MILK
The principal constituents of milk are water, fat, proteins, lactose (milk sugar) and minerals (salts). Milk also contains trace amounts of other substances such as pigments, enzymes, vitamins, phospholipids and gases.
The residue left when water and gases are removed is called the dry matter(DM) or total solids content of the milk.
Milk is defined as the lacteal secretion of the mammary glands of mammals which is obtain five days after calving's.
Milk is a complex biological fluid consisting of seven main components : water, fat, protein, sugar(lactose), minerals, vitamins and enzymes.
The color varies from white to yellow depending on the carotene content of the fat. Milk has a pleasant, slightly sweet taste, and pleasant odour. It is an excellent source of calcium, phosphates and riboflavin.
The capacity of milk is due to its content of suspended particles of fat, proteins and certain minerals.
Oxytocin hormone is responsible for emptying under of cow.
 |
COMPOSITION OF MILK
1-Milk fat
Milk and cream are examples of fat in water(or oil in water)emulsions. The milk fat exist as small globules or droplets dispersed in the milk serum. Their diameters range from 0.1 to 20 mm. The average size is 3-4 mm and there are some 15 million globules per ml.
Milk fat consist of triglycerides, di- and mono glycerides, fatty acids, sterols, carotenoids(the yellow color of fat), vitamins(A, D, E, and K)and all the others, trace elements, are minor components.
2- Proteins in milk
Proteins are essential part of our diet. The proteins we are eat are broken down into simpler compounds in the digestive system and in the liver. Milk contains hundred types of proteins, Most of them in very small amounts. The proteins can be classified in various ways according to their chemical or physical properties and their biological functions.
a- Casein_ Casein is a group name for the dominant class of proteins in milk. The casein easily from polymers containing polymers containing several identical or different types of molecules. Due to the abundance of ionisable hydrophobic and hydrophilic sites in the casein molecule, the molecular polymers formed by the caseins are very special.
b- Whey proteins_ Whey proteins is the name commonly applied to milk serum proteins. If the casein is removed from skimmed milk by some precipitation method, such as the addition of mineral acid, there remains in solution a group of proteins which are called milk serum proteins. As long as they are not denatured by heat, they are not precipitated at their isoelectric points.
whey proteins derivatives are widely used in the food industry.
* alpha lactalbumin
This protein may be considered to be the typical whey protein component of milk from cows.
* beta lactoglobulin
This protein is the major whey protein component of milk from cows.
3. Lactose or milk sugar
Is the distinctive carbohydrate of milk. It is a disaccharide composed of glucose and galactose. Lactose is a reducing sugar.
4. Mineral
Milk contains a number of minerals. The total concentration is less than 1%. Mineral salts occur in solution 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 increase and gives the milk a salty taste, while the amounts of other salts are correspondingly reduced.
5. Enzymes in milk
Enzymes are a group of proteins produced by living organisms. They have the ability to trigger chemical reactions and to affect the course and speed of such reactions. Enzymes do this without being consumed. Several of the enzymes in milk are utilized for quality testing and control. Among the more important ones are peroxidase, catalase, phosphatase and lipase.
PHYSICAL PROPERTIES OF MILK
1. Color and optical properties
Optical properties are influenced by the manner of scattering of light by the molecules. Light scattering occurs when the wavelength of light matches the magnitude of the particle. Thus, smaller particles scatter light of shorter wavelengths and vice versa.
Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more then the red.
Beta-carotene, the carotenoid precursor of vitamin A, is responsible for the creamy color of cow milk. The greenish tinge in whey is due to the presence of riboflavin.
Refractive index of milk is an optical property and ranges from 1.3440 to 1.3485 at 20 degree celsius.
2- Flavor of milk
The natural sweet flavor of milk is due to the combined effect of its components.
Off - flavors are very quickly developed in milk owing to several factors.
The feed consumed by animals may lead to some undesirable flavors.
Bacterial growth in milk causes fruity, barny, malty or acid flavors.
Enzyme activities also may lead to unnatural flavors, rancidity due to lipase action being a classic example.
Oxidative reactions may cause a cardboard flavor in milk. Processing of milk may produce cooked flavors.
3- Specific gravity and density
* Milk is heavier than water. The specific gravity of cow milk varies from 1.018 to 1.036 and of buffalo milk from 1.018 to 1.036 and of buffalo milk from 1.018 to 1.038. Though specific gravity varies with temperature and vice versa, (lower at higher temperature and vice versa), the rate of this variation is not uniform.
* The density of milk is related within the range of 1.027 to 1.033 kg/cm3 at 20 degree celsius. The density of milk is used to estimate the solids content, to convert volume into mass and vice versa and calculate other physical properties such as dynamic viscosity.
4- Viscosity
Viscosity of milk depends on the temperature and the and the amount and state of dispersion of the solid constituents, mainly casein ad fat. Viscosity of the whole milk at 25 degree celsius is about 20 cP.
5- Surface tension
The surface activity of milk is related to proteins, fat, phospholipids and fresh fatty acids present in it. Homogenization and heat sterilization increase the surface tension of milk. Milk has a surface tension of 50 dyne/cm at 20 degree celsius.
6- Freezing and boiling points if milk
The freezing points of cow and buffalo milk vary from -0.512 to 6.7 and from -0.521 to -0.575 degree celsius respectively. Freezing point of milk is mainly used to determine added to water. The boiling point of milk is 100.17 degree celsius.
7- Acidity and pH
Freshly drawn milk has a pH value in the range of 6.5 to 6.7nand contains 0.14 to 0.18% titratable acid and calculated as lactic acid. There is no development acidity in freshly drawn milk, the slightly lower in than the neutral pH being attributed to the presence of carbon dioxide, citrate, casein etc.
8- Heat stability of milk
Heat stability is defined as the length of time required to induce coagulation at a given temperature or the temperature required to induce coagulation in a given time.
Casein and salt balance of milk governs it heats stability. Added citrates, phosphates and calcium have a great impact the heat stability.
Comments
Post a Comment