milk composition

1. DAIRY TECHNOLOGY

2. Dr Aysha Sameen
MILK COMPOSITION

3. Defination of Milk
 Milk may be defined as the whole, fresh,
clean, lacteal secretion obtained by the
complete milking of one or more healthy
milch animals, excluding that obtained
within 15 days before or 5 days after
calving or such periods as may be
necessary to render the milk practically
colostrums-free, and containing the
minimum prescribed percentage of milk fat
and milk solid not fat

4. MILK
True fat ( 98% TGs +
MG+ DG+ FFA
Phospholipids
(Lecithin, Cephalin,
Sphyngomylin)
Water
Total Solids
Fat (Lipids)
Emulsion form (50-
100 nm dia.)
SNF
Associated
Substance
Lactose
(solution form
.01 -1 nm)
Nitrogen
Substance
Mineral
matter
Other
constituents
Cholesterol Carotene
Vitamins
(A,D,E,K)
Non protein
Protein
(Suspension, 1-100nm dia.)
Caseins
(α,β,γ,κ) α- Lactalbumin
β Lactglobulin Proteose Peptones
PO4, citrates ,
Chlorides of Na, K,
Ca, Mg + traces of
Fe, Cu, I etc.
•Pigments
•Dissolved Gases
•Vit. C &
B Complex.
•Enzymes etc

5. Milk COMPOSITION
 The principal constituents of milk are water, fat,
proteins, lactose (milk sugar) and minerals (salts).
 Milk also contains trace amounts of other
substances such as pigments, enzymes, vitamins,
phospholipids
(substances with fatlike properties), and gases.
 The residue left when water and gases are removed
is called the dry matter (DM) or total solids content of
the milk

6. Composition of milk from
various animals.

8. Milk Fat
 The milk fat exists as small globules or
droplets dispersed in the milk serum,
 Their diameters range from 0,1 to 20 µm (1 µm
= 0,001 mm).
 The average size is 3 – 4 µm and there are
some 15 billion globules per ml.
 The emulsion is stabilised by a very thin
membrane only 5 – 10 nm thick (1 nm = 10–9
m ) which surrounds the globules and has
complicated composition.

9. The composition of milk fat.

10. Milk fat consists of :
 triglycerides (the dominating components),
 di- and monoglycerides,
 fatty acids,
 sterols,
 carotenoids (giving the yellow colour of
the fat)
 vitamins (A, D, E, and K)

11. The fat globule membrane consists of
 phospholipids,
 lipoproteins,
 cerebrosides,
 proteins,
 nucleic acids,
 enzymes,
 trace elements (metals) and bound water.

12. Fat globules have the
lightest density at15,5
°C 0,93 g/cm3, they
tend to rise to the
surface when milk is
left to stand in a
vessel.

13.  The rate of rise follows Stokes’ Law, but the
small size of the fat globules makes creaming a
slow process.
 Cream separation can, however, be accelerated
by aggregation of fat globules under the
influence of a protein called agglutinin.
 These aggregates rise much faster than
individual fat globules
 The aggregates are easily broken up by heating
or mechanical treatment.
 Agglutinin is denatured at time-temperature
combinations such as 65 °C/10 min or 75 °C/2

14. PRINCIPAL FATTY ACID IN
MILK

15. FATTY ACID IN MILK
 four most abundant fatty acids in milk are
myristic,
palmitic, stearic and oleic acids.
 This variation of fatty acids affects the
hardness of the fat.
 Fat with a high content of high-melting fatty
acids, such as palmitic acid, will be hard;
 but on the other hand, fat with a high content
of low-melting oleic acid makes soft butter.

16. Iodine value
 The iodine value states the percentage of
iodine that the fat can bind.
 Iodine is taken up by the double bonds of the
unsaturated fatty acids.
 Since oleic acid is by far the most abundant of
the unsaturated fatty acids, which are liquid at
room temperature, the iodine value is largely a
measure of the oleic-acid content and thereby
of the softness of the fat.
 The iodine value of butterfat normally varies
between 24 and 46.

17. Refractive index
 The amount of different fatty acids in fat also
affects the way it refracts light.
 This is a quick method of assessing the
hardness of the fat.
 The refractive index normally varies between
40 and 46.

18. Cow milk proteins
Concentration g/kg %
Total proteins 33.0 100.0
Total caseins 26.0 79.5
as1-casein 10.0 30.6
2.6 8.0
b-casein 9.3 28.4
-casein 3.3 10.1
Whey proteins 6.3 19.3
a-lactalbumin 1.2 3.7
b-lactoglobulin 3.2 9.8
Bovine Serum Albumin 0.4 1.2
Immunoglobulins 0.7 2.1
Other (proteoses-peptones) 0.8 2.4
Proteins of fat globule membrane 0.4 1.2
as2-casein

19. Composition of casein micelles
 93 % caseins : 4 phosphoproteins
 as1-CN : 36 %
 as2-CN : 10 %
 b-CN : 34 %
 -CN : 12 %
 7 % : colloidal mineral complex
containing phosphate, calcium,
magnesium and citrate
In milk the whey proteins are in colloidal solution

20. Amino acids
 1.Negatively charged in alkaline solutions
 2. Neutral at equal + and – charges
 3. Positively charged in acid solutions

21.  If the side chain is polar, the water-attracting
properties of the basic and acid groups, in
addition
to the polar side chain, will normally dominate
and
the whole amino acid will attract water and
dissolve in water. hydrophilic
 A long hydrocarbon chain repels water and
makes the amino acid less soluble or
compatible with water. Hydrophobic
 If there are certain radicals such as hydroxyl (–
OH) or amino groups (–NH2) in the
hydrocarbon chain, its hydrophobic properties
will be modifiedtowards more hydrophilic.

22.  Hydroxyl groups in the chains of some amino
acids in casein may be esterified with
phosphoric acid.
 Such groups enable casein to bind calcium
ions or colloidal calcium hydroxyphosphate,
forming strong bridges between or within the
molecules.

23. The electrical status of milk
proteins
 The side chains of some amino acids in milk
proteins carry an electric charge which is
determined by the pH of the milk.
 When the pH of milk is changed by addition of
an acid or a base, the charge distribution of
the
proteins is also changed.

27. Casein
 Casein is a group name for the dominant class
of proteins in milk.
 The caseins easily form polymers containing
several identical or different types of
molecules.
 Abundance of ionisable groups and
hydrophobic and hydrophilic sites the
molecular polymers formed.

28. Casein micelles.
 The polymers are built up of hundreds and
thousands of individual molecules and form a
colloidal solution
These molecular complexes are known as
casein micelles.
 Such micelles may be as large as 0.4 microns,
and can only be seen under an electron
microscope
 A medium-sized micelle consists of about 400
to 500 sub-micelles which are bound together

30. Subgroups of casein
 The three subgroups of casein, αs-casein, κ-
casein and β-casein,
 All heterogeneous and consist of 2 – 8 genetic
variants.
 Genetic variants of a Protein differ from each
other only by a few amino acids.
 The three Sub- Groups have in common the fact
that one of two amino acids containing hydroxy
groups are esterifies to phosphoric acid.
 The phosphoric acid binds calcium and
magnesium and some of the complex salts to
form bonds between and within molecules.

31. Casein micelles

32.  The content of α-, β- and κ-casein is
heterogeneously distributed in the different
micelles.
 Calcium salts of αs-casein and β-casein are al-
most insoluble in water, while those of κ-casein
are
readily soluble.
 Due to the dominating localisation of κ-casein
to the surface of the micelles, the solubility of
calcium κ-caseinate prevails over the
insolubility of the other two caseins in the
micelles,

33.  The α- and β-caseins are mainly concentrated
in the middle of the sub-micelles, while κ-
casein predominates on the surface.
 The hydrophilic protruding chain of the κ-
casein protrudes from the surface of the sub-
micelles forming a hairy layer ( 5 – 10 nm).
 The κ-casein-deficient sub-micelles are
mainly located in the centre of the micelle,

34.  whereas the κ-casein-rich sub-micelles
predominate on the surface, giving the whole
micelle a hairy surface layer.
 The hairy layer of the κ-casein’s protruding
chain is partially responsible for the micelle’s
stability through a major contribution to the
negative charge of the micelles


35.  The calcium phosphate and hydrophobic
interactions between sub-micelles are
responsible for the integrity of the casein
micelles.
 Adding an excess of Ca and phosphate results
in aggregation of sub-micelles into larger units
– micelles.
 The reason for this aggregation is presumably
the
deposition of Ca-phosphate in the sub-micelles,
which lowers their electric charge and makes

36. Casein curd
 If the hairy layer is removed, e.g. by acid addition
or rennet – induced hydrolysis, the colloidal
stability of the micelle is destroyed and the
micelles coagulate or precipitate.
 In an intact micelle there is surplus of negative
charges, therefore they repel each other.
 Water molecules held by the hydrophilic sites of k-
casein form an important part of this balance.
 When the hydrophilic sites are removed, water will
start to leave the structure.
 This gives the attracting forces room to act

37. Casein curd
 New bonds are formed, one of the salt type,
where calcium is active, and the second of the
hydrophobic type
 These bonds will then enhance the expulsion
of water and the structure will finally collapse
into a dense curd.

38. Low temperature effect on β-
casein
 The micelles are adversely affected by low
temperature
 β-casein chains start to dissociate and the CCP
leaves the micelle structure, where it existed in
colloidal form, and goes into solution.
 The explanation of this phenomenon is that β-
casein is the most hydrophobic casein and that
the hydrophobic interactions are weakened when
the temperature is lowered.
 The loss of CCP causes a weaker attraction
between sub-micelles and individual casein
molecules in the sub-micelles.

39.  β-casein is then also more easily hydrolysed
by various proteases
 Hydrolysis of β-casein to γ-casein and
proteose-peptones means lower yield at
cheese production because the proteose-
peptone fractions are lost in the whey.
 The breakdown of β-casein may also result in
formation of bitter peptides, causing off-flavour
problems in the cheese.

41. Precipitation by casein
 One characteristic property of casein is its
ability to precipitate.
 Due to the complex nature of the casein
molecules, and that of the micelles formed
from them, precipitation can be caused by
many different agents.

42. Precipitation by acid
 The pH will drop if an acid is added to milk or if
acid-producing bacteria a allowed to grow in milk.
This will change the environment of the casein
micelles in two ways.
1. Firstly colloidal calcium hydroxyphosphate,
present in the casein micelle, will dissolve and
form ionised calcium, which will penetrate the
micelle structure
and create strong internal calcium bonds.
2. Secondly the pH of the solution will approach the
isoelectric points of the individual casein
species

43.  Both methods of action initiate a change within
the micelles.
 Growth of the micelles through aggregation
and ending with a more or less dense
coagulum.
 The isoelectric points of the casein
components depend on the ions of other kinds
present in the solution.
 Theoretical values, valid under certain
conditions, are pH 5.1 to 5.3.

44. Precipitation by enzymes
 The amino-acid chain forming the κ-casein
molecule consists of 169 amino acids.
 From an enzymatic point of view the bond
between amino acids 105(phenylalanin) and
106 (methionin) is easily accessible to many
proteolytic enzymes.
 The soluble amino end contains amino acids
106 to 169, which are dominated by polar
amino acids and the carbohydrate, which give
this sequence hydrophilic properties.

45.  This part of the κ-casein molecule is called the
glycomacro-peptide and is released into the whey
in cheesemaking.
 The remaining part of the κ-casein, consisting of
amino acids 1 to 105, is insoluble and remains in
the curd together with αs- and β-casein.
 This part is called para-κ-casein.
 The formation of the curd is due to
1. Sudden removal of the Hydrophilic
Macropeptides
2. Imbalance in intermolecular forces.

46.  Bonds between hydrophobic sites start to develop
and are enforced by calcium bonds which develop
as the water molecules in the micelles start to
leave the structure.
 This process is usually referred to as the phase of
coagulation and syneresis.
 The splitting of the 105 – 106 bond in the κ-casein
molecule is often called the primary phase of the
rennet action,
 while the phase of coagulation and syneresis is
referred to as the secondary phase.

47.  There is also a tertiary phase of rennet action,
where the rennet attacks the casein
components in a more general way. This
occurs during cheese ripening.
 The durations of the three phases are
determined mainly by pH and temperature.

48. Whey proteins
 Whey protein is the name commonly applied
to milk serum proteins.
 they are not precipitated at their isoelectric
points.
 They are, precipitated by polyelectrolytes such
as carboxymethyl cellulose.
 When milk is heated, some of the whey
proteins denature and form complexes with
casein, thereby decreasing the ability of the
casein to be
attacked by rennet and to bind calcium.

49. α-lactalbumin
 Whey proteins in general, and α-lactalbumin in
particular, have very high nutritional values.
 Their amino acid composition is very close to
that which is regarded as a biological
optimum.
 α - Lactalbumin contains 123 amino acids and
represents about 25% of the serum proteins in
milk

50. α-lactalbumin
 It is present in milk from all mammals and
plays a significant part in the synthesis of
lactose in the udder
 The protein has a very compact, globular
structure that is nearly spherical in shape.
 α - Lactalbumin is the most heat stable serum
protein in milk. 50% of the α -Lactalbumin will
not be denatured even after 30 minutes of
heating at 77oC.

51. β-lactoglobulin
 β - Lactoglobulin contains 162 amino acids.
 It is the major milk serum protein. It is about
50% of the serum protein and 8% of the
protein in milk.
 There is no β -Lactoglobulin present in
human milk.
 β - Lactoglobulin can be irreversibly
denatured by heat.
 This stress causes rupture of intramolecular
disulfide bonds and precipitation

52.  If milk is heated to over 60 °C, denaturation is
initiated where the reactivity of the sulphur-amino
acid of β-lactoglobulin plays a prominent part.
 Sulphur bridges start to form
1. between the β-lactoglobulin molecules,
2. between β-lactoglobulin molecule and a κ-casein
3. between β-lactoglobulin and α-lactalbumin.
 At high temperatures, sulphurous compounds
such as hydrogen sulphide are gradually
released.
 These sulphurous compounds are responsible for
the
“cooked” flavour of heat treated milk.

53. Immunoglobulins and related
minor proteins
 Immunoglobulins are antibodies synthesised in
response to stimulation by specific antigens.
 They are specifically present in blood.
 Their content in cows’ milk is low, but some of
them are present in higher levels in colostrum
and human milk.
 They can also act against “particles” such as
bacteria, viruses and even fat globules, and
flocculate them, a reaction called agglutination

54. Serum albumin
• Comes from blood
• Role in the transport of bile salts, fatty acids

55. Membrane proteins
 Membrane proteins are a group of proteins
that form a protective layer around fat globules
to stabilise the emulsion
 Some of the proteins contain lipid residues
and are
called lipoproteins.

56. Denatured proteins
 As long as proteins exist in an environment
with a temperature and pH within their limits of
tolerance, they retain their biological functions.
 If they are heated to temperatures above a
certain maximum their structure is altered.
 The same thing happens if proteins are
exposed to acids or bases, to radiation or to
violent agitation.
 The proteins are denatured and lose their
original solubility.

57. Denatured proteins
 When proteins are denatured, their biological
activity ceases.
 Enzymes, a class of proteins whose function is
to catalyse reactions, lose this ability when
denatured.
 The reason is that certain bonds in the
molecule are
broken, changing the structure of the protein.
 After a weak denaturation, proteins can
sometimes revert to their original state, with
restoration of their
biological functions.

58. Milk Enzymes
 Enzymes in milk occur in various states:
 (1) as unassociated forms in solution,
 (2) associated or an integral part of membrane
fractions, such as the fat globule membrane
 (3) associated with casein micelles,
 (4) as part of the microsomal particles.
 The origin of these enzymes in milk is from:
 cow’s udder (synthesized enzymes)
 or from bacterial enzymes (bacterial source).

59. PLASMIN
 This enzyme hydrolyzes proteins.
 Limited proteolysis of B-casein by this enzyme is
responsible for the presence in milk of large
polypeptides derived from this protein, known as the
gamma-caseins.
 Activity of this enzyme is also important in cheese
ripening and the stability of casein micelles in various
products such as UHT milk.
 Nearly, 80% of its proteolytic activity is lost when milk is
pasteurized.
 Microbial derived proteases are more heat stable than
native
 proteases in milk and they tend to survive even UHT

60. Lactoperoxidases
 Peroxidase transfers oxygen from hydrogen
peroxide (H2O2) to other readily oxidisable
substances.
 This enzyme is inactivated if the milk is heated
to
80 °C for a few seconds,

61. Catalase
 catalase splits hydrogen peroxide into water and
free oxygen.
 By determining the amount of oxygen that the
enzyme can release in milk, it is possible to
estimate the catalase content of the milk and learn
whether or not the milk has come from an animal
with a healthy udder.
 Milk from diseased udders has a high catalase
content, while fresh milk from a healthy udder
contains only an insignificant amount.
 Catalase is destroyed by heating at 75 °C for 60
seconds.

62. Phosphatase
 Phosphatase split certain phosphoric-acid
esters into phosphoric acid and the
corresponding alcohols.
 Phosphatase is destroyed by ordinary
Pasteurisation (72 °C for 15 – 20seconds), so
the phosphatase test can be used to
determine whether the Pasteurisation
temperature has actually been attained.
 The phosphatase test should preferably be
performed immediately after heat treatment.

63. Lipase
 Lipase splits fat into glycerol and free fatty acids.
Excess free fatty acids in milk and milk products
result in a rancid taste.
 The action of this enzyme seems, in most cases,
to be very weak, though the milk from certain
cows may show strong lipase activity.
 The quantity of lipase in milk is believed to
increase towards the end of the lactation cycle.
 Lipase is, to a great extent, inactivated by
pasteurisation, but higher temperatures are
required for total inactivation.

64.  Beneficial effects of its activity include the
possible aid in initial digestion and absorption
of milk lipids in the intestinal tract and flavor in
certain cheeses made from raw milk.
 sodium and magnesium tend to stimulate the
lipase activity, while calcium and magnesium
show an inhibitory effect.

65. Mineral in milk
 Mineral salts occur in solution in milk serum or
in casein compounds.
 The most important salts are those of calcium,
sodium, potassium and magnesium.
 They occur as phosphates, chlorides, citrates
and caseinates.
 Potassium and calcium salts are the most
abundant in normal milk.
 The amounts of salts present are not constant.

66.  Towards the end of lactation, and even more
so in the case of udder disease, the sodium
chloride
content increases and gives the milk a salty
taste.

67. Mineral Concentration (mg/kg)
Total calcium 1250
Soluble calcium 350
Total magnesium 115
Soluble magnesium 70
Total sodium 425
Soluble sodium 400
Total potassium 1600
Soluble potassium 1500
Total chloride 1100
Soluble chloride 1100
Total phosphorus 950
Soluble phosphorus 420
Total inorganic phosphate (in phosphorus) 720
Soluble inorganic phosphate (in phosphorus) 300
Total citrate 1650
Soluble citrate 1500
Mineral composition (total and soluble) of milk
[Total] – [Soluble] = [Micellar] or [Colloidal]

68. Partition of Major Minerals in Colloidal and
Soluble Phases (% of Total Minerals)

69. Carbohydrates in milk : two
types
Free
• lactose
• glucose
• galactose
• N-acetylated glucose
• N-acetylated galactosamine

70.  Linked to proteins
• glycoproteins
• -casein
• Lactoferrin
• Lactoperoxidase
•

71.  Disaccharide composed of one glucose unit
and one galactose unit
 Major sugar of the most milks from different
mammals
 Lactose is one of the least soluble of the
common sugars, having solubility in water of
only 17.8% at 25◦C
 Cow and human milks contain about 4.8 %
and 7 % of lactose respectively

72.  When lactose undergoes dehydration to form
lactulose. It stimulates the growth of
Bifidobacterium bifidum and is thus beneficial
in establishing healthy commensal microbiota
in the gut.
 Lactose is a good source of energy and may
promote calcium absorption.

73.  Digestion of lactose presents a problem in
some people as they lack beta-d-
galactosidase enzyme in their GIT.
 Consequently, dietary lactose is not
hydrolyzed and reaches colon where it is
metabolized by colonic bacteria forming gases
(methionine and hydrogen).
 Accumulation of gas leads to discomfort
caused by bloating and diarrhea.

74.  Such lactose malabsorption is aggravated by
yogurt containing live cultures,
 the culture furnishes the lactose-hydrolyzing
enzyme beta-d-galactosidase and normal
digestion pattern is restored.

75. • Sucrose : 100
• Fructose : ~ 150
• Glucose : ~ 75
• Maltose : ~ 40
• Galactose : ~ 35
• Lactose : ~ 20
Sweet value of lactose
Low sweet value of lactose compared to other sugar

76.  Contribute to the nutritive value of milk and dairy products
 Essential component for the fermentation of some dairy
products (yoghurts, cheeses, …)
 Affects the texture of some concentrated and frozen
products (viscosity, cristals, …)
During rapid drying,amorphous lactose is formed. This
form of lactose is very hygroscopic and causes caking in
dried products containing moisture levels of 8% or more.
 Participate to the color and flavor changes of dairy
products during heat treatments and storage (Maillard
reaction)
Role of lactose in milk and dairy products

77. Food Applications
 Infantile food
 Glazing, prevent crystallisation of other sugars in mixture, ...
(confectionery - bakery)
 Appearance and taste of bakery products, fried foods...
(Maillard)
 Exhauster of taste (sauces, french dressing,...)
 Stabiliser of aroma
 Agent of encapsulation (confectionery

78. Vitamins in milk
 Vitamins are organic substances that occur in
very small concentrations in both plants and
animals.
 They are essential to normal life processes,
but
cannot be synthesised by the body.
 Among the best known vitamin in milk are A,
the vitamin B group, vitamin C and D.

79. Vitamins in Bovine Milk

80. Physical properties of milk
Appearance
 the opacity of milk is due to its content of
suspended particles of fat, proteins and certain
minerals.

81. Density formula

82. Osmotic pressure
 Osmolality is a measure of the total number of
dissolved particles in a given volume of
solution given in osmol/KJ.
 Osmotic pressure is controlled by the number
of molecules or particles, not the weight of
solute; thus 100 molecules of size 10 will have
10 times the osmotic pressure of 10 molecules
of size 100

83. Freezing point
 The freezing point of milk is lower than that of
pure water due to the dissolved components
such as lactose and soluble salts.
 The freezing point of milk is the only reliable
parameter to check for adulteration with water.
 The freezing point of milk from individual cows
has
been found to vary from –0.54 to –0.59°C.
 Adulterated milk will show increased freezing
point due to lower molal concentration of
lactose and salts.

84. .
Boiling Point
 The boiling point of milk is higher than that of pure
water due to dissolved components.
 The boiling point of milk is 100.17◦C.
Density
 Milk density at 20◦C ranges from 1.027 to 1.033
with an average of 1.030 g/cm3
 Temperature affects density because milk
expands when heated and so it becomes less
dense as temperature rises.

85. Specific Gravity
 It is the ratio of the mass of a solution or a
substance to the mass of a similar volume of
water.
 fresh whole milk has specific gravity in range
of
1.030–1.035, with an average of 1.032.

86. Other constituents of milk
 Milk always contains somatic cells (white
blood corpuscles or leucocytes).
 The somatic cell content of milk from healthy
animals is as a rule lower than 200 000
cells/ml, but counts of up to 400 000 cells/ml
can be accepted.
 Milk also contains gases, some 5 – 6 % by
volume in milk fresh from the udder, but on
arrival at the dairy, the gas content may be as
high as 10 % by volume.

87.  The gases consist mostly of carbon dioxide,
nitrogen and oxygen.
 Dissolved in the milk
 Bound and non-separable from the milk
 Dispersed in the milk

88. FACTORS AFFECTING
COMPOSITION
 1. Species
 2. Breed
 3. Individuality
 4. Interval of milking
 5. Completeness of milking
 6. Frequency of milking
 7. Irregularity of milking
 8. Day-to-day milking
 9. Diseases and abnormal conditions

89. Effects of Heat on Milk

90. Heat Stability of Milk
“The heat stability time (HCT) is defined as the
time
taken for milk to coagulate while being agitated
in
a sealed tube at a set temperature (usually
between
120 and 140◦C).”
 Determination of the heat stability (by HCT
methods) showed the milk was least stable in
June but most stable in October

91. Milk Mineral and Heat Stability
 Calcium and phosphate have been found to
play a
major role in the heat stability of milk.
 addition of calcium salts to milk causes an
increase in serum calcium ions, an increase in
calcium in the colloidal phase, a decrease in
pH and a decrease in heat stability
 Alteration of calcium concentration by the
addition of phosphates or citrates generally
causes an
increase in heat stability.

92.  In concentrated milks addition of either whey
protein or purified b-lactoglobulin has a
detrimental effect on heat stability of
concentrated milk.
 Addition of casein to milk increased the
stability of concentrates produced from
adjusted milk (adjustment of casein
components of milk protein).
 Modification of the casein by hydrolysis
causes loss of heat stability.