PHYSICAL, CHEMICAL AND SPECTROSCOPIC ANALYSIS OF MILK SAMPLES COLLECTED FROM DIFFERENT AREAS
INTRODUCTION
Milk is whitish fluid secreted by female mammals for the purpose of feeding their offspring. The glands in which milk is secreted are called memory glands animals possessing such glands are classified as mammalian. It is formed four blood of the mother mammal, is a biological fluid of couple properties and composition. Milk is the only single flood known that is able to support life without any supplement.
Although the constituents of milk are deliver from those of the blood, yet they differ from then in chemical composition and physical properties. The casein lactose and fat present in milk are absent from blood.
The work “Milk” used without qualification is generally understand to mean cow’s milk. Other milks are usually qualified by the name of the animal from which they are obtained, e.g goat’s milk, sheep’s milk etc(1).
DEFINITION OF MILK
According to the milk ordinance and code of recommendation by the USA
“The natural secretion of the mammary glands of female mammals for the feeding of their young”
“The normal post part nutrition secretion of the healthy udder”
“It is an emulsion of liquid fat dispersed in water”
“It is the only material provided by nature to serve as a balanced food, mankind has used the, milk of various mammals(2)”
PROPERTIES OF MILK(3)
Physical properties of Milk
Milk is not only of a complex chemical nature, but due to the various constituents, its physical nature is also very complicated. Among the constituents of milk which in dispersion or true solution may be listed as lactose, a portion of minerals salts and possibly some of the lact-albumin. The proteins of milk are dispersed .The fat of milk is only constituent that occurs in the from of coarse dispersion.
Taste and odour of milk
Normal freshly drawn milk tastes slightly sweet to most people and has a characteristic although not pronounced odour. The pleasing flavor of milk may be correlated with high lactose and relbatively low chloride content.
The color of milk
Milk ranges in color from a bluish whit to a golden yellow, depending upon the animal, the kind of feed the amount of fat and solids present. The white color of milk is due to the reflection of light by the dispersed fat globules, the calcium caseinate and the colloidal phosphate.
Specific gravity and specific heat
Milk is a complex colloidal system in which the dispersion medium water contains salts and sugar in solution. It is therefore heavier than water. The specific gravity of milk is 1.032. Milk has heat of 0.920 at 32 F (0 C)
Boliling and freezing points of milk
Milk is slightly heavier than water, so it boils at temperature slightly above that of water. Milk boils at 212.3 F (100.17 C). Milk freeze at a temperature slightly lower than that of water, the average being 31.01 F
(-0.55 C)
Viscosity of milk
Milk is approximately 1.5 to 1.7 times more viscous, owing to the presence of solids in milk, some of which are in solution, other in suspension and still others in the form of emulsion.
Water
Milk contains a high percentage of water. In average milk, about 87% is water. This water is not different from ordinary water and serves to hold in solution the soluble constituent of milk. Water is the medium in which all of the other components of milk are dissolved or suspended. The percentage of water varies from 84 to 89%, although and individual sample of authentic milk may exceed that limit. Any variation in the amount of other constituents is also reflected upon the water percentage.
Fat
Milk fat often called “butterfat” is commercially, the most valuable constituent of milk. The agreeable flavor of rich milk, and large extent of other dairy product, is largely due to milk fat. Milk fat exists in milk in the form of minute globules in a true emulsion of the oil in water type, the fat globules being in the dispersed phase. The fat globules are invisible to the naked eye, but are seen readily under low power of microscope. It is estimated that single drop of average milk contained about 100,000,000 fat globules. Milk fat not a single chemical compounds but a variable mixture of several different triglycerides that makes up to 98 to 99% of milk fats. The remaining 1 to 3% of milk fat is composed phospholipids, steroids, carotenoides, fat soluble vitamin A, D, E, K and some traces of free fatty acids. The fat of the milk varies from 3.10% in human milk to 7% in buffalo milk. Milk fat is good source of retinal and vitamin D. Fat is one of the major components of milk and the most variable. It is influenced by nutrition and environment to a greater extent than the other milk components. There is more variation in the percentage of milk fat among animals within a species than in any other major milk constituent. There is also considerable variation in the percentage and composition of milk fat among species. Milk fat is composed primarily of triglycerides. Bovine milk fat, for instance, contains only 1% other lipids. The general formula for a triglyceride is shown in the figure in which three fatty acids , R-1, R-2 and R-3 are esterified to glycerol. The most striking difference between the ruminants and the non-ruminants milk fat is relatively high percentage of short chain fatty acids in ruminant milk fat. Human and pig milk fat also contain higher amounts of unsaturated fatty acids than that of ruminants. Milk fats from other non-ruminants have fatty acid compositions comparable to those of human.
Protein
In milk of average composition, the protein contains will be approximately 3.8% nearly all samples of normal milk would come between 2.8 to 4%. The protein of milk is not a single compound but includes two major proteins and small quantities of others. Casein constitutes about 80% in milk. A third protein recognized as present in milk lactglobulin probably about 0.05 to 0.07%. Casein is a yellowish white granular substance. It contributes to the whiteness of milk. In milk it is found in combination with calcium and calcium caseinate. Casein exists in milk in the form of small gelatinous particles in suspension. Lacto-albumin when dried is tasteless powder and has extremely complex chemical composition. The ratio of casein to albumin is nearly 1:1 in human milk as compared to 7:1 in animal milk. Cow, buffalo and goat milk contain nearly 3 times a much protein as human milk.
Carbohydrates
Milk sugar, commonly designated as “lactose” is a reducing disaccharide. It is six times less than sucrose. Milk contains on the average about 4.8% of lactose. Lactose has an important relation with the manufacturing of milk products, due to which it is decomposed by bacteria (lactic and bacilli) into lactic acid.
C12H22O11+H2O+BACTERIA_______4C3H6O3(LACTIC ACID)
Minerals matter
In milk, minerals % is about 0.7%. This ash is complicated mixture containing a number of metallic elements, in fact the same ones founds in the animals body. Milk ash is known to contain potassium, sodium, calcium, magnesium, chloride, phosphorus and sulfur in relatively large amounts. The major mineral constituents of cow’s milk are shown in table. The two most important minerals from a nutritional standpoints are calcium and phosphorus. Only 25% of the calcium, 20% of the magnesium and 44% of the phosphorus are in a soluble form where as, the total amount of other major constituents is in insoluble form. The calcium and magnesium in the insoluble form are in chemical or physical combination with caseinate, phosphate, and citrate. This provides a mechanism by which milk can contain a high concentration of calcium and at the same maintain normal osmotic equilibrium with the blood. The Buffering capacity of milk is due to its composition of citrates, phosphates, bicarbonates, and protein. The action of these substances keep the hydrogen concentration of milk close to a pH of 6.6. The calcium in milk comes from the diet as well as the skeleton. The blood serum calcium is in equilibrium with that of the skeleton. For this reason, it is difficult to increase the calcium content of milk by increasing the calcium content of the feed. The inorganic phosphate of the blood serum is the precursor of the phosphates in milk. There is considerable evidence to indicate that dairy animals free from infection have constant amounts of lactose, Potasium, Sodium and chlorine in their milk. The constant amount is characteristic of each individual animal and the molar sum of the Sodium and potassium contents. In addition, there is a close inverse relationship between the lactose content of milk and molar sum of the sodium and potassium contents of the milk. It appears that the water moves into milk to maintain osmotic equilibrium with that of blood. Consequently, the secretion of lactose, potassium, sodium and chlorine determines the volume of milk produced. The mechanisms that control the amount of these ions and the lactose that go into the milk have not been described. Milk also contains trace amounts of other minerals. On a quantitative basis however, there are relatively small amounts with the exception of zinc. Except for copper and iron, feeding high amounts of trace minerals increase the amount in cow’s and ewe’s milk. Only slight increase occur with zinc, but marked increase can occur in the iodine content of milk with feeding large amounts of iodine. Amount of mineral constituents of cow’s milk in table (1.2)
Trace elements
There are small amounts of iron, copper, zinc, aluminum manganese, cobalt and iodine, while traces of silicon, boron, titanium, vanadium, rubidium, lithium and strontium have also been reported in milk as presented in table (1.3)
The table (1.3) presents the trace mineral composition of milk; in addition aluminum, barium, chromium, lead nickel, radium, selenium, silicon, sliver, strontium, tin, titanium and vanadium can pass into milk. Most trace minerals are combined in organic complex with the SNF and fatty portion of the milk. Iron and copper appear to be portion bound on surfaces of fat globules; these two minerals are not altered by feeding. All other trace elements very with feeding of supplements. Iodine is markedly increased in milk when increased in feed.
Phospholipids
The principle phospholipids present in milk is lecithin, a fat like substance that contains nitrogen and phosphorus. Lecithin is closely associated with the fat, doubtless making a portion of the concentrated colloidal layer surrounding the fat.
Cholesterol
Milk has contained cholesterol. These authors report cholesterol as occurring in milk in direct proportion to the amount of fat present. They report milk as containing from 105 to 176 parts per million of cholesterol.
Gases
Milk contains from 7 to 10% by volume of gas. Carbon dioxide (CO2) is present in milk as it comes from the udder, while nitrogen oxygen, are taken up by the milk during milking. On standing the amount of gas becomes less. Gases may be formed n milk by certain bacteria. The detrimental effects of these gases, however are not apparent for several hours after milk is produced.
Non-protein nitrogenous substances
Milk contains a group of nitrogenous in addition to the proteins. These substances do not occur in large quantities but they are known to be present. The Non-protein nitrogenous substances of milk may be listed as follow:
Urea, nitrogen, amino nitrogen, uric acid, adenine and guanine. These milk substances are measured in ppm, ranging from 1.5 to 10 in milk.
An enzyme is biological catalyst elaborated by a living cell where as milk enzymes as being elaborated by the cell of mammary tissues. The enzymes produced by bacteria in milk.
Vitamins
Milk contains vitamin A,Vitamin B1 (thiamin) vitamin B12 (riboflavin), nicotinic acid (niacin) vitamin E (α-tocopherol), vitamin C (ascorbic acid) and vitamin K. The feed in known to be a factor influencing the amount of some vitamins in milk.
Enzymes in milk(5)
Milk contains a number of enzymes. An enzymes is biological catalyst collaborated by a living cell where milk enzymes are being elaborated by the cells of mammary tissues. The enzymes produced by the bacteria in milk can not be considered as normal components of milk. Enzymes are protein, they are inactivated by high temperature, and they posses a pH of optimum activity and exhibit specificity for certain substrate. There are different enzymes in milk, some are given as follow:
Lipase:
It is an enzymes which the hydrolysis of fats glycerol and fatty acid.
Esterase
It is an enzyme which catalyzes the hydrolysis of esters.
Alkaline phosphate
This enzyme is also called as alkaline phosphomonoesterase, catalyzes the hydrolsis of organic phosphate, yielding an alcohol or native enzymes in milk always present but destroyed by pasteurization.
Acid phosphates
It is reported in low concentration. It is found in skim milk and its optimum pH is about 4.0.
Lactoperxidase
It is an enzyme which catalyzes the transfer of oxygen from peroxide especially hydrogen peroxide to the other substances. All milk contains peroxides; it is a heme protein with an iron of about 0.007% and its optimum pH is 6.8.
Protease
It is an enzyme which catalyses hydrolysis of the peptide linkage of protein to produce smaller proteins fragments i.e peptones, proteases amino acids and ammonia.
Amylase
Amylase is an enzyme, which catalysis the hydrolysis of starch to dextrin or maltose. It catalyzes the hydrolysis of glucosidal linkage in starch molecule.
Ribonuclease
It is hydrolysis nucleic acid to its components nucleotides and it is present in the milk relatively in large amounts. Similarly other enzymes like lysozyme, aldolase, catalase, xanthineoxidase etc are present in milk perform their specific function.
Pigments in milk
Milk contains fat soluble and water soluble pigments which are largely the carotenes and riboflavin called earlier lacto-flavin. The yellow appearance of milk fat is caused by the fat soluble carotenes. Water soluble riboflavin produces a slight yellow or greenish tint in skim milk which becomes a distinct green fluorescence in whey removal of light reflecting components in cheese making.
FACTORS WHICH INFLUENCE THE COMPOSITION OF MILK(6)
Season
The most pronounced change is the sharp decrease in the percentage of total solids in the milk from May to June cows receiving exactly the same ratio throughout the year with only slight variation still exhibited this seasonal low late spring or early Summer. The solid fats are so constant throughout the year.
Age of the cow
The average fat percentage of the milk decreases with the age of the cow. A gradual, through irregular decrease in solid fat contents is also noted with age. Lactose and casein are the solid and these affected by the aging of the cows.
Weather conditions
In dry years, the total yield of milk tends to decrease together an increase in solid and increase in fat percentage. In wet years, there is a tendency for both solid and fat percentage to decrease without an increase in pounds of the milk depending upon the level of nutrition.
Exercise
The dairy cows are benefited by mild exercise. During a period of exercise feed consumption is increased while milk production is maintained and the percentage of fat is increased over a corresponding period of rest.
Stage of lactation
Consumption of milk varies considerably during lactation, with the major changes usually occurring soon after start of lactation. The consumption of colostrums secretion gradually changes to that of milk. Lactose increases in percentage during initial hour’s post-claving.
Breed of mammals
The consumption of milk also varies within the same species. Breed differentiate milk consumption, especially milk fat and protein have been the basis for the premium payments that dairy producers have received for quality of the milk they produce. Milk consumption also varies substantially among individuals of the same breed.
Complex physiochemical system
Milk is a very complex physiochemical system because of its consumption. It is a heterogeneous mixture of ions strong salts, ions of weak salts, the salts themselves, ampholytes such as protein and non-electrolyte that is fats and some minor components. The principle physiochemical to be considered are as,
Coagulation
Milk coagulates naturally through the action of the enzyme reins or because of the lactic acid produced by the lactose fermenting organisms. Heat is also a factor that causes coagulation. Two factors are involved in the coagulation of milk by heat. One is the chemical denaturation of proteins and other is the physical coagulation of the proteins. The formation of skin by milk when heated is phenomenon of coagulation.
Acidity of milk
Fresh is slightly acidic in nature having ph of 6.5 to 6.7.The production of acid in milk commonly termed “souring” and the sour taste of such taste of such milk in due to the presence of lactic acid that is caused by the action of bacteria which break down lactose to lactic acid.(2)
TYPES OF MILK
Following are different types of milk.
Whole Milk
It is commercially considered here as cows which consist on the average of 3.5% fat, 55% lactose, 3.5% protein and 7% ash. It has a bland slightly sweet flavor, a yellowish white color. It functions as a base for ice cream, yogurt, beverages and cheese. It is also source of milk, cream whey, casein, lactose and milk solid.
Flavored Milk
It is the whole milk that contains added flavoring material, such as chocolate, vanilla and fruit flavors. Chocolate milk in the most popular flavored milk. In milking this chocolate flavor it contains 1% coco and 6% sucrose added to the whole milk.
Low fat milk
In low fat milk, the fat content is reduced up to 1 to 2% milk fat. It is 99% fat free and containing only 1% of the fat.
Skim Milk
Removing the most or all milk fat from whole milk processes skim milk. Vitamin A and D may be used fortified. Skim milk in states, where it contains a maximum fat contents of 0.1 to 1.5% and minimum solid not fat content from 8.0 to 8.25%.
Low sodium Milk
Low sodium Milk is available in some areas where consumer requires a diet of low sodium food. In the processing procedure normal milk is passed over an ion-exchange resin in which the sodium of the milk is replaced by potassium ions. The normal sodium contents of milk are reduced from about 50mg of sodium per 100ml approximately 3mg per 100ml. The other components of milk remain essentially the same after processing.
Fermented milk is usually fluid or semi fluid in consistency and all contain lactic acid in varying proportions. Fermented milk are cultured dairy product made from skim, whole or slightly concentrated milk that require specific lactic acid, bacteria to develop their characteristic flavor and texture.
Condensed Milk
The sweetened condensed milk is defined as a liquid or semi liquid food made by the evaporating a mixture of sweet milk and refine sugar (sucrose) or any combination of refined sugar and refine cane sugar (dextrose) such a point that finished the sweetened product, which contain not less than 28% of the total milk solids and less than 8.5% of milk fat. The quantity of sucrose and dextrose be sufficient to prevent spoilage.
Dry Milk(7)
Dry milk is produced by the drying of milk, by removing the moisture contents to around 4% or less. The drying offers the conveniences of transportation, utility and stability. It contains not less than 26% fat and more than 5% mixture. It is used in dry mixtures such as pudding and desert mixtures.
There are three main methods of drying to obtain milk powder:
1. Freeze drying
Freeze drying is another but expensive alternative. Depending upon the intensity of heat treatment, powder is classification low, medium and high heat powders. Low heat powder used in all types of recommended milk products and baby milk, formulas, medium heat powder in used in all types of recombined condensed milk product and high heat powder is used in bakery and chocolate industry. The quality is determined as a measure of non-denature protein.
2. Roller drying
Milk is distributed on to rotating roller, the water evaporated and the layer of the dried milk is continuously removed by knives which scrap the dried milk from the rollers. This is ground into flakes subsequently into the powder.
3. Spray process
In the spray process fine streams of fluid milk, previously heat treated or pasteurized, are forced into a hot chamber through which a current of hot air in blown. Drying is almost instantaneous. Because the fat of dried milk becomes rancid so easily in the presence of customer to pack dried of milk in an atmosphere of nitrogen for commercial distribution. The drying of milk apparently produces little changes in its nutritive properties. For spray dried milk, the loss of vitamin B1 is approximately the same as for evaporated milk slightly greater than milk dried by the roller process. Except of some loss of vitamin C, the vitamin seen little affected by the processing. The treatment of milk incident to drying by the various processes results in a product that from small curds in the stomach. Dried whole milk contains moisture from 1.5 to 2.0%, ash from 5.8 to 6% protein from 32 to 36.5%, fat from 1 to 12%. Caloric values ranges from 3.25 to 3.62% calories to the also. Dry milk is used in baked products, ice-creams and biscuits flours. It is also useful for special diet from infant feeding.¹
TABLE (1.1)
Composition of milk of various animals(8)
Sr. No. | Constituents(gm) | Milk derived from | |||
Skimmed Milk | Cow milk | Goat milk | |||
1. | Moisture | 81 | 92.1 | 87.5 | 86.6 |
2. | Protein | 4.3 | 2.50 | 3.20 | 3.30 |
3. | Fat | 6.5 | 0.10 | 4.10 | 4.50 |
4. | Minerals | 0.8 | 0.70 | 0.80 | 0.80 |
5. | Carbohydrates | 5.0 | 4.60 | 4.40 | 4.60 |
6. | Energy(Kcal) | 117 | 29 | 67 | 72 |
7. | Calcium | 210 | 120 | 120 | 170 |
8. | Phosphorus | 130 | 90 | 90 | 120 |
9. | Iron | 0.20 | 0.20 | 020 | 0.30 |
TABLE (1.2)
Major minerals constituents of cow’s milk(9)
Mineral/Elements | %of total composition | % in soluble from |
Calcium | 0.12 | 25 |
Phosphorus | 0.10 | 44 |
Potassium | 0.15 | 100 |
Chlorine | 0.11 | 100 |
Magnesium | 0.01 | 20 |
Sodium | 0.05 | 100 |
TABLE (1.3)
Trace minerals composition in milk(11)
Minerals | Ppm |
Arsenic | 0.05 |
Boron | 0.2 |
Cobalt | 0.001 |
Copper | 0.13 |
Fluorine | 0.15 |
Iodine | 0.04 |
Iron | 0.45 |
Manganese | 0.03 |
Molybdenum | 0.05 |
Zinc | 3.7 |
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