MILK

Milk is the natural food for mammals in the first stages of life. It is the near-perfect single food containing protein, carbohydrate, fat, minerals and vitamins. However, it is lacking in iron, vitamin C and D. It is one of the most complete foods, because it contains the nutrients required for growth and development. Milk obtained from several species of animals is used for human consumption, but the most popular are cows, buffaloes and goats’ milk.

Composition

Components Cow’s milk Buffalo’s milk
Water 85-87% 80%
Carbohydrate 5% 5%
Protein 3% 4.5%
Fat 4% 9%
Vitamins and Minerals 1% 1.5%

However, the composition doesn’t always remain the same. This will vary slightly, depending upon the species, the age and the feed of the animal. It will also depend upon the lactation period and as animal advances in age, the fat content will also increase. Animals who feed on grass pastures produce a high yield of milk, richer in Vitamin A.

Nutritive Value
Protein
Some of the most complete forms of protein are found in milk. Casein is the main milk protein. It is insoluble in water and is very easily precipitated by acids. This results in the curdling of milk. Lactoglobulin and Lactalbumin are the other two proteins that are found in milk. They coagulate easily on heating and are responsible for the precipitate found at the bottom and the sides of the pan used for the heating of milk. They are not easily precipitated by acid.

Carbohydrates
The main carbohydrate found in milk is Lactose or milk sugar. Lactose is not very soluble in water and it is responsible for the sweet taste in milk. Lactose also tends to crystallize in milk powder during storage and results in lump formation and caking. It isn’t water soluble and is responsible for the sandy or gritty texture in condensed milk and ice-cream. In the souring of milk, lactic acid bacteria converts lactose into lactic acid, giving sour milk its characteristic flavour.

Fat
The fat constituent of milk is present in the form of cream. It is dispersed as fine globules. When heated, it rises to the surface, where it forms a layer. It contains fat-soluble vitamin A and D. Together with lactose, fat provides the energy in the diet.

Minerals and Vitamins
Milk is rich in calcium, phosphorus, sodium, potassium and all the vitamins known to be essential for human nutrition are present in milk. Milk is rich in Vitamin A, S, E and K.
Since milk contains a great deal of water, it is a very dilute food. Because of this, it is easy to consume and digest. Milk is an ideal food for all, especially infants and young children. There is no evidence that it is an indispensable constituent of the adult diet.

Processing
From the time it is milked from the animal to the time of sale, milk has to undergo processing to improve the keeping quality and to make it fit for consumption. The various stages are:
1. Collection
Milk is brought to the dairy in clean sterilized vessels, preferably stainless steel.
2. Holding tanks
The milk is immediately transferred to holding tanks and is held at 10°C to keep it safe. Cooling is done either in a tank, jacketed with pipes in which runs a brine solution. Else the milk is run over very cold water pipes.
3. Filtration
The milk is passed through a series screens and filters to remove sediment and floating particles.
4. Pasteurization
It is the process of heating milk to 63.7°C and holding it at that temperature for 30 minutes. This is known as the “Holder Process of Pasteurization”. Nowadays, the Flash Pasteurization is more commonly used. It is also called the HTST or High Temperature Short Time method, where the milk is heated to 71.6°C for only 15 seconds. Pasteurization makes milk safe for human consumption by destroying pathogenic germs (pathogens). It also helps to increase the shelf life. Flavour of the milk remains unaffected at pasteurization temperature.
5. Homogenization
At temperature of 60°C, milk is passed under high pressure through small opening of a machine called homogenizer. The main purpose is to subdivide the fat globules in milk and disperse them evenly in the entire mass. Fat has a low density and tends to rise to the surface during heating. Homogenization prevents this by first breaking up the fat into tiny particles and then dispersing them throughout the milk. This adds to the flavour and results in a better body.
6. Bottling
The bottles of selected and uniform size have to be sterilized by steam and hot water and then they are filled with milk, which are capped automatically. Nowadays milk is filled in plastic pouches and these are more economical, easily transported and save storage space. Plastic pouches are easily disposed and are safe to handle.
7. Sterilization
The sealed bottles are now heated for 30 to 40 minutes at temperatures ranging from 104-110°C in steam chambers called autoclaves and then allowed to cool. Milk can also be sterilized before bottling. It is subjected to temperatures of 135-150°C for just 1 second. This is called the UHT or Ultra Heat Treatment. This process kills off all microorganisms and the very short holding temperature reduces the changes in colour and lined with aluminum foil. Milk is then distributed through various outlets.

Effect of heat on Milk
Digestibility is slightly improved in the heated milk as it tends to form smaller, tenderer curds in the stomach than are formed from raw milk. Heating to higher temperatures than those used in pasteurization effects a greater variety of changes in milk. The tendency of milk to curdle is diminished by the use of low or moderate temperatures. Casein is not coagulated at the usual temperatures. It coagulates when heated at too high temperatures or for long period of time at boiling temperature.
The coagulation of milk protein by heat is accelerated by an increase in acidity. Heating milk brings about an increase in acidity. The addition of acid foods to milk may also result in coagulation.
The browning reaction, non-enzymatic browning or Millard reaction occurs when sugars and proteins are heated together. The prolonged heating of condensed milk in the can results in a product of brown colour, caramel flavour and thickened consistency.
Minerals like calcium phosphate is precipitated by heat and sometimes gets collected at the bottom of the pan with coagulated albumin and some of it gets entangled in the scum on the top surface of the milk.
Skin formation is one of the most troublesome of reactions that occurs during the heating of milk. It may be prevented by covering the pan, by diluting the milk, or by presence of fat floating on the surface. As the temperature is increased, a tough scum forms, which is insoluble and can be removed from the surface. The scum is composed of coagulated albumin with some precipitated salts and fat globules. Scum formation can be prevented by beating the cooked milk with a beater. The formation of scum on boiled milk is the principal reason for the behaviour of milk in boiling over. A certain amount of pressure develops under the scum, which later forces the scum upward and the milk flow over the sides of the pan i.e. spill.
The flavour of milk that has been heated to boiling temperatures or above is described as Flat. It is due to the loss of dissolved gases, carbon dioxide and oxygen. Some change that occurs in the protein also influences the flavour of the cooked milk.
The layer of fat that may form on milk that is boiled is the result of the breaking of the films of protein that surround the fat globules in unheated milk. The breaking of the films of emulsifying agent permits the firm coalescence of fat globules.
The scorching of milk placed over direct heat is due in part to the film of coagulated albumin that collects on the bottom and sides of the pan. Stirring the milk while heating lessens the thickness of the film, but may not prevent scorching, particularly if large quantity of milk is heated. Millard reaction is responsible for the brown colour of the scorched milk. Heating milk over hot water will retard or completely prevent scorching.

Acid coagulation of milk
Acid - either that which is formed by bacterial action in milk, or by added acid – precipitates casein as curd. The acid curdling the milk is a desired reaction in the making of some cheese and curd, but is undesirable in food products such as curdling of tomato soup, fruit cream and causes the casein to precipitate.
Rennin is an enzyme present in the stomach of man and animals which helps to clot milk during digestion. Commercially the enzyme is available in the form of rennet tablets or extract, which are prepared from the inner lining of the stomach of calves and lambs.
The action of vegetable enzyme – bromelin – from pineapple also clots milk, but later digests the clot. Other enzymes in fruits are responsible for some of the curdling action that occurs when milk or cream and certain fruits are combined. All fruits and vegetables contain some organic acids but not always in sufficient concentration to cause the curdling of milk. Destroying the enzymes before combining the fruit with the milk will prevent curdling caused by enzyme action. Curdling of milk may occur if tannin containing foods, such as potatoes are cooked in milk. Tannins are also present in brown sugar and in cocoa product.
Besides enzymes, acids and tannins, salts present in the milk, in the food combined with the milk, or added sodium chloride may also influence coagulation of the casein. Of the meats commonly cooked in milk, ham usually causes more coagulation than chicken, veal or pork. The high sodium content of ham may be responsible for the excessive curdling that occurs when cooked with milk.

Type of Milk

A. Fluid Milk
1. Whole milk
Milk that has none of its fat removed. It generally contains 3.25% fat. This fat is not homogenized so that the milk will have cream line.
2. Skimmed Milk
This type of milk became popular because of the emphasis on low-fat diet. Cream is separated by machine. Skimmed milk resembles fresh whole milk, except that the fat content is less than 0.5%, but rest of the nutrients remains the same, except for fat soluble vitamins. It is often fortified with vitamin D and minerals. Skimmed milk is usually pasteurized and may also be homogenized.
3. Homogenized Milk
It has the same composition as the whole fluid milk, but the fat content is dispersed throughout the mass through the process of halogenations. There won’t be any cream line when the milk is heated.
4. Toned Milk
Toning is done to make buffalo milk resemble in appearance and flavour to cow’s milk. It is done by dilution and addition of skimmed milk powder. 40 % of skimmed milk is added to 60% of buffalo milk. The addition of skimmed milk powder makes up for the dilution of the nutrients, the fat content remains diluted and equal to that of cow’s milk.
5. Fortified Milk
Additional nutrients such as Vitamin D may be added to milk to improve the quality.
6. Flavoured Milk
Addition of flavouring agents such as essences like vanilla, strawberry and masalas to improve the flavour, appearance and to add more variety.

B. Concentrated Milk
1. Evaporated Milk
This is unsweetened concentrated milk. After pasteurization. The milk is evaporated under reduced pressure in steam heated vacuum pans, till the volume has been reduced to 60%. It is then homogenized, cooled and canned.
2. Condensed Milk
The term usually implies whole milk, which has been partially evaporated and to which sugar has been added. This is sweetened concentrated milk. 27% moisture remains out of 80%. It has a 62% sugar concentration. The milk is cooled very carefully, so that the crystallization of the lactose is in very fine crystals before being canned.
3. Dried Milk or Milk Powder
In this case, the water content is removed completely and thereby the bulk is reduced. It requires no special storage facilities and is an outlet for the skimmed milk from cream and butter production. There are two processes for drying milk:
a. Roller Drying
The milk is usually concentrated in an evaporator until it has 97% total solids as against 11.5% in normal fluid milk. The milk is then fed into the very smooth surface of single or twin heated drums, operating at temperatures of 150°C (303°F). The water content of the milk evaporates very quickly, leaving behind a thin film of dried milk, which is then scraped off by a blade that is attached to the drums. This is then cooled before being ground into a very fine powder. It is then sieved and packed into airtight containers. Milk powder is hydroscopic and will quickly absorb moisture from the air and the powder will deteriorate, becoming stale in flavour and less soluble in reconstitution. Although roller drying is cheaper and produces a product that is practically free from bacteria owing to the severe heat treatment it receives, it does develop a cooked flavour and will not reconstitute easily. The structure of the fat globules in milk is destroyed, causing an oily layer on the surface when reconstituted in warm water.
b. Spray Drying
The milk is pre-heated to 80-90°C for 10 seconds. It is homogenized and concentrated at 43°C to about 40% total solids. The milk is then atomized to a fine mist in a drying chamber containing hot air at 165°C (330°F). The very minute milk particles give off their moisture content almost instantaneously and drop to the bottom of the drying chamber as tiny grains of dried milk. The powder is removed to cool as quickly as possible, before being packed into airtight containers. This method is more expensive than Roller drying, but has 98% solubility with less pronounced flavour changes than Roller dried milk.
c. Instant Milk Powder
This type of powder will dissolve easily and completely when added to water, tea or coffee. The process is a modified spray-drying method in which the dried milk powder is moistened with steam to give a lumpy porous structure; the lumps of powder are being dried, cooled and then reduced to a more standard size. When added to a liquid, they quickly absorb it and dissolve completely. The cost of this type of milk powder is high and it is mainly used in hot beverages

The concentration of milk serves three main purposes:
1. It reduces bulk by the reduction of the water content.
2. The heat treatment and high sugar content in condensed milk gives the product a good shelf life.
3. It is convenient outlet for skimmed milk from the production of butter and cream.

Substitute milk powder
It doesn’t contain any milk constituent at all, but are especially processed from products such as dried glucose, syrup and vegetable fat.

Imitation milk
Also known as Substitute milk, it is a recent development. The products don’t contain any milk constituent, but contain glucose syrup and vegetable fat. It has a very low calorific value. Another substitute being promoted as a healthier alternative to milk is Soya milk. Soya milk is much cheaper, but leaves an unpleasant after-taste and flavour. Imitation and substitute milk are ideal for those who are lactose intolerant i.e. allergic to milk.

C. Cultured Milk
The process consists of deliberately souring the milk by adding specific harmless bacteria, which will then produce an acid and so control the growth of possible harmful bacteria. The process originated by man’s attempts to prevent milk from being totally unusable and develops into a variety of products, which are nutritious and easily digestible. Cultured milk can be classified into three groups, as listed:
1. Butter Culture
These include butter-milk and cultured cream.
Butter-milk is a byproduct from the manufacture of butter. It is made from pasteurized skimmed milk, which is incubated with an acid producing butter culture.
Cultured cream is prepared from pasteurized single cream, which is incubated with an acid producing butter culture. It is often marketed as “fresh soured cream” to distinguish it clearly from cream, which has soured by accident or carelessness. It may be used in recipes instead of sour cream.
2. Yogurt Culture
These include yogurt (in many forms and flavours), curdled milk and soured milk drinks. Yogurt can be made from whole, skimmed, evaporated or dried milk or a mixture of these. Various types of yogurt are available – natural yogurt made from milk products, natural yogurt sweetened with sugar, fruit yogurt, which may be flavoured with pieces of fruits or just fruit juice or a flavouring essence.
3. Weak alcoholic beverages
This group differs from the previous groups in that yeast cultures, which produce alcohol, are added, in addition to bacterial cultures. This group includes:
a. Kefir (Poland)
It is made from whole or skimmed cow’s milk to which the kefir grain is added. The milk is incubated for three days, during which the alcohol and acid percentage increases. The finished product resembles single cream in appearance and has a lactic acid flavour.
b. Koumiss/Kumis (Russia)
It was originally made from mare’s milk and is now produced from cow’s milk. Two cultures are added to milk and is incubated at 37°C for about 4-6 hours and cooled to 30°C and yeast added and incubated for 4 hours at 25°C. Matured for 3 days during which the alcoholic and acid percentage increases. The product resembles single cream in appearance, but is slightly grey in colour and has smell gas bubbles throughout, sour in taste and has a faint alcoholic smell.
c. Whey Champagne (Poland)
This is made from clarified way with the addition of yeast and caramel. The product is of a clear, light amber colour with a slight sparkle to it and a faint caramel flavour.
d. Feliwsoka (Poland)
It is made from buttermilk, which is fermented at 15-18°C for 8-10 hours, until the required acidity is reached. The liquid that has separated to the top is removed and sugar and yeast are added. The buttermilk is then bottled, sealed and kept at 18-20°C for 4 hours, cooled below 8°C and stored. The final product appears foamy due to carbon dioxide production and has a sweet, yeasty and refreshing taste.

Uses of Milk
1. Served as a refreshing drink – both hot and cold.
2. Served as a beverage with tea and coffee.
3. The main ingredient in some sauces like Béchamel.
4. Used to enrich dishes like soups and to give a smooth texture.
5. The main ingredient in several desserts like ice-cream, puddings and custards.
6. The first food for new born infants.
7. Acts as a cooking medium for Indian sweets like Kheer or Payasams.

Storage of Milk
The rich nutritional composition of milk makes it readily infected with micro-organisms, which may be derived from the animal, the equipment or the people handling the milk. Fresh milk should be purchased daily. Old and new milk should never be mixed. Leftover milk can be easily converted into curds. Fresh milk should be stored at refrigerated temperatures of 2°C after boiling and cooling. Milk should never be stored near strong smelling foods such as cut onions and peeled garlic as milk readily absorbs flavours and odour. Milk must never be exposed to direct sunlight as riboflavin is very easily lost. UTH and canned (evaporated and condensed) milk must be stored at slightly lower than room temperatures. When opened, they must be refrigerated immediately and used within two days. Dried milk must be stored in airtight containers in a cool and dry area of the storeroom.