Definition and Composition of milk (The ‘major’ and ‘minor’ constituents of milk)
CHEMICAL COMPOSITION:
It is important that the student first become familiar the chief constituents of milk, leaving until later a consideration of those which are present in small quantities, even though they may be important.
A chemist might describe milk as, essential an emulsifying of fat in a watery solution of sugar and mineral salts, and w
|
Percentage
|
|
|
Water |
87.25
|
|
Dry
matter |
12.75
|
|
Fat |
3.80
|
|
Protein |
3.50
|
|
Sugar |
4.80
|
|
Ash |
0.65
|
|
Total
|
100.00 |
Milk fat or butter fat refers to the “fat of milk’’.There are certain terms used in connection with the
above constituents when referring to them singly and in groups.
The protein sugar and ash or salts of milk are termed
“milk solids-not-fat” or “solids-not-fat”. Those in dairy often refer to this
group of solids as the”serum solids” of milk.
The dry matter is commonly designated as “total
solids”.
The ‘major’constituents of milk are :Water, Fats, Protein, Lactose or Sugar, ash or mineral matter.
The ‘minor’constituents of milk are : Phospholipids, Cholestrol, Pigments, Enzymes, Vitamines, Gases, Non-protein Nitrogenous substances
.
Table
Chemical composition of milk of different species
|
Name of
species |
Percentage
compositions |
||||
|
Water |
Fat |
Protein |
Lactose |
Ash |
|
|
Cow(foreign) |
86.6 |
4.6 |
3.4 |
4.9 |
0.7 |
|
Buffalo |
84.2 |
6.6 |
3.9 |
5.2 |
0.8 |
|
Goat |
86.5 |
4.5 |
3.5 |
4.7 |
0.8 |
|
Camel |
86.5 |
3.1 |
4.0 |
5.6 |
0.8 |
|
Ass |
90.0 |
1.3 |
1.7 |
6.5 |
0.5 |
|
Elephant |
67.8 |
19.6 |
3.1 |
8.8 |
0.7 |
Major Milk Constituents:
(1) Water in Milk:
Water constitutes the medium in which the other
milk constituents are either dissolved or suspended. Most of it is ‘Free’, and
only a few very small portion is in the ‘bound’ form, being firmly bound by
milk proteins, phospholipids etc.
In average milk ,about 87 in each
pounds is water. This water is not different from ordinary water and serve to
hold in solution the soluble constituents of milk.
The percentage of water varies from
84.0 to 89.0 per cent,although occasionally , an individual sample of authentic
milk may exceed these limits.
Any variation in the amount of other
constituents is also reflected upon the water percentage.
The fluid nature and the high water
percentage as given above, often lead to an erroneous conception of the food
value of milk.
The facts that many other articles of
food, including radishes, carrots and pumpkins, contain even more water than
milk, while apple and potatoes rank but little above the richer milk samples in
this respect.
Among cattle feeds, fresh grass and
root crops have less dry matter than milk and some silages but little more.
(2)Fat of Milk:
The bulk of the fat in milk exists in
the form of small globules, which average approximately 2 to 5 microns in size
( range 0.1 to 22 microns).
This is an oil-in-water type emulsion.
The surface of these fat globules is
coated with an absorbed layer of material commonly know as the fat globule
membrane.
This membrane contain phospholipids
and proteins in the form of a complex, and stabilize the fat elusion.
In other words, the membrane present
the fat globules from coalescing but keeps separated from one another.
The emulsion may, however be broken
by agitation ( at a low temperature) , heating ,freezing, etc.
When milk is held undisturbed, the
fat globules tends to rise to the surface to form a cream layer.
The thickest cream layer is secured
from milks which have a higher fat content and relatively large fat globules(
such as buffalo’s milk when compared with cow’s milk)
Chemically, milk fat is composed of a
number of glyceride-esters of fatty acids; on hydrolysis, milk fat furnishes a
mixture of fatty acids and glycerol.
The fatty acids are saturated and
unsaturated .
Saturated fatty acids are relatively
stable.
On the other hand, the unsaturated
ones play an important role in the physic-chemical properties of milk fat.
Milk often called “butter fat” is
commercially the most valuable constituent of milk.
It is also of great importance from
the stand point of the food value of the milk.
The agreeable flavor of rich milk, to
a large extent of other dairy products is largely due to the milk fat.
The cream separator concentrate the
milk fat into a small portion of the milk. The product is know as “cream”.
Normally cream for table use contain
18 to 25 per cent of milk fat.
When cream is churned , the milk is
gathered into a concentrated mass, and after working ,it is know as “butter”.
Butter is , therefore a concentrated substance containing from 80
to 85 per cent of milk fat
(3)Protein of Milk
Protein are among the most complex of organic substances.
They contain Carbon, hydrogen,
oxygen, nitrogen,s ulphur and sometimes phosphorus.
They are Characterized chiefly by the
fact that they contain nitrogen. Protein is indispensable part of the food of
animals as it is chief constituents of the protoplasm which forms the vital
part of every living cells.
Proteins are composed of a large
number of amino-acids, some ‘essential’ and other ‘non-essential’. The
essential amino acids are necessary in the diet for the formation of body
proteins.
On hydrolysis , protein furnish a
mixture of amino acids.
The proteins of milk consists mainly
of casein, β-lactoglobuline, α-lactalbumin,
Albumin,other protein etc.
(i) Casein
There is no protein in nature similar
to casein.
As it appears in commerce, casein is
a yellowish-white granular substance.
In its pure state it is snow-white ,
odourless, and tasteless.
It contributes to the whiteness of
milk.
In milk it is found in combination
with calcium and is often designated as “Calcium caseinate”.
It is a well- established fact that
casein exists in milk in the form of small gletinous particles in suspension.
The chemist refers to this state of dispersion as “colloidal”.
Casein can be removed from milk by
filtering through a porcelain filter or through an animal membrane.
Casein may be precipitated from milk
by dilute acid , rennin, and alcohol.
The casein precipitated with alcohol is calcium caseinate.
The casein precipitated with week acids is free of calcium.
When precipitated with rennin,
paracasin is formed, it contain more calcium than calcium casenate.
Pure casein is not precipitated by
heat, but in fresh milk prolonged heating at high temperature (121⁰F.(100 ⁰C)).
For 12 or more hours or 248 ⁰F.(120 ⁰ C) .under pressure will cause the
precipitation of casein.
On boiling fresh milk , a thin layer
of finely precipitated casein , together with other milk constituents including
fat, forms a thin layer over the surface of the milk.
The
application of heat to milk that is slightly acid will cause the
precipitation of casein. This fact is know to housewife who has had it in home.
She has learned too, that when combining milk with acid foods, such as
tomatoes, care must be exercised or the milk will ”curdle”.
Casein forms more than 8 % of the
total protein in milk.
Casein itself is composed of α, β, γ fractions.
The heterogeneous nature of α- casein can be observed
through electrophoresis. α- casein is the component in casein
micelle that is responsible for the stabilization of the micelle in milk.
Later studies have also revealed that
α-
casein is composed of at least two
sub-fractions, viz, α s- casein
precipitated by calcium-ion under certain conditions and also called
çalcium-sensitive casein; and K-casein, also called ‘calcium-insensitive
casein’,not precipitated by calcium-ion.K—casein is the richest repository of
carbohydrates as against other casein frcation . It is also a site for rennin
action.
(ii) β-lactoglobuline, α-lactalbumin
β-lactoglobuline and α-lactalbumin are also know as whey or serum-proteins.
They are also
present in collidal state and are easliy
coagulable by heat
(iii)Albumin
The term ‘albumin’ is used commonly
to designated the protein of milk known chemically as “Lactalbumin”.
The albumin content of the milk
remain fairly uniform at about 0.5 % .
It resembles but is not identical
with blood albumin.
When dried, lactalbumin is a
tasteless powder.
Like other proteins albumin has an
extremely complex chemical composition.
It contains carbon, oxygen, hydrogen,
nitrogen, and small quantity of sulfur.
The sulphur content, through small,
is about twice that of casein.
Albumin contains no phosphorous.
Unlike casein, albumin is coagulated by heat,
but not by rennet.
There is some question as to whether
albumin exist in milk in the colloidal state or in true
solution.
Albumin may be separated from milk by
first precipitating the casein by the use of week acid , and heating the
filtrate whey to a temperature of 158⁰F.( 70 ⁰C). Or above ,for a short period.
The turbid appearance of the heated
filtrate shows the presence of coagulated albumin.
By filtering the heated material the
albumin may be recovered on the filter.
This method however, does not result
in a complete precipitation of the albumin.
Lactalbumin precipitated by the
action of heat is insoluble.
(iv) Other Proteins:
In addition to casein and
lactalbumin, milk is frequently credited with containing traces of other
proteins namely, lactoglobulin, fibrin, and alcohol-soluble proteins.
Lactoglobulin was first isolated from
milk by Sibelin and Emmerling, who work independently.
The occurrence of globulin as
independent protein has been questioned
by chemist through its properties and freely discussed.
Milk does not contain more than 0.1 %
of lactoglobulin.
Only a trace of fibrin occurs in
milk, and under some condition it is totally absent.
Evidence indicate that milk contain an alcohol-soluble protein, the characteristics of which differ from the alcohol-soluble proteins of cereals in which these proteins usually occur.
(4) Lactose or Milk Sugar:
Milk sugar, commonly designated by
the chemist as “lactose”, is found only in milk.
It is reducing disaccharides which
upon hydrolysis yield 1 molecule of galactose and 1 molecule of glucose.
It has the same molecular
formula,C12H22O11,
As ordinary cane sugar or sucrose.
It differs from sucrose , however in
molecular configuration, relative sweetness, solubility and chemical
reactivity.
Sucrose is about six time as sweet as
lactose.
Lactose is in true solution in the
milk serum.
On crystallization from water ,it
form hard hard gritty crystals.
Its solubility , however is one-third
that of sucrose at 212⁰F.(0 ⁰ C).
Prolong heating of aqueous solution
of lactose at temperature from 212 to 226 ⁰F.(100-130 ⁰C) result in a
decomposition which is indicated by a light-brown or “caramel” colour.
In heat-treated milk , in the
presence of protein and certain mineral salts, brown colour development occurs
quite readily and give to the browning
of sterilized milk and of certain condensed and dried milk products
during storage.
The greatest chemical reactivity of
lactose, as compared with sucrose ,is due to the presence of a potentially free
aldehyde group in the glucose portion of the molecules.
Lactose is responsible ,under certain
condition for the defect know as ‘sandiness’in ice cream and sweetened
condenced milk.
Chemically speaking ,lactose is known
to occur in two forms, the alpha and the more soluble beta,both of which may
occur as either the hydrate or anhydride.
Milk contain an average about 4.8% of
lactose.
Lactose has an important relative to
the manufacture of the milk products, due to the fact that it is easily
decomposed by bacteria.The lactic acid formed in milk, which gives to milk its
characteristic sour taste, is the result of bacterial action on milk sugar.
Many other products may result from
bacterial action ,the nature and amount depending upon a variety of factors.
It has been found that 1 gram of
lactose may form on decomposition of 0.8 gram of lactic acid.
In the ordinary souring of milk ,the fermentation continues until an acid % of 0.8 to 1.0is present at which point the acid itself inhibits further growth of the organism which produce it.
(5) Ash or Mineral Matter ( Salts of Milk) :
If milk is dried first, then the
residue burned, a white powder ,which is the ash or mineral part of the
original milk, remains.
The mineral salts of milk are usually
determined after ashing. Although milk is acidic ,ash is distinctly basic.
If
careful weight are taken it will
be found that the percentage of mineral is about 0.7.
Further chemical study would reveal
that this ash is a complicated mixture containing a number of metallic
elements, in fact the same one found in the animal body.
The procedure followed in determining
the ash content of milk makes it extremely difficult to determine the exact
combination that exist in the original product .
Milk ash is known to content potassium, sodium, calcium, magnesium, chlorine,
phosphrous, and sulfur in relatively large amount.
There are present small amounts of
iron, copper, zinc, aluminum, manganese, cobalt, and iodine while trace of
silicon, boron, titanium, vanadium, rubidium, lithium, and strontium have been
reported as present.
Parts of calcium and phosphorus are
combined with the proteins.
The remaining portion of these
elements, together with the magnesium, are partly in suspension and partly in
solution.
The potassium , sodium, and chlorine
are entirely in solution.
The small quantity of sulfur enters
into the composition of the casein and lactoglobulin.
Minor Milk Constituents:
(1) Phospholipids:
In milk , there are three types of
phospholipids, viz. lecithin, cephalin, and sphingomylin.
The principal phospholipid contained
in milk is lecithin, a fat-like substance which contains nitrogen and
phosphorus.
Lecithin is closely associated with
the fat, doubtless making a portion of the concentrated colloidal layer
surrounding the fat.
Lecithin, which forms an important
constituents of the fat globule membrane, contributes to the ‘richness’ of
flavour of milk and other dairy products. i t is highly sensitive to oxidative
changes, giving rise to oxidized metallic flavours.
Phospholipids are excellent
emulsifying agents, and no doubt serve to stabilize the milk fat emulsion.
Upon the separation of milk the lecithin becomes more concentrate in the cream, a trace being left in the skim milk.
During agitation of cream in
churning, the lecithin is rubbed off the fat globule, and as the fat globules
coalesce most of the lecithin remains in the buttermilk.
A small portion only is retained in
the butter , which contains but a trace (0.03 to 0.05 5) of lecithin.
Buttermilk however has been shown to
contain approximately 0.2% owing to the increased concentration which takes
place.
Though butter contains but small
amount of lecithin , it of of great significance.
Fishy flavour in butter , a rather
common defect at times responsible for large losses on the market, has been
shown to be due to the decomposition of lecithin.
In the splitting of lecithin,
trimethylamine is formed. This is the compound responsible for the fishy
flavour. This same flavour is sometimes produced in dairy products such as
evaporated milk.
(2) Cholesterol
Milk has been shown by Denis and
Minot to contain cholesterol.
These authors report cholesterol as
occurring in milk in direct proportion to the amount of fat present .
They report milk as containing from
105 to 176 parts per million of cholesterol.
(3) Pigments
Milk contain two pigments, one of
which is fat-soluble and the other water-soluble.The fat-soluble pigment of
milk belong to a group know as “carotenoids”and has been designated as
“Carotene”.
Carotene ( the pure substance of
which has a reddish –brown colour) is fat soluble and responsible for the
yellow colour of milk, cream, butter, ghee, and other fat-rich dairy
products.Besides contributing to the colour of cow milk, carotene acts as an anti-oxidant and also as a precursor of
vitamn A.One molecule of β-Carotene yield two molecules of
vitamin A, while α-carotene yield only one .
It does not occur in the milk of all
species, as the milk of the goat, ewe, and camel ,for example are colourless.
Carotene is not synthesized by the cow ,
but by plant and find its way from the feed into the blood stream and thus into
the milk. This pigment is deposited in the body fat of cattle, the amount varying
with the breed as does the amount in the milk fat.
Dairy animals differ in their capacity to
transfer carotene from feeds to milk fat; This varies with species, breed and
individuality. Cow in general , and some breeds in particular (such as jersey)
, can transfer more carotene from their feed to the milk fat than buffaloes,
who do not seem to posses this capacity. Hence buffalo milk is white in colour.
The carotinoid content of buffalo milk varies from 0.25 to 0.48/ug/g. While
that of cow milk may be as high as 30/ug/g.
The water soluble pigment of milk has
been shown to be riboflavin, previously called “lactoflavin”or “lactochrome”,
and occur in the serum or whey, contributing a yellow – green tint to the
portion of milk. The amount of this pigment occurring in milk is very small,
amounting to but 0.05 to 0.1%.
Riboflavin occurs in the milk not
only of the cow but of most mammals. Its presence in milk cannot be connected
directly with the fed the animal receives. There is some relationship, however
between the riboflavin content of milk and high-protein feeds.
(4) Enzymes of Milk
Substance know as “enzymes “ are apparently normal constituents of milk.
Enzymes are chemical substances secreated by living plant and animal cells which stimulate chemical reaction without becoming a part of comounds formed.
Commom examples of enzyme are ptyalin of the saliva, which act upon starch, forming maltose and Pepsin found in the stomach, which assists in the breaking down of proteins,
One of the first enzymes demonstrated as occuring in milk was galactose, a proteolytic enzyme.
Russell and Hastings were the first to contend that galactase was present as a normal constituents of milk .The enzyme thought to play an important part in the ripening of cheese.
Lipase, a fat –splitting enzyme, has been shown to be a normal constituents of the milk of cows that have been in for an abnormally long period.When this enzyme in the occurs in milk it may be the cause of a bitter flavour developing.Extream cases of rancidity in butter , also have been attributed to the action of lipase.
It has been established definitely that milk contains catalase,an oxidizing enzyme, and reductase, a reducing enzyme.
There is some evidence too, that milk contains lactase, a sugar-fermenting enzyme,
Diastase, a starch-splittting enzymes, and peroidase, an oxidizing enzyme.
Raw milk contains Phosphatase, which is inactivated by proper pasteurization.This inactivation serves as the basis of a test for evaluating the effectiveness of pasteurizaation.
(5)Vitamins:
As of today,over 25 vitamins have been reported.Those found in milk are ; fat-soluble vitamins A,D,E and K; and
water- soluble
vitamins of the ‘B complex’ such as:
- vitamin B1 or (thiamin),
- vitamin B2 or G or (riboflavin),
- Nicotinic acid (niacin),
- Vitamin B 6 or (Pyridoxine),
- Panthenic acid,
- Vitamin C ( ascorbic acid),
The feed is know to be a factor influencing the amount of some vitamins in milk.
(6)Gases :
Milk contain from 7 to 10 % by volume of gas.
Carbon dioxide is present in milk as it comes from the udder, while nitrogen and oxygen are taken up by the milk during milking.
In addition to the above gases,milk may absorb gaseous odors from the air and although such conditions are abnormal, The fact remains that the market quality of milk may be seriously handicaped by absorbed gases.
Gases may also be formed in milk by certain bacteria.
(7) Non-protein
Nitrogenous Substances:
Milk contain a group of nitrogenous substance in addition to the proteins.
These substances do not occur in large quantities but they are known to be present.
The non- protein nitrogenous substance of milk may be listed as fallows:
- Urea nitrogen,
- Amino nitrogen,
- Creatine,
- Creatinine,
- Uric acid,
- Adenine and guanine.
These substance are measured in parts per million , ranging from 1.5 to 10 in milk.
References:
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