Enzymes are a group of proteins produced by living organisms. They have the ability to start chemical reactions and to affect the course and speed of such reactions. Enzymes do this without being consumed. They are therefore sometimes called biocatalysts. An enzyme probably takes part in a reaction, but is released again when it has completed its job. 

The action of enzymes is specific; each type of enzyme only catalyses one type of reaction. 

Two factors which strongly affect enzymatic action are temperature and pH. Enzymes are usually most active in an optimum temperature range between 25 and 50 C. The activity drops if the temperature increases beyond optimum, stopping altogether somewhere between 50 and 120 C. At these temperatures the enzymes are more or less completely destroyed (inactivated). The temperature of inactivation varies from one type of enzyme to another - a fact which has been widely utilized for the purpose of finding the degree of pasteurization of milk. Enzymes also have their optimum pH ranges; some function best in acid solutions, others in alkaline solutions. 

The enzymes in milk come either from the cow's udder or from bacteria. The former are normal constituents of milk and are called original enzymes. The latter, bacterial enzymes, vary in type and number according to the nature and size of the bacterial population. Several of the enzymes in milk are utilized for quality testing and control. Among the more important ones are peroxidase, catalase, phosphatase and lipase. 

Lipase (30-31) 

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 HTST pasteurization, but higher temperatures are required for total inactivation. Many microorganisms produce lipase. This can cause serious problems as this enzyme is very resistant to heat.
 

Enzymes start chemical changes: 

- one substance in milk changes

to other substances.

the udder.

Pasteurization usually destroys them.

from bacteria,
e.g. on dirty equipment, and change the milk
to have a bad taste and smell.

Pasteurization does not always destroy them.

Lactose is a sugar and belong to the group of organic chemical compounds called carbohydrates. 

Carbohydrates are the most important source in our diet.  Bread and potatoes, for example, are rich in carbohydrates, an provide a reservoir of nourishment.  They break down into high-energy compounds which can take part in all biochemical reactions, where they provide the necessary energy.  Carbohydrates also supply material for the synthesis of some important chemical compounds in the body.  They are present in muscles as muscle glycogen and in liver as liver glycogen.  Blood sugar is also composed of carbohydrates. 

Glycogen is an example of a carbohydrate with a very large molecule.  Other examples are starch and cellulose.  Such composite hydrocarbons are called polysaccharides and have giant molecules made up of many glucose molecules.  In glycogen and starch the molecules are often branched, while in cellulose they are in the form of long, straight chains. 

The lactose content of milk varies between 3.6 and 5.5 %.  Figure 2 shows what happens when lactose is attacked by lactic acid bacteria.  These bacteria contain an enzyme called lactase which attack lactose, splitting its molecules into glucose and galactose.  Other enzymes from the lactic acid bacteria then attack the glucose and galatose, converting them into various acids of which lactic acid is the most important.  This is what happens when milk goes sour, i.e. fermentation of lactose to lactic acid.  Other micro-organisms in the milk generate other breakdown products. 

Figure 2: Break Down of Lactose to Lactic Acid

Lactose is water soluble, occurring as a molecular solution in milk.  In cheese-making most of the lactose remains dissolved in the whey.  Evaporation of whey in the manufacture of whey cheese increases the lactose concentration further.  Lactose is not as sweet as other sugars; it is 30 times less sweet that cane sugar, for example.
 

milk contains 4.6 - 4.7%.

It gives milk a slightly sweet taste and is soluble in water.

gives our body energy.

lactic acid bacteria and
produces the fresh sour taste in sour milk, yoghurt, cheese and other products.

a brownish colour and
a caramel taste.

The vitamins in milk can be divided into two groups, those soluble in fat and those soluble in water. Vitamins are organic compounds, whose presence in the organism is necessary for its normal functions. They cannot be produced by the organism itself, and therefore they must be taken in through the food. 

The fat soluble vitamins in milk are A, D, E, and K. Milk fat also contains carotene which can be changed to vitamin A. The B and C vitamins make up the water soluble vitamins. The B vitamin complex consists of a series of vitamins of which the following can be found in milk: Thiamin (B₁), Riboflavin (B₂), Niacin, Folic acid, Pyridoxin (B₆), Biotin, Kolin (B₁₂) and Inositol. 

The water soluble vitamins are partly destroyed by heat treatment. Among the fat soluble vitamins only vitamin A can be denatured in milk. This is done by oxidation. 

Vitamin A (37) 

1 glass of full-fat milk provides about 8 % of an adult's and about 10 % of child's daily vitamin A requirement. 

Deficiency of vitamin A can cause eye diseases.
 

Vitamin B₂ (38) 

1 glass of fresh full-fat milk provides about 12 % of a child's and of an adult's daily vitamin B₂ requirement.

Some vitamins are soluble in cream ( A, D, E, K ).

Other vitamins are soluble in skim milk ( B, C ) i.e. they are water soluble.

Milk contains a number of minerals. The total concentration is less than 1 %. 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. 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, while the amounts of other salts are correspondingly reduced. 

Calcium (40)

Calcium is necessary to build up children's teeth and bones and to maintain adult's bones. Calcium also serves many other functions in the body. Other calcium sources are fish (where the bones are eaten as well) and leafy vegetables. 

1 glass of milk provides about 40 % of an adult's and 30 % of a child's daily calcium requirement. 

White corpuscles (41-42) 

Milk always contains white blood corpuscles (leucocytes). The content is low in milk from a healthy udder, but increases if the udder is diseased - usually in proportion to the severity of the disease.

ash remains.

The ash contains minerals
which make up about 1% of the milk.

few white corpuscles in their milk.

many white corpuscles.

Do not use milk from diseased udders
for drinking or for making products.

The quality of milk is largely determined by the number and the types of bacteria present in the milk. 

Each bacterium consists of only one cell. Bacteria multiply by dividing into new cells. This process can be very rapid. Some bacteria need only twenty minutes to divide into two new bacteria. 

So, after forty minutes there may be four bacteria, after one hour eight, etc. After four hours, one single bacterium may have multiplied to four thousand! 

Milk is a very good medium for bacteria and in a relatively short time large numbers of bacteria can develop in milk if it is not properly cooled. Cooling of the milk prevents the growth of most types of bacteria to a considerable degree. If milk has to be stored for 12 - 18 hours only, cooling down to 8^oC is enough. If milk must be stored for one day or longer, cooling down to 4^oC is necessary. 

Types of bacteria

Lactic acid bacteria change milk sugar into lactic acid. They are found everywhere: in the air, the stable, on clothes and on milking equipment (especially if not properly cleaned). Milk is easily contaminated with these bacteria. 

Coli bacteria also change milk sugar, but not only into lactic acid. They also form gases. Coli bacteria are mostly found in faeces. 

Infectious bacteria are commonly found in soil and water, but also on milking equipment which is not properly cleaned. In the latter case they can infect milk. They affect the protein and sometimes the fat in milk, giving it a bad flavour. 

Butyric acid bacteria are commonly found in (bad quality) silage. By feeding this during milking, the milk will be contaminated. Therefore, silage should only be fed after milking is finished. The presence of butyric acid bacteria in milk causes problems if the milk is used for cheese making. 

Mastitis bacteria are present in milk when the udder of the cow is not healthy and has developed udder infection. 

On the farm, proper cleaning and disinfection of the milk equipment helps to prevent the development of a large number of the above mentioned bacteria. 

Note: Dust or dried cow dung may contain as many as 1,000,000,000 bacteria per gram!
 

Some bacteria cause useful changes

e.g. in making cheese, yoghurt
and other milk products.

and bad keeping quality.

-on dirty udders, hands and containers

-in diseased udders

at warm temperatures.

Try to slow the reproduction of bacteria by keeping your milk:

-cool
-clean.