- COMPOSITION OF MILK
Any two single samples of milk will differ quite noticeably when analysed in detail. It should be pointed out, however, that from a large number of such analyses the average composition of milk can be determined, and it is the average composition which is given below.
Before giving the composition of milk, it should be noted that, for the first few months of their lives and the same applies to all other mammals, calves depend entirely on milk for their nourishment. Milk is, therefore, a highly nutritious food which provides all the nutrients the young need. Not only is it a nutritious food for infants, it is also an exceptionally good growth medium for micro-organisms called bacteria which have the ability to spoil milk. It will also be seen that milk consists of a large amount of water, and this is essential for the digestion the young mammal which is not capable of digesting roughage or fibrous foods. It is only later in life that the young mammal, in this case the calf, can digest more complex types of foods.
The list which is given below should be learnt thoroughly as it is the basis for quality control of the product. There are basic legal requirements which have to be observed when we come to consider the properties of milk, in particular, the composition of milk. This is to ensure that producers do not add anything to the milk – such as water; or that they do not remove anything from the milk, such as fat.
The composition of the milk can also be used to determine the effect of certain feeds and the effect of rapid changes in the food. It can also be used to check on the general health of the animal. Although this is not used on a regular basis, it can be an exceptionally good indication of the general health of the animal. For these reasons then, it is important to know in detail the composition of the milk. The diagram below illustrates the main divisions which occur in the product.
Whole Milk Sample divided into
Water and Total Solids divided into
Total Solids Not Fat and Milk Fat
Total Solids-Not-Fat divided into
(often abbreviated SNF)
Lactose Protein and Ash
It is suggested that you learn this simple breakdown and then learn the composition in detail by remembering the percentages, which are as follows:
|Solids-not-fat (SNF)||8.5%||Total 12%|
Notice very carefully how the totals are arrived at. This is very important. If you study this, it should become clear to you.
We should spend a little time talking about the protein as these forms the basis of a lot of cheese production. You have probably heard about ‘Little Miss Muffet who sat on her tuffet eating her curds and whey’. Well, she is in fact eating milk in a changed form. When milk is mixed with acid (of which there is a lot in the stomach of a mammal) the protein will coagulate. That is, it will become solid and separate out a solution. The solids are the curds and the liquid is the whey. You have probably noticed this before. It you let milk stand for a period, it will also become solid; the spoilage bacteria produce an acid which causes the milk to curdle. The main protein we find in milk is called casein of which there is about 2.5%. The other proteins are called Albumin and Globulin.
Proteins are extremely complicated biochemical compounds which are made up of Amino Acids. These are the basic building-blocks for growth and development. They also have other valuable properties.
The other important constituent of milk is sugar or Lactose. This gives milk its characteristic sweet taste. It is the lactose which is broken down fairly quickly by micro-organisms to lactic acid which causes milk to go sour. As you can appreciate, sugar is an important nutrient as it provides the energy requirements of the young mammal. Already you can see how valuable milk is as a food for mammals, including ourselves.
The ash portion is that part of the milk which is left behind when you burn the total solids, and the ash contains all the chemicals which are required for good healthy growth and development. You should try to remember at least some of these chemicals as you will come across them later in your nutrition course. You will have to learn what the deficiency symptoms are: you might do yourself a favour by remembering in what form you can obtain these important chemicals.
- Calcium – for bone development
- Potassium – for blood development
- Sodium – for muscle development
- Magnesium – for marrow development
- Iron – for blood development
- Copper – for blood development
- Zinc – for metabolism
- Chlorine – for bone development
- Phosphates – for nervous system
- Iodine – for goitre metabolism
This list includes, as you can see, some of the important functions of these components: this will make it easier for you to recall them.
There are a number of factors which can affect the composition of milk and these are conveniently divided into:
- Environmental Factors
- Physiological Factors.
The following is a list of factors in the two groups mentioned. You should at least be able to recall some of the details, since some of the factors in the former, can be controlled. Where this is possible, it should form part of a basic management policy.
Remember, in some cases, the changes are only short-lived, until the animal has adapted to the changed conditions and then it will revert to normal, this is particularly noticeable with the fat. Always look at the outline of the composition when you revise this section: it will help you to understand it better.
|Environmental Factors||Physiological Factors|
|Feed||Individuality of the cow|
|Stage of milking||Course of lactation|
|Frequency of milking||Age of the cow|
Many of these factors affect not only the composition of milk, but also the quantity of milk produced by the cow. As a general rule, anything that reduces the amount of milk given by the cow will increase the composition, particularly the butterfat. One theory is the cow produces a certain amount of fat, and this is diluted with the milk; anything reducing the amount of milk will increase the percentage of fat in the milk.
The food that a cow eats has a great influence on the amount of milk produced, but much less influence on milk composition; it does affect the composition in the following ways, however:
- Does not alter the percentage of Fat permanently;
- Any feed with a high proportion of oil or fat will increase the Fat percentage of milk for a short time only;
- Fish liver oils will reduce Fat percentage;
- Certain feeds can alter the nature of the Fat in milk by changing the fatty acids;
- A reduction or change in feed will reduce the milk yield of the cow and so increase the Fat
- Feeding can influence the Vitamin content of milk and;
- Poor feeding, or feeding a ration that is not balanced for milk production, can reduce the
Solids-not-fat content of the milk.
Prolonged cold weather tends to reduce milk production and so increase the Fat percentage.
Stage of Milking
When a cow is milked, the first milk taken from the udder, called the Foremilk, is low in Fat. The milk taken last from the udder, the Strippings, is high in fat.
Failure to milk out properly will reduce the Fat percentage.
Frequency of Milking
When cows are milked twice a day at equal intervals, that are every 12 hours, the fat content of the milk will be the same at each milking. When they are milked at unequal intervals, say at 6 a.m. and 3
- the Fat content of the afternoon milking will be higher than, that of the morning milking.
When cows are disturbed during milking, or just before milking, their milk yield is reduced and they do not ‘let down’ their milk. This has the effect of raising the Fat content of the milk.
Cows get used to the routine of dipping and this should have no real effect provided it is done quietly.
Individuality of the Cow
Individual cows vary a great deal in both composition and quantity of milk produced. Selection and a sound breeding programme within a herd should help to even out the performances of individual cows.
Milk composition goes with the breed of the cow although there are great variations within each breed. Breed averages are:
Course of Lactation
At the start of a cow’s lactation, the Fat is fairly high, and it then drops until the peak milk production has passed. It then rises steadily until the end of the lactation. The lowest Fat percentage occurs when the cow is giving the most milk, and the highest Fat percentage is when she is giving the least milk. Solids-not-fat, however, starts off at a high level, drops rapidly to peak milk production, remains fairly constant, and then rises rapidly during the last 6 weeks of the lactation.
This is the period when the cow is in-calf. Once the cow has been served by the bull and has conceived, the milk production tends to fall off, and the Fat percentage starts to rise.
Age of Cow
This has little effect, although very old cows give less milk and therefore a higher Fat percentage.
This is when the cow is bulling, and because she is excited and disturbed, she will give less milk for a short period, 24 hours, and the fat will be higher.
The main disease with dairy cows is mastitis. This lowers milk yields, lowers S.N.F. and slightly increases the Fat percentage.