Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
Environmental impacts on and of animals
Challenge of artificial environment to domestic animals:
problems to be solved in relation to quality issues of local
food products
Milk composition
Nutritional role of milk and milk products
Part I. Milk composition
[E-B 1] Milk is a complex biological fluid consisting practically of all chemical
components (carbohydrates, proteins, lipids, minerals, and vitamins etc.
(Table 1), necessary for building and functioning of living cells. The number of
milk ingredients increases due to technical improvement of analysis methods. At
present about 100 000 components have been identified.
Chemical composition and physical properties of milk vary within wide ranges.
The variability can be resulted from the following factors:
1. genetical difference between breeds and individuals;
2. physiologically specific features resulted from lactation stage, heat, age
of an animal and gestation;
3. environment, especially feeding- and keeping conditions, but also climate.
The main constituent of milk is WATER (87%) in which other components are
dispersed. Milk components as sugars, minerals and vitamins are dissolved
there. In water emulgated milk fat and suspended proteins are present.
[E-B 2] LACTOSE, or milk sugar (5%) is a disaccharide consisting of glucose
and galactose. The amount of other carbohydrates in milk is presented in milligrams. Lactose is the energy and carbon source for most microbes growing in
milk. The osmotic pressure of milk, the decrease of freezing point and the increase of boiling temperature depend mainly on lactose. During heating, lactose
reacts with the proteins’ amino groups. Lactose forms soluble complexes with
Ca-salts, thus favouring Ca assimilation. Fermentation of lactose by lactic acid
bacteria forms basis for the technologies for producing sour milk products.
[E-B 3] Milk FAT (4%) is composed of many different lipophilic substances. Fat
content can vary from 2 to 8%, according to nutrition and breed.
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
Table 1 Composition and structure of milk (approximate average quantities in 1
kg milk) (Walstra and Jenness, 1984)
Fat Globule
Glycerides
triglycerides
diglycerides
monoglycerides
Fatty acids
Sterols
Carotenoids
Vitamins A, D, E, K
Water
Others
Serum
Water
Carbohydrates
lactose
others
Minerals
Ca
Mg
K
Na
chloride
phosphate
sulphate
bicarbonate
Trace elements
Zn
Fe
Cu
many others
Globule Membrane
Casein Micelle
Water
80 mg Protein
38 g Protein
350 mg
casein
0.1 g Lipids
proteose peptone
10 mg
phospholipides
210 mg Salts
25 mg
cerebosides
30 mg Ca
100 mg
gangliosides
5 mg
phosphate
0.4 mg
sterols
15 mg
citrate
2 mg
natural gly+
Mg, K, Na, Zn,
cerides
etc.
60 mg Enzymes
+
Enzymes
30 mg Cu
4 μg Water
Fe
100 μg
870 g Organic acids
citrate
46 g formate
0.1 g
acetate
lactate
370 mg oxalate
75 mg
others
1340 mg Gases
460 mg oxygen
1060 mg nitrogen
1080 mg Lipides
100 mg natural glycerides
100 mg fatty acids
phospholipides
400 μg cerebosides
100 μg sterols
20 μg others
Vitamins
B vitamins
ascorbic acid
Leucocytes
Enzymes
Nucleic acids
26 g
0.4 g
800 mg
950 mg
140 mg
150 mg
+
+
Protein
casein
+
β-lactoglobulin
3200 mg
α-lactalbumin
1200 mg
serum albumin
400 mg
immuno750 mg
globulins
20 mg
proteose pep200 mg
tone
others
400 mg
6 mg Nonprotein nitrogenous
15 mg compounds
urea
300 mg
+
peptides
200 mg
1600 mg
40 mg
30 mg
30 mg
20 mg
15 mg amino acids
110 mg others
10 mg Phosphoric esters
15 mg Enzymes
Alcohol
300 mg
300 mg
+
3 mg
200 mg
20 mg
Lipoprotein Particle
Polar lipids
Protein
Enzymes
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
Triglycerides form more than 98% of total fat weight. Of other components free
fatty acids, cerebrosides, phospholipids and cholesterol should be mentioned. In
milk fat about 400 different fatty acids are represented, however the content of
only 14 of them is over one per cent in total fatty acids by weight. As compared to
other animal fats, milk fat content of long and medium C-chain fatty acids and
polyunsaturated fatty acids is relatively high. Short chain fatty acids inhibit the development of pathogenic microbes. As to non-essential fatty acids, arachidonic
and linolenic acids should be pointed out. The content of individual fatty acids in
milk fat depends on ration, lactation stage, season, breed etc. Altering of the fatty
acid composition of milk fat affects favour, odour, consistence and shelf life of
butter. In milk fat the contents of myristic, palmitic, stearic and oleic acids are the
highest. Milk fat includes soluble vitamins A, D and E as well.
Milk fat is dispersed in the plasma in the form of small globules. Each fat globule
(0.1-10µm) is surrounded by a membrane. The membrane of fat globule is consisting of phospholipids, lipoproteins, cerebrosides, proteins, nucleic acids, enzymes, trace elements and water. Lipids comprise 60% and proteins 40% of the
membrane. In natural untreated milk the membrane inhibits lipolysis. When the
structure of the membrane is broken, degradation of fat into free fatty acids
quickly begins, causing rancid flavour of milk. Thus, before homogenisation milk
must be pasteurised in order to inactivate active lipoprotein lipases.
[E-B 4] Cow’s milk contains 2.8 to 3.5% PROTEIN (Table 2). As milk also contains non-protein nitrogen compounds (urea, creatin, free amino acids), the protein content cannot be directly determined by nitrogen content. Cow’s milk content of protein hormone prolactin is 50µg/kg. In milk also steroid hormones
cortisol, corticosterol, progesterone and estradiol can be found. The content of
steroid hormones do not exceed one microgram per litre.
Milk proteins are divided into caseins (precipitate at pH 4.6) and whey proteins.
Casein is a phosphoprotein consisting of 4 gene products: αs1-, αs2-, β- and κcaseins. Isoelectric point of caseins is pH 4.6, at which micelles lose their electric
stability and aggregate. Solubility of caseins at this pH value differs from that of
whey proteins so that they can be separated. In all caseins there is at least one
phosphate group attached by ester bonds, whey proteins are not phosphorylized.
Caseins contain quite few sulphur-containing amino acids: αs1-casein and β-casein do not contain cystein, αs2-casein and κ-casein both contain two cystein
molecules. The content of proline in caseins is quite high and thus no ordered
secondary sructure exists.
κ-casein (MW 19025) is the only protein in casein complex containing carbohydrates. κ-casein binds about 2 mol of calcium per 1 mol of protein. As to technological aspect, it is important that κ-casein is hydrolysed by renneting enzyme
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
chymosin. In technologies caseins are converted to coagulants by acids (sour
milk products) and by renneting enzymes (cheeses).
Table 2 Approximate composition of milk protein
Protein
Casein
αs1-casein
αs2-casein
β-casein
κ-casein
Whey proteins
α-lactalbumin
β-lactoglobulin
Serumalbumin
Immunoglobulins
Other whey proteins
Globule membrane proteins
Total
Content g/kg
26.0
10.0
2.6
10.1
3.3
6.3
1.2
3.2
0.4
0.7
0.8
0.4
32.7
% of total protein
79.5
30.6
8.0
30.8
10.1
19.3
3.7
9.8
1.2
2.1
2.5
1.2
100
Whey proteins consist of two proteins synthesized in the mammary gland – βlactoglobulin and α-lactalbumine. Serum albumine and immunoglobulines originate from the blood (Table 2). Whey proteins include some products of β-casein
decomposition known as proteoses and peptones. Whey proteins are thermolabile – it is possible to precipitate them by heating whey.
As to technological aspect, the most important whey protein is β-lactoglobulin
that exists in two main genetic variants – A and B. In the milk of some cow
breeds variants C and D exist. Genetic variants differ from each other mainly by
the composition of amino acids sequence. β-lactoglobulin comprises up to 50%
of the total whey proteins. The other important whey protein is α-lactalbumin,
comprising about 20% of whey proteins in cow’s milk. This protein is with higher
thermostability than β-lactoglobulin.
Serum albumine takes the third place by amount, originating from the blood. Albumines differ from caseins mainly due to comparatively high sulphur content in
their molecule, phosphor is practically lacking.
The content of immunoglobulines, important if immunity is considered, is high in
colostrum that is necessary for passive immunisation. In the milk of the first milking in which protein content may reach to 16%, immunoglobulines comprise
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
about 50% of the protein. Another essential whey protein is lactoferrin, bactericidial iron-binding glycoprotein, which content in milk is 0.2 mg per litre. Its content is considerably higher in colostrum and mastitic milk. Lactoferrin binds 2 mol
of Fe3+ ions per 1 mol.
[E-B 5] Cow’s milk contains 7 to 8 g MINERALS per one litre. The salts of organic
as well as anorganic acids are presented in milk. The majority of them are potassium-, calcium-, magnesium- and sodium phosphates, citrates, chlorides, hydrogen carbonates (Fox, McSweeney, 1998; Laht, Olkonen, 2001). Table 3 represents macro elements found in milk and their content.
Table 3 Mineral content of milk, macro elements
(Fox, McSweeney, 1998)
Element
Average, g/l
Range, g/l
Calcium
Phosphorus
Potassium
Sodium
Chlorine
Magnesium
1.20
0.95
1.45
0.50
1.00
0.13
0.65...2.65
0.47...1.44
1.15...2.00
0.11...1.15
0.54...2.42
0.02...0.23
Micro amounts of more than 20 chemical elements can be found in milk. Most of
them are very important as to metabolic aspect. Milk is a good source of Zn and
I, covering the needs of the organism. Besides them, milk may contain other minerals, which are not very important in nutrition (Li, B, Si, Br, Al, Sr, Co, As, Ag,
Pb, Hg, Cd, Rb, Cs, V), some of them may be toxic. The concentration of toxic
elements exceeds the limits very rarely as the udder serves as a efficient biofilter
(Laht, Olkonen, 2001).
Mineral components are in the form of salts or are bound to casein micelles. Very
small part of minerals is bound to fat globules (Walstra, Jenness, 1984) and
about 0.15% of Ca is attached to α-lactalbumin. One-valence cations are mostly
in solution, 66% of Ca and 57% of P is in the composition of colloids (Walstra,
Jenness, 1984; Fox, McSweeney, 1998).
Phosphorus is present in milk only in several compounds. Phosphate group has
an important role in giving milk its buffering capacity (Nielsen, Ullum, 1989). Equilibrium between colloidal and dissolved salts in milk is affected by several factors
as acidity of milk, temperature, dilution and freezing (Fox, McSweeney, 1998).
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
1. Acidifying milk, colloidal calcium phosphate dissolves from casein (at pH≤
4.9 whole calcium phosphate is dissolved) (Walstra, Jenness, 1984; Fox,
McSweeney, 1998).
2. With the increasing of temperature the solubility of calcium phosphate decreases and in cooling the percentage of calcium phosphate increases at
the expense of colloidal calcium phosphate. At high temperatures this reaction is irreversible. Calcium phosphate precipitates if milk is heated at high
temperature. The dependence of the equilibrium of Na, K, Mg and citrate
on temperature has not been found (Walstra, Jenness, 1984; Fox, McSweeney, 1998).
3. Dilution of milk decreases the concentration of dissolved calcium phosphate and this is compensated at the expense of colloidal calcium phosphate. With the increase of milk concentration the percentage of colloidal
calcium phosphate increases as well as milk acidity (Walstra, Jenness,
1984; Fox, McSweeney, 1998).
4. Freezing of milk results in water crystallization and increase of salt concentration. The co-effect of the decreased percentage of Ca-ions caused by
concentration and the decrease of milk pH (pH 5.8 at -20°C) result in the
destabilization of casein micelles (Fox, McSweeney, 1998).
Milk contains all known VITAMINS that are soluble in fat or water.
Usually raw milk contains carbon dioxide, nitrogen and oxygen. The contents of
oxygen and nitrogen increase when milk is exposed to air. Oxygen affects the
development of starter culture, in modern yoghurt manufacturing systems milk is
degased.
SOMATIC CELLS are leucocytes and different types of epithelial cells. The number of them in the milk of healthy cows ranges from a thousand to several hundred thousand per ml of milk. Inflammatory processes increase the number of somatic cells to millions.
References and recommended reading
1. Fox P.F., McSweeney P.L.H. Dairy Chemistry and Biochemistry. Chapman
& Hall, 1998, 465.
2. Laht, T.-M., Olkonen, A. Piima koostis, füüsikalis-keemilised omadused. Piimanduse Käsiraamat. Koost. A. Olkonen. EPMÜ LKI, Tartu, 2001, 102–130.
3. Nielsen, E.W., Ullum, J.A. Dairy technology I. Danish Turnkey Dairies Ltd,
1989, 110.
4. Roginski, H., Fuquay, J.F., Fox, P.F. Encyclopedia of Dairy Sciences. 2002,
Vol. 3, Academic Press.
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
5. Walstra, P., Jenness, R. Dairy Chemistry and Physics. John Wiley & Sons,
1984, 467.
Part II. Nutritional role of milk and milk products
Foodstuff’s nutritional quality is assessed and evaluated by
•
•
•
amount of energy derived from it;
content of essential amino acids, fatty acids, minerals and vitamins;
biological availability of nutrients.
[E-B 6] Milk is the mammary gland secret of the mammals which contains all nutrients needed by offspring in a balanced proportion. Over thousands of years
people have learned to use animal milk as a valuable food. In Northern countries
cow’s milk plays a central role, in other cultures milk of goat, sheep, camel and
horse are used as well.
Milk and sour milk contain about 3.2% protein. The main milk proteins are casein
(80%) and serum (or whey) proteins (20%). Casein in milk is in the form of small
globules, which are containing about 80% of calcium found in milk and also much
phosphorus. The primary function of casein is to supply offspring with amino
acids needed for building up body proteins. Casein is rich in essential amino
acids in favourable proportions. Half a litre of milk per day almost covers the minimal need for essential amino acids.
[E-B 7] Milk carbohydrate − lactose − comprises about 30% of the amount of energy assimilated from milk. Lactose is a disaccharide consisting of galactose and
glucose. In nature it can be found in milk only.
Lactose
•
•
favours the absorption of calcium and many other minerals in the digestive
tract, favours the development of lactic acid bacteria which are favourable
for the digestive tract,
lactic acid derived from lactose increases the level of acidity in the digestive
tract and inhibits the development of pathogenic bacteria.
Some people may have lactase deficiency or hypolactasia. In the digestive tract
such people lack the lactose degrading enzyme lactase, or it is synthesized in
small amounts. Undegraded lactose goes into the large intestine where microbes
use it for nutrition, resulting in pain in the stomach, diarrhoea and formation of
gases. Sour milk products cause fewer symptoms than milk as lactose in them
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
has turned into lactic acid. People having hypolactasia are recommended to use
kefir, sour milk, yoghurt or cheese.
[E-B 8] Milk fat is primarily source of energy. Compared to other animal origin
fats, the content of short and medium C-chain fatty acids and polyunsaturated
fatty acids in milk is comparatively high. Butyric acid is found only in milk fat of
the ruminants. Short chain fatty acids inhibit the development of pathogenic microbes. As to non-essential amino acids, in milk fat arahhidonic and linolenic
acids can be found which are mainly needed to build up nervous tissue. Milk fat
contains fat-soluble vitamins A, D, E, F and K. Due to the high content of short
chain and unsaturated fatty acids, milk fat is in a liquid state and thus easily assimilated as liquid triglycerides are hydrolyzed in the small intestine by pancreatic
lipase. The digestibility of milk fat is 99%. Comparing to other animal fats, the
cholesterol content of milk fat is low. Nowadays researchers are in opinion that
45% of fatty acids of milk fat reduce cholesterol level and only 14% increase it.
The effect of 41% fatty acids on plasma cholesterol is not known yet. The effect
of the most important milk fatty acids on blood cholesterol level is presented in
Table 1.
Table 1 The effect of most important milk fatty acids on blood cholesterol level
(Laht, 2001)
Concentration %
Effect*
Butyric acid
C 4:0
4
0
Caproic acid
C 6:0
2
0
Caprylic acid
C 8:0
1
0
Capric acid
C 10:0
3
0
Lauric acid
C 12:0
3
+
Myristic acid
C 14:0
11
+
Palmitic acid
C 16:0
28
0
Stearic acid
C 18:0
10
Oleic acid
C 18:1
26
Linoleic acid
C 18:2
3
*
0 = neutral, + = increases cholesterol level, - = decreases cholesterol level
Fatty acid
[E-B 9] Milk contains almost all micro- and macro elements needed for the human organism. Milk and milk products are very good sources of calcium, phosphorus, sodium, potassium, zinc and iodine in our food. This is not only due to
their high mineral content but also due to the fact that minerals present in these
products are easily absorbable. By the data of different researchers, milk and
milk products should cover 55 to75% of our daily calcium requirement (Schaafsma, 1984; Hazell, 1985) and 30 to 45% of daily phosphorus requirement
(Hazell, 1985; Flynn, Cashman, 1997). Figure 1 shows the percentage of our
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
daily calcium and phosphorus content, which milk and milk products should cover.
Fish,
meat,
eggs
7%
Vegetable
food
7%
A
Other
sources
11 %
Eggs
5%
Vegetables
8%
Beans
6%
Grain
Milk and
12 %
milk products
75 %
Other
sources
9%
Milk and
milk products
35 %
Meat, fish 25 %
B
Figure 1 Sources of calcium (A) and phosphorus (B) in average mixed foods
(Teesalu, 1997)
Without consuming milk it is almost impossible to cover the daily calcium requirement. For skeleton formation 100 to 150 mg calcium per day is needed up to the
age of 20. The critical minimum limit per day is 600 mg. Consumption of calcium
below that limit directly increases risk of thinning of bones or osteoporosis
(Teesalu, 1998). Very intense muscle work, stress and depression increase calcium requirement even more (Zilmer et al., 1996). Absorption of calcium from milk
is favoured by other milk components as lactose, vitamin D and phosphorus, but
also vitamins A and C. If the calcium requirement of the organism is high, all calcium present in cow’s milk is absorbable. Although in skimmed milk almost all
calcium is remained, calcium absorption from skimmed milk and the products
made from it is low. This is caused by removing fat and together with it fat-soluble vitamin D (Teesalu, 1998).
Milk contains all known vitamins and is an important source of them in our food.
Milk is especially rich in vitamins B2 and B12, but also covers our daily need for vitamins A, K and B7 (Schaafma, 2002) Pasteurization of milk can result in the loss
of temperature-sensible vitamins (C, B1, B6, folic acid) by 5 to 20%. Further loss
(oxidation) of vitamins from milk products depends on storage conditions (exposure to light, temperature, and wrapping material). By removing fat, fat-soluble vitamins are removed from milk as well, thus milk products with low fat content are
poor in fat-soluble vitamins.
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Estonian University of Life Sciences, Institute for Veterinary Medicine, Tartu, Estonia
References
1. Flynn, A., Cashman, K. Nutritional aspects of minerals in bovine and human
milk. Ed. by In: Fox, P. F. (Ed.). Advanced Dairy Chemistry. Volume 3:
Lactose, water, salts and vitamins, 2nd Edition. Chapman & Hall, London,
1997, 257–302.
2. Hazell, T. Minerals in foods: dietary sources, chemical forms, interactions,
bioavailability. World Rev. Nutr. Diet., 46, 1985, 1–123.
3. Laht, T.-M. Piim ja piimatooted toiduna. Piimanduse Käsiraamat. Koost. A.
Olkonen. EPMÜ LKI, Tartu, 2001, 75–101.
4. Schaafma, G. The significance of milk as a source of dietary calcium. IDF
Bulletin 166, 1984, 19–32.
5. Schaafma, G. Vitamins. General introduction. In: Roginski, H., Fuquay, J. F.,
Fox, P. F. (Eds.). Encyclopaedia of Dairy Sciences, 4. Ed by Roginski, H.,
Fuquay, J.F., Fox. Academic Press, 2002, 2653–657.
6. Teesalu, S. Piim ja piimatooted meie toidulaual. Piimast ja eesti piimandusest. Koost. M. Karelson, S. Teesalu, A. Tammisto. EPMÜ LKI, Tartu, 1997,
7–19.
7. Teesalu, S. Laste ja noorukite osteoporoos - luude hõrenemine. Tartu, 1998,
110.
8. Zilmer M., Karelson E., Vihalem T. Meditsiiniline biokeemia I. Biomolekulid:
biokeemilised ja meditsiinilised aspektid. Tartu, 1996, 321.
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