
• Serum albumin
• Immunoglobulines
• Laktoferin
• Proteoso-peptones
Whey proteins

β-lactoglobulin

Heat treatment
Interaction of denatured whey protein and caseins

• Denaturation begins at 65 °C
• Recommended milk heat treatment at 72 °C for 20 s for
hard and semi-hard cheese
• Total denaturation at 90 °C for 5 minutes (suitable for
fermented dairy products)
Fat
• Milk fat → fat globules
Fat globules
• Different composition of inner and outer
part
• Membrane on the surface
– Phospholipids and proteins
– Protection against
• aggregation
• lipolysis
• Inside hydrophobic parts of fat
Fat
• Triglycerides (98 % of milk fat)
– 70 % saturated FA
– 25 % unsaturated FA
– 5 % polyunsaturated FA
• Monoglycerides, diglycerides, free fatty
acids, phospholipids, sterols,
carbohydrates, vitamins A, D, E, K
Fat
• The most abundant FA: myristic, palmitic,
stearic and oleic acids
• Differences depending on composition of
fat during season (more unsaturated FA in
summer)
Fat
• Lower fat content in milk for cheese
production → lower yield
• Important for sensory properties
– Taste/flavour – directly and also develop
during ripening
– Texture – lower fat content, higher hardness
– Meltability – higher fat content, better
meltability
Lactose
• Main saccharide in milk
• Disascharide
– Glucose and galactose (β-(1,4)-glycosidic
bond)
– α- and β-anomeres
Lactose
• Production in lactoc gland
• Soluble in water
• Sweet taste of milk (only 33 % of the
sweetness of sucrose)
• Effect on osmotic pressure in milk
Lactose
• Fermentation by LAB (lactic acid bacteria)
– Main product is lactic acid
– Ripening of cheese (naturaly lactose free)
– Decreasing pH of cheese
• Syneresis of protein gel
• Longer shelf-life
Lactose
• Energy source
• Decomposition to monosaccharides by
lactase (enzyme)
• Lactose intolerance
– Low production of lactase
– Increased osmotic preassure in intestine
– Flatulence,
diarrhoea
Minerals
• Up to 1 %
• Mainly calcium, phosphorus, potassium,
sodium, chlorine, magnesium, sulphur, etc.
• Different forms
– Solution
– Colloid form
– Linkage with milk constituents
Calcium and phosphorus
• Forms
– Soluble form (30 %)
– Colloid calcium phosphate (40–50 %)
– Bonded in proteins (20 %)

• Soluble form of Ca: necessary for second

phase od precipitation of casein by
enzymes
Calcium and phosphorus
• Changes during heat treatment of milk

• Cheese production
– Weak curd – less bond between protein
chains
– Addition of CaCl2 (E509) for increasing Ca2+
ions
Vitamins
• Source of all vitamins, but variable content
• Water-soluble vitamins
– B-complex (thiamine, riboflavine, niacine,
pyridoxine, folacine, kobalamine, panthoteic
acid)
– Vitamin C
• Fat-soluble vitamins
– A, D, E, K
– Provitamine A (β-carotene)
Vitamins
• Riboflavine
– Yellow/green colour of whey
• Vitamin C
– Thermolabile (degradation during heat
treatment)
– Antioxidant
• Carotenoids β-carotene
– Milk colour
Enzymes
• Distribution in milk
– Fat globules surface
– Milk proteins association
• Sources
– Endogenic (lactic gland)
– Exogenic (microbial
enzymes)
Enzymes
• Lipases
– Hydrolysis of
triglycerides →
glycerol + FA
– Protection against
lipolysis by
phospholipid
membrane on the
fat globule surface
Enzymes
• Alcalic phosphatase
– Hydrolysis of estericaly bound phosphoric
acid

– Inactivation: 70 °C/15 s → control of correctly

performed pasteurization
Enzymes
• Proteases
– Low activity in mature milk
– High activity in contamining microorganisms
– Plasmine: proteolysis of caseins (cheese from
raw milk)
Hormones
• Catalysis of metabolic pathways
• Transfer from blood into milk
– Possible risk for consumer
– Rules for veterinary treatment and use of
such milk
Gases
• 6–9 % in fresh milk
• Especially CO2, N2 and O2
• Role of O2
– Oxidation of vitamin C and fat
– Stimulation of aerobic microorganisms
– Inhibition of anaerobic microorganisms
(bifidobacterium)
Milk properties
• Organoleptic properties
• Density
• Freezing point
• Acidity
• Fermentation ability
• Curd-forming ability
• Heat stability
• Surface tension
Organoleptic properties
• Odour
– No strong smell
– Acceptation of odours from environment
(bond with fat globules)
– Good hygienic praxis!
• Colour
– White opaque liquid (fat globules and caseine
micelles)
– Content of riboflavine and carotens
Organoleptic properties
• Consistency
– High amount of water
– Homogenic structure (polydisperse system)
Density
• Usually in the range 1.028–1.032 g/cm3
• Dependent on the basic constituents
content
– Lowering with increasing fat content
– Increasing with increasing protein, lactose
and mineral content
Freezing point
• Usually in the range from -0.54 to -0.57 °C
• Lower value (compared to water) caused
by
– Lactose content
– Soluble mineral content
• Quick consideration of technologic
integrity of raw milk
– Addition of 1 % of water → increase in
freezing point by 0.005 °C
– Intended/unintended addition
Acidity
• Titration acidity
– Consumption of 0.25M NaOH needed for
neutralization of 100 ml milk
– Normal value 6.2–7.8°SH (mmol/l)

• Active acidity (pH)

– Normal value 6.4–6.8
– Buffering capacity of milk
Fermentation ability

• Ability to form lactic acid by fermentation

of lactose
– Lactic acid bacteria
• very sensitive
• importance of hygiene (milking and milk treatment)
• inhibitory substances residues (cleansing and
disinfectant agents, pesticides, veterinary
pharmaceuticals, etc.)
Curd forming ability

• Ability to coagulate with rennet and form

curd with desired properties
• Elementary criteria of milk quality for
cheese making
Heat stability

• Resistance of milk proteins to precipitation

by heat
• Important for sterilisation and UHT
treatment
• Effect of pH
– decrease of heat stability at pH <6.2
Surface tension

• Energy needed to create as small as

possible surface
– decrease of heat stability at pH <6.2
• Water at 20 °C: 47.10-3 N.m-1
• Milk at 20 °C: 72.10-3 N.m-1
– difficulties with cleaning of porous devices
– need of tensides
Thank you for your attention