Milk Composition
Milk Composition
Milk Composition
WHAT IS MILK?
Milk is the normal product of mammary gland secretion. The average composition of milk from cows and buffalo is
presented in Table 1. Milk is a complex, nutritious product that contains more than 100 substances that are either
in solution, suspension or emulsion in water. For example:
Casein, the major protein of milk, is dispersed as a great number of solid particles so tiny that they do not
settle, but remain in suspension. These particles are called micelles, and the dispersion of the micelles in the
milk is referred to as a colloidal suspension;
The fat and fat soluble vitamins in the milk are in the form of an emulsion; that is, a suspension of small liquid
globules that do not mix with the water in milk;
Lactose (milk sugar), some proteins (whey protien), mineral salts and other substances are soluble; that is,
they are entirely dissolved in the water in milk.
Cow
88.0
61.0
3.2
3.4
4.7
0.72
Buffalo
84.0
97.0
3.7
6.9
5.2
0.79
Human
87.5
70.0
1.0
4.4
6.9
0.20
The casein micelles and the fat globules give milk most of its physical characteristics, and give taste and flavor to
dairy products such as butter, cheese, yogurt, etc. The composition of milk varies considerably with the breed of
cow, stage of lactation, feed, season of the year, and many other factors. However, some relationships between
constituents are very stable and can be used to indicate whether any tampering with the milk composition has
occurred. For example, a milk of normal composition has a specific gravity that varies normally from 1.023 to 1.040
(at 20oC) and a freezing point that varies from -0.518 to -0.534oC. Any alteration, by addition of water for example,
can be easily identified because these characteristics of milk will no longer be in the normal range.
Milk is a highly perishable product that should be cooled to about 4oC as soon as possible after collection.
Extremes of temperature, acidity (pH) or contamination by microorganisms can rapidly decrease its quality.
In a significant portion of the human population, the deficiency of the enzyme lactase in the digestive tract results in
the inability to digest lactose. Most individuals with low lactase activity develop symptoms of intolerance to large
doses of lactose, but the majority can consume moderate amounts of milk without discomfort. Not all dairy products
contain similar proportions of lactose. The fermentation of lactose during processing lowers its concentration in
many dairy products, especially in yogurts and cheeses. In addition, milk pre-treated with lactase, which minimizes
the problems associated with lactose intolerance, is now available.
Proteins
Most of the nitrogen in the milk is found in the form of protein (Figure 2). The building blocks of all proteins are the
amino acids. There are 20 amino acids that are commonly found in proteins. The order of the amino acids in a
protein, which is determined by the genetic code, gives the protein a unique conformation. In turn, the spatial
conformation of the protein gives it a specific function.
Figure 2: Structure of proteins (R1, R2, etc., are radicals specific to each
amino acid. The number of amino acids in the caseins of milk varies
from 199 to 209).
The concentration of protein in milk varies from 3.0 to 4.0% (30-40 grams per liter). The percentage varies with the
breed of the cow and in proportion to the amount of fat in the milk. There is a close relationship between the
amount of fat and the amount of protein in milk-the higher the fat, the higher the protein.
The protein falls into two major groups: caseins (80%) and whey proteins (20%). Historically, this classification
followed the process of cheesemaking, which consists of separating the casein curd from the whey after the milk
has clotted under the action of rennin or rennet (a digestive enzyme collected from the stomach of calves).
The behavior of the different types of caseins (a, b and k) in milk when treated with heat, different pH (acidity) and
different salt concentra-tions provide the characteristics of cheeses, fermented milk products and different forms of
milk (condensed, dried, etc.).
Occasionally, infants or young children are allergic to milk because their bodies develop a reaction to the proteins
in the milk. The allergy causes rash, asthma, and/or gastrointestinal disorders (colic, diarrhea, etc.). In cases of
allergies, goat milk is often used as a substitute; however, sometimes hydrolyzed casein milks must be used.
Fat
Normally, fat (or lipid) makes up from 3.5 to 6.0% of milk, varying between breeds of cattle and with feeding
practices. A ration too rich in concentrates that do not elicit rumination in the cow may result in milk with a
depressed percentage of fat (2.0 to 2.5%).
Fat is present in milk in small globules suspended in water. Each globule is surrounded by a layer of phospholipids,
which prevents the globules from clumping together by repelling other fat globules and attracting water. As long as
this structure is intact, the milk fat remains as an emulsion.
The majority of milk fat is in the form of triglycerides formed by the linking of glycerol and fatty acids (Figure 3). The
proportions of fatty acids of different lengths determine the melting point of fat and thus the consistency of the
butter derived from it. Milk fat contains predominantly short-chain fatty acids (chains of less than eight carbon
atoms) built from acetic acid units derived from fermentation in the rumen. This is a unique feature of milk fat
compared with other kinds of animal and plant fats. The long chain fatty acids in milk are primarily the unsaturated
(hydrogen deficient) acids, with the predominant one being oleic (18-carbon chain), and polyunsaturated linoleic
and linolenic acids.
Minerals and vitamins
MINERALS
mg/100 ml
VITAMINS
mg/100 ml
Potassium
138
Vit. A
30.0
Calcium
125
Vit. D
0.06
Chloride
103
Vit. E
88.0
Phosphorus
96
Vit. K
17.0
Sodium
58
Vit. B1
37.0
Sulfur
30
Vit. B2
180.0
12
Vit. B6
46.0
Vit. B12
0.42
Magnesium
Trace
minerals2
<0.1
Vit. C
1.7
Milk is an excellent source of most minerals required for the growth of the young. The digestibility of calcium and
phosphorus are unusually high, in part because they are found in association with the casein of the milk. As a
result, milk is the best source of calcium for skeletal growth in the young and maintenance of bone integrity in
adults. Another mineral of interest in the milk is iron. The low iron concentration in milk cannot meet the needs of
the young, but this low level turns out to have a positive aspect because it limits bacterial growth in milk-iron is
essential for the growth of many bacteria.
Additional water;
Antibiotics;
Pesticides or insecticides;
Bacteria.
The vigilance of producers in following instructions for the use of chemicals, as well as good milking, cleaning and
storage procedures are not only essential to their own success, but also to the success of the dairy industry as a
whole.